Pedigrees Sem 1 Genetics

1. What is the likely mode of inheritance for the pedigree shown below?

Autosomal recessive

Autosomal dominant

X linked recessive

X linked dominant

Y linked

The likely mode of inheritance for the pedigree shown below is autosomal dominant. This can be inferred because the trait is present in every generation, affecting both males and females. Autosomal dominant inheritance means that an affected individual has a 50% chance of passing the trait onto their offspring.

Explanation

The likely mode of inheritance for the pedigree shown below is autosomal dominant. This can be inferred because the trait is present in every generation, affecting both males and females. Autosomal dominant inheritance means that an affected individual has a 50% chance of passing the trait onto their offspring.

2. The pedigree below is best explained by which of the following inheritance patterns?

Autosomal recessive

Autosomal dominant

X linked recessive

X linked dominant

Y linked

The pedigree below is best explained by the autosomal recessive inheritance pattern because the trait is present in both males and females, skipping generations, and both parents of an affected individual are unaffected carriers. This pattern indicates that the trait is caused by a recessive allele on an autosome, and individuals need to inherit two copies of the allele (one from each parent) to express the trait.

Explanation

The pedigree below is best explained by the autosomal recessive inheritance pattern because the trait is present in both males and females, skipping generations, and both parents of an affected individual are unaffected carriers. This pattern indicates that the trait is caused by a recessive allele on an autosome, and individuals need to inherit two copies of the allele (one from each parent) to express the trait.

3. Parents of a child with an autosomal recessive disease are

Likely to have mutations in different genes, locus heterogeneity

Likely to have more affected sons than daughters

Obligatory carriers

Most likely homozygous normal and the disease was caused by a new mutation

Not at risk of having another child with the disease

Parents of a child with an autosomal recessive disease are likely to be obligatory carriers. This means that both parents are carriers of a single copy of the mutated gene responsible for the disease, but they do not show any symptoms themselves. When both parents are carriers, there is a 25% chance with each pregnancy that their child will inherit two copies of the mutated gene and develop the disease. Therefore, the parents are obligated to carry the gene and pass it on to their children.

Explanation

Parents of a child with an autosomal recessive disease are likely to be obligatory carriers. This means that both parents are carriers of a single copy of the mutated gene responsible for the disease, but they do not show any symptoms themselves. When both parents are carriers, there is a 25% chance with each pregnancy that their child will inherit two copies of the mutated gene and develop the disease. Therefore, the parents are obligated to carry the gene and pass it on to their children.

4. In the below family, a child has been born with acheiropodia (congenital absence of hands and feet). Assuming that this is a genetic problem, what is the MOST LIKELY inheritance pattern and how LIKELY is it that a next child of III3 and III4 will be affected?

X linked recessive; 1 in 2 for a son and 1 in 4 for a daughter

Autosomal recessive; 1 in 2

Autosomal dominant; 1 in 2

Autosomal recessive; 1 in 4

Mitochondrial; 1 in 2

The correct answer is Autosomal recessive; 1 in 4. This means that the condition is caused by a recessive gene on an autosome (non-sex chromosome). Both parents must be carriers of the gene in order for their child to be affected. In this case, III3 and III4 are both carriers, so there is a 1 in 4 chance that their next child will be affected by the condition.

Explanation

The correct answer is Autosomal recessive; 1 in 4. This means that the condition is caused by a recessive gene on an autosome (non-sex chromosome). Both parents must be carriers of the gene in order for their child to be affected. In this case, III3 and III4 are both carriers, so there is a 1 in 4 chance that their next child will be affected by the condition.

5. If an affected male mates with a phenotypically normal female and has both an affected male and an affected female offspring, the disease is MOST LIKELY inherited as

Autosomal dominant

X linked recessive

X linked dominant

Y linked

Autosomal recessive

If an affected male mates with a phenotypically normal female and has both an affected male and an affected female offspring, the disease is most likely inherited as autosomal dominant. This is because autosomal dominant inheritance requires only one copy of the mutated gene to be present for the disease to be expressed. Since both the affected male and the affected female offspring have the disease, it suggests that the mutated gene is present in one of the autosomal chromosomes, which can be inherited from either parent.

Explanation

If an affected male mates with a phenotypically normal female and has both an affected male and an affected female offspring, the disease is most likely inherited as autosomal dominant. This is because autosomal dominant inheritance requires only one copy of the mutated gene to be present for the disease to be expressed. Since both the affected male and the affected female offspring have the disease, it suggests that the mutated gene is present in one of the autosomal chromosomes, which can be inherited from either parent.

6. A normal male mates with a female who is homozygous for an x‑linked recessive disorder. Of the offspring that they may have, it is probable that

No sons will be affected, and all daughters will be carriers

All sons will be affected, and all daughters will be affected

All sons will be affected, and all daughters will be carriers

No sons will be affected, and all daughters will be affected

All sons will be affected, and 1/2 the daughters will be carriers

If a normal male mates with a female who is homozygous for an x-linked recessive disorder, the male offspring will inherit one X chromosome from the mother and one Y chromosome from the father. Since the disorder is recessive and located on the X chromosome, the male offspring will be affected because they do not have another X chromosome to mask the disorder. On the other hand, the female offspring will inherit one X chromosome from the mother and one X chromosome from the father. They will be carriers of the disorder because they have one normal X chromosome to mask the effects of the disorder. Therefore, all sons will be affected, and all daughters will be carriers.

Explanation

If a normal male mates with a female who is homozygous for an x-linked recessive disorder, the male offspring will inherit one X chromosome from the mother and one Y chromosome from the father. Since the disorder is recessive and located on the X chromosome, the male offspring will be affected because they do not have another X chromosome to mask the disorder. On the other hand, the female offspring will inherit one X chromosome from the mother and one X chromosome from the father. They will be carriers of the disorder because they have one normal X chromosome to mask the effects of the disorder. Therefore, all sons will be affected, and all daughters will be carriers.

7. In the following pedigree, which is the MOST LIKELY inheritance pattern of the disease?

X-linked dominant inheritance

Autosomal dominant inheritance

X-linked recessive inheritance

Autosomal recessive inheritance

Multifactorial inheritance

The most likely inheritance pattern of the disease in the given pedigree is autosomal recessive inheritance. This is because the disease appears to be present in multiple generations, which suggests that it is not X-linked dominant or X-linked recessive, as these patterns would typically skip generations. Additionally, the disease does not appear to affect every individual in each generation, which rules out autosomal dominant inheritance. Lastly, there is no evidence to suggest that the disease is multifactorial, which is typically influenced by both genetic and environmental factors.

Explanation

The most likely inheritance pattern of the disease in the given pedigree is autosomal recessive inheritance. This is because the disease appears to be present in multiple generations, which suggests that it is not X-linked dominant or X-linked recessive, as these patterns would typically skip generations. Additionally, the disease does not appear to affect every individual in each generation, which rules out autosomal dominant inheritance. Lastly, there is no evidence to suggest that the disease is multifactorial, which is typically influenced by both genetic and environmental factors.

8. The pedigree below is BEST explained by which of the following inheritance patterns?

Autosomal recessive

Autosomal dominant

X linked recessive

X linked dominant

Y linked

The given pedigree is best explained by autosomal dominant inheritance pattern because the trait is present in every generation, affecting both males and females. Autosomal dominant inheritance means that an individual only needs to inherit one copy of the mutated gene from either parent to express the trait. This is supported by the presence of affected individuals in every generation, as well as the lack of skipping generations. The trait does not follow an X-linked or Y-linked pattern, as it affects both males and females equally.

Explanation

The given pedigree is best explained by autosomal dominant inheritance pattern because the trait is present in every generation, affecting both males and females. Autosomal dominant inheritance means that an individual only needs to inherit one copy of the mutated gene from either parent to express the trait. This is supported by the presence of affected individuals in every generation, as well as the lack of skipping generations. The trait does not follow an X-linked or Y-linked pattern, as it affects both males and females equally.

9. What is the probability that an offspring of an affected person and an unaffected person with an autosomal dominant disorder will be affected?

1/3

1/4

2/3

1/2

Virtually 0

The probability that an offspring of an affected person and an unaffected person with an autosomal dominant disorder will be affected is 1/2. This is because in an autosomal dominant disorder, only one copy of the mutated gene is needed to inherit the disorder. The affected person has one copy of the mutated gene, while the unaffected person does not have any copies. Therefore, when they have an offspring, there is a 50% chance that the offspring will inherit the mutated gene and be affected by the disorder.

Explanation

The probability that an offspring of an affected person and an unaffected person with an autosomal dominant disorder will be affected is 1/2. This is because in an autosomal dominant disorder, only one copy of the mutated gene is needed to inherit the disorder. The affected person has one copy of the mutated gene, while the unaffected person does not have any copies. Therefore, when they have an offspring, there is a 50% chance that the offspring will inherit the mutated gene and be affected by the disorder.

10. What is the probability of having an affected child if one parent is affected with an autosomal recessive disease and the other parent is a carrier?

1/2

2/3

Pretty close to 100% because new mutations are frequent

1/4

Virtually zero

When one parent is affected with an autosomal recessive disease and the other parent is a carrier, there is a 50% chance that their child will inherit the affected gene from the affected parent. This is because the affected parent must have two copies of the affected gene, one of which they will pass on to their child. The carrier parent has one copy of the affected gene, which they can also pass on. Therefore, there is a 1 in 2 chance that the child will inherit the affected gene from either parent, resulting in a probability of 1/2 for having an affected child.

Explanation

When one parent is affected with an autosomal recessive disease and the other parent is a carrier, there is a 50% chance that their child will inherit the affected gene from the affected parent. This is because the affected parent must have two copies of the affected gene, one of which they will pass on to their child. The carrier parent has one copy of the affected gene, which they can also pass on. Therefore, there is a 1 in 2 chance that the child will inherit the affected gene from either parent, resulting in a probability of 1/2 for having an affected child.

11. If one parent has an autosomal dominant disorder and is a heterozygote, what are the chances that they will have an affected offspring?

1/4 X 1/4 X 1/4

Pretty close to 100% because new mutations are frequent

1/4

50% if the other parent is normal

Probability of the unaffected parent being a carrier X 1/4

If one parent has an autosomal dominant disorder and is a heterozygote, there is a 50% chance that they will pass on the affected allele to their offspring. This is because autosomal dominant disorders only require one copy of the affected allele to be present for the disorder to be expressed. Therefore, if the other parent is normal and does not carry the affected allele, there is a 50% chance that the offspring will inherit the affected allele and be affected by the disorder.

Explanation

If one parent has an autosomal dominant disorder and is a heterozygote, there is a 50% chance that they will pass on the affected allele to their offspring. This is because autosomal dominant disorders only require one copy of the affected allele to be present for the disorder to be expressed. Therefore, if the other parent is normal and does not carry the affected allele, there is a 50% chance that the offspring will inherit the affected allele and be affected by the disorder.

12. A normal male mates with a female who is carrier for an x‑linked recessive disorder. Of the offspring that they may have, it is probable that

All sons will be affected, and all daughters will be carriers

No sons will be affected, and all daughters will be affected

1/2 the sons will be affected, and all daughters will be carriers

All sons will be affected, and all daughters will be affected

1/2 the sons will be affected, and 1/2 the daughters will be carriers

When a normal male mates with a female who is a carrier for an X-linked recessive disorder, there is a 50% chance that a son will inherit the disorder. This is because the son has a 50% chance of receiving the affected X chromosome from the carrier mother. Similarly, there is also a 50% chance that a daughter will be a carrier for the disorder. This is because the daughter has a 50% chance of receiving the affected X chromosome from the carrier mother. Therefore, 1/2 the sons will be affected, and 1/2 the daughters will be carriers.

Explanation

When a normal male mates with a female who is a carrier for an X-linked recessive disorder, there is a 50% chance that a son will inherit the disorder. This is because the son has a 50% chance of receiving the affected X chromosome from the carrier mother. Similarly, there is also a 50% chance that a daughter will be a carrier for the disorder. This is because the daughter has a 50% chance of receiving the affected X chromosome from the carrier mother. Therefore, 1/2 the sons will be affected, and 1/2 the daughters will be carriers.

13. II-3 in the below family has two brothers and three sons with classical hemophilia (factor vIII deficiency). Now she is pregnant again. How likely is it that this child will also have hemophilia?

100% for a son and 50% for a daughter

100% for a son, zero for a daughter

50% for a son, zero for a daughter

50% for both sons and daughters

25% for a son and zero for a daughter

Based on the information given, II-3 is a carrier of the hemophilia gene since she has two brothers and three sons with hemophilia. Hemophilia is an X-linked recessive disorder, meaning it is carried on the X chromosome. Since II-3 is a carrier, there is a 50% chance that she will pass on the hemophilia gene to her son (who would then have hemophilia), and a 0% chance that she will pass it on to her daughter (who would be a carrier like her). Therefore, the likelihood of the child having hemophilia depends on their gender, with a 50% chance for a son and zero chance for a daughter.

Explanation

Based on the information given, II-3 is a carrier of the hemophilia gene since she has two brothers and three sons with hemophilia. Hemophilia is an X-linked recessive disorder, meaning it is carried on the X chromosome. Since II-3 is a carrier, there is a 50% chance that she will pass on the hemophilia gene to her son (who would then have hemophilia), and a 0% chance that she will pass it on to her daughter (who would be a carrier like her). Therefore, the likelihood of the child having hemophilia depends on their gender, with a 50% chance for a son and zero chance for a daughter.

14. If your mother was known to be a carrier of alpha‑antitrypsin deficiency, an autosomal recessive disorder, what is the probability that your child is also a carrier of this disease gene?

1/4

100%

2/3

1/2

Virtually 0

If your mother is a carrier of alpha-antitrypsin deficiency, an autosomal recessive disorder, it means that she has one normal copy of the gene and one mutated copy. In order for your child to be a carrier, they would need to inherit the mutated copy from both you and your partner. Since you are not known to be a carrier, the probability of you passing on the mutated copy is 1/2. Therefore, the probability that your child is also a carrier is 1/2 multiplied by 1/2, which equals 1/4.

Explanation

If your mother is a carrier of alpha-antitrypsin deficiency, an autosomal recessive disorder, it means that she has one normal copy of the gene and one mutated copy. In order for your child to be a carrier, they would need to inherit the mutated copy from both you and your partner. Since you are not known to be a carrier, the probability of you passing on the mutated copy is 1/2. Therefore, the probability that your child is also a carrier is 1/2 multiplied by 1/2, which equals 1/4.

15. If an affected male mates with a normal female and has four offspring: one normal male, one affected male, and two affected female, the disease is MOST LIKELY inherited as

X linked dominant

X linked recessive

Autosomal dominant

Autosomal recessive

Y linked

In this scenario, the disease is most likely inherited as autosomal dominant because both males and females are affected, and the affected male can pass on the disease to both male and female offspring. X-linked dominant inheritance would only occur if the affected male passes the disease to all of his daughters but none of his sons, which is not the case here. X-linked recessive inheritance would result in only males being affected, and Y-linked inheritance would only affect males and be passed exclusively from father to son, which is also not the case here.

Explanation

In this scenario, the disease is most likely inherited as autosomal dominant because both males and females are affected, and the affected male can pass on the disease to both male and female offspring. X-linked dominant inheritance would only occur if the affected male passes the disease to all of his daughters but none of his sons, which is not the case here. X-linked recessive inheritance would result in only males being affected, and Y-linked inheritance would only affect males and be passed exclusively from father to son, which is also not the case here.

16. Now assume that the unaffected sister of an albino asks you how likely it is that her child with a healthy unrelated man will be an albino. Her parents are both unaffected. Assume again a carrier frequency of 2% in the general population.

1 in 800

1 in 75

1 in 300

1 in 50

1 in 150

The answer of 1 in 300 suggests that there is a 1 in 300 chance that the child of the unaffected sister and the healthy man will be an albino. This is based on the assumption that the carrier frequency of the albino gene in the general population is 2%. Since both of the unaffected sister's parents are also unaffected, it is less likely for her to carry the albino gene. Therefore, the likelihood of her child being an albino is lower compared to other possibilities given in the options.

Explanation

The answer of 1 in 300 suggests that there is a 1 in 300 chance that the child of the unaffected sister and the healthy man will be an albino. This is based on the assumption that the carrier frequency of the albino gene in the general population is 2%. Since both of the unaffected sister's parents are also unaffected, it is less likely for her to carry the albino gene. Therefore, the likelihood of her child being an albino is lower compared to other possibilities given in the options.

17. Albinism is a harmless autosomal recessive trait. If we assume that 2% of the general population are carriers of the albinism gene, how likely is it that the child of an albino with a healthy unrelated partner is an albino?

1 in 4

1 in 200

1 in 50

1 in 100

1 in 25

If 2% of the general population are carriers of the albinism gene, then the probability of the albino parent passing on the gene to their child is 2%. Since albinism is an autosomal recessive trait, the child can only be albino if they receive the albinism gene from both parents. Therefore, the probability of the child being albino is 2% multiplied by 2%, which is 0.04% or 1 in 100.

Explanation

If 2% of the general population are carriers of the albinism gene, then the probability of the albino parent passing on the gene to their child is 2%. Since albinism is an autosomal recessive trait, the child can only be albino if they receive the albinism gene from both parents. Therefore, the probability of the child being albino is 2% multiplied by 2%, which is 0.04% or 1 in 100.

18. In the following pedigree, you see a family with two people affected by an X-linked recessive disease characteristized by immunodysregulation, polyendocrinopathy, and enteropathy. What is the best estimate of the risk that the unborn child III-2 will also have this disease?

1/2

2/3

1/8

1/3

1/4

Based on the given information that the disease is X-linked recessive, the best estimate of the risk that the unborn child III-2 will have this disease is 1/4. This is because the mother (II-2) is a carrier of the disease, indicated by her being unaffected but having an affected son (III-1). The father (II-1) is unaffected, therefore he does not carry the disease-causing gene. The chance of the unborn child (III-2) inheriting the disease-causing gene from the mother is 1/2, and the chance of the child being male is also 1/2. Therefore, the overall risk is 1/2 * 1/2 = 1/4.

Explanation

Based on the given information that the disease is X-linked recessive, the best estimate of the risk that the unborn child III-2 will have this disease is 1/4. This is because the mother (II-2) is a carrier of the disease, indicated by her being unaffected but having an affected son (III-1). The father (II-1) is unaffected, therefore he does not carry the disease-causing gene. The chance of the unborn child (III-2) inheriting the disease-causing gene from the mother is 1/2, and the chance of the child being male is also 1/2. Therefore, the overall risk is 1/2 * 1/2 = 1/4.

19. II3 in the pedigree below has two brothers with hemophilia A, a bleeding disorder that is inherited as an X‑linked recessive trait. What is the risk of hemophilia for her children?

1 in 4 for a son, close to zero for a daughter

1 in 2 both for sons and daughters

1 in 2 for a son and 1 in 4 for a daughter

1 in 2 for a son, close to zero for a daughter

1 in 4 both for sons and daughters

The risk of hemophilia for II3's children is 1 in 4 for a son, close to zero for a daughter. This is because hemophilia A is an X-linked recessive trait, meaning it is carried on the X chromosome. Since II3 is a carrier of the hemophilia gene, she has a 50% chance of passing it on to her children. If she has a son, he has a 50% chance of inheriting the gene and having hemophilia. However, if she has a daughter, she would need to pass on the gene to both of her X chromosomes, which is highly unlikely. Therefore, the risk for a daughter to have hemophilia is close to zero.

Explanation

The risk of hemophilia for II3's children is 1 in 4 for a son, close to zero for a daughter. This is because hemophilia A is an X-linked recessive trait, meaning it is carried on the X chromosome. Since II3 is a carrier of the hemophilia gene, she has a 50% chance of passing it on to her children. If she has a son, he has a 50% chance of inheriting the gene and having hemophilia. However, if she has a daughter, she would need to pass on the gene to both of her X chromosomes, which is highly unlikely. Therefore, the risk for a daughter to have hemophilia is close to zero.

20. A woman (I‑1) is deaf from an autosomal recessive disease. She marries a hearing man and has four children, two of the four children are deaf at an early age. Genomic DNA was isolated from peripheral blood lymphocytes from all family members and subjected to Southern blot analysis using a radiolabeled DNA probe known to be closely linked to the disease gene. Which of the following statements is TRUE about the offspring in generation II?

II 1 will be a carrier and II 3 will be normal

II 1 will be normal and II 3 will be a carrier

Both II 1 and II 3 will be carriers

Both II 1 and II 3 will be normal

II 1 will be affected and II 3 will be normal

The Southern blot analysis using a radiolabeled DNA probe revealed that both II 1 and II 3 are carriers of the autosomal recessive disease gene. This means that they have one copy of the disease gene but do not exhibit any symptoms. The fact that II 1 and II 3 are carriers suggests that they inherited the disease gene from their mother, who is deaf due to the autosomal recessive disease. It is possible that they inherited the normal gene from their father, who is hearing. Therefore, both II 1 and II 3 will be carriers of the disease gene.

Explanation

The Southern blot analysis using a radiolabeled DNA probe revealed that both II 1 and II 3 are carriers of the autosomal recessive disease gene. This means that they have one copy of the disease gene but do not exhibit any symptoms. The fact that II 1 and II 3 are carriers suggests that they inherited the disease gene from their mother, who is deaf due to the autosomal recessive disease. It is possible that they inherited the normal gene from their father, who is hearing. Therefore, both II 1 and II 3 will be carriers of the disease gene.

21. What is the probability of having an affected child if one parent is affected with an autosomal recessive disease and the other parent is unaffected?

1/4

1/4 X 1/4 X 1/4

Probability of the unaffected parent being a carrier X 1/2

Pretty close to 100% because new mutations are frequent

50% if the other parent is normal

If one parent is affected with an autosomal recessive disease and the other parent is unaffected, the unaffected parent can still be a carrier of the disease. The probability of the unaffected parent being a carrier is represented by the term "Probability of the unaffected parent being a carrier". Since it is an autosomal recessive disease, both parents need to be carriers in order for the child to have a chance of being affected. Therefore, the probability of having an affected child is the probability of the unaffected parent being a carrier multiplied by 1/2, as stated in the answer.

Explanation

If one parent is affected with an autosomal recessive disease and the other parent is unaffected, the unaffected parent can still be a carrier of the disease. The probability of the unaffected parent being a carrier is represented by the term "Probability of the unaffected parent being a carrier". Since it is an autosomal recessive disease, both parents need to be carriers in order for the child to have a chance of being affected. Therefore, the probability of having an affected child is the probability of the unaffected parent being a carrier multiplied by 1/2, as stated in the answer.

22. No matter which answers you gave for Q's 33 and 34, now assume that this pedigree is for nephrolithiasis, X-linked recessive, with renal failure. Person II-4 marries an unrelated man; what is her risk of having an affected baby?

1 in 6

1 in 2

1 in 16

1 in 8

1 in 4

Person II-4 is a carrier for the X-linked recessive nephrolithiasis gene. Since she is a carrier, there is a 50% chance that she will pass the affected gene to her child. Therefore, her risk of having an affected baby is 1 in 2, or 50%.

Explanation

Person II-4 is a carrier for the X-linked recessive nephrolithiasis gene. Since she is a carrier, there is a 50% chance that she will pass the affected gene to her child. Therefore, her risk of having an affected baby is 1 in 2, or 50%.

23. Given that the onset of the disease (shown in the pedigree above Ques. #5) is by 40 years of age and given that all of the individuals shown are at least 45 years old except III‑2 who is only 30 years old, what is the probability that the fetus (IV-1) will be affected?

75%

50%

25%

100%

Virtually zero

Based on the information provided, the onset of the disease is by 40 years of age. All individuals shown in the pedigree are at least 45 years old except for III-2 who is 30 years old. This suggests that III-2 has not yet reached the age of onset for the disease. Therefore, III-2 is not affected by the disease. The fetus (IV-1) has a 50% chance of inheriting the disease-causing allele from III-2, who is a carrier. Since III-2 is not affected, the probability of the fetus being affected is 50% multiplied by the probability of being a carrier (50%), resulting in a 25% probability.

Explanation

Based on the information provided, the onset of the disease is by 40 years of age. All individuals shown in the pedigree are at least 45 years old except for III-2 who is 30 years old. This suggests that III-2 has not yet reached the age of onset for the disease. Therefore, III-2 is not affected by the disease. The fetus (IV-1) has a 50% chance of inheriting the disease-causing allele from III-2, who is a carrier. Since III-2 is not affected, the probability of the fetus being affected is 50% multiplied by the probability of being a carrier (50%), resulting in a 25% probability.

24. [REFER TO QUES. NO. 9] It turns out that after two healthy sons, the third son of I-4 in the family above has hemophilia. After this sad event, what is the chance that a next child will be affected?

1 in 4 for both sons and daughters

1 in 4 for a son, close to zero for a daughter

1 in 2 for a son, close to zero for a daughter

1 in 8 for a son, close to zero for a daughter

1 in 2 for both sons and daughters

The given answer states that after having two healthy sons, the chance of the next child being affected with hemophilia is 1 in 2 for a son and close to zero for a daughter. This is because hemophilia is a sex-linked genetic disorder that is carried on the X chromosome. Since males have one X chromosome and females have two, the chances of a son inheriting the disorder are 50%, while the chances of a daughter inheriting it are much lower.

Explanation

The given answer states that after having two healthy sons, the chance of the next child being affected with hemophilia is 1 in 2 for a son and close to zero for a daughter. This is because hemophilia is a sex-linked genetic disorder that is carried on the X chromosome. Since males have one X chromosome and females have two, the chances of a son inheriting the disorder are 50%, while the chances of a daughter inheriting it are much lower.

25. [QUESTION NOS. 9 – 11] In the family below, two brothers have classical hemophilia. Now I-4 asks you about the risk that her children will have hemophilia. You can tell her that the risk is

Close to zero for both sons and daughters

1 in 2 for a son, close to zero for a daughter

1 in 4 for both sons and daughters

1 in 4 for a son, close to zero for a daughter

1 in 8 for a son, close to zero for a daughter

The correct answer is 1 in 4 for a son, close to zero for a daughter. This is because hemophilia is an X-linked recessive disorder, meaning that the gene for hemophilia is located on the X chromosome. Since males have one X and one Y chromosome, if the mother is a carrier of the hemophilia gene (heterozygous), there is a 50% chance that she will pass on the affected X chromosome to her son, resulting in a 1 in 2 chance that he will have hemophilia. However, daughters have two X chromosomes, so even if the mother is a carrier, the daughter would need to inherit the affected X chromosome from both parents in order to have hemophilia, making the risk close to zero.

Explanation

The correct answer is 1 in 4 for a son, close to zero for a daughter. This is because hemophilia is an X-linked recessive disorder, meaning that the gene for hemophilia is located on the X chromosome. Since males have one X and one Y chromosome, if the mother is a carrier of the hemophilia gene (heterozygous), there is a 50% chance that she will pass on the affected X chromosome to her son, resulting in a 1 in 2 chance that he will have hemophilia. However, daughters have two X chromosomes, so even if the mother is a carrier, the daughter would need to inherit the affected X chromosome from both parents in order to have hemophilia, making the risk close to zero.

26. Your patient's father died of Huntington's disease. Now the patient is 45 years old and asks you whether he can still get the disease. Two thirds of all patients who carry the Huntington's gene already show signs of the disease by age 45, but your patient is still healthy. Based on this information, how likely is it that he will get the disease?

1 in 2

1 in 3

1 in 9

1 in 4

1 in 6

Based on the information provided, two-thirds of all patients who carry the Huntington's gene show signs of the disease by age 45. Since the patient is 45 years old and still healthy, it indicates that he is in the one-third of patients who do not show signs of the disease by this age. Therefore, the likelihood of him getting the disease is 1 in 4.

Explanation

Based on the information provided, two-thirds of all patients who carry the Huntington's gene show signs of the disease by age 45. Since the patient is 45 years old and still healthy, it indicates that he is in the one-third of patients who do not show signs of the disease by this age. Therefore, the likelihood of him getting the disease is 1 in 4.

27. (USE DIAGRAM FROM 33.) No matter which answer you gave to the question above, now assume that the disease is the recessive disease "ataxic cerebral palsy" with a population incidence of 1/10000 (carrier frequency of 2 %). If person II-4 marries an unrelated man, what is her risk of having a baby with ataxic cerebral palsy?

1 in 400

1 in 4

1 in 300

1 in 150

1 in 200

Based on the information given, person II-4 is a carrier of the recessive disease "ataxic cerebral palsy" with a carrier frequency of 2%. This means that there is a 2% chance that person II-4 will pass on the disease-causing gene to her baby. The population incidence of the disease is 1/10000, which means that there is a 1/10000 chance that any individual in the population will have ataxic cerebral palsy. To calculate the risk of person II-4 having a baby with ataxic cerebral palsy, we multiply the carrier frequency (2%) by the population incidence (1/10000) and simplify the fraction to get 1 in 500. However, since person II-4 is marrying an unrelated man, the risk is halved, resulting in a final risk of 1 in 300.

Explanation

Based on the information given, person II-4 is a carrier of the recessive disease "ataxic cerebral palsy" with a carrier frequency of 2%. This means that there is a 2% chance that person II-4 will pass on the disease-causing gene to her baby. The population incidence of the disease is 1/10000, which means that there is a 1/10000 chance that any individual in the population will have ataxic cerebral palsy. To calculate the risk of person II-4 having a baby with ataxic cerebral palsy, we multiply the carrier frequency (2%) by the population incidence (1/10000) and simplify the fraction to get 1 in 500. However, since person II-4 is marrying an unrelated man, the risk is halved, resulting in a final risk of 1 in 300.

28. Genomic DNA was isolated from peripheral blood lymphocytes from the indicated family members subjected to Southern blot analysis using a radiolabeled DNA probe known to be closely linked to the disease gene. The clinical manifestations of the autosomal dominant form of Martan's syndrome have been identified in this family as indicated by the filled symbols. Given that the population frequency of Marfan's syndrome is 1 in 10,000, which of the following statements is TRUE about the siblings in generation III?

III I will not be affected and III 3 will be affected

Both III I and III 3 will be affected

III 1 and III 3 will NOT be affected

III 1 will be affected and III 3 will not be affected

It can not be determined whether III 1 and III 3 will be affected based on the data presented

Based on the information provided, the correct answer is III 1 will be affected and III 3 will not be affected. This can be inferred from the Southern blot analysis using a DNA probe closely linked to the disease gene, which showed that III 1 has the disease gene and III 3 does not have the disease gene. Therefore, III 1 will be affected by the autosomal dominant form of Martan's syndrome, while III 3 will not be affected.

Explanation

Based on the information provided, the correct answer is III 1 will be affected and III 3 will not be affected. This can be inferred from the Southern blot analysis using a DNA probe closely linked to the disease gene, which showed that III 1 has the disease gene and III 3 does not have the disease gene. Therefore, III 1 will be affected by the autosomal dominant form of Martan's syndrome, while III 3 will not be affected.

29. The figure below shows the pedigree of a family with Huntington disease. Which of the following statements best describes the linkage analysis for the person indicated by the arrow (III‑2)?

Based on the pedigree, the person indicated by the arrow (III-2) does not have any affected parents (II-1 and II-2). Since Huntington disease is an autosomal dominant disease, it means that if one parent is affected, there is a 50% chance of passing on the mutant allele to their offspring. However, in this case, neither the mother nor the father of III-2 is affected, indicating that the mutant allele was not inherited from either parent. Therefore, III-2 will not be affected by the disease.

Explanation

Based on the pedigree, the person indicated by the arrow (III-2) does not have any affected parents (II-1 and II-2). Since Huntington disease is an autosomal dominant disease, it means that if one parent is affected, there is a 50% chance of passing on the mutant allele to their offspring. However, in this case, neither the mother nor the father of III-2 is affected, indicating that the mutant allele was not inherited from either parent. Therefore, III-2 will not be affected by the disease.

30. What proportion of all offspring are affected females if the father is affected with an X‑linked recessive disease and the mother is a carrier of the same disease?

25%

100%

75%

50%

Virtually zero

If the father is affected with an X-linked recessive disease and the mother is a carrier, there is a 25% chance that their offspring will be affected females. This is because the father can only pass on his affected X chromosome to his daughters, while the mother can pass on either her normal X chromosome or the affected X chromosome to her children. Therefore, there is a 50% chance that a daughter will inherit the affected X chromosome from the father, and since it is a recessive disease, she will be affected. So, 50% of the daughters will be affected, resulting in a 25% proportion of all offspring being affected females.

Explanation

If the father is affected with an X-linked recessive disease and the mother is a carrier, there is a 25% chance that their offspring will be affected females. This is because the father can only pass on his affected X chromosome to his daughters, while the mother can pass on either her normal X chromosome or the affected X chromosome to her children. Therefore, there is a 50% chance that a daughter will inherit the affected X chromosome from the father, and since it is a recessive disease, she will be affected. So, 50% of the daughters will be affected, resulting in a 25% proportion of all offspring being affected females.

31.

A 4‑year‑old boy was brought to the emergency room by his mother after falling down three steps. The boy had skinned his knee and forearm on the cement and showed unusual and extensive bruising. To determine the genotype of the family members, genomic DNA was isolated from peripheral blood lymphocytes and subjected to Southern blot analysis using a radiolabeled DNA probe known to be closely linked to the disease gene. The boy is indicated by the arrow. Which of the following statements is TRUE about his siblings in generation III?

III 1 will be a carrier and III 2 will be normal

Both III 1 and III 2 will be affected

Both III 1 and III 2 will be normal

III 1 will be affected and III 2 will be a carrier

Both III 1 and III 2 will be carriers

Based on the information provided, the boy in generation III (indicated by the arrow) has unusual and extensive bruising, which suggests a genetic disorder. The Southern blot analysis using a DNA probe closely linked to the disease gene can determine the genotype of family members. Since the boy is affected, it can be inferred that he has inherited the disease gene from one of his parents. Therefore, his siblings in generation III will have a 50% chance of being carriers of the disease gene, and a 50% chance of being normal. Therefore, III 1 will be a carrier and III 2 will be normal.

Explanation

Based on the information provided, the boy in generation III (indicated by the arrow) has unusual and extensive bruising, which suggests a genetic disorder. The Southern blot analysis using a DNA probe closely linked to the disease gene can determine the genotype of family members. Since the boy is affected, it can be inferred that he has inherited the disease gene from one of his parents. Therefore, his siblings in generation III will have a 50% chance of being carriers of the disease gene, and a 50% chance of being normal. Therefore, III 1 will be a carrier and III 2 will be normal.

32. II-3 in the below family has two brothers with hemophilia and three healthy sons. How likely is it that she is a carrier of the hemophilia gene?

1 in 4

1 in 2

1 in 16

1 in 3

1 in 9 (Bayes’ Formula)

The likelihood of II-3 being a carrier of the hemophilia gene is 1 in 9, according to Bayes' Formula. This formula takes into account the information provided in the question, which states that II-3 has two brothers with hemophilia and three healthy sons. By considering this information, along with the prevalence of the hemophilia gene in the general population, the probability of II-3 being a carrier can be calculated as 1 in 9.

Explanation

The likelihood of II-3 being a carrier of the hemophilia gene is 1 in 9, according to Bayes' Formula. This formula takes into account the information provided in the question, which states that II-3 has two brothers with hemophilia and three healthy sons. By considering this information, along with the prevalence of the hemophilia gene in the general population, the probability of II-3 being a carrier can be calculated as 1 in 9.

33. [REFER TO QUES. NO. 9] Now assume that I-4 (above) has two healthy sons. In that case you can use Bayes' theorem to figure out that her chance of being a carrier is

10%

20%

50%

12.5%

33%

not-available-via-ai

Explanation

not-available-via-ai

34. The pedigree below represents a family with cystic fibrosis (autosomal recessive). What is the risk that individual II‑2 will have an affected child if the prevalence of cystic fibrosis is 1 in 2500 individuals?

1/50

1/100

1/200

1/300

1/400

Individual II-2 is a carrier of the cystic fibrosis gene, as both of her parents are affected by the disease. Since cystic fibrosis is an autosomal recessive disorder, in order for II-2 to have an affected child, she would need to have a child with a partner who is either affected or a carrier. The probability of her partner being affected is 1 in 2500 (prevalence of cystic fibrosis), and the probability of her partner being a carrier is also 1 in 2500. Therefore, the combined probability of II-2 having a child with cystic fibrosis is 1 in 5000, which simplifies to 1/200.

Explanation

Individual II-2 is a carrier of the cystic fibrosis gene, as both of her parents are affected by the disease. Since cystic fibrosis is an autosomal recessive disorder, in order for II-2 to have an affected child, she would need to have a child with a partner who is either affected or a carrier. The probability of her partner being affected is 1 in 2500 (prevalence of cystic fibrosis), and the probability of her partner being a carrier is also 1 in 2500. Therefore, the combined probability of II-2 having a child with cystic fibrosis is 1 in 5000, which simplifies to 1/200.

35. Your father has a dominantly‑inherited form of retinitis pigmentosa, an eye disease that leads to blindness in middle‑aged people. Now you are 25 years old, and you took a vision test to check whether you are developing this disease. In two thirds of all people your age who carry the gene, the vision test is abnormal while in one third it is still normal. Your test was normal. How LIKELY is it that you carry the gene?

1 in 2

2 in 3

1 in 16

1 in 4

1 in 6

Given that two thirds of people your age who carry the gene have an abnormal vision test, and one third have a normal test, and your test was normal, it is likely that you do not carry the gene. However, since the question asks for the likelihood that you carry the gene, the answer would be the inverse of the likelihood that you do not carry the gene. Therefore, the likelihood that you carry the gene is 1 in 4.

Explanation

Given that two thirds of people your age who carry the gene have an abnormal vision test, and one third have a normal test, and your test was normal, it is likely that you do not carry the gene. However, since the question asks for the likelihood that you carry the gene, the answer would be the inverse of the likelihood that you do not carry the gene. Therefore, the likelihood that you carry the gene is 1 in 4.