Genetics Quiz

1. Why can't a color-blind father pass this condition on to his sons?

The father gives only his Y chromosome to his sons.

The father gives only his X chromosome to his sons.

Children can receive the color deficiency allele only from their mother.

Fathers pass on dominant alleles for color vision only to their sons.

A color-blind father cannot pass this condition on to his sons because he gives only his Y chromosome to his sons. Color blindness is typically a sex-linked recessive trait located on the X chromosome. Since males have one X and one Y chromosome (XY), the father contributes the Y chromosome to his sons, determining their male sex. The sons receive their single X chromosome from their mother. Therefore, if the mother carries the allele for color blindness, the sons can inherit it from her, not from the father. This genetic mechanism explains why a color-blind father does not pass the condition to his sons.

Explanation

A color-blind father cannot pass this condition on to his sons because he gives only his Y chromosome to his sons. Color blindness is typically a sex-linked recessive trait located on the X chromosome. Since males have one X and one Y chromosome (XY), the father contributes the Y chromosome to his sons, determining their male sex. The sons receive their single X chromosome from their mother. Therefore, if the mother carries the allele for color blindness, the sons can inherit it from her, not from the father. This genetic mechanism explains why a color-blind father does not pass the condition to his sons.

2. Genes are composed of __________.

Traits

Nucleotides

Proteins

Alleles

Amino acids

Genes are composed of nucleotides. Nucleotides are the building blocks of DNA and RNA, the molecules that carry genetic information in cells. Each nucleotide consists of a sugar, a phosphate group, and a nitrogenous base. The sequence of these nucleotides encodes the genetic instructions for building and maintaining an organism. While proteins and amino acids are important in the expression of genes, and alleles represent variations of a gene, it is the nucleotides that form the fundamental structure of genes. Traits are the characteristics that result from gene expression.

Explanation

Genes are composed of nucleotides. Nucleotides are the building blocks of DNA and RNA, the molecules that carry genetic information in cells. Each nucleotide consists of a sugar, a phosphate group, and a nitrogenous base. The sequence of these nucleotides encodes the genetic instructions for building and maintaining an organism. While proteins and amino acids are important in the expression of genes, and alleles represent variations of a gene, it is the nucleotides that form the fundamental structure of genes. Traits are the characteristics that result from gene expression.

3. A mother has two X chromosomes. A father has an X and a Y. What are the odds that they produce a female child?

25%

50%

75%

100%

The odds that a mother (XX) and a father (XY) will produce a female child are 50%. During reproduction, the mother always contributes one of her X chromosomes, while the father can contribute either his X or his Y chromosome. If the father contributes his X chromosome, the resulting child will have two X chromosomes (XX), making them female. If the father contributes his Y chromosome, the resulting child will have one X and one Y chromosome (XY), making them male. Therefore, there is an equal 50% chance of having a female child (XX) and a 50% chance of having a male child (XY).

Explanation

The odds that a mother (XX) and a father (XY) will produce a female child are 50%. During reproduction, the mother always contributes one of her X chromosomes, while the father can contribute either his X or his Y chromosome. If the father contributes his X chromosome, the resulting child will have two X chromosomes (XX), making them female. If the father contributes his Y chromosome, the resulting child will have one X and one Y chromosome (XY), making them male. Therefore, there is an equal 50% chance of having a female child (XX) and a 50% chance of having a male child (XY).

4. Two healthy parents give birth to a child with cystic fibrosis, a recessive disorder. What can we say about both parents?

Both parents are heterozygous for the disease allele.

Only one parent must be heterozygous for the disease allele.

One parent must be homozygous for the disease allele.

Both parents are homozygous for the disease allele.

One parent must be heterozygous for the disease allele. The other must be homozygous healthy.

For two healthy parents to give birth to a child with cystic fibrosis, a recessive disorder, both parents must be heterozygous for the disease allele. This means each parent carries one normal allele (C) and one cystic fibrosis allele (c). Since cystic fibrosis is recessive, the child must inherit two copies of the disease allele (cc) to exhibit the disorder. The parents, having one normal allele, do not show symptoms but can pass on the recessive allele to their offspring. When both parents are heterozygous (Cc), there is a 25% chance with each pregnancy that their child will inherit the disorder (cc).

Explanation

For two healthy parents to give birth to a child with cystic fibrosis, a recessive disorder, both parents must be heterozygous for the disease allele. This means each parent carries one normal allele (C) and one cystic fibrosis allele (c). Since cystic fibrosis is recessive, the child must inherit two copies of the disease allele (cc) to exhibit the disorder. The parents, having one normal allele, do not show symptoms but can pass on the recessive allele to their offspring. When both parents are heterozygous (Cc), there is a 25% chance with each pregnancy that their child will inherit the disorder (cc).

5. Which of the following is true about dominant alleles?

Dominant alleles mask the presence of recessive alleles.

Dominant alleles are more important than recessive alleles.

Dominant alleles are more common in the population than recessive alleles.

Dominant alleles are healthier than recessive alleles.

Dominant alleles are passed on more often than recessive alleles.

Dominant alleles mask the presence of recessive alleles. This means that if an individual has one dominant allele and one recessive allele for a particular trait, the dominant allele will determine the individual's appearance for that trait. The presence of the recessive allele will not be visible in the phenotype unless the individual has two recessive alleles. Dominant alleles are not necessarily more common, more important, healthier, or passed on more often than recessive alleles. Their primary characteristic is their ability to mask the expression of a recessive allele when both are present.

Explanation

Dominant alleles mask the presence of recessive alleles. This means that if an individual has one dominant allele and one recessive allele for a particular trait, the dominant allele will determine the individual's appearance for that trait. The presence of the recessive allele will not be visible in the phenotype unless the individual has two recessive alleles. Dominant alleles are not necessarily more common, more important, healthier, or passed on more often than recessive alleles. Their primary characteristic is their ability to mask the expression of a recessive allele when both are present.

6. In the 1800s, it was commonly believed that the traits of both parents for any feature blended to create the offspring and that the blended trait was passed on. Which of the following observations was most important to Mendel's understanding of genes as unchanged units of information?

Recessive traits are not expressed in children.

Pure breeding lines always produce the same offspring.

Recessive traits can reappear in later generations.

Dominant traits are always expressed in children.

Dominant traits are more common in the population.

The observation that recessive traits can reappear in later generations was most important to Mendel's understanding of genes as unchanged units of information. Mendel's experiments with pea plants showed that traits do not blend but are inherited as discrete units (genes). He noticed that while dominant traits can mask recessive traits in the immediate offspring, the recessive traits can reappear in subsequent generations. This observation led to the formulation of Mendel's laws of inheritance, including the concepts of dominant and recessive alleles and the idea that genes remain unchanged as they are passed from parents to offspring.

Explanation

The observation that recessive traits can reappear in later generations was most important to Mendel's understanding of genes as unchanged units of information. Mendel's experiments with pea plants showed that traits do not blend but are inherited as discrete units (genes). He noticed that while dominant traits can mask recessive traits in the immediate offspring, the recessive traits can reappear in subsequent generations. This observation led to the formulation of Mendel's laws of inheritance, including the concepts of dominant and recessive alleles and the idea that genes remain unchanged as they are passed from parents to offspring.

7. What are the possible genotypes of a person with type O blood?

Ao, Bo, oo

Ao, Bo

Oo, oo

Oo

A person with type O blood has the genotype oo. Blood type is determined by the ABO gene, which has three alleles: A, B, and O. Type O blood means the person has two O alleles (oo), which is why their blood type is O. The O allele is recessive, so for a person to have type O blood, they must inherit an O allele from both parents. As a result, the only possible genotype for type O blood is oo. This genotype leads to the absence of A and B antigens on the surface of red blood cells, characterizing the O blood type

Explanation

A person with type O blood has the genotype oo. Blood type is determined by the ABO gene, which has three alleles: A, B, and O. Type O blood means the person has two O alleles (oo), which is why their blood type is O. The O allele is recessive, so for a person to have type O blood, they must inherit an O allele from both parents. As a result, the only possible genotype for type O blood is oo. This genotype leads to the absence of A and B antigens on the surface of red blood cells, characterizing the O blood type

8. Why do males suffer from sex-linked disorders more often than females?

The diseases that are sex-linked require the expression of testosterone.

Males can express single recessive alleles from their single X chromosome.

Males can express single recessive alleles from their single Y chromosome.

Fathers are more likely to pass disease alleles on to sons than to daughters.

The disease genes are only located on the Y chromosome of males.

Males suffer from sex-linked disorders more often than females because they can express single recessive alleles from their single X chromosome. Males have one X and one Y chromosome (XY), while females have two X chromosomes (XX). If a male inherits a recessive allele for a disorder on the X chromosome, he will express the disorder because he does not have a second X chromosome to mask the effect of the recessive allele. In contrast, females would need to inherit two copies of the recessive allele (one on each X chromosome) to express the disorder, making such conditions less common in females.

Explanation

Males suffer from sex-linked disorders more often than females because they can express single recessive alleles from their single X chromosome. Males have one X and one Y chromosome (XY), while females have two X chromosomes (XX). If a male inherits a recessive allele for a disorder on the X chromosome, he will express the disorder because he does not have a second X chromosome to mask the effect of the recessive allele. In contrast, females would need to inherit two copies of the recessive allele (one on each X chromosome) to express the disorder, making such conditions less common in females.

9. A person is heterozygous for hair texture. Which of these statements is correct about this person's DNA?

This person has two of the same gene for hair texture.

This person has two different genes for hair texture.

This person has the same allele at the loci for hair texture.

This person has two different alleles at the loci for hair texture.

This person has two different homologues for hair texture.

This person has the same homologues for hair texture.

A person who is heterozygous for hair texture has two different alleles at the loci for hair texture. This means that for the gene responsible for hair texture, the person has inherited different versions (alleles) of the gene from each parent. For example, if one parent contributes an allele for curly hair and the other contributes an allele for straight hair, the individual is heterozygous for that trait. This genetic variation is crucial for diversity in traits among individuals. Homologues refer to pairs of chromosomes that contain the same genes, but being heterozygous specifically refers to having different alleles at the same gene loci.

Explanation

A person who is heterozygous for hair texture has two different alleles at the loci for hair texture. This means that for the gene responsible for hair texture, the person has inherited different versions (alleles) of the gene from each parent. For example, if one parent contributes an allele for curly hair and the other contributes an allele for straight hair, the individual is heterozygous for that trait. This genetic variation is crucial for diversity in traits among individuals. Homologues refer to pairs of chromosomes that contain the same genes, but being heterozygous specifically refers to having different alleles at the same gene loci.

10. A purple-flowered pea plant self-fertilizes and produces both purple and white offspring. What is its genotype?

Pp x PP

PP

Pp

Pp x Pp

Pp

Pp x pp

The genotype of the purple-flowered pea plant that self-fertilizes and produces both purple and white offspring is Pp. This is because the plant must have one dominant allele (P) for purple flowers and one recessive allele (p) for white flowers. When this heterozygous plant (Pp) self-fertilizes, it can produce offspring with the genotypes PP (purple), Pp (purple), and pp (white). The presence of both purple and white offspring indicates that the parent plant carries both the dominant and recessive alleles, confirming a heterozygous (Pp) genotype.

Explanation

The genotype of the purple-flowered pea plant that self-fertilizes and produces both purple and white offspring is Pp. This is because the plant must have one dominant allele (P) for purple flowers and one recessive allele (p) for white flowers. When this heterozygous plant (Pp) self-fertilizes, it can produce offspring with the genotypes PP (purple), Pp (purple), and pp (white). The presence of both purple and white offspring indicates that the parent plant carries both the dominant and recessive alleles, confirming a heterozygous (Pp) genotype.

11. Considering pea flowers, where purple is dominant to white, describe the genotype of a plant with the alleles Pp.

Homozygous

Homozygous dominant

White

Heterozygous

Heterozygous dominant recessive

Purple

A plant with the alleles Pp is described as heterozygous. This means it has one dominant allele (P) for purple flowers and one recessive allele (p) for white flowers. In this case, the dominant allele (P) will mask the effect of the recessive allele (p), so the plant will have purple flowers. However, because it has both a dominant and a recessive allele, it is heterozygous. This genetic makeup allows the plant to produce both purple and white offspring if it self-fertilizes, as it can pass on either the dominant or recessive allele to its progeny.

Explanation

A plant with the alleles Pp is described as heterozygous. This means it has one dominant allele (P) for purple flowers and one recessive allele (p) for white flowers. In this case, the dominant allele (P) will mask the effect of the recessive allele (p), so the plant will have purple flowers. However, because it has both a dominant and a recessive allele, it is heterozygous. This genetic makeup allows the plant to produce both purple and white offspring if it self-fertilizes, as it can pass on either the dominant or recessive allele to its progeny.

12. During which phase of meiosis does the law of segregation of alleles occur?

Metaphase II

Anaphase II

Metaphase

Metaphase I

Anaphase

Anaphase I

The law of segregation of alleles occurs during anaphase I of meiosis. This is the phase where homologous chromosomes are separated and pulled to opposite poles of the cell. Each homologous chromosome carries different alleles of the same gene. During anaphase I, the pairs of homologous chromosomes separate, ensuring that each gamete receives only one allele from each gene pair. This segregation of alleles ensures genetic diversity in the resulting gametes and ultimately in the offspring. This fundamental process underpins Mendel's first law, explaining how alleles are separated into different gametes during the formation of reproductive cells.

Explanation

The law of segregation of alleles occurs during anaphase I of meiosis. This is the phase where homologous chromosomes are separated and pulled to opposite poles of the cell. Each homologous chromosome carries different alleles of the same gene. During anaphase I, the pairs of homologous chromosomes separate, ensuring that each gamete receives only one allele from each gene pair. This segregation of alleles ensures genetic diversity in the resulting gametes and ultimately in the offspring. This fundamental process underpins Mendel's first law, explaining how alleles are separated into different gametes during the formation of reproductive cells.

13. A pure-breeding plant with red flowers is crossed with a pure-breeding plant with white flowers. The offspring all have pink flowers. What pattern of inheritance does this involve?

Pleiotropic gene

Codominance

Multi-gene phenotype

Incomplete dominance

Multi-allele gene

The pattern of inheritance described involves incomplete dominance. In incomplete dominance, the phenotype of the heterozygous offspring is a blend of the phenotypes of the two homozygous parents. In this case, a pure-breeding plant with red flowers (RR) is crossed with a pure-breeding plant with white flowers (rr), resulting in offspring with pink flowers (Rr). The pink flowers are an intermediate phenotype, showing that neither the red nor the white allele is completely dominant over the other. This blending of traits is characteristic of incomplete dominance, where the heterozygous phenotype is distinct from either homozygous phenotype.

Explanation

The pattern of inheritance described involves incomplete dominance. In incomplete dominance, the phenotype of the heterozygous offspring is a blend of the phenotypes of the two homozygous parents. In this case, a pure-breeding plant with red flowers (RR) is crossed with a pure-breeding plant with white flowers (rr), resulting in offspring with pink flowers (Rr). The pink flowers are an intermediate phenotype, showing that neither the red nor the white allele is completely dominant over the other. This blending of traits is characteristic of incomplete dominance, where the heterozygous phenotype is distinct from either homozygous phenotype.

14. What error is responsible for creating Jacob syndrome?

Sex-linked inheritance

Incomplete dominant

Nondisjunction of chromosomes

Dominant mutation

Trisomy 21

Recessive mutation

Jacob syndrome, also known as 47,XYY syndrome, is caused by nondisjunction of chromosomes. Nondisjunction is an error in cell division where chromosomes fail to separate properly during meiosis. In the case of Jacob syndrome, this error leads to a male having an extra Y chromosome, resulting in the karyotype 47,XYY. This condition occurs when the father's sperm cell has an extra Y chromosome, which then combines with the mother's X chromosome. Individuals with Jacob syndrome typically have a normal appearance and may have mild symptoms such as increased height, learning difficulties, and slightly delayed development of motor skills.

Explanation

Jacob syndrome, also known as 47,XYY syndrome, is caused by nondisjunction of chromosomes. Nondisjunction is an error in cell division where chromosomes fail to separate properly during meiosis. In the case of Jacob syndrome, this error leads to a male having an extra Y chromosome, resulting in the karyotype 47,XYY. This condition occurs when the father's sperm cell has an extra Y chromosome, which then combines with the mother's X chromosome. Individuals with Jacob syndrome typically have a normal appearance and may have mild symptoms such as increased height, learning difficulties, and slightly delayed development of motor skills.

15. During which phase of the cell cycle does the law of independent assortment occur?

Anaphase II

Metaphase II

Anaphase

Anaphase I

Metaphase I

Metaphase

The law of independent assortment occurs during metaphase I of meiosis. In this phase, homologous chromosome pairs line up along the metaphase plate in a random orientation. This random alignment allows for the independent segregation of different gene pairs located on different chromosomes. As a result, the combination of alleles that end up in the gametes is due to chance, contributing to genetic diversity. This process explains how alleles of different genes assort independently of one another, as each homologous pair's orientation is independent of the others. This randomness is a key aspect of genetic variation in sexually reproducing organisms.

Explanation

The law of independent assortment occurs during metaphase I of meiosis. In this phase, homologous chromosome pairs line up along the metaphase plate in a random orientation. This random alignment allows for the independent segregation of different gene pairs located on different chromosomes. As a result, the combination of alleles that end up in the gametes is due to chance, contributing to genetic diversity. This process explains how alleles of different genes assort independently of one another, as each homologous pair's orientation is independent of the others. This randomness is a key aspect of genetic variation in sexually reproducing organisms.

16. Considering pea flowers, where purple is dominant to white, describe the genotype of a white flower.

White

Pp

Purple

PP

Heterozygous recessive

Homozygous recessive

The genotype of a white flower, given that purple is dominant to white, is homozygous recessive. This means that the flower must have two copies of the recessive allele (represented as "pp") for the white color to be expressed. In pea flowers, the dominant allele for purple (represented as "P") would mask the presence of a recessive allele for white if it were present. Therefore, only a flower with two recessive alleles (pp) will exhibit the white phenotype, indicating a homozygous recessive genotype.

Explanation

The genotype of a white flower, given that purple is dominant to white, is homozygous recessive. This means that the flower must have two copies of the recessive allele (represented as "pp") for the white color to be expressed. In pea flowers, the dominant allele for purple (represented as "P") would mask the presence of a recessive allele for white if it were present. Therefore, only a flower with two recessive alleles (pp) will exhibit the white phenotype, indicating a homozygous recessive genotype.

17. What feature controls the tightness of the linkage of two genes?

The importance of the genes in a metabolic pathway

The length of the two genes

Whether both genes are dominant or recessive

The distance between the two genes

The distance between the two chromosomes during meiosis

The tightness of the linkage between two genes is controlled by the distance between the two genes on a chromosome. Genes that are located close to each other on the same chromosome tend to be inherited together because there is a lower chance of a recombination event (crossing over) occurring between them during meiosis. Conversely, genes that are farther apart are more likely to be separated by recombination, leading to a higher likelihood of independent assortment. This concept is the basis for genetic linkage mapping, which estimates the distance between genes based on the frequency of recombination events.

Explanation

The tightness of the linkage between two genes is controlled by the distance between the two genes on a chromosome. Genes that are located close to each other on the same chromosome tend to be inherited together because there is a lower chance of a recombination event (crossing over) occurring between them during meiosis. Conversely, genes that are farther apart are more likely to be separated by recombination, leading to a higher likelihood of independent assortment. This concept is the basis for genetic linkage mapping, which estimates the distance between genes based on the frequency of recombination events.

18. In a family pedigree, what does the following symbol stand for?

Homozygous male

Incomplete dominant female

Homozygous female

Carrier female

Carrier male

Incomplete dominant male

In a family pedigree, a carrier female is typically represented by a circle (indicating female) that is half-shaded or has a dot in the center. This symbol denotes that the female carries one copy of a recessive allele for a particular trait or disorder but does not express the disorder herself. Pedigree charts use standardized symbols to represent different genetic statuses: circles for females, squares for males, shaded shapes for individuals expressing the trait, and half-shaded or dot-in-center shapes for carriers of recessive traits. Understanding these symbols helps trace the inheritance patterns of genetic conditions within a family.

Explanation

In a family pedigree, a carrier female is typically represented by a circle (indicating female) that is half-shaded or has a dot in the center. This symbol denotes that the female carries one copy of a recessive allele for a particular trait or disorder but does not express the disorder herself. Pedigree charts use standardized symbols to represent different genetic statuses: circles for females, squares for males, shaded shapes for individuals expressing the trait, and half-shaded or dot-in-center shapes for carriers of recessive traits. Understanding these symbols helps trace the inheritance patterns of genetic conditions within a family.

19. A pea plant with purple flowers is crossed with a pea plant with white flowers. One-half of the offspring have purple flowers and one-half have white flowers. What are the genotypes of the parent plants?

PP x pp

Pp x Pp

Pp x pp

Pp x pp - where one parent is heterozygous purple (Pp) and the other is homozygous recessive white (pp). This pairing correctly predicts that half of the offspring will have purple flowers and half will have white flowers.

Explanation

Pp x pp - where one parent is heterozygous purple (Pp) and the other is homozygous recessive white (pp). This pairing correctly predicts that half of the offspring will have purple flowers and half will have white flowers.

20. In studying the karyotypes of a random sample of 10,000 people, you find the following disorders related to nondisjunction of chromosomes:

11 people have trisomy 21
3 women have XO
10 men have XXY
9 women have trisomy X
11 men have XYY

You find no other cases of extra or missing chromosomes. What might you conclude about nondisjunction disorders in general?

Extra or missing copies of most of the chromosomes are common but must cause no symptoms.

Nondisjunction of chromosome 21 must be directly related to the age of the mother.

An XY homologue pair must be extremely susceptible to nondisjunction.

Extra or missing copies of most of the autosomes must be lethal early in life.

The data indicate that nondisjunction disorders related to sex chromosomes (like XO, XXY, trisomy X, and XYY) are relatively more common and generally compatible with life, although they can lead to various health issues. In contrast, there are very few cases of trisomy 21 (Down syndrome), and no other autosomal trisomies or monosomies are observed in the sample. This suggests that extra or missing copies of most autosomes are likely lethal early in life, resulting in miscarriage or early pregnancy loss. Only a few specific trisomies, like trisomy 21, are viable, though they still cause significant developmental disorders.

Explanation

The data indicate that nondisjunction disorders related to sex chromosomes (like XO, XXY, trisomy X, and XYY) are relatively more common and generally compatible with life, although they can lead to various health issues. In contrast, there are very few cases of trisomy 21 (Down syndrome), and no other autosomal trisomies or monosomies are observed in the sample. This suggests that extra or missing copies of most autosomes are likely lethal early in life, resulting in miscarriage or early pregnancy loss. Only a few specific trisomies, like trisomy 21, are viable, though they still cause significant developmental disorders.