1.
Genes are composed of __________.
Correct Answer
B. Nucleotides
Explanation
Genes are composed of nucleotides, which are the building blocks of DNA. Nucleotides are made up of a sugar, a phosphate group, and a nitrogenous base. These nitrogenous bases (adenine, thymine, cytosine, and guanine) form the genetic code that determines the sequence of amino acids in a protein. Traits, proteins, alleles, and amino acids are all related to genes, but they are not the fundamental components that make up genes.
2.
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?
Correct Answer
C. Recessive traits can reappear in later generations.
Explanation
Mendel's understanding of genes as unchanged units of information was most influenced by the observation that recessive traits can reappear in later generations. This observation contradicted the commonly held belief that traits were blended in offspring, as it demonstrated that certain traits could be present in one generation and then reappear in subsequent generations. This led Mendel to propose the concept of discrete hereditary units, which he called "factors" (now known as genes), that were passed down unchanged from parents to offspring.
3.
A person is heterozygous for hair texture. Which of these statements is correct about this person's DNA?
Correct Answer
D. This person has two different alleles at the loci for hair texture.
Explanation
A person who is heterozygous for hair texture has two different alleles at the loci for hair texture. Heterozygous means that the person has inherited two different versions of the gene for hair texture, one from each parent. Therefore, this person's DNA contains two different alleles at the specific location (loci) that determines hair texture.
4.
Which of the following is true about dominant alleles?
Correct Answer
A. Dominant alleles mask the presence of recessive alleles.
Explanation
Dominant alleles mask the presence of recessive alleles, meaning that if an individual has one dominant allele and one recessive allele for a particular trait, only the dominant allele will be expressed in the phenotype. This is because the dominant allele is able to produce a functional protein or enzyme, while the recessive allele is not. Therefore, the presence of a dominant allele will override the expression of the recessive allele.
5.
Considering pea flowers, where purple is dominant to white, describe the genotype of a white flower.
Correct Answer
F. Homozygous recessive
Explanation
The genotype of a white pea flower is homozygous recessive. This means that both alleles for flower color are the same and recessive, represented by "pp". Since purple is dominant to white, a white flower must have two copies of the recessive allele for white color.
6.
A purple-flowered pea plant self-fertilizes and produces both purple and white offspring. What is its genotype?
Correct Answer
C. Pp
Explanation
The genotype of the purple-flowered pea plant is Pp. This is because it produces both purple and white offspring, indicating that it carries both the dominant allele (P) for purple flowers and the recessive allele (p) for white flowers. The presence of both alleles in the genotype suggests that the plant is heterozygous, hence the genotype is Pp.
7.
Considering pea flowers, where purple is dominant to white, describe the genotype of a plant with the alleles Pp.
Correct Answer
D. Heterozygous
Explanation
The genotype of a plant with the alleles Pp is heterozygous. This means that the plant has two different alleles for the gene that determines flower color. In this case, the dominant allele is P (purple) and the recessive allele is p (white). Since the plant has one dominant allele and one recessive allele, it will display the dominant trait, which is purple.
8.
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?
Correct Answer
D. Pp x pp
Explanation
When a pea plant with purple flowers (genotype Pp) is crossed with a pea plant with white flowers (genotype pp), the offspring have a 50% chance of inheriting the purple flower allele (P) and a 50% chance of inheriting the white flower allele (p). This indicates that the parent plants must have been heterozygous (Pp) for flower color. Therefore, the correct answer is Pp x pp.
9.
During which phase of meiosis does the law of segregation of alleles occur?
Correct Answer
F. Anaphase I
Explanation
During anaphase I of meiosis, the homologous chromosomes separate and move towards opposite poles of the cell. This is when the law of segregation of alleles occurs, as each chromosome carries different alleles for a particular gene and they are separated, ensuring that each resulting gamete receives only one allele from each pair. Therefore, anaphase I is the phase in which the law of segregation of alleles is observed.
10.
During which phase of the cell cycle does the law of independent assortment occur?
Correct Answer
D. Anaphase I
Explanation
During anaphase I of the cell cycle, the law of independent assortment occurs. This is because anaphase I is the stage where homologous chromosomes separate and move to opposite poles of the cell. The law of independent assortment states that the alleles for different traits segregate independently of one another during the formation of gametes. Therefore, during anaphase I, the chromosomes containing different alleles for different traits separate and assort independently, leading to genetic variation in the resulting gametes.
11.
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?
Correct Answer
D. Incomplete dominance
Explanation
This pattern of inheritance involves incomplete dominance. Incomplete dominance occurs when the heterozygous offspring show a phenotype that is intermediate between the phenotypes of the two homozygous parents. In this case, the red-flowered plant and the white-flowered plant are both pure-breeding, meaning they are homozygous for their respective traits. When they are crossed, the heterozygous offspring have pink flowers, which is an intermediate phenotype between red and white. This demonstrates incomplete dominance, where neither allele is completely dominant over the other.
12.
What are the possible genotypes of a person with type O blood?
Correct Answer
D. Oo
Explanation
A person with type O blood can only have the genotype "oo" because type O blood is determined by having two recessive alleles for the blood type. The other options include either one dominant allele (A or B) or a combination of a dominant and recessive allele, which would result in a different blood type. Therefore, the correct answer is "oo".
13.
What feature controls the tightness of the linkage of two genes?
Correct Answer
D. The distance between the two genes
Explanation
The tightness of the linkage of two genes is controlled by the distance between the two genes. Genes that are located close together on the same chromosome are more likely to be inherited together, while genes that are located far apart are more likely to undergo recombination and be inherited independently. Therefore, the distance between the two genes determines the likelihood of genetic linkage.
14.
Two healthy parents give birth to a child with cystic fibrosis, a recessive disorder. What can we say about both parents?
Correct Answer
A. Both parents are heterozygous for the disease allele.
Explanation
If both parents give birth to a child with cystic fibrosis, a recessive disorder, it means that they both carry the disease allele but do not exhibit the disorder themselves. This indicates that both parents are heterozygous for the disease allele, meaning they have one copy of the disease allele and one copy of the healthy allele. This is because cystic fibrosis is a recessive disorder, so for a child to have the disorder, they must inherit two copies of the disease allele, one from each parent.
15.
A mother has two X chromosomes. A father has an X and a Y. What are the odds that they produce a female child?
Correct Answer
B. 50%
Explanation
The odds that a mother and father will produce a female child is 50%. This is because the mother can only contribute an X chromosome, while the father can contribute either an X or a Y chromosome. Since the father has an equal chance of passing on either chromosome, there is a 50% chance that the child will receive an X chromosome from the father and therefore be female.
16.
Why can't a color-blind father pass this condition on to his sons?
Correct Answer
A. The father gives only his Y chromosome to his sons.
Explanation
Color blindness is a sex-linked genetic disorder that is carried on the X chromosome. Since the father only passes on his Y chromosome to his sons, and color blindness is not present on the Y chromosome, the sons cannot inherit the condition from their father. Color deficiency alleles can only be received from the mother, who can pass on either a normal X chromosome or a color deficiency allele on her X chromosome.
17.
Why do males suffer from sex-linked disorders more often than females?
Correct Answer
B. Males can express single recessive alleles from their single X chromosome.
Explanation
Males have one X and one Y chromosome, while females have two X chromosomes. If a male inherits a recessive allele for a sex-linked disorder on his X chromosome, he will express the disorder because he does not have another X chromosome to mask the effects of the allele. In contrast, females have two X chromosomes, so even if they inherit a recessive allele for a sex-linked disorder, they may have another X chromosome with a normal allele that can compensate for the effects of the disorder allele. Therefore, males are more likely to suffer from sex-linked disorders.
18.
In a family pedigree, what does the following symbol stand for?
Correct Answer
E. Carrier male
Explanation
The symbol "carrier male" in a family pedigree represents a male individual who carries a genetic trait but does not express it. This means that the male has one copy of a recessive gene for a particular trait, but since it is recessive, it is not manifested in his phenotype. However, the male can pass on this gene to his offspring, increasing the chances of the trait being expressed in future generations.
19.
What error is responsible for creating Jacob syndrome?
Correct Answer
C. Nondisjunction of chromosomes
Explanation
Nondisjunction of chromosomes is responsible for creating Jacob syndrome. Nondisjunction occurs when chromosomes fail to separate properly during cell division, leading to an abnormal number of chromosomes in the resulting cells. In the case of Jacob syndrome, there is an extra copy of the Y chromosome in males, resulting in the presence of XYY instead of the usual XY chromosomes. This condition can lead to physical and developmental abnormalities.
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?
Correct Answer
D. Extra or missing copies of most of the autosomes must be lethal early in life.
Explanation
The given information states that there are no other cases of extra or missing chromosomes, except for the specific disorders mentioned. This suggests that extra or missing copies of most of the autosomes (non-sex chromosomes) are likely to be lethal early in life, as they are not observed in the sample. This conclusion is supported by the fact that trisomy 21 (Down syndrome) is the only disorder mentioned, which affects chromosome 21, a smaller autosome. The presence of other sex chromosome disorders does not provide enough evidence to conclude that an XY homologue pair is extremely susceptible to nondisjunction or that nondisjunction is directly related to the age of the mother.