Question 1

Which of the following best describes codominance?

Accepted Answer

Both alleles are fully expressed in the heterozygote, resulting in a joint expression of both phenotypes.

Question 2

A woman with blood type AB and a man with blood type O have a child. What is the probability that the child will have blood type A?

Accepted Answer

$0.5$

Question 3

Which of the following is an example of a gain-of-function mutation?

Accepted Answer

A mutation that increases the quantity or activity of the gene product beyond the wild-type level.

Question 4

In the case of the Bombay phenotype, which genetic event prevents the expression of A or B antigens on red blood cells?

Accepted Answer

A homozygous recessive mutation at the FUT1 locus, preventing the production of the H substance.

Question 5

A cross between two heterozygous yellow mice (A$^Y$a) produces offspring in a 2:1 ratio of yellow to agouti. What is the most likely explanation for the absence of a 3:1 ratio?

Accepted Answer

The yellow allele is a recessive lethal allele; homozygotes (A$^Y$A$^Y$) do not survive.

Question 6

Which of the following best describes pleiotropy?

Accepted Answer

A single gene affects multiple, seemingly unrelated phenotypic traits.

Question 7

A man with hemophilia (an X-linked recessive disorder) and a woman who is a carrier for hemophilia have a child. What is the probability that their son will have hemophilia?

Accepted Answer

$0.5$

Question 8

Which of the following is an example of dominant epistasis?

Accepted Answer

A dominant allele at one locus masks the expression of alleles at a second locus, as seen in summer squash fruit color.

Question 9

Which of the following best explains the concept of penetrance?

Accepted Answer

The percentage of individuals with a particular genotype that actually display the associated phenotype.

Question 10

A geneticist is studying two mutations that cause white eyes in Drosophila. She crosses two homozygous mutants and the offspring all have red eyes. What does this result indicate?

Accepted Answer

The mutations are in different genes (complementation has occurred).

Question 11

Which of the following is true regarding X-linked inheritance in humans?

Accepted Answer

Mothers can pass X-linked traits to both sons and daughters.

Question 12

Which of the following scenarios best illustrates the concept of expressivity?

Accepted Answer

Individuals with the same mutant genotype show a range of phenotypes, from mild to severe.

Question 13

A child is born with phenylketonuria (PKU), a disorder caused by a loss-of-function mutation. Which dietary intervention can prevent the severe symptoms of PKU?

Accepted Answer

A diet low in phenylalanine.

Question 14

Which of the following is an example of a temperature-sensitive mutation?

Accepted Answer

A mutation that causes red flowers at $23^	ext{o}$C and white flowers at $18^	ext{o}$C in evening primrose.

Question 15

Which of the following best describes the genetic phenomenon observed in Marfan syndrome?

Accepted Answer

A single gene mutation leads to multiple, unrelated phenotypic effects (pleiotropy).

Question 16

A male with genotype $Bb$ for a gene showing sex-influenced inheritance (where $B$ is dominant in males and recessive in females) will show which phenotype?

Accepted Answer

The phenotype associated with the $B$ allele, because it is dominant in males.

Question 17

Which of the following is an example of genetic anticipation?

Accepted Answer

A genetic disease shows earlier onset and increased severity in each successive generation.

Question 18

In a dihybrid cross involving recessive epistasis (e.g., mouse coat color), what phenotypic ratio is expected in the $F_2$ generation?

Accepted Answer

$9:3:4$

Question 19

Which of the following best explains why a wild-type allele is not always dominant?

Accepted Answer

Wild-type alleles are the most common in a population, but dominance is determined by the relationship between alleles, not frequency.

Question 20

A person with genotype $I^A i$ for the ABO blood group has which blood type?

Accepted Answer

Type A

Extensions of Mendelian Genetics: Chapter 4 Study Notes

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