Q: A monohybrid cross (Aa × Aa) yields 792 dominant-phenotype and 208 recessive-phenotype offspring (total = 1000). What is the expected number of dominant-phenotype offspring under a 3:1 ratio?

$$E = \frac{3}{4} \times 1000 = 750$$

Question 1

Which of the following best describes incomplete dominance, and how does it differ from complete dominance?

Accepted Answer

Incomplete dominance occurs when the phenotype of the heterozygote is intermediate between the two homozygotes. In complete dominance, the dominant allele completely masks the recessive allele.

Question 2

A cross between two heterozygous snapdragon plants (Rr × Rr), where red (RR) and white (rr) alleles show incomplete dominance, produces what phenotypic ratio in the F2 generation?

Accepted Answer

$$1:2:1 ($$red:pink:white)

Question 3

In a complementation test, two mutant strains are crossed and the F1 offspring display the wild-type phenotype. What does this indicate about the mutations?

Accepted Answer

The mutations are in different genes.

Question 4

Which of the following best defines epistasis?

Accepted Answer

A gene whose product masks or modifies the expression of another gene at a different locus.

Question 5

A Labrador retriever’s coat color is determined by two genes: one for pigment color (B = black, b = brown) and one for pigment deposition (E = allows color, e = no color, resulting in yellow). What is the expected phenotypic ratio in the F2 generation from a dihybrid cross (BbEe × BbEe)?

Accepted Answer

$$9:3:4 ($$black:brown:yellow)

Question 6

Which statement correctly distinguishes penetrance from expressivity?

Accepted Answer

Penetrance is the proportion of individuals with a genotype who express the phenotype, while expressivity is the degree to which the phenotype is expressed among those individuals.

Question 7

A temperature-sensitive mutation in yeast causes the cells to grow normally at $$25^\circC $$but not at $$37^\circC. $$Which term best describes $$25^\circC in $$this context?

Accepted Answer

Permissive condition

Question 8

A monohybrid cross (Aa × Aa) yields 792 dominant-phenotype and 208 recessive-phenotype offspring (total = 1000). What is the expected number of dominant-phenotype offspring under a 3:1 ratio?

Accepted Answer

$$E = \frac{3}{4} 	imes 1000 = 750$$

Question 9

Given the observed and expected values from the previous question (O = 792, E = 750 for dominant; O = 208, E = 250 for recessive), calculate the chi-squared ($$\$$chi^2$$) value for this cross.

Accepted Answer

$$\$$chi^2$$ = \frac{(792-750)^2$}{750} + \frac{(208-250)^2$}{250} = \frac{42^2$}{750} + \frac{(-42)^2$}{250} = \frac{1764}{750} + \frac{1764}{250} = 2.352 + 7.056 = 9.408$$

Question 10

In pedigree analysis, which inheritance pattern is suggested if an affected father passes the trait to all his daughters but none of his sons?

Accepted Answer

X-linked dominant

Question 11

If both parents are carriers for an autosomal recessive disorder (Aa × Aa), what is the probability that an unaffected child is a carrier?

Accepted Answer

$$\frac{2}{3}$$

Question 12

Which of the following best describes pleiotropy?

Accepted Answer

A single gene affects multiple, seemingly unrelated phenotypic traits.

Question 13

A recessive lethal allele is detected in a cross when the expected Mendelian ratio is $$3:1$$, but the observed ratio is closer to $$2:1. $$What is the most likely explanation?

Accepted Answer

Homozygous recessive individuals die before birth.

Question 14

Which of the following is true regarding the use of the chi-squared ($$\$$chi^2$$) test in genetics?

Accepted Answer

It is used to determine if observed data fit expected Mendelian ratios.

Question 15

Which rule should you use to calculate the probability of two independent events both occurring in a pedigree analysis?

Accepted Answer

The product rule

Question 16

Which of the following is a classic example of codominance?

Accepted Answer

ABO blood group in humans, where both IA and IB alleles are expressed in the phenotype.

Question 17

Which of the following statements about essential genes is correct?

Accepted Answer

An essential gene is one whose loss-of-function mutation results in lethality.

Question 18

In a pedigree, which pattern suggests autosomal recessive inheritance?

Accepted Answer

The trait often skips generations and unaffected parents can have affected children.

Question 19

Which of the following best explains why pleiotropic effects make sense mechanistically?

Accepted Answer

The gene product is expressed in multiple tissues or cell types, affecting different traits.

Question 20

If a gene shows 100% penetrance but variable expressivity, what does this mean?

Accepted Answer

All individuals with the genotype show the phenotype, but the severity or nature of the phenotype varies among individuals.

Genetics Exam Study Guide: Patterns of Inheritance, Gene Interactions, and Analysis

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