Exam 3 Textbook Questions

Which shows greater deviation?

Cross (b), because it has the larger χ2\chi^2χ2 (0.39 > 0.064).

both show no significant difference

Using p = 0.10 as the critical value is more stringent about not rejecting the null hypothesis. Increasing the critical p value means that a smaller χ² value is sufficient to reject the null hypothesis, so less difference between observed and expected values is needed for rejection. Therefore, it becomes harder to fail to reject the null hypothesis, increasing the stringency of failing to reject it.

a. consistent

b. consistent

c. not consistent

The creeper allele is dominant for the short-leg phenotype but lethal when homozygous.

Let C = creeper, c = normal.

• Creeper × normal: Cc × cc → 1/2 Cc (creeper), 1/2 cc (normal) ✅ matches 1:1

• Creeper × creeper: Cc × Cc → 1/4 CC (lethal, dies), 1/2 Cc (creeper), 1/4 cc (normal)
  Among living chicks: 2/3 creeper : 1/3 normal ✅

So creepers “never breed true” because CC doesn’t survive.

This trait shows incomplete dominance.

• CC = chestnut

• C Cᶜʳ = palomino

• CᶜʳCᶜʳ = cremello

Cremello × chestnut → F₁: all palomino
F₁ × F₁ → F₂: 1/4 chestnut : 1/2 palomino : 1/4 cremello

16a. AACC × aacc

• F₁: all AaCc → all agouti

• F₂ genotypes (simplified):
  9 A_C_, 3 aaC_, 4 __cc

• F₂ phenotypes:
  9 agouti : 3 black : 4 white

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16b. Genotypes of agouti females (crossed to aacc males)

1. 8 agouti : 8 white → female = AaCc

2. 9 agouti : 10 black (no white) → female = AaCC

3. 4 agouti : 5 black : 10 white → female = AaCc

Here are the F₁ phenotypic ratios (short, final answers only).
Key rule: cc = albino (epistatic); color only appears if C_ is present.

Phenotypes:

• A_B_ → gray

• A_bb → yellow

• aaB_ → black

• aabb → cream

• __cc → albino

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a. AAbbCC × aaBBcc

All offspring: AaBbCc
→ 100% gray

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b. AaBBCC × AABbcc

All offspring are C_, no albino.
A and B segregate.

→ 3/4 gray : 1/4 yellow

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c. AaBbCc × AaBbcc

• 1/2 cc → albino

• Remaining 1/2 C_ show A/B colors

Final ratio:

• 9/32 gray

• 3/32 yellow

• 3/32 black

• 1/32 cream

• 16/32 albino

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d. AaBBCc × AaBBCc

Only gray, black, albino possible.

→ 9/16 gray : 3/16 black : 4/16 albino

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e. AABbCc × AABbcc

• All A_ (no cream)

• 1/2 cc → albino

Final ratio:

• 3/8 gray

• 1/8 yellow

• 1/2 albino

a. Mode of inheritance

This is two independently assorting genes with a 9:3:3:1 ratio.

Let:

• A_B_ = yellow

• A_bb = blue

• aaB_ = red

• aabb = mauve

All colors can be true-breeding (homozygous).

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b. Cross: true-breeding red × true-breeding mauve

• Red = aaBB

• Mauve = aabb

F₁:
aaBB × aabb → aaBb → all red

F₂ (aaBb × aaBb):

• 3/4 red (aaB_)

• 1/4 mauve (aabb)

“Phenotypic expression is dependent on the genome of the organism, the immediate molecular and cellular environment of the genome, and numerous interactions among a genome, the organism, and the environment.”

Penetrance refers to the percentage of individuals among a population who express some degree of a mutant phenotype, whereas expressivity refers to the range of expression of a given phenotype and can vary from individual to individual.”

Labrador coat color is controlled by two genes with recessive epistasis.

Model (gene symbols):

• A_B_ = black

• aaB_ = brown

• __bb = golden (bb is epistatic and blocks pigment)

True-breeding genotypes:

• Black: AABB

• Brown: aaBB

• Golden: any bb genotype (AAbb or aabb)

This two-gene epistatic model explains all the listed mating outcomes and ratios.

Crossing over occurs more often between genes that are far apart because there is more physical space along the chromosome for a crossover to happen between them. Genes that are very close together are less likely to be separated by a crossover, since a single crossover event is unlikely to occur in the small interval between them.

A single crossover involves only two of the four chromatids in a homologous chromosome pair. Even if crossing over always occurs between two linked genes, only half of the chromatids become recombinant, while the other half remain parental. Therefore, the maximum possible recovery of single-crossover (recombinant) products is 50%, which is the upper limit.