Question

Unicorn horns can be smooth (S), spiralled (P), or bent (B). Note: letters in brackets represent the phenotype.

You cross true breeding lines of the different phenotypes to one-another, then intercross the F₁s (F₁ X F₁) and find the following (letters represent phenotypes)

• Cross 1 - Parental: S X B; F₁ = B; F₂ = 102B: 34S
• Cross 2 - Parental: P X B; F₁ = P; F₂ = 117P: 39B
• Cross 3 - Parental: P X S; F₁ = P; F₂ = 96P: 24B: 8S

You are not the only scientist working on unicorn horn genetics and while you're performing your crosses a paper is published showing that two genes are involved in unicorn horn shape.

We will call these genes A, and B. There are two alleles of each - A and a; B and b.

Using the cross results above what were the parental genotypes and the genotype of the F1 for each cross (Cross 1, Cross 2 and Cross 3), above. Make sure in your answer it's clear how genotype (genes A and B) and phenotype (spiralled, smooth, bent) are related.

Hint: Start with Cross 3. Review how we got a 9:7 phenotypic ratio when two genes complement each other (instead of 9:3:3:1); we calculated 7 by adding multiple categories from the Mendelian 9:3:3:1 ratio together based on how the genes were interacting -> 3 + 3+ 1 = 7. What might a 96:24:8 ratio suggest (95:25:8 = 12:3:1)?

Answer 1

* Cross 3 - Parental: P X S; F₁ = P; F₂ = 96P: 24B: 8S

Phenotypic ratio of F2- 12:3:1 { 12/16 A_B_: 3/16 aaB_: 1/16 aabb}

This ratio suggest DOMINANT EPISTASIS

Parental: AABB X aabb

(genotype of S: aabb, genotype of P: AABB)

F1: AaBb

F2: 96 A_B_, 24 aaB_, 8aabb

* Cross 1 - Parental: S X B; F₁ = B; F₂ = 102B: 34S

Parental: aabb X aaBB

(Genotype of S: aabb, geotypge of B: aaBB)

F1: aaBb

F2: 102 aaB_, 34 aabb

* Cross 2 - Parental: P X B; F₁ = P; F₂ = 117P: 39B

Parental: AABB X aaBB

(genotype of P: AABB, genotype of B: aaBB)

F1: AaBB

F2: 117 A_BB, 39 aaBB

Here, Unicorns dominant for both genes (A and B) have spiralled horns (A_B_)

Unicorns homozygous recessive for gene A and dominant for gene B have bent horns

Unicorns homozygous recessive for both genes (A and B) have smooth horns (aabb)

Thus, the dominant allele A is epistatic to both B and bb combinations i.e it blocks the function of allele B.