FLASHCARDS – Lecture 3.2: Dihybrid Cross

What is a dihybrid cross?

A genetic cross that tracks inheritance of two genes at once

What genotype is used in a classic dihybrid cross?

RrYy × RrYy (heterozygous for both traits)

What does independent assortment mean?

Each gene segregates into gametes independently of the other genes

How many gametes does a RrYy individual produce?

4 gametes: RY, Ry, rY, ry

What is the phenotypic ratio in a dihybrid cross (RrYy × RrYy)?

9:3:3:1 (9 dominant/dominant, 3 dominant/recessive, 3 recessive/dominant, 1 recessive/recessive)

What does a 9:3:3:1 ratio indicate?

That the two genes are unlinked and assort independently

What gametes are produced by TtYyRr?

8 gametes: TYR, TYr, TyR, Tyr, tYR, tYr, tyR, tyr

What is a dihybrid testcross?

Crossing a heterozygous individual (AaBb) with a double homozygous recessive (aabb)

What is the expected ratio in a dihybrid testcross?

1:1:1:1 phenotypic ratio if genes assort independently

What does a deviation from 1:1:1:1 in a testcross suggest?

The genes are likely linked

How do you calculate the number of gamete combinations from a genotype?

Use 2^n, where n = number of heterozygous genes

What is the formula for binomial combinations?

(\binom{n}{k} = \frac{n!}{k!(n-k)!})

What does \binom{n}{k} represent in genetics?

The number of ways to arrange k successes in n trials (e.g., 4 girls out of 7 kids)

Are \binom{7}{4} and \binom{7}{3} equal?

Yes, because \binom{n}{k}=\binom{n}{n-k}