Question

Assignment: Using Punnett Squares Exploration

Answer 1

You should draw these simple Punnett Squares as it's easy to see the answers:

1. Crossing RR x Rr results in all (100%) red cow offspring. 50% will be RR (homozygous dominant), 50% will be Rr (heterozygous).

2. Crossing Rr x Rr results in 3 red offspring (75%) and 1 white (25%).  Genotypes though will be 25% RR, 50% Rr, and 25% rr.

3. Crossing rr x rr will result in 100% white offspring. All offspring will be rr.

Answer 2

Answer:

a) First cross is between a a homozygous red bull (RR) and a heterozygous red cow (Rr)

This shows that "Red" color is a dominant trait

So the cross between Rr and RR is

R R

R RR RR

r Rr Rr

Genotypic ratio:  RR : Rr = 2 : 2 = 1: 1

Phenotypic ratio: homozygous red bull (RR)  :  heterozygous red cow (Rr) = 2:2 = 1: 1

b) Cross between a heterozygous red bull (Rr) and a heterozygous red cow (Rr)

R r

R RR Rr

r Rr rr

Genotypic ratio:  RR : Rr:rr = 1:2:1

Phenotypic ratio: homozygous red bull (RR)  :  heterozygous red cow (Rr) : homozygous white bull (rr) = 1: 2: 1

c) Cross between a homozygous white bull (rr) and a homozygous white cow (rr)

Xr Yr

Xr Xr Xr Xr Yr

Xr Xr Xr Xr Yr

Genotypic ratio:  Xr Xr: Xr Yr  = 2: 2 = 1: 1

Phenotypic ratio: homozygous white cow (Xr Yr )  :  heterozygous white bull (Xr Yr ) = 2: 2 = 1: 1

Explanation:

a) First cross is between a a homozygous red bull (RR) and a heterozygous red cow (Rr)

This shows that "Red" color is a dominant trait

So the cross between Rr and RR is

R R

R RR RR

r Rr Rr

Genotypic ratio:  RR : Rr = 2 : 2 = 1: 1

Phenotypic ratio: homozygous red bull (RR)  :  heterozygous red cow (Rr) = 2:2 = 1: 1

b) Cross between a heterozygous red bull (Rr) and a heterozygous red cow (Rr)

R r

R RR Rr

r Rr rr

Genotypic ratio:  RR : Rr:rr = 1:2:1

Phenotypic ratio: homozygous red bull (RR)  :  heterozygous red cow (Rr) : homozygous white bull (rr) = 1: 2: 1

c) Cross between a homozygous white bull (rr) and a homozygous white cow (rr)

Xr Yr

Xr Xr Xr Xr Yr

Xr Xr Xr Xr Yr

Genotypic ratio:  Xr Xr: Xr Yr  = 2: 2 = 1: 1

Phenotypic ratio: homozygous white cow (Xr Yr )  :  heterozygous white bull (Xr Yr ) = 2: 2 = 1: 1