Question

In shorthorn cattle, the polled (hornless) condition is dominant over the horned condition. Heterozygotes produced by mating red and white cattle are roan-colored; and long legs are dominant over short. A homozygous red, polled, long-legged bull is mated to a herd of cows that are all homozygous for the alternative alleles at all three loci. a) What will the progeny look like? (1) b) When the F1 progeny are mated, what phenotypic combinations, and in what ratio, should be expected in the F1 if independent assortment occurs? (2)

Answer 1

Answer (a)

A trihybrid cross between two cattles that are homozygous for three different traits.

Our trihybrid example:

HHRRLL X hhrrll is a trihybrid cross.

The horn of the cattle is controlled by one set of alleles, where H ( Hornless condition) is completely dominant over (h) Horn condition.

HH : Hornless condition

Hh : Hornless condition

hh : Horn condition.

The color of the cattle is controlled by second set of allele (example of incomplete dominance) where:

RR : Red

Rr : Roan color

rr : White

The third set of alleles controlled the leg size of the cattle where L (long leg) is completely dominant over l (short leg)

LL : Long legged

Ll : Long legged

ll : Short legged

F1 progeny that will be produced after the cross between two homozygous cattles

Parental : HHRRLL X hhrrll

F1 progeny : HhRrLl (Hornless, Roan colored and Long legged)

So, the phenotype of the progeny that are produced after the above parental cross should have following features:

Hornless condition (H dominant over h), Roan colored (Incomplete dominance) and Long legged (L is dominant over l).

Answer (b)

When F1 progeny are mated i.e HhRrLl

F1 progeny have following 8 gametes :

HRL, HRl, HrL, Hrl, hRL, hRl, hrL and hrl

f1 cross	HRL	HRl	HrL	Hrl	hRL	hRl	hrL	hrl
HRL	HRRLL	HHRRLl	HHRrLL	HHRrLl	HhRRLL	HhRRLl	HhRrLL	HhRrLl
HRl	HHRRLl	HHRRll	HHRrLl	HHRrll	HhRRLl	HhRRll	HhRrLl	HhRrll
HrL	HHRrLL	HHRrLl	HHrrLL	HHrrLl	HhRrLL	HhRrLl	HhrrLL	HhrrLl
Hrl	HHRrLl	HHRrll	HHrrLl	HHrrll	HhRrLl	HhRrll	HhrrLl	Hhrrll
hRL	HhRRLL	HhRRLl	HhRrLL	HhRrLl	hhRRLL	hhRRLl	hhRrLL	hhRrLl
hRl	HhRRLl	HhRRll	HhRrLl	HhRrll	hhRRLl	hhRRll	hhRrLl	hhRrll
hrL	HhRrLL	HhRrLl	HhrrLL	HhrrLl	hhRrLL	hhRrLl	hhrrLL	hhrrLl
hrl	HhRrLl	HhRrll	HhrrLl	Hhrrll	hhRrLl	hhRrll	hhrrLl	hhrrll

Phenotype Ratio :

Hornless, Red and Long : 9

Hornless, Red and small : 3

Hornless, Roan and Long : 14

Hornless, Roan and small: 6

Hornless, white and long : 9

Hornless, white and small: 3

Horn , Red and long : 3

Horn, Red and short : 1

Horn , Roan and long : 6

Horn , Roan and short : 2

Horn , white and long : 3

Horn , white and short : 1

Genotypic ratio:

1: HHRRLL

2: HHRRLl

1: HHRRll

2: HHRrLL

4: HHRrLl

2 : HHRrll

1: HHrrLL

2: HHrrLl

1: HHrrll

2: HhRRLL

4: HhRRLl

2 : HhRRll

4: HhRrLL

8: HrRrLl

4: HhRrll

2: HhrrLL

4: HhrrLl

2: Hhrrll

1: hhRRLL

2: hhRRLl

1: hhRRll

2: hhRrLL

4: hhRrLl

2: hhRrll

1:hhrrLL

2: hhrrLl

1:hhrrll