In holstein cattle, the allele for black hair color (b) is dominant over the allele for red hair color (b), and the allele for polled (p), or lacking horns, is dominant over the allele for having horns(p). what is the expected phenotypic ratio of the offspring of a bbpp × bbpp cross if these alleles sort independently?††

We need to work out a Punnett square to determine the genotypes resulting from the cross between BbPp and BbPp. The dyhybrid cross results in 16 different genotypes, of which a few have the same phenotypes.

It can be seen that there are 4 different kinds of phenotypes present, in the ratio 9:3:3:1.

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marianaegarciaperedo marianaegarciaperedo · Answer 2 · 2021-11-10T16:37:54+01:00

We assume that these genes assort independently, have no interaction between them, and express complete dominance. By crossing two dihybrid individuals, it is expected a phenotypic ratio of 9:3:3:1 of the progeny.

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Available data:

Two diallelic genes involved in hair color and horns development

Gene for Hair color

B is the dominant allele and codes for black
b is the recessive allele and codes for red

Gene for horns development

P is the dominant allele and codes for polled or lacking horns
p is the recessive allele and codes for horns

We will assume that these genes assort independently and express complete dominance.

Cross: bbpp × bbpp

F1) 100% bbpp ⇒ red haired animals with horns

We will perform another cross, just in case, for you to know how to proceed in the presence of dominant and recessive alleles.

Cross: Between two dihybrid individuals

Parentals) BbPp x BbPp

Gametes) BP, Bp, bP, bp

BP, Bp, bP, bp

Punnett square) BP Bp bP bp

BP BBPP BBPp BbPP BbPp

Bp BBPp BBpp BbPp Bbpp

bP BbPP BbPp bbPP bbPp

bp BbPp Bbpp bbPp bbpp

F1) Genotypes

BBPP ⇒ 1/16
BBPp ⇒ 2/16
BbPP ⇒ 2/16
BBpp ⇒ 1/16
BbPp ⇒ 4/16
Bbpp ⇒ 2/16
bbPP ⇒ 1/16
bbPp ⇒ 2/16
bbpp ⇒ 1/16

Phenotypes

9/16 ⇒ B-P- ⇒ Black-haired with polled or lacking horns
3/16 ⇒ B-pp ⇒ Black-haired with horns
3/16 ⇒ bbP- ⇒ Red-haired with polled or lacking horns
1/16 ⇒ bbpp ⇒ Red-haired with horns

Note: The symbol - represents either a dominant or a recessive allele.

So, according to this last cross, the expected phenotypic ratio of the offspring is 9:3:3:1.

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