Question

Black hair (B) is dominant over blonde hair (b); similarly, brown eyes (H) are dominant over blue eyes (h). Suppose that a man who is heterozygous for black hair and brown eyes marries a blond woman with blue eyes. Answer the following question based on the given situation.
a) What is the phenotypic ratio of F1 generation?
b) What percentage of offspring will have blonde hair and blue eyes?
c) What will be the percentage of homozygous dominant offspring?

Answer 1

Hint:- The monohybrid cross is made when the only single character needs to be determined. A dihybrid cross is made when two characters are studied together in an organism. To know the expected offspring produced from the cross Punnett square is made and the results are studied from it.

Complete answer:
In the above-given question the homozygous dominant genes for the hair type would be ‘BB’ as black hair type is dominant. In heterozygous conditions it would be considered as ‘Bb’ and for the homozygous recessive hair type, it would be ‘bb’.
So, if the parent has heterozygous black hair and brown eyes then its genotype would be ‘BbHh’ and the parent with blonde hair and blue eyes would be ‘bbhh’.
The gametes of the parent of BbHh would be – BH, Bh, bH, and bh.
The gametes of the parent of bbhh would be - bh, bh, bh, and bh.
The dihybrid cross between the parents would be:

| BH| Bh| bH| bh
---|---|---|---|---
bh| BbHh| Bbhh| bbHh| bbhh
bh| BbHh| Bbhh| bbHh| bbhh
bh | BbHh| Bbhh| bbHh| bbhh
bh | BbHh| Bbhh| bbHh| bbhh

So, the progeny obtained from the cross has black hair and brown eyes (BbHh), black hair and blue eyes (Bbhh), blonde hair and brown eyes (bbHh) and blonde hair and blue eyes (bbhh).
a) The phenotypic ratio obtained of the progeny is 1:1:1:1.
b) The percentage of the blonde hair blue eyes offspring is 25 percent.
c) The progeny obtained doesn’t have homozygous dominant genes; they only have heterozygous dominant genes.

Note:- Mendel knew about the dominant relationship that each allele possessed for each trait. But to determine any dependence of two traits on each other dihybrid cross was made. A Dihybrid cross is made for those organisms who have an identical hybrid of two traits.