Question

In humans, hair color is controlled by the interaction of two genes. The same pigment, melanin, is present in chestnut-haired and blond-haired people, but chestnut people have much more. Brown hair (B) is dominant of blonde (r). Feeling melanin depends on another gene. The dominant form of this second gene (M) allows the synthesis of melanin; the recessive form (m) prevents such synthesis. Recessive homozygous (mm) are albinos. For each of the following junctions mention the phenotype of the parents.

a. BBMM X BrMm

b. BrMm X BrMm

c. BrMm X rrmm

Answer 1

The given data states that people with brown/Chestnut colored hair have more melanin than people with blond hair. Synthesis of melanin depends on M gene where M denoted melanin presence. However absence of melanin in the recessive form (mm) gives no colour or albino phenotype. So individuals with mm will neither be blond or chestnut but albinos. When MM or Mm is present only then, the B allele will show increased melanin production or less melanin production. So, the phenotypes for the following will be:  

a. BBMM(Brown haired) and BrMm (Brown haired).

In both the cases M allele denotes presence of melanin that influences B allele to produce more melanin resulting in brown hair.

b. BrMm (Brown hair) X BrMm (Brown hair)

c. BrMm (Brown hair) X rrmm (albino)

Absence of M allele means no melanin will be formed and so irrespective of B or r, these individuals will have albino phenotype.

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