Question

Suppose that a gene for coat color in mice has two alleles, B and b, where B is completely dominant and encodes a black coat color, and b is recessive and encodes a brown coat color. A mouse that is heterozygous has genotype _____, and its phenotype is _____.

Answer 1

Answer:

Genotype= Bb

Phenotype= Black coat

Explanation:

A single gene coding for coat color in mice is involved here. An allele is a contrasting or variant form of a gene. Most genes possess two alleles.

According to Mendel's findings in his law of dominance, an allele is capable of masking the expression of another gene. The allele masking or being expressed is called a DOMINANT allele while the allele being masked is called a RECESSIVE allele.

The alleles of a particular gene can either be in a homozygous or heterozygous state. The alleles are homozygous when they are the same in that gene while it is heterozygous when they contain two different alleles i.e. a combination of the dominant and recessive allele.

In this case of a gene coding for coat color in mice, The allele coding for Black coat (B) is dominant over that coding for brown coat (b). This means that in an heterozygous state, allele B will mask the expression of allele b.

The genotype of an organism is the actual genetic make-up of that organism while its phenotype is just its phyaically expressed trait.

Since a heterozygous organism has a combination of two different alleles, hence, the mouse in an heterozygous state for coat color will have a Bb genotype.

Also, since the allele for black coat coloration (B) is dominant over the brown coat coloration (b), thus, in a heterozygous state, the black colour allele will mask the expression of the brown color allele, and express itself (exercise its dominance).

Therefore, a Bb genotypic mouse will produce a black coat phenotype.

N.B: A recessive trait can only be expressed when the organism is homozygous recessive for a particular gene i.e. have the same recessive genes but a dominant phenotype can have a homozygous or heterozygous genotype.