Question

1. in a certain strain of mice, black coat (b) is dominant over white coat (b). describe what you would do to determine the genotype of a male with a black coat and how this would enable you to choose between the genotypes bb or bb.

Explanation:

To determine the genotype of the black - coated male mouse (either \(BB\) or \(Bb\)), we can use a test cross. A test cross involves crossing the individual with an unknown genotype (the black - coated male) with an individual that is homozygous recessive for the trait. In this case, the homozygous recessive individual would be a white - coated mouse with the genotype \(bb\).

Step 1: Understand the principle of test cross

The test cross works based on the principle of Mendelian genetics. If the unknown individual is homozygous dominant (\(BB\)), when it is crossed with a homozygous recessive (\(bb\)) individual, all of the offspring will inherit one dominant allele from the unknown parent and one recessive allele from the \(bb\) parent. So, all the offspring will have the genotype \(Bb\) and will show the dominant phenotype (black coat).

If the unknown individual is heterozygous (\(Bb\)), when it is crossed with a \(bb\) individual, half of the offspring will inherit the dominant allele \(B\) from the unknown parent and the recessive allele \(b\) from the \(bb\) parent (genotype \(Bb\), black coat), and the other half will inherit the recessive allele \(b\) from the unknown parent and the recessive allele \(b\) from the \(bb\) parent (genotype \(bb\), white coat).

Step 2: Perform the cross and observe the offspring

1. Cross the black - coated male (unknown genotype: \(BB\) or \(Bb\)) with a white - coated female (\(bb\)).
2. Observe the phenotypes of the offspring:

• If all the offspring have black coats, then the genotype of the black - coated male is most likely \(BB\). This is because when \(BB\) (black) is crossed with \(bb\) (white), the Punnett square for the cross \(BB\times bb\) is:

\[

$$\begin{array}{|c|c|} \hline & b & b \\ \hline B & Bb & Bb \\ \hline B & Bb & Bb \\ \hline \end{array}$$

\]
All the offspring have the genotype \(Bb\) and thus a black coat.

• If some of the offspring have white coats (and some have black coats), then the genotype of the black - coated male is \(Bb\). The Punnett square for the cross \(Bb\times bb\) is:

\[

$$\begin{array}{|c|c|} \hline & b & b \\ \hline B & Bb & Bb \\ \hline b & bb & bb \\ \hline \end{array}$$

\]
Here, 50% of the offspring have the genotype \(Bb\) (black coat) and 50% have the genotype \(bb\) (white coat).

Answer:

To determine the genotype of the black - coated male mouse, we perform a test cross by crossing it with a white - coated (\(bb\)) mouse. If all offspring are black, the male's genotype is \(BB\); if some offspring are white, the male's genotype is \(Bb\).