In mice, a gene controlling coat colour has 2 alleles; B (black) and b (white). A pure breeding black mouse is crossed to a white mouse and all the offspring are black. The offspring are then crossed to white mice. The phenotypes of the offspring of this second cross are expected to be
all black.
1/4 black, 3/4 white.
all white.
1/2 black, 1/2 white.
3/4 black, 1/4 white.
The mice would be half heterozygous for black, and half white.
1/2 black, 1/2 white
To determine the phenotypes of the offspring in the second cross, we need to consider the genotypes of the parents and use Punnett squares.
Let's assign the following symbols for the genotypes:
- Black (pure breeding) mouse: BB
- White mouse: bb
In the first cross, a pure breeding black mouse (BB) is crossed to a white mouse (bb). Since black color is dominant over white, all the offspring would be black (Bb).
Now, in the second cross, the black offspring (Bb) are crossed to white mice (bb). Let's set up a Punnett square to see the possible combinations:
| Bb | Bb |
_______________________
bb | Bb | Bb |
_______________________
bb | bb | bb |
From the Punnett square, we can see that there are two possible genotypes of the offspring: Bb (black) and bb (white).
Therefore, the phenotypes of the offspring in the second cross would be 1/2 black and 1/2 white.
The correct answer is: 1/2 black, 1/2 white.
To determine the phenotypes of the offspring in the second cross, we need to understand how the alleles for coat color are inherited.
In this case, the gene controlling coat color has two alleles: B (black) and b (white). A pure breeding black mouse is crossed with a white mouse, indicating that the black mouse is homozygous for the black allele (BB) and the white mouse is homozygous for the white allele (bb).
When two animals reproduce, each parent contributes one allele to form the offspring's genotype. In this scenario, the black mouse (BB) will always pass the black allele (B), while the white mouse (bb) can only pass the white allele (b).
When the pure breeding black mouse (BB) is crossed with the white mouse (bb), all the offspring will be heterozygous (Bb) because each offspring receives one black allele (B) from the black parent and one white allele (b) from the white parent. As a result, all the offspring will exhibit the black coat color phenotype.
In the second cross, when the heterozygous offspring (Bb) are crossed with white mice (bb), the possible genotypes of the offspring will be:
- 1/2 of the offspring will be Bb, resulting in the black phenotype.
- 1/2 of the offspring will be bb, resulting in the white phenotype.
Therefore, the phenotypes of the offspring in the second cross are expected to be 1/2 black and 1/2 white. Hence, the correct answer is "1/2 black, 1/2 white."