Question

Corn snakes show variety in their skin color pattern. While the complete genetics of corn

snake color are complex, the most common colors on normal corn snakes—red and
black— are each coded by one gene.
For the red gene, the allele for the presence of red pigment (R) is dominant and the allele
for the absence of red pigment (r) is recessive. Likewise, for the black gene, the allele for
the presence of black pigment (B) is dominant and the allele for the absence of black
pigment (b) is recessive.
a. Draw the Punnett square for the cross of a snake that is homozygous dominant for
the red color with a snake that is heterozygous for the red color. What percentage
of the offspring is expected to have red pigment in their skin?
b. Draw the Punnett square for the cross of two snakes that are heterozygous for the
black color. What percentage of the offspring are expected to have black pigment
in their skin?
c. The parent snakes in part (b) that are heterozygous for black color are both
homozygous recessive for the red gene. Each parent has genotype rr for the red
gene. Based on this information, what percentage of their offspring are expected
to lack both the red and black pigments in their skin? Explain your reasoning.

Answer 1

a. Punnett square for the cross of a snake that is homozygous dominant for the red color (RR) with a snake that is heterozygous for the red color (Rr):

```

| R | R |

----------------

| RR | RR | RR |

----------------

| Rr | Rr | Rr |

```

In this cross, all the offspring (100%) will have the red pigment in their skin because the presence of red pigment (R) is dominant over its absence (r).

b. Punnett square for the cross of two snakes that are heterozygous for the black color (Bb):

```

| B | b |

----------------

| BB | BB | Bb |

----------------

| Bb | Bb | bb |

```

In this cross, 75% of the offspring are expected to have black pigment in their skin (BB and Bb genotypes), and 25% of the offspring are expected to lack black pigment (bb genotype) due to the absence of the dominant black allele (B).

c. Both parent snakes are homozygous recessive for the red gene (rr), meaning they lack the red pigment. Since the red gene and black gene are independent of each other, the absence of red pigment does not affect the inheritance of the black pigment. Therefore, the percentage of offspring expected to lack both red and black pigments in their skin would be the same as the percentage of offspring lacking the black pigment (bb genotype) in the Punnett square from part (b). Thus, 25% of their offspring are expected to lack both the red and black pigments in their skin.