Part a: Phenotypes and Genotypes of Rabbits in Crosses
We are working with dark fur (R) as the dominant allele and white fur (r) as the recessive allele in rabbits. Here’s how different crosses would play out:
1. Parent Cross: Homozygous Dominant × Homozygous Recessive (RR × rr)
- Parent 1 (RR): Homozygous dominant, dark fur
- Parent 2 (rr): Homozygous recessive, white fur
When these two rabbits are crossed (RR × rr), the offspring will inherit one allele from each parent. All offspring will inherit R from the first parent and r from the second, resulting in Rr for all F1 rabbits.
- F1 Genotype: Rr (heterozygous)
- F1 Phenotype: Dark fur (since R is dominant over r)
2. Parent Cross: Heterozygous × Homozygous Recessive (Rr × rr)
- Parent 1 (Rr): Heterozygous, dark fur
- Parent 2 (rr): Homozygous recessive, white fur
When crossed (Rr × rr), the offspring can inherit the following combinations:
-
50% chance of Rr (dark fur)
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50% chance of rr (white fur)
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F1 Genotype: 50% Rr (heterozygous), 50% rr (homozygous recessive)
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F1 Phenotype: 50% dark fur, 50% white fur
3. Parent Cross: Heterozygous × Heterozygous (Rr × Rr)
- Parent 1 (Rr): Heterozygous, dark fur
- Parent 2 (Rr): Heterozygous, dark fur
When crossed (Rr × Rr), the offspring can inherit the following combinations:
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25% RR (homozygous dominant, dark fur)
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50% Rr (heterozygous, dark fur)
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25% rr (homozygous recessive, white fur)
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F1 Genotype: 25% RR (homozygous dominant), 50% Rr (heterozygous), 25% rr (homozygous recessive)
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F1 Phenotype: 75% dark fur, 25% white fur
Part b: Explanation of the Law to Which These Crosses Belong
These crosses follow Mendel’s Law of Segregation.
Mendel’s Law of Segregation states that:
- Every individual has two alleles for a given gene (one inherited from each parent).
- These alleles separate (segregate) during the formation of gametes (egg and sperm).
- Each gamete carries only one allele for each gene.
- When gametes combine during fertilization, the resulting offspring will inherit one allele from each parent, restoring the pair of alleles.
In the cases described, for the fur color gene in rabbits:
- Each parent contributes one allele to their offspring.
- The R (dominant dark fur) and r (recessive white fur) alleles segregate during gamete formation.
- The offspring inherit one allele from each parent, and the combination of alleles determines the phenotype (fur color) of the offspring.
The observed ratios in the F1 generation (like 3 dark fur: 1 white fur in the Rr × Rr cross) are direct evidence of Mendel’s Law of Segregation.