Question

In rabbits, the color of body fat is controlled by a single gene with two alleles, designated \(Y\) and \(y\). The outcome of this trait is affected by the diet of the rabbit. When raised on a standard vegetarian diet, the dominant \(Y\) allele confers white body fat, and the \(y\) allele confers yellow body fat. However, when raised on a xanthophyll-free diet, a homozygote \(y y\) rabbit has white body fat. If a heterozygous rabbit is crossed to a rabbit with yellow body fat, what are the proportions of offspring with white and yellow body fat when raised on a standard vegetarian diet? How do the proportions change if the offspring are raised on a xanthophyll-free diet?

Short answer

When raised on a vegetarian diet, 50% offspring will have white body fat and 50% will have yellow body fat. However, if the offspring are raised on a xanthophyll-free diet, all offspring will have white body fat irrespective of their genotype.

Step-by-step solution

Identify the Genotype of Parents

First identify the genotypes of the parents. The rabbit with white body fat is heterozygous (\(Y y\)) and the other with yellow body fat is homozygous recessive (\(y y\)).

Perform a Punnett Square

Next, create a Punnett Square to determine the genotype of the offspring. The \(Y\) and \(y\) alleles from the heterozygote parent will each combine with the \(y\) allele from the homozygous recessive parent. The resultant genotypes will be \(Y y\) and \(y y\).

Predict Offspring Phenotypes under a Vegetarian Diet

Using the genotypes obtained from the punnett square, determine the phenotype under a vegetarian diet. \(Y y\) (white body fat) and \(y y\) (yellow body fat) are the genotypes. Hence in a population, 50% of the offspring will have white body fat and 50% will have yellow body fat when raised on a vegetarian diet.

Predict Offspring Phenotypes under a Xanthophyll-free Diet

Under a Xanthophyll-free diet, the \(y y\) phenotype changes from yellow body fat to white body fat. Therefore all of the offspring, both \(Y y\) and \(y y\) genotypes, will have white body fat when raised on a Xanthophyll-free diet.

Key concepts

Punnett Square

Understanding how genetics work can sometimes be complicated, but there's a handy tool called the Punnett Square that makes it easier to visualize how alleles are inherited. The Punnett Square is a diagram that is used to predict the genotype of the offspring resulting from a cross between two organisms. In our rabbit example, a heterozygous parent (carrying alleles Y and y) is crossed with a homozygous recessive parent (carrying alleles y y). By drawing a box divided into four squares, each square represents a possible genotype of the offspring.

We place the alleles of one parent along the top and the alleles of the other parent along the side. The squares are filled in by combining the alleles along the row and column of each square. This simple visualization allows us to see that the possible genotypes for the offspring are either Y y or y y. Understanding the use of the Punnett Square is foundational for genetics, as it helps predict the distribution of inherited traits.

Allele Inheritance

Alleles are different forms of a gene that determine distinct traits that can be passed from parents to offspring. The way alleles are inherited follows certain patterns, with dominant alleles being expressed over recessive ones. In the context of our rabbit scenario, the allele for white body fat (Y) is dominant over the allele for yellow body fat (y).

A rabbit with a genotype Y y will have white body fat because the dominant Y allele masks the presence of the recessive y allele. Conversely, only when a rabbit inherits two copies of the recessive y y will it show yellow body fat, demonstrating the concept of complete dominance in allele inheritance. However, this inheritance can be influenced by environmental factors, as evidenced by the effect of diet on the phenotype, which we will explore further in the next section.

Phenotype Diet Influence

While genetics play a crucial role in determining an organism's characteristics, external factors such as diet can also significantly affect phenotypes. A phenotype is the observable physical property of an organism, and it can be influenced by both genetic and environmental components. Our rabbits’ body fat color illustrates this well. On a standard vegetarian diet, the inheritance of alleles Y and y lead to white or yellow body fat, depending on the combination.

However, when the rabbits are raised on a xanthophyll-free diet, the expression of the yellow body fat phenotype is altered. Interestingly, this diet causes homozygous y y rabbits, which would normally exhibit yellow body fat under a standard diet, to develop white body fat instead. This showcases how diet can modify the expression of certain genes, and underscores the complexity of phenotypes beyond simple genetic inheritance. This concept is essential for understanding how organisms can have different physical traits even with the same genotypes in varied environments.