Rabbit Body Weight Inheritance

This question describes the inheritance of body weight in rabbits, which is controlled by two gene loci, 'a' and 'b'. The alleles at these loci have additive and equal effects on body weight. This means that the more 'additive' alleles an individual has, the greater its body weight will be, above a certain base weight.

Understanding Additive Gene Effects

• A quantitative trait like body weight is often influenced by multiple genes, each contributing a small amount to the overall phenotype.
• In this case, alleles denoted by '+' are assumed to be additive alleles that increase weight, while alleles denoted by '-' are non-additive or contribute to the base weight.
• The effect of these additive alleles is cumulative.

Parental Genotypes and Weights

We are given the genotypes and average weights for the two extreme phenotypes:

• Parental Genotype 1: a- a- b- b- has an average body weight of 1 kg. This genotype has zero additive alleles (assuming '+' is additive). This represents the base weight.
• Parental Genotype 2: a+ a+ b+ b+ has animals averaging 3.4 kg in weight. This genotype has four additive alleles (two a+ and two b+).

Calculating the Effect of One Additive Allele

The total increase in weight from the genotype with 0 additive alleles to the genotype with 4 additive alleles is the difference between their weights:

\(\text{Total weight increase} = \text{Weight of a+ a+ b+ b+} - \text{Weight of a- a- b- b-}\)

\(\text{Total weight increase} = 3.4 \text{ kg} - 1 \text{ kg} = 2.4 \text{ kg}\)

This total increase of 2.4 kg is due to the presence of 4 additive alleles. Since the effects are equal and additive, the effect of one additive allele can be calculated as:

\(\text{Effect of one additive allele} = \frac{\text{Total weight increase}}{\text{Number of additive alleles}}\)

\(\text{Effect of one additive allele} = \frac{2.4 \text{ kg}}{4} = 0.6 \text{ kg}\)

Predicting F1 Progeny Genotype and Weight

A male rabbit with genotype a- a- b- b- is crossed with a female of genotype a+ a+ b+ b+.

• The male (a- a- b- b-) can only produce gametes with the alleles a- b-.
• The female (a+ a+ b+ b+) can only produce gametes with the alleles a+ b+.

When these gametes combine, the F1 progeny will all have the same genotype:

F1 Genotype = Gamete from male + Gamete from female = (a- b-) + (a+ b+) = a+ a- b+ b-

Now, let's determine the number of additive alleles in the F1 genotype (a+ a- b+ b-). Assuming a+ and b+ are the additive alleles:

• At locus 'a', the genotype is a+ a-. This contributes 1 additive allele (a+).
• At locus 'b', the genotype is b+ b-. This contributes 1 additive allele (b+).

Total number of additive alleles in the F1 genotype = 1 (from locus 'a') + 1 (from locus 'b') = 2 additive alleles.

Finally, we can calculate the predicted average weight of the F1 progeny. The weight is the base weight plus the combined effect of the additive alleles:

\(\text{F1 average weight} = \text{Base weight} + (\text{Number of additive alleles in F1} \times \text{Effect of one additive allele})\)

\(\text{F1 average weight} = 1 \text{ kg} + (2 \times 0.6 \text{ kg})\)

\(\text{F1 average weight} = 1 \text{ kg} + 1.2 \text{ kg}\)

\(\text{F1 average weight} = 2.2 \text{ kg}\)

Therefore, the predicted average weight of the F1 progeny of this cross is 2.2 kg.