Problem 3 - Campbell Biology

Question

A man has six fingers on each hand and six toes on each foot. His wife and their daughter have the normal number of digits. Remember that extra digits is a dominant trait. What fraction of this couple's children would be expected to have extra digits?

Step-by-Step Solution

Answer

50% of the couple's children would be expected to have extra digits.

Step 1 - Understanding Traits

Extra digits (polydactyly) is a dominant trait, which means it can be represented as 'P', while the normal number of digits (recessive trait) is 'p'.

Step 2 - Genotype of the Man

Since the man has extra digits, his genotype could be either 'PP' or 'Pp'. However, given that polydactyly is not more common in the population, it's likely that he is 'Pp' (heterozygous).

Step 3 - Genotype of the Wife

The wife has the normal number of digits, so her genotype is 'pp'.

Step 4 - Eliciting Possible Offspring Genotypes

Using the Punnett Square method, the man (Pp) can produce gametes 'P' or 'p', and the wife (pp) can produce gametes 'p' or 'p'. The possible combinations for their children are: Pp and pp.

Step 5 - Predicting Offspring's Traits

The combinations 'Pp' (extra digits) and 'pp' (normal digits) show that 50% of the children will have extra digits, and 50% will have a normal number of digits.

Key Concepts

polydactyly

Polydactyly is a condition characterized by having extra fingers or toes. It's a genetic trait that can be inherited from one or both parents. In many cases, the extra digits are fully functional, although they might be smaller or less developed than the usual five fingers or toes. Polydactyly can occur as an isolated condition or as part of other genetic syndromes.

This trait follows a dominant inheritance pattern, which means that only one copy of the dominant gene (represented as 'P') is needed for a person to exhibit the trait. Thus, an individual with a genotype of either 'PP' (homozygous dominant) or 'Pp' (heterozygous) will have this trait.

dominant and recessive traits

In genetics, traits are determined by specific genes, which come in different versions called alleles. These alleles can be dominant or recessive.

• Dominant Alleles: Represented by a capital letter (like 'P' for polydactyly), these alleles express the trait even if only one copy is present.
• Recessive Alleles: Represented by a lowercase letter (like 'p' for the normal number of digits), these alleles express the trait only if two copies are present (homozygous recessive 'pp').

When a dominant allele is paired with a recessive allele (heterozygous 'Pp'), the dominant trait will be displayed. In the case study, the man with polydactyly likely has the genotype 'Pp'. His wife, who has the normal number of digits, has the genotype 'pp'.

Punnett Square

A Punnett Square is a useful tool in genetics to predict the possible genotypes of offspring based on the genotypes of the parents. It shows how alleles from each parent can combine in their children.

In this case, the father has the genotype 'Pp', and the mother has the genotype 'pp'. To create a Punnett Square:

• List the possible gametes (sperm or eggs) from each parent. For 'Pp', the possible gametes are 'P' and 'p'. For 'pp', the only possible gametes are 'p' and 'p'.
• Draw a grid and combine the gametes in each cell. The combinations are: 'Pp' and 'pp'.

Based on the Punnett Square, there is a 50% chance ('Pp') for a child to have polydactyly and a 50% chance ('pp') to have a normal number of digits.