Genetic linkage mapping for a large number of families identifies 4% recombination between the genes for Rh blood type and elliptocytosis (see Problem 18). At the Rh locus, alleles R and r control Rh+ and Rh- blood types. Allele E producing elliptocytosis is dominant to the wild-type recessive allele e. Tom and Terri each have elliptocytosis, and each is . Tom's mother has elliptocytosis and is Rh- while his father is healthy and has Rh+. Terri's father is Rh+ and has elliptocytosis; Terri's mother is Rh- and is healthy.

What is the probability that a child of Tom and Terri who is Rh+ will have elliptocytosis?

Key Concepts

Genetic Linkage

Genetic linkage refers to the tendency of genes located close to each other on a chromosome to be inherited together during meiosis. The degree of linkage can be measured by recombination frequency, which indicates how often crossing over occurs between these genes. In this case, a 4% recombination frequency between the Rh blood type and elliptocytosis genes suggests they are relatively close on the same chromosome.

Alleles and Dominance

Alleles are different forms of a gene that can exist at a specific locus on a chromosome. In this scenario, the Rh blood type is determined by alleles R (Rh+) and r (Rh-), while elliptocytosis is controlled by alleles E (dominant) and e (recessive). Understanding the dominance relationships between these alleles is crucial for predicting the phenotypes of offspring based on parental genotypes.

Punnett Square and Probability

A Punnett square is a tool used in genetics to predict the genotypes and phenotypes of offspring from parental crosses. By filling out a Punnett square with the possible gametes from Tom and Terri, one can calculate the probability of their child being Rh+ and having elliptocytosis. This method allows for a systematic approach to understanding inheritance patterns and calculating the likelihood of specific traits in offspring.