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All of the conclusions regarding heredity (dominant/recessive;

codominant) we have discussed so far have been obtained from

analyzing the results of controlled crosses.

In some situations, we do not have the opportunity to perform controlled crosses. Rather we need to analysis

an existing population.

Scientists have devised another approach, called pedigree analysis, to study the inheritance of genes in humans. Pedigree analysis is also useful when studying any population when progeny data from several generations is limited.

A series of symbols are used to represent different aspects of a

pedigree. Below are the principal symbols used when drawing a

pedigreeNormal Male

Affected Male

Normal Female

Affected Female

Mating

Parents

Offspring

Goals of Pedigree Analysis

• 1. Determine the mode of inheritance:

dominant, recessive, sex-linked, autosomal, maternal effect.

• 2. Determine the probability of an affected offspring for a given cross.

Is this a dominant or a recessive disorder?

This disorder is recessive. If it were dominant, it would show up in at

least one parent in each of the first two generations.

Is this a sex-linked disorder?The disorder is located on one of the 22

autosomes in humans. In autosomal disorders, the disorder is found equally

as often in both sexes

There are a few individuals in this pedigree that we can say for certain are heterozygous for the mutated gene. Which individuals do your

believe are heterozygous for the disorder?

1 2 3 4

5 6

We know that both of the individuals in generation 2 must carry a mutated

recessive allele because the couple has produced homozygous recessive

offspring.

1 2 3 4

5 6

Is the mutated gene in this family autosomal or sex linked?

Sex linked because is shows up only in males.

The mutated allele is located

on the X chromosome.

The affected male in the first generation passes his mutated X chromosome to all of his daughters, but none of his sons. These daughters are carriers and pass the mutated chromosome to half of their offspring- both males and females. The males that receive the mutated X chromosome are all affected. Remember males have only one X chromosome. Females can have a normal X to override the disorder.