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3.3: Pedigree

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    What's a pedigree?

    When you are talking about a pedigree dog, it means the dog is purebred. Through selective breeding, the dog has all the traits of that particular breed. When talking about genetics, however, a pedigree is a chart that helps show family relationships.

    Pedigree Analysis

    A pedigree is a chart that shows the inheritance of a trait over several generations. A pedigree is commonly created for families, and it outlines the inheritance patterns of genetic disorders and traits. A pedigree can help predict the probability that offspring will inherit a genetic disorder.

    Pictured below is a pedigree displaying recessive inheritance of a disorder through three generations (Figure below). From studying a pedigree, scientists can determine the following:

    • If the trait is sex-linked (on the X or Y chromosome) or autosomal (on a chromosome that does not determine sex).
    • If the trait is inherited in a dominant or recessive fashion.

    Sometimes pedigrees can also help determine whether individuals with the trait are heterozygous (two different alleles) or homozygous (two of the same allele). Some points to keep in mind when analyzing a pedigree are:

    1. With autosomal recessive inheritance, all affected individuals will be homozygous recessive.
    2. With dominant inheritance, all affected individuals will have at least one dominant allele. They will be either homozygous dominant or heterozygous.
    3. With sex-linked inheritance, more males (XY) than females (XX) usually have the trait. Sex-linked inheritance is usually recessive.
    Symbols of a pedigree, and an example pedigree
    Figure \(\PageIndex{1}\): In a pedigree, squares symbolize males, and circles represent females. A horizontal line joining a male and female indicates that the couple had offspring. Vertical lines indicate offspring which are listed left to right, in order of birth. Shading of the circle or square indicates an individual who has the trait being traced. In this pedigree, the inheritance of the recessive trait is being traced. A is the dominant allele, and a is the recessive allele.

    Summary

    • A pedigree is a chart which shows the inheritance of a trait over several generations.
    • From studying a pedigree, scientists can determine if a trait is sex-linked or autosomal.

    Review

    1. What is a pedigree?
    2. How might a pedigree aid a scientist?
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