Patterns of Heredity

Pedigrees

March 19, 2010

• Interpret a pedigree.

Section Objectives:

• Identify human genetic disorders caused by inherited recessive alleles.

• Predict how a human trait can be determined by a simple dominant allele.

• A pedigree is a graphic representation of genetic inheritance.

• It is a diagram made up of a set of symbols that identify males and females, individuals affected by the trait being studied, and family relationships. It is like a family tree for your genes.

Pedigrees illustrate inheritancePedigrees illustrate inheritance

Key for Understanding

Pedigrees

Key for Understanding

Pedigrees

Male

Female

Affected male

Affected female

Mating

Parents

Siblings

Known heterozygotes for recessive allele

Death

• In a pedigree, a circle represents a female; a square represents a male.

Pedigrees illustrate inheritance

Pedigrees illustrate inheritance

Female Male

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• Highlighted circles and squares represent individuals showing the trait being studied.

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Pedigrees illustrate inheritance

Pedigrees illustrate inheritance

Pedigrees illustrate inheritance

Pedigrees illustrate inheritance

• Circles and squares that are not highlighted designate individuals that do not show the trait.

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• A half-shaded circle or square represents a carrier, a heterozygous individual.

Pedigrees illustrate inheritancePedigrees illustrate inheritance

Simple Recessive HereditySimple Recessive Heredity

• Most genetic disorders are caused by recessive alleles.

Cystic fibrosisCystic fibrosis

• Cystic fibrosis (CF) is a fairly common genetic disorder among white Americans.

Another is

• Tay-Sachs (tay saks) disease. It is a recessive disorder of the central nervous system.

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Typical Pedigree

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Typical Pedigree

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Tay-Sachs

Who had this

disease?

Tay-Sachs

Who had this

disease?

Huntington’s diseaseHuntington’s disease

• Huntington’s disease is a lethal genetic disorder caused by a rare dominant allele.

• It results in a breakdown of certain areas of the brain.

Huntington’s diseaseHuntington’s disease

• Ordinarily, a dominant allele with such severe effects would result in death before the affected individual could have children and pass the allele on to the next generation.

• But because the onset of Huntington’s disease usually occurs between the ages of 30 and 50, an individual may already have had children before knowing whether he or she is affected.

Typical Pedigree of Huntington’s DiseaseTypical Pedigree of Huntington’s Disease

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