Mendel and the Gene Idea

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Recessively inherited disorders show up only in individuals homozygous for the allele

Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal (i.e., pigmented)

Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair

Slide 77

Fig. 14-16

Parents

Normal

Normal

Sperm

Eggs

Normal

Normal

(carrier)

Normal

(carrier)

Albino

Aa

Aa

A

A

AA

Aa

a

Aa

aa

a



Slide 78

If a recessive allele that causes a disease is rare, then the chance of two carriers meeting and mating is low

Consanguineous matings (i.e., matings between close relatives) increase the chance of mating between two carriers of the same rare allele

Most societies and cultures have laws or taboos against marriages between close relatives

Slide 79

Cystic Fibrosis

Cystic fibrosis is the most common lethal genetic disease in the United States,striking one out of every 2,500 people of European descent

The cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes

Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine

Slide 80

Sickle-Cell Disease

Sickle-cell disease affects one out of 400 African-Americans

The disease is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells

Symptoms include physical weakness, pain, organ damage, and even paralysis

Slide 81

Dominantly Inherited Disorders

Some human disorders are caused by dominant alleles

Dominant alleles that cause a lethal disease are rare and arise by mutation

Achondroplasia is a form of dwarfism caused by a rare dominant allele

Slide 82

Fig. 14-17

Eggs

Parents

Dwarf

Normal

Normal

Normal

Dwarf

Dwarf

Sperm

Dd



dd

d

D

Dd

dd

dd

Dd

d

d

Slide 83

Huntington’s Disease

Huntington’s disease is a degenerative disease of the nervous system

The disease has no obvious phenotypic effects until the individual is about 35 to 40 years of age

Slide 84

Multifactorial Disorders