Some terms
Consanguineous marriage: between related individuals
Proband, or propositus: index case or case that originally attracts attention of the geneticist to a family
Sibship: a family
Penetrance: the number of people with the gene that actually exhibit the trait.
Characteristics of autosomal dominant inheritance:
1. A gene is dominant if it is expressed when heterozygous
2. An affected individual has a 50% chance of having an affected child. 3. An affected child will have one affected parent
4. The affected parent can be either the mother or the father
5. Autosomal dominant traits have low frequencies in the population 6. Autosomal dominant traits are usually lethal when homozygous 7. No skipping of generations
aa
Aa
aa
Aa
aa
Aa
aa
Aa
Exceptions to strict dominant inheritance: 1. Variable expressivity: can occur in different forms or locations 2 Reduced penetrance: not all individuals with the gene express the trait.
Autosomal recessive inheritance:
1. Unaffected parents have affected children
2. Affected individual has unaffected children unless they marry a heterozygote or an affected person
3. Sexes are affected equally 4. All normal children of an affected individual will be heterozygotes or “carriers” 5. Often there has been a consanguineous marriage
Cc
Cc
C_
cc
Cc Cc
C_ Cc
C_
C_
cc
Sex linked recessive inheritance
1. Gene is on the X chromosome 2. Males are affected more often than females 3. Carrier females have affected and normal sons at a 1:1 ratio