Mendelian Inheritance Patterns

Study Notes

Pattern of Inheritance

Predicting genetic risk in pregnancy outcomes, adult-onset disorders, and recurrence risks in future offspring
Collection, storage, and analysis of pedigree information can be done manually or using software

Mendelian Inheritance Patterns

Classified into five basic patterns based on gene location and expression
Autosomal recessive inheritance
- Result from defect gene located on an autosome and expressed in homozygous state
- Heterozygotes unaffected, homozygotes affected
- Risk of affected child: 25%, phenotypically unaffected carrier child: 50%, normal child: 25%
- Affected individuals usually in one single generation
- Consanguineous marriage plays an important role in the appearance of this disorder
Autosomal dominant inheritance
- Result from defect gene located on an autosome and expressed in heterozygous state
- Heterozygotes affected
- Very rare in homozygous state
- 50% chance of transmitting the trait to offspring
- Males and females are at equal risk
- Every affected individual usually has an affected parent
Co-dominance
- Co-dominant alleles are fully expressed in heterozygotes
- Example: ABO blood types
X-linked recessive inheritance
- Result from defect gene located on X chromosome and expressed in homozygous state
- Only hemizygous males usually affected
- Transmitted through unaffected carrier females
- Risk of affected son: 25%, normal son: 25%, unaffected carrier daughter: 25%, normal daughter: 25%
- Affected males transmit the disorder to daughters (all carriers) but not to sons
X-linked dominant inheritance
- Result from defect gene located on X chromosome and expressed in heterozygous state
- Males and females affected
- Females less severely affected than males
- Affected males can transmit the disorder to all their daughters (all affected) but not to sons
- Affected females have a 50% chance of affected children, irrespective of sex
Y-linked inheritance
- Result from defect gene located on Y chromosome and inherited directly from father to son
- Only males are affected
- Affected males must transmit the disorder to their sons

Complementation and Linkage

Complementation: more than one gene can be involved in producing a phenotype
Linkage: genes on the same chromosome are linked when they are close together
- Linked genes do not show independent assortment at meiosis but tend to be inherited together
- The frequency of recombination between linked genes gives an indication of distance between them

Genetic Map

Arrangement and distance between genes on a chromosome deduced from studies of recombination
- 1 map unit (centimorgan, cM) = 1% recombination
- Map unit or centimorgan (cM): a measure of the genetic (or linkage) distance between two loci