Chapter 12 Mendelian inheritance in humans
A Mendelian character is one in which a particular genotype at one locus is both necessary and sufficient for the character to be expressed, given the normal range of human genetic and environmental backgrounds.
Also referred to as “monogenic traits”
12.1 Five Basic Mendelian Patterns in Humans
12.1.1 Autosomal
12.1.1.1 Autosomal dominant inheritance
- Affected person has at least one affected parent
- Daughters and sons equally affected
- Transmitted by either sex
- A mating between an affected and unaffected has a 50% chance of producing and affected offspring
- P(affected) = 0.5
12.1.1.2 Autosomal recessive inheritance
- Affected people born to unaffected parents
- Parents are carriers (heterozygotes)
- Male and female children both affected
- Often associated with increased consanguinity (cousin marriage)
- Two carriers have a 25% chance of having an affected offspring
- P(affected) = 0.25
12.1.2 Sex linked
12.1.2.1 X-linked dominant inheritance
- Affects either sex, but females more frequently
- At least one parent is affected
- Females affected more mildly and more variably
- Child of affected female has 50% chance of being affected (regardless of sex)
- For an affected male, all daughters are affected, but no sons are affected
12.1.2.2 X-linked recessive inheritance
- Affects mainly males
- Affected males born to unaffected parents
- Mother is usually unaffected but may have affected male relatives
- Female affected if father is affected and mother is a carrier
- No male-male transmission
12.1.2.3 Y-linked inheritance
- Affects only males
- Affected males have an affected father
- All sons of affected man are affected
12.1.3 Special cases
12.1.3.1 Pseudoautosomal inheritance
- Indistinguishable from an autosomal dominant pedigree
- Affected person has at least one affected parent
- Daughters and sons equally affected
- Transmitted by either sex
- A mating between an affected and unaffected has a 50% chance of producing and affected offspring
12.1.3.2 Mitochondrial inheritance
- Affects both sexes
- Inherited from an affected mother
- Not transmitted by father to children
- High variable clinical manifestation
12.2 Variations on Mendelian Inheritance in Humans
12.2.1 Incomplete Dominance
12.2.2 Co-dominance
12.2.3 Expressivity
Variable expressivity refers to traits that are expressed to different degrees in different individuals.
12.2.4 Penetrance
Penetrance refers to the proportion of organisms whose phenotype matches their genotype for a given trait.