FOR FRESHERS Mendelian Inheritance

Mendelian inheritance There are two alleles of a gene on different sister chromosomes. Dominant alleles trump recessive alleles phenotypically (what they look like…). Disorders can be linked to a dominant allele or a recessive allele, which can be on an autosomal or sex-linked chromosome.

Punett Square

Autosomal dominant Vertical inheritance: present in every generation Both sexes equally affected and likely to pass on Unaffected individuals produce normal offspring Affected individuals produce 50% affected children

Autosomal recessive Horizontal inheritance: skips generations Both sexes equally affected and likely to pass on Affected individuals produce 50% carriers Carriers produce 50% carriers

X-linked dominant Affected males produce normal males and affected females Affected females produce 50% affected offspring Affects both sexes, but usually more females since males die before birth

X-linked recessive Affects mainly males Affected males are usually born from unaffected parents Disease is usually transmitted by a female carrier No male-male transmission

Y-linked Affects males only Affected male produces all affected males

Complications to Mendelian inheritance Genomic imprinting  Differential expression depending on parental origin (i.e. is either maternal or paternal)  E.g. Prader-Willie, Angelman Polygenic inheritance  Condition affected by many genes at different loci  Varying clinical presentations Multi-factorial disorder  Disorder due to a combination of genetic and environmental factors Mitochondrial inheritance  Inherited from female only, DNA in mitochondria