Patterns of Inheritance

• Autosomal recessive - Allele on chromosomes 1-22; expressed in phenotype if genotype is homozygous for recessive allele.
• Autosomal dominant - Allele on chromosome 1-22; always expressed in phenotype.
• Homozygous - Having two identical alleles of the same gene e.g. EE
• Heterozygous - Having two different alleles of the same gene e.g. Ee
• Incomplete dominance - When an allele is not completely masked by a dominant allele, thus affecting an individual’s phenotype.
• Sex-linked recessive - Recessive allele carried on the x chromosome
• Autosomal Recessive Inheritance

  Trait :

  – expressed relatively rarely

  – May skip generations

  – Males and females equally affected

  – All sufferers homozygous recessive

  – Non-sufferers homozygous dominant or heterozygous
• Autosomal Dominant Inheritance

  Trait:

  – Appears in every generation

  – Each sufferer has an affected parent

  – When a branch of the family does not express the trait it fails to reappear in future generations of that branch

  – Males and females affected equally

  – All non sufferers homozygous recessive

  – Sufferers homozygous dominant or heterozygous
• Autosomal Incomplete Dominance

  • The fully expressed form of the disorder occurs relatively rarely

  • Partially expressed form occurs more frequently

  • Males and females equally affected

  • Non sufferers homozygous for one incompletely dominant allele

  • Sufferers of the fully expressed form of the disorder are homozygous for the other incompletely dominant allele

  • Sufferers of the partly expressed form are heterozygous for the two

  alleles
• Sex Linked Recessive

  • Many more males are affected than females

  • None of the sons of an affected male show the trait

  • Some grandsons of an affected male show the trait

  • Sufferers of the trait are homozygous recessive (normally male XhY rarely female XhXh)

  • Non sufferers are homozygous dominant XHY XHXH

  • Or heterozygous females are carriers XHXh