Autosomal recessive trait
  1. Usually appears in both sexes with equal frequency.
  2. Tends to skip generations.
  3. Affected offspring are usually born to unaffected parents.
  4. When both parents are heterozygous, approximately one-fourth of the offspring will be affected.
  5. Appears more frequently among the children of consanguine marriages.
Autosomal dominant trait
  1. Usually appears in both sexes with equal frequency.
  2. Both sexes transmit the trait to their offspring.
  3. Does not skip generations.
  4. Affected offspring must have an affected parent unless they possess a new mutation.
  5. When one parent is affected (heterozygous) and the other parent is unaffected, approximately half of the offspring will be affected.
  6. Unaffected parents do not transmit the trait.
X-linked recessive trait
  1. Usually more males than females are affected.
  2. Affected sons are usually born to unaffected mothers; thus, the trait skips generations.
  3. Approximately half of a carrier (heterozygous) mother’s sons are affected.
  4. Never passed from father to son.
  5. All daughters of affected fathers are carriers.
X-linked dominant trait
  1. Both males and females are usually affected; often more females than males are affected.
  2. Does not skip generations. Affected sons must have an affected mother; affected daughters must have either an affected mother or an affected father.
  3. Affected fathers will pass the trait on to all their daughters.
  4. Affected mothers (if heterozygous) will pass the trait on to half of their sons and half of their daughters.
Y-linked trait
  1. Only males are affected.
  2. Passed from father to all sons.
  3. Does not skip generations.
Table 6.1: Pedigree characteristics of autosomal recessive, autosomal dominant, X-linked recessive, X-linked dominant, and Y-linked traits