Genetic Family Histories Pedigrees: Genetic Family Histories

Women are represented with a circle. Men are represented with a square Women are represented with a circle. Men are represented with a square. Affected individuals are shaded.

Pedigree for attached ear lobes. Is the trait dominant or recessive? Recessive: Trait appears in 2nd generation but not parents.

What is the genotype of II-3? ll What is the genotype of II-5? Pedigree for short eyelashes. Long eyelashes are dominant (L). Short eyelashes is recessive (l). What is the genotype of II-3? ll What is the genotype of II-5? LL or Ll What is the genotype of III-7? Ll

(In many cases could be homozygous dominant or heterozygous) Can’t always determine the genotype of an individual with the dominant trait. (In many cases could be homozygous dominant or heterozygous) Must be heterozygous if: Has a child with the recessive trait. Has a parent with the recessive trait.

Can it be determined from this pedigree if the PTC tasting gene is sex-linked (X chromosome) or autosomal? Must be autosomal. For a sex-linked trait, female can only inherit recessive trait if father exhibits trait. For sex-linked trait, if female has recessive trait, all sons must exhibit trait.

Errors in Meiosis Nondisjunction is a failure of a chromosome pair to separate properly during meiosis. Recall that: Homologous pairs separate during Meiosis I Sister chromatids separate during Meiosis II

Nondisjunction can occur in either meiosis I or Meiosis II Nondisjunction can occur in either meiosis I or Meiosis II. (The difference is that if it occurs in Meiosis I all the egg/sperm produced have an abnormal number.)

Aneuploidy- the condition having an abnormal number of chromosomes. When nondisjunction occurs half of the eggs/sperm produced will have 2 copies of the chromosome and half will not have the chromosome. (recall that gametes normally have one copy) If the egg or sperm with 2 copies is fertilized, the resulting zygote (fertilized egg) will have 3 copies of that chromosome. This type of aneuploidy is called trisomy. If the egg or sperm with no copies of a chromosome is fertilized, the resulting zygote will have only one copy. This type of aneuploidy is called monosomy.

Down’s Syndrome is caused by trisomy 21.