PEDIGREE CHARTS and Test Crosses Biology 12 Review

PEDIGREE CHARTS (and Test Crosses) Biology 12

Review: Each Pair of Gene Alleles Is Separated and Two Alleles End Up in Different Gametes

A Testcross Can Reveal Genotypes: • Used if one is unsure if there is a homozygous dominant or heterozygous individual o In order to determine the genotype of these individuals a test cross must be performed • Cross unknown (TT or Tt) with a homozygous recessive (tt) o If any offspring show the recessive trait then the unknown must be heterozygous. o If all offspring show the dominant trait the unknown must be homozygous.

Example Test Cross: TT x tt Tt x tt t t T Tt Tt Offspring: all dominant phenotype t t T Tt Tt t tt tt Offspring: 1/2 dominant, ½ recessive

What is a pedigree chart? • Pedigree charts show a record of the family of an individual • They can be used to study the transmission of a hereditary condition • They are particularly useful when there are large families and a good family record over several generations.

Goals of Pedigree Analysis 1. Determine the mode of inheritance: dominant, recessive, partial dominance, sex-linked, autosomal, mitochondrial, maternal effect. 2. Determine the probability of an affected offspring for a given cross.

What symbols are used?

o Example: family with 5 children. • The pedigree chart shows that the second son is affected by the condition.

Organising the pedigree chart: o Children are list oldest to youngest from left to right o Generations are identified by Roman numerals I II IV

o Individuals in each generation are identified by both a. b. Roman numerals (indicated generation) Numbered from the left Example: The affected individuals below are II-3, IV-2 and IV-3

Some other symbols: Dizygotic twins Adopted Monozygotic twins Consanguineo us marriage Sex unspecified Divorced Number of children of each sex indicated Multiple marriage Affected Extra marital union

Example: • A pedigree chart of a family showing 20 individuals

To do: o Go back and add a pedigree chart to #7 on the Monohybrid Cross Assignment o Pedigree Studies Activity o Page 142 #1, 2 o Page 148 # 3 (Dominant allele for PKU, so if a person has one “big letter” they show symptoms of the disease)