1. PEDIGREE CHARTS (and Test Crosses)
Biology 12

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

3. A Testcross Can Reveal Genotypes:
Used if one is unsure if there is a homozygous dominant or heterozygous individual
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)
If any offspring show the recessive trait then the unknown must be heterozygous.
If all offspring show the dominant trait the unknown must be homozygous.

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

5. 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.

6. 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.

7. What symbols are used?

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

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

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

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

12. Example:
A pedigree chart of a family showing 20 individuals

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