Independent Assortment, Test Crosses, & Linked Genes

1. Independent Assortment, Test Crosses, & Linked Genes

2. Read the question Write possible genotypes Use a test cross Cross unknown (TT or Tt) with homozygous recessive If recessive offspring, then the unknown was heterozygous Example: In peas, the gene for tall (T) is dominant over short (t). What are the parents’ genotypes if 100% of F1 generation peas are tall? T= tall t= short Test Crosses • Individual has dominant phenotype. How do we know what its genotype is??

3. In humans, the gene for brown eyes (B) is dominant over blue (b). What are the parents’ genotypes if 50% of the F1 generation are blue-eyed? Show your work! Practice Problem

4. Independent Assortment • Mendel’s 3rd Law • Alleles segregate independently of each other during meiosis • Example: The allele for hair color has no effect on eye color

5. Independent Assortment Problem The height allele has no affect on the round allele 2. Make a Punnett Square Tr tr TR tR Example: TtRr x TtRr T = tall R = round Tt= short rr= wrinkled 1. Go through all possible combinations TR Tr tR tr Phenotypes: 3. Write ratio of genotype & phenotype Genotype: 9: 3: 3: 1: Phenotypes:

6. Linked Genes • Batteson & Punnett continued Mendel’s work with sweet peas • P = purple p= red L= long pollen l=round pollen • Crossed PpLl x PpLl  should get 9:3:3:1 ratio based on law of independent assortment pl pL PL Pl Ratio: 9 purple long 3 purple round 3 red long 1 red short PL Pl pL pl

7. Linked Genes Results • NO 9:3:3:1 RATIO! • Most purple peas had round pollen • Reason: Genes are linked • Alleles located close together are NOT independent of each other • Gives unexpected results during crossing over in meiosis