1. Dihybrid crosses a study of inheritance patterns for organisms differing in two traits… more realistic

2. Dihybrid cross • Monohybrid: heterozygous (having 2 different alleles for one gene pair). • Ex: Aa • So what is a dihybrid???? • Di: meaning two • Hybrid: means heterozyous • instead of one trait we are looking at two traits at the same time • AaBb represents the genotype of a dihybrid

3. Remember Mendels laws: • Law of segregation. • Only one allele for each trait can be passed at a time. Only pass on one ‘A’ AND one ‘B’ and one… • Law of independent assortment. • How each type of letter (allele) sorts or is passed on to the next generation is independent • i.e. Which “A” is passed on doesn’t affect which “B” is passed on

4. What combinations of alleles are possible? • If an organism has a genotype of AaBb • What are all the different combinations of A’s and B’s? • AB • Ab • aB • ab • These combinations represent the possible gametes (sperm or egg) that could form during meiosis.

5. An example of a dihybrid cross • Key: • E = Two Eyes, e = one eye • T = Triangular Head, t = pentagonal head • So, lets have two dihybrid parents; therefore their genotypes are: • EeTt x EeTt: What do they look like? • Phenotype= two eyes and triangular heads. • Phenotype: physical characteristics of an organism: what you “see”. • What types of combinations are possible? • ET, Et, eT, et. Is this all?

6. Punnett squares

7. Incomplete Dominance • Incomplete Dominance = neither allele is dominant. Heterozygous individuals show an intermediate (in between) phenotype. • Example: petal color in carnations RR = red RW = pink WW = white

8. Codominance • Codominance = both alleles contribute to the phenotype. Heterozygous individuals show both traits separately (not blended like in incomplete dominance). • Example: coat color in cattle • RR = red • WW = white • RW = roan

9. Multiple Alleles • Multiple Alleles = genes that have more than two alleles. • Example: blood type in humans