Biology 12

These are crosses that involve genes for two different characteristics

The natural progression for Mendel was to study two characteristics at the same time. Thus, the study of 2 pairs of contrasting traits at the same time = a dihybrid cross ex.. round yellow seeds X wrinkled green seeds

Principle of Independent Assortment When more then one trait is studied in the same cross, the genes for each trait sort into gametes independently of the genes of the other traits

Lets cross a homozygous round/yellow (AABB) with a homozygous wrinkled/green (aabb) plant….

1.Determine Genotypes of Parents 2.Determine Genotypes of Gametes (remember gametes only have 1 copy of each gene) and think FOIL (from math class!) 3.Punnett Square: need 16 squares now, 4 gametes from each parent 4.Identify Phenotype Ratios in Offspring

o In snapdragons… o Tallness (T) is dominant to dwarfness(t) o Red color is due to gene (R) and white to its corresponding allele (r). o The heterozygous condition results in pink (Rr) flower color. o A dwarf pink snapdragon is crossed with a plant homozygous for tallness and red flowers. Give the possible genotypes and corresponding phenotypes for all of the possible F 1 generation.

A dwarf pink snapdragon is crossed with a plant homozygous for tallness and red flowers. Parent 1:Dwarf pink = ttRr Parent 2:Homozygous tall, red = TTRR

Dwarf pink = ttRr Homozygous tall, red = TTRR tRtr tR tr TR

Gametes from TALL red parent tR tr tR tr pink Gametes from dwarf pink parent The Punnett square determines the genotypes of the offspring

TtRR TtRr TtRR TtRr TtRR TtRr TtRR TtRr TtRR Tall, red = 8/16 Tall, pink = 8/16 Tall, white = 0/16 Short, red = 0/16 Short, pink = 0/16 Short, white = 0/16

Page 154 questions 1, 3 (and then try #2)