Mendel’s Second Law The Law of Independent Assortment Mendel also crossed plants of two traits. because two traits are involved in these crosses they are called a dihybrid cross. Mendel crossed true breeding tall plants that had green pods (TTGG) with true breeding short plants that had yellow pods (ttgg) to produce the F 1 generation

in this case the true breeding plants will produce only one type of gametes TTGG → will produce gametes with the TG genes ttgg → will produce gametes with the tg genes ♀♂♀♂ tg TG TtGg TG TtGg the phenotypic ratio of the F 1 generation: 100% tall and green pods the genotypic ratio of the F 1 generation 100% heterozygous

Mendel then crossed the F 1 generation to produce an F 2 generation in this case the plants of the F 1 generation produce four different types of gametes TtGg → will produce gametes with the: TG genes (tall, green) Tg genes (tall, yellow) tG genes (short, green) tg genes (short, yellow)

TtGg → will produce gametes with the: TG genes Tg genes tG genes tg genes ♀♂♀♂ TGTgtGtg TG TTGGTTGgTtGGTtGg Tg TTGgTTggTtGgTtgg tG TtGGTtGgttGGttGg tg TtGgTtggttGgttgg

PhenotypesTally Tall & Green Pods 9 Tall & Yellow Pods 3 Short & Green Pods 3 Short & Yellow Pods 1 TT = tallGG = green Tt = tallGg = green tt = shortgg = yellow

for every dihybrid cross that Mendel carried he got the 9:3:3:1 ratio (when he crossed the F 1 generation). this ratio is what is expected if the segregation of alleles for one gene had no influence on the segregation of alleles of another gene. Law of Independent Assortment The two alleles of one gene segregate (assort) independently of the alleles for other genes during gamete formation

Pleiotropic Genes a gene that affects more than one characteristic example: Sickle-cell anemia the normal hemoglobin is produced by the allele Hb A in sicke-cell anemia the individual has two copies of the mutated allele Hb s

Pleiotropic Genes the mutation cause abnormally shaped hemoglobin that cannot deliver oxygen to the cells. causes fatigue, enlarged spleen, pneumonia and major organ damage. a heterozygous individual has resistance to malaria but an increased chance of having homozygous recessive offspring.

Homework “5.Dihybrid Cross” from your workbook (questions 1-5) Journal questions Read pages