R Mother Rr Female gametes R r Father Rr Male gametes R r RrRR Rrrr Resulting genotypes: 1/4 RR : 1/2 Rr : 1/4 rr Resulting phenotypes: 3/4 : 1/4 Figure 10.7 R = Dominant allele for seed shape (round) r = Recessive allele for seed shape (wrinkled) = Round-seeded phenotype = Wrinkled-seeded phenotype

Crosses Between Plants that Differ in One Trait Identifies A Fundamental Pattern in Genetics n Mendel develops Principle of Segregation The two alleles of each gene must segregate into different gamete cells during the formation of eggs and sperm in the parent.

Dominant allele for seed shape (round) Rr Recessive allele for seed shape (wrinkled) Chromosomes replicate RRrr Meiosis I rrRR Meiosis II rrRR Gametes Figure 10.11a Principle of segregation

Figure 10.8a R = Dominant allele for seed shape (round) r = Recessive allele for seed shape (wrinkled) Y = Dominant allele for seed color (yellow) y = Recessive allele for seed color (green) Father Mother Parental generation Gametes Meiosis Fertilization F 1 generation Cross peas that differ in two traits

F 2 generation phenotype Number = 556 Fraction of progeny 9/163/16 1/16 = 1 Figure 10.8b Self-fertilize F 1 peas and count F 2 offspring

3 : : : Round seeds : Wrinkled seeds How does the 9 : 3 : 3 : 1 ratio observed for two traits relate to the 3 : 1 ratio observed for one trait? 416 : : Yellow seeds : Green seeds Figure 10.8c

Parental generation F 1 generation ALL RrYy rryy RrYy Female gametes RRYY 1/4 RY1/4 Ry1/4 rY1/4 ry R = Dominant allele for seed shape (round) r = Recessive allele for seed shape (wrinkled) Y = Dominant allele for seed color (yellow) y = Recessive allele for seed color (green) 1/4 RY 1/4 Ry 1/4 rY 1/4 ry RrYy RRYY RrYy rryyrrYyRryyRrYy rrYy Rryy RrYyRrYYRRYy RRyyRRYy RrYYRrYyrrYY Resulting genotypes: 9/16R-Y- : 3/16R-yy :3/16rrY- :1/16rryy 9/163/16 1/16 Resulting phenotypes: Male gametes Figure 10.9

Crosses Between Plants that Differ in Two Traits Identifies A Second Fundamental Pattern in Genetics n Mendel develops Principle of Independent Assortment Each pair of alleles segregates into gametes separately from every other pair of alleles. A testcross to a homozygous recessive confirms independent assortment of alleles. (Fig )

F1 parent Homozygous recessive parent rryy RrYy All ry RY Ry rY ry1/4 1/4 RrYy1/4 Rryy1/4 rrYy1/4 rryy Figure 10.10

Chromosome Theory of Inheritance n Mendel's rules of inheritance can be explained by independent assortment of chromosomes during meiosis. (Fig b)

Rr r Y y Meiosis I Alleles for seed shape Alleles for seed color Rr y Y R R Meiosis II y Y r Y r Y 1/4 RY1/4 ry1/4 Ry1/4 rY RR yyY r y Gametes Figure 10.11b Principle of independent assortment

Figure 10.12

Thomas Hunt Morgan

Thomas Hunt Morgan discovered a male fly with white eyes in one of his Drosophila cultures n Mated true-breeding red-eyed flies with the white-eyed mutant male n Bred P flies to produce an F 1 generation n F 1 generation all had red eyes Concluded gene for white eyes is recessive n Bred F 1 flies to produce an F 2 generation Got ratio of 3:1 red-eyed flies to white-eyed flies However, all of the females had red eyes and half of the males had white eyes

n To explore this unexpected result, Morgan next bred some of the F 1 females (red-eyed) with white-eyed males Some of the female offspring had white eyes Now Morgan could do the reciprocal cross

n Cross white-eyed females with wild type (red-eyed) males All of the female offspring had red eyes All of the male offspring had white eyes!!! Clearly, the reciprocal cross did not produce the same results as the male white-eyed x female red-eyed cross