R

Mother

Rr

Female gametes

R r

Father

Rr

Mal

e g

ame

tes R

r

RrRR

Rr rr

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

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 allelefor seed shape(round)

R r Recessive allelefor seed shape(wrinkled)

Chromosomes replicate

R R r r

Meiosis I

r rR R

Meiosis II

r rRR

Gam

etes

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 MotherParentalgeneration

Gametes

Meiosis

Fertilization

F1 generation

Cross peas that differ in two traits

F2 generationphenotype

Number 315 101 108 32 = 556

Fraction ofprogeny 9/16 3/16 3/16 1/16 = 1

Figure 10.8b

Self-fertilize F1 peas and count F2 offspring

3 : 1

3 : 1

423 : 133

315 + 108 : 101 + 32

Round seeds : Wrinkled seeds

How does the 9 : 3 : 3 : 1 ratio observed for two traitsrelate to the 3 : 1 ratio observed for one trait?

416 : 140

315 + 101 : 108 + 32

Yellow seeds : Green seeds

Figure 10.8c

Parentalgeneration

F1

generationALL

RrYy

rryy

RrYy

Female gametes

RRYY

1/4 RY 1/4 Ry 1/4 rY 1/4 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

RrYY RrYy rrYY

Resulting genotypes: 9/16R-Y- : 3/16R-yy : 3/16rrY- : 1/16rryy9/16 3/16 3/16 1/16Resulting phenotypes:

Ma

le g

am

ete

s

Figure 10.9

Crosses Between Plants that Differ in Two Traits Identifies A Second Fundamental Pattern in Genetics

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. 10.10)

F1 parent

Homozygousrecessiveparent

rryy RrYy

All ry

RY Ry rY ry1/4 1/4 1/4 1/4

1/4 RrYy 1/4 Rryy 1/4 rrYy 1/4 rryy

Figure 10.10

Chromosome Theory of Inheritance

Mendel's rules of inheritance can be explained by independent assortment of chromosomes during meiosis. (Fig. 10.11b)

R r

rY y

Meiosis I Meiosis I

Alleles for seed shape

Alleles for seed color

R r

y YRR

Meiosis II Meiosis II

yY

r

Y

rY

1/4 RY 1/4 ry 1/4 Ry 1/4 rY

RRyyY

r

y

Gam

etes

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

Mated true-breeding red-eyed flies with the white-eyed mutant male

Bred P flies to produce an F1 generation

F1 generation all had red eyes

• Concluded gene for white eyes is recessive

Bred F1 flies to produce an F2 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

To explore this unexpected result, Morgan next bred some of the F1 females (red-eyed) with white-eyed males

• Some of the female offspring had white eyes

• Now Morgan could do the reciprocal cross

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