Monohybrid & Dihybrid Crossesturnerclassroom.weebly.com/uploads/2/3/1/8/...Mendel and Dihybrid Crosses •Mendel began his experiments with purebred (homozygous) parent plants.RRYY

Monohybrid & Dihybrid Crosses

Probability and Genetics!

Why did Mendel use Pea Plants?

- easy to control pollination

Pea Flower

Stamens

(male)

produce

pollen

Pistil

(female)

produces

ovules

- seven distinct, inherited traits

Plant size

Flower location

Flower color

Pod color

Pod shape

Seed shape

Seed color

P1 F1

F1 F2

Cross Fertilization of Parents

Purebred

Purple-flowered

Parent

Purebred

White-flowered

Parent

Cross-Fertilize

All Purple-flowered

Offspring

Pollen

Pollen

P

F1

P

F1

Self-Fertilize

F2 F2 F2 F2

75% Purple

25% White

Self-fertilization of F1

Punnett squares •The probability the next generation will exhibit a certain trait can be shown with a Punnett square.

• The genotype of the mother is shown on the top of the square (TT) • The genotype of the father is shown on the side of the square (tt) • The various combinations of genes (genotypes) are shown in the squares. All of these offspring will be (T) tall and carry the recessive (t) short allele.

T T

t

t

Tt

Tt Tt

Tt

Monohybrid Cross (crosses that examine the inheritance of

only ONE specific trait)

•What happens if you cross a tall (heterozygous) plant (Tt) with a short plant (tt)?

Genotypic Ratio:

TT : Tt : tt

0 : 2 : 2

Phenotypic Ratio:

Tall : Short

2 : 2

T t

t

t

Tt

tt Tt

tt

Punnett squares • If deafness in dogs is recessive (dd), show what offspring could result from two dogs that are heterozygous (Dd)

3 possible genotypes: DD, Dd, dd Genotypic Ratio: DD : Dd : dd 1 : 2 : 1 2 possible phenotypes: Not deaf or Deaf Phenotypic Ratio: Not Deaf : Deaf 3 : 1

D d

D

d

DD

dd Dd

Dd

Test Cross

•Farmer Dan breeds guinea pigs.

•There is a gene in guinea pigs for teeth length. Teeth can either be normal (T) or bucked (t).

•Farmer Dan buys what he thinks is a normal TT guinea pig. How can he be sure it is “pure” (TT) and not a heterozygote (Tt)?

Test Cross: Breed the unknown genotype with a homozygous recessive.

A homozygous dominant guinea pig will result in offspring that all have normal teeth.

T T

t Tt Tt

t Tt Tt

T t

t Tt tt

t Tt tt

A heterozygous guinea pig will result in a 2:2 or 1:1 ratio of normal to buck-teethed offspring.

Dihybrid Cross (crosses that examine the inheritance of

only TWO different traits)

Mendel and Dihybrid Crosses

• Mendel also studied dihybrid crosses.

– He crossed a plant with yellow round peas with a plant with green wrinkled peas.

Mendel and Dihybrid Crosses

• Mendel began his experiments with purebred (homozygous) parent plants. RRYY x rryy

– Remember each trait has 2 genes due to getting half your genes from mom and half from dad.

• How many gamete options do you have for RRYY??

RY

• How many gamete options do you have for rryy?

ry

• These are the gametes from the “P” (parent) generation.

Mendel and Dihybrid Crosses

“P” generation: RRYY x rryy

RRYY

rryy

RY RY RY RY

ry RrYy RrYy RrYy RrYy

ry RrYy RrYy RrYy RrYy

ry RrYy RrYy RrYy RrYy

ry RrYy RrYy RrYy RrYy

Mendel and Dihybrid Crosses

What is the genotypic ratio?

16 RrYy:0 anything else

Phenotypic ratio?

16 Round Yellow:

0 anything else

RY RY RY RY

ry RrYy RrYy RrYy RrYy

ry RrYy RrYy RrYy RrYy

ry RrYy RrYy RrYy RrYy

ry RrYy RrYy RrYy RrYy

Mendel and Dihybrid Crosses He then crossed two “F1” organisms:

RrYy x RrYy

RY Ry rY ry

RY RRYY RRYy RrYY RrYy

Ry RRYy RRyy RrYy Rryy

rY RrYY RrYy rrYY rrYy

ry RrYy Rryy rrYy rryy

RrYy

RrYy

Mendel and Dihybrid Crosses

1 RRYY: 2 RRYy: 1 RRyy: 2 RrYY: 4 RrYy: 2 Rryy: 1 rrYY: 2 rrYy: 1 rryy

RY Ry rY ry

RY RRYY RRYy RrYY RrYy

Ry RRYy RRyy RrYy Rryy

rY RrYY RrYy rrYY rrYy

ry RrYy Rryy rrYy rryy

What is the genotypic ratio for the F2 generation?

Mendel and Dihybrid Crosses

Phenotypic ratio for the F2 generation? RY Ry rY ry

RY RRYY RRYy RrYY RrYy

Ry RRYy RRyy RrYy Rryy

rY RrYY RrYy rrYY rrYy

ry RrYy Rryy rrYy rryy

9:16 Round Yellow, 3:16 Round Green, 3:16 Wrinkled Yellow, 1:16 Wrinkled Green

Mendel and Dihybrid Crosses

9:16 yellow and round,

3:16 green and round,

3:16 wrinkled and yellow,

1:16 wrinkled and green.

Mendel and Dihybrid Crosses

• Mendel continued to find this approximately 9:3:3:1 phenotypic ratio in the F2 generation, regardless of what 2 different traits he chose.

Mendel and Dihybrid Crosses

–This is due to his Law of Independent Assortment.

• Each allele pair separate independently from other allele pairs during meiosis (gamete formation)…

–different traits are inherited separately.

Dihybrid Cross Practice

• Suppose you are studying the color and texture of kernels on a corn cob.

– Kernels can be either purple (R) or yellow (r).

– They can also be smooth (T) or wrinkled (t).

– Predict the phenotypic and genotypic outcome of crossing a dihybrid (heterozygous) purple (heterozygous) smooth with a yellow wrinkled.

Answer: phenotypic and genotypic outcome of crossing

a dihybrid (heterozygous for both) purple smooth with a yellow wrinkled

Cross: RrYy x rryy

RrYy

rryy

RY rY Ry ry

ry RrYy rrYy Rryy rryy

ry RrYy rrYy Rryy rryy

ry RrYy rrYy Rryy rryy

ry RrYy rrYy Rryy rryy

4:16 Purple Smooth, 4:16 Purple Wrinkled, 4:16 Yellow Smooth, 4:16 Yellow Wrinkled