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 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.


“P” generation: RRYY x rryy

RRYY

rryy

RY RY RY RY

ry RrYy RrYy RrYy RrYy





What is the genotypic ratio?

16 RrYy:0 anything else

Phenotypic ratio?

16 Round Yellow:

0 anything else

RY RY RY RY





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


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

RY Ry rY ry





What is the genotypic ratio for the F2 generation?


Phenotypic ratio for the F2 generation? RY Ry rY ry





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


9:16 yellow and round,

3:16 green and round,

3:16 wrinkled and yellow,

1:16 wrinkled and green.


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


–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




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

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

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