Monohybrid & Dihybrid Crosses

Monohybrid & Dihybrid Crosses

Probability and Genetics!

Why did Mendel use Pea Plants?

- easy to control pollination

Pea FlowerStamens(male)producepollen

Pistil(female)producesovules

- seven distinct, inherited traits

Plant size

Flower location

Flower color

Pod colorPod shape

Seed shapeSeed color

P1 F1

F1 F2

Cross Fertilization of Parents

PurebredPurple-flowered

Parent

Purebred White-flowered

Parent

Cross-Fertilize

All Purple-floweredOffspring

Pollen

Pollen

P

F1

P

F1

Self-Fertilize

F2 F2 F2 F2

75% Purple25% 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

TtTt

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 : tt0 : 2 : 2

Phenotypic Ratio:Tall : Short 2 : 2

T t

t

t

Tt

ttTt

tt

Punnett squares• If deafness in dogs are 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

ddDd

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 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 RYry RrYy RrYy RrYy RrYyry RrYy RrYy RrYy RrYyry RrYy RrYy RrYy RrYyry RrYy RrYy RrYy RrYy

Mendel and Dihybrid CrossesWhat is the genotypic ratio? 16 RrYy:0 anything elsePhenotypic ratio? 16 Round Yellow: 0 anything else

RY RY RY RYry RrYy RrYy RrYy RrYyry RrYy RrYy RrYy RrYyry RrYy RrYy RrYy RrYyry RrYy RrYy RrYy RrYy

Mendel and Dihybrid CrossesHe then crossed two “F1” organisms: RrYy x RrYy

RY Ry rY ryRY RRYY RRYy RrYY RrYy

Ry RRYy RRyy RrYy RryyrY RrYY RrYy rrYY rrYy

ry RrYy Rryy rrYy rryy

RrYy

RrYy

Mendel and Dihybrid Crosses

What is the genotypic ratio for the F2 generation? 1 RRYY:2 RRYy: 1 RRyy: 2 RrYY: 4 RrYy: 1 rrYY: 2 Rryy: 2 rrYy: 1rryy

RY Ry rY ryRY RRYY RRYy RrYY RrYy

Ry RRYy RRyy RrYy RryyrY RrYY RrYy rrYY rrYyry RrYy Rryy rrYy rryy

Mendel and Dihybrid Crosses

Phenotypic ratio for the F2 generation?

RY Ry rY ryRY RRYY RRYy RrYY RrYy

Ry RRYy RRyy RrYy RryyrY 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 smooth with a yellow wrinkled.

• Don’t forget to answer the questions on the bottom of your note template.

Answer: phenotypic and genotypic outcome of crossing

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

Cross: RrYy x rryyRrYy

rryy

RY rY Ry ryry RrYy rrYy Rryy rryyry RrYy rrYy Rryy rryyry RrYy rrYy Rryy rryyry RrYy rrYy Rryy rryy

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

• Parent generation (AABB x aabb) are purebred for both traits because they are homozygous for both traits.

• Possible gametes

AaBb -> AB, Ab, aB, ab

• Genotype: AAbb -> Phenotype: Purple, short