Genetics. Dihybrid Crosses Dihybrid crosses are the comparison of genetic makeup for two characteristics. EX: color and shape. Mendel used dihybrid crosses.

Genetics

Dihybrid Crosses

• Dihybrid crosses are the comparison of genetic makeup for two characteristics.

• EX: color and shape.• Mendel used dihybrid crosses to

determine the principle of independent assortment: this principle tells us that genes for two characteristics can be passed on independently of each other.

Dihybrid Punnett Squares

1. One capital/lowercase letter is assigned to one of the traits and a different capital/lowercase letter is assigned to the other trait.

2. Find the possible combinations that each parents’ gamete could contain.

3. Fill in the Punnett Square and work as you normally would.

Dihybrid Punnet Squares: EX

• Peas can be yellow or green.– Yellow is dominant Y and green is recessive

y.

• Peas can be round or wrinkled.– Smooth is dominant R and wrinkled is

recessive r.

• Ex: mom is RrYy and dad is RrYy (both display round and yellow characteristics)

Dihybrid Punnett square:

Mom Dad

RY rY Ry ry

RY RRYY RrYY RRYy RrYy

rY RrYY rrYY RrYy rrYy

Ry RRYy RrYy RRyy Rryy

ry RrYy rrYy Rryy rryy

Polygenic Traits

• Polygenic traits are traits that are determined by the combined effects of more then one gene.

• They do not follow the normal inheritance laws and for some the Punett Square is not effective for predicting inheritance.

Codominance

• Genetic traits that are codominant have no recessive trait.

• This means that both the capital form and lowercase form of the gene are FULLY expressed.

• Example: HH, Hh, hh from “making baby” lab. ABO blood type.

Codominance: Determining the offspring• Use all capital letters, choose a letter and then

give it a superscript.• Ex, the gene C for flower color white and

yellow is codominant. Cw = white and Cy = yellow.

• You can still use the traditional punnet square, however, the heterozygous individual will be white with large yellow spots.

Codominance

Cw Cw = white

Cy Cy = yellow

Cw Cy = white with yellow spots

Cw Cw

Cy Cw Cy Cw Cy

Cy Cw Cy Cw Cy

Incomplete dominance

• The dominant gene cannot completely “beat” the recessive gene.

• Results in a blending of the dominant and recessive trait.

• Ex: sickle cell disease. Individuals with SS = normal blood cell shape, ss= severe sickle cell blood cell shape, Ss= a minor change in blood cell shape.

Incomplete dominance: determining the offspring• Assign a capital letter to the dominant trait and

a lowercase letter to the recessive trait. • Complete the punnett square in the exact

wayyou have learned. • The heterzygous individuals will display a trait

that is some of the dominant form mixed with some of the recessive form. (neither is completely expressed)

Incomplete dominance

RR = red petal plant

rr= white petal plant

Rr=pink petal plant

R R

R RR RR

r Rr Rr

Multiple Alleles

• Traits controlled by more then two alleles (gene types) are said to be controlled by multiple alleles.

• Even if there are 100 alleles, you can only carry two of them!

• Example: ABO blood type.

ABO blood type: determining the offspring• A, and B are dominant while O is the recessive form of

the red blood cell proteins. • AA and AO are the “A blood type”• BB and BO are the “B blood type”• AB is the “AB blood type” where both the A proteins

and the B proteins are expressed.• The ABO blood types are an example of

codominance and multiple alleles.• A and B are two different dominant alleles while O is

the recessive allele.

Multiple Alleles

A=dominant A blood type

B=dominant B blood type

O= recessive blood type

A A

B AB AB

O AO AO