Revisiting Dihybrid Punnett SquaresRevisiting Dihybrid Punnett Squares

• What exactly is a dihybrid cross?– Predicting the outcomes from crossing 2

traits.

• Why do we use them?– Instead of doing to monohybrid crosses, you

can combine them to see all the possible combinations from that particular mother and father.

Dihybrid Crosses Cont’dDihybrid Crosses Cont’d• What do the tops and sides of a Punnett Square

represent?– The gametes from the mother and father that are going

to mix (fertilize) to make the possible offspring in the middle.

Webbed feet (W) in ducks are dominant to un-webbed (w) feet. If a mother duck is heterozygous for webbed feet, and the father is homozygous for webbed feet, will any of their ducklings have un-webbed feet?

Mother = Ww, Father = WW

W W

W

w

W W W W

W w W w

Each gamete is haploid. The mother’s eggs can have either W or w; father’s sperm can have either W or W.

Dihybrid Crosses Cont’dDihybrid Crosses Cont’d• Why do we have a Punnett Square with 4 boxes on

each side instead of 2?– There will be 2 possible alleles that can be passed on for

each trait. You will have 4 total options to include all the possibilities of combining the 2 traits.

Short beaks (b) in ducks are recessive to long beaks (B) feet. If a father duck has a short beak, and the mother is a carrier for short beaks, will any of their ducklings have the same type of beak as their mother??

Mother = bb, Father = Bb

The mother can pass on a B or a b; the father can pass on a b or a b towards the beak trait.

Remember, the mother can pass on a W or a w allele for webbed feet and the father can pass on a W or a W…

SO, all the possible gamete combinations are:

Mother: WB,Wb, wB, wb

Father: Wb, Wb, Wb, Wb

Putting it all together…Mother’s gamete choices: WB, Wb, wB, wb

Father’s gamete choices: Wb, Wb, Wb, Wb

By taking all the daughter cell possibilities, we’re covered no matter what gametes end up getting fertilized to restore diploidy.

WB

Wb

wB

wb

Wb Wb Wb Wb

WWBb WWBb WWBb WWBb

WWbb WWbb WWbb WWbb

WwBb WwBb WwBb WwBb

Wwbb Wwbb Wwbb Wwbb

Codominance, Codominance, Polygenic Inheritance Polygenic Inheritance

and Blood Typesand Blood Types

Continuing our exploration of patterns of inheritance with…

CodominanceCodominance

• Heterozygote expresses both alleles’ conditions– Ex. A black rooster bred with a white hen

produces a black and white checkered chicken.

• Usually uses both capital letters (black= B, white=W , checkered = BW)

Codominance practice: cross a black rooster with a white hen

B

B

WW

Results:

100% checkered (BW)

BW BW

BW BW

Key:

B = black

W = white

BW = checkered

Now let’s cross a checkered hen with a checkered rooster.

B

W

WBResults:

25% Black

50% Checkered

25% White

(1:2:1)

BB BW

BW WW

Polygenic InheritancePolygenic Inheritance

• Trait controlled by 2 or more genes

• May be on the same or different chromosomes

• Shows a range, intermediate is most common phenotype

• Upper and lower case letters used

Skin color: A polygenic traitSkin color: A polygenic traitDetermining # of Genes Involved in Skin Color

Observed distribution of skin color

Expected distribution- 1 gene

Expected distribution- 4 genes

Expected distribution- 3 genes

Range of skin colorLight Right

Nu

mb

er o

f in

div

idu

als

And the answer is…And the answer is…

• Skin color is actually due to 5 genes

• Genotypes darkest to lightest:–AABBCCDDEE would be darkest

skinned

–AaBbCcDdEe would be medium skinned

–aabbccddee would be lightest skinned

Influence of External EnvironmentInfluence of External Environment

• In arctic foxes temperature has an effect on the expression of coat color. In winter, fur is white; in summer, fur is brown

• Leaves can have different sizes, thicknesses, and shapes depending on the amount of light they receive.

Influence of External EnvironmentInfluence of External Environment

Influence of Internal EnvironmentInfluence of Internal Environment

• The internal environments of males and females are different because of hormones and structural differences.

• An organism’s age can also affect gene function.

– Ex. Adult male lion’s manes

Peacock (male) Peahen (female)

Coloration may also be caused by differences in hormones between sexes

Blood type quick factsBlood type quick facts

• Red blood cells are called erythrocytes

• Proteins on their surfaces are called antigens, controlled by genes

• Antigens make antibodies to foreign substances, which includes RBCs with different antigens on their surface

• 4 phenotypes: A, B, AB, O

• 3 alleles: IA, IB, i

• The lA allele is dominant to i, so inheriting either the lAi alleles or the lA lA alleles from both parents will give you type A blood.

Phenotype APhenotype A

• Surface molecule A is produced.

Surface molecule A

• The lB allele is also dominant to i.

Phenotype BPhenotype B

• To have type B blood, you must inherit the lB allele from one parent and either another lB allele or the i allele from the other.

• Surface molecule B is produced.

Surface molecule B

• The lA and lB alleles are codominant.

Phenotype ABPhenotype AB

• If you inherit the lA allele from one parent and the lB allele from the other, your red blood cells will produce both surface molecules and you will have type AB blood.

Surface molecule B

Surface molecule A

Phenotype OPhenotype O

•No antigens produced

Check your chart!Check your chart!Blood Group

Antigens Antibodies Can receive

from

Can give to

A A B A or O A or AB

B B A B or O B or AB

AB A and B Both A, B, AB, O

AB

O None None O A, B, AB, O

Possible genotypes for each Possible genotypes for each phenotype:phenotype:

• A = IA IA or IAi

• B = IBIB or IBi

• AB = IAIB

• O = ii

Now let’s put your skills to the test…Now let’s put your skills to the test…

• Can you save these patients?