Dihybrid CrossesIncomplete Dominance

CodominanceMultiple Alleles

In a nutshell!

Review: Dominant/Recessive

• One allele is dominant over the other (capable of masking the recessive allele)– Example: Purple flowers are dominant over white

PP = purple pp = white Pp = purple

Review Problem: Dominant/Recessive

• In pea plants, purple flowers (P) are dominant over white flowers (p) show the cross between two heterozygous plants.

P

p

P p

pp

Pp

Pp

PP- PP (1); Pp (2); pp (1)- ratio 1:2:1

- purple (3); white (1)- ratio 3:1

GENOTYPES:

PHENOTYPES:

Review of Monohybrid Crosses

• Remember, monohybrid crosses involve only ONE trait

• Practice… In fruit flies, red eyes are dominant over white eyes.– Cross a white-eyed fly with a homozygous

dominant red-eyed fly.– Cross two heterozygous red-eyed flies.

Draw a Punnett square for each cross,

and determine the genotypic and phenotypic ratios.

• Can you determine the genotype of a white-eyed fly just by looking at it? Why?

– Yes- it’s rr– There is only one genotype for white eyes

• Can you determine the genotype of a red-eyed fly just by looking at it? Why?

– No- it could be RR or Rr– There are two genotypes for red eyes

• How could you determine the genotype of the red-eyed fly?– Perform a test cross

r r

R

R

Rr Rr

Rr Rr

Rr

Rr rr

RR

R

R

r

r

Review of Monohybrid Crosses

• What two organisms would you cross in the test cross?– The unknown red-eyed fly with a white eyed fly

• If the red-eyed fly was RR, what would the offspring of the test cross look like?– All of the offspring would have red eyes

• If the red-eyed fly was Rr, what would the offspring of the test cross look like?– Half of the offspring would have red eyes, half would

have white eyes

Dihybrid Cross:a cross that shows the possible offspring

for two traits

Fur Color:B: Black b: White

Coat Texture:R: Roughr: Smooth

In this example, we will cross a heterozygous individual with

another heterozygous individual.

Step 1: Determine the parents’ genotypes

Step 2: You must find ALL possible gametes that can be made from each parent.

Remember, each gamete must have one B and one R.

Parent 1: BbRr Parent 2: BbRr

Parent 1: BbRr Parent 2: BbRr

Parent 1

Possible gametes:

BR

Br

bR

br

Parent 2

Possible gametes:

BR

Br

bR

br

Step 3: arrange all possible gametes for one parent along the top of your Punnett Square, and all possible gametes for the other parent down the side of your Punnett Square…

Fur Color:B: Black b: White

Coat Texture:R: Roughr:

Smooth

BbRr x BbRr

BR

bR

br

bR

Br

BR

br

Br

Step 4: Find the possible genotypes of the offspring

Fur Color:B: Black b: White

Coat Texture:R: Roughr:

Smooth

BbRr x BbRr

BR

bR

br

bR

Br

BR

br

Br

BBRR

BbRR BbRr

BBRr BBrr BbRr Bbrr

BbRR BbRr bbRR bbRr

BbRr Bbrr bbRr bbrr

BBRr

BR bR

br

bR

Br

BR

br

Br

BBRR

BbRR

BbRr

BBRr BBrr BbRr Bbrr

BbRR

BbRr bbRR

bbRr

BbRr Bbrr bbRr bbrr

BBRr

How many of the offspring would have a black, rough coat?

How many of the offspring would have a black, smooth coat?

How many of the offspring would have a white, rough coat?

How many of the offspring would have a white, smooth coat?

Fur Color:B: Black b: White

Coat Texture:R: Roughr:

Smooth

BR bR

br

bR

Br

BR

br

Br

BBRR

BbRR

BbRr

BBRr BBrr BbRr Bbrr

BbRR

BbRr bbRR

bbRr

BbRr Bbrr bbRr bbrr

BBRr

How many of the offspring would have black, rough coat?

How many of the offspring would have a black, smooth coat?

How many of the offspring would have a white, rough coat?

How many of the offspring would have a white, smooth coat?

Fur Color:B: Black b: White

Coat Texture:R: Roughr:

Smooth

Phenotypic Ratio

9:3:3:1

• On a blank sheet of paper…In pea plants, yellow seeds (Y) are dominant over

green seeds (y), and rounded peas (R) are dominant over wrinkled peas (r).

Cross a plant that is heterozygous for both traits with a plant that is homozygous recessive for both traits. Draw a Punnett square to show all possible offspring, and determine the phenotypic ratio.

YyRr X yyrr

YR yR

yr

yr

yr

yr

yrYr

YyRr yyRr yyrr

YyRr Yyrr yyRr yyrr

YyRr Yyrr yyRr yyrr

YyRr Yyrr yyRr yyrr

Yyrr

Incomplete Dominance• A third (new) phenotype appears in the

heterozygous condition.

• Flower Color in 4 O’clocks

RR = red rr = white Rr = pink

Problem: Incomplete Dominance

• Show the cross between a pink and a white flower.

- Rr (2); rr (2); RR (0)- ratio 2:2:0

- pink (2); white (2) Red (0)- ratio 2:2:0

R r

r

r

rrRr

rrRr

GENOTYPES:

PHENOTYPES:

Codominance

• The heterozygous condition, both alleles are expressed equally

• Sickle Cell Anemia in Humans

NN = normal cells

SS = sickle cells NS = some of each

Problem: Codominance• Show the cross between an individual with

sickle-cell anemia and another who is a carrier but not sick.

N S

S

S

NS

NS

SS

SS

- NS (2) SS (2)- ratio 2:2:0

- carrier (2); sick (2)- ratio 2:2:0

GENOTYPES:

PHENOTYPES:

Multiple Alleles

• There are more than two alleles for a trait

• Blood type in humans

• Blood Types?– Type A, Type B, Type AB, Type O

• Blood Alleles?– A, B, O (in book – IA, IB, I)

Rules for Blood Type

• A and B are codominant– AA = Type A– BB = Type B– AB = Type AB

• A and B are dominant over O– AO = type A– BO = type B– OO = type O

Problem: Multiple Alleles• Show the cross between a mother who has type O

blood and a father who has type AB blood.

- AO (2) BO (2)

- type A (2); type B (2)- ratio 2:2:0:0

GENOTYPES:

PHENOTYPES:

O O

A

B

AO

BO

AO

BO

Problem: Multiple Alleles• Show the cross between a mother who is

heterozygous for type B blood and a father who is heterozygous for type A blood.

-AB (1); BO (1); AO (1); OO (1)- ratio 1:1:1:1

-type AB (1); type B (1) type A (1); type O (1)- ratio 1:1:1:1

GENOTYPES:

PHENOTYPES:

A O

B

O

AB

OO

BO

AO