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.

All possible gamete combinations Dihybrid Cross Traits: Seed shape & Seed color Alleles: R round r wrinkled Y yellow y green RrYy x RrYy RY Ry rY ry RY Ry rY ry All possible gamete combinations

Dihybrid Cross RY Ry rY ry RY Ry rY ry

Dihybrid Cross RY Ry rY ry Round/Yellow: 9 Round/green: 3 wrinkled/Yellow: 3 wrinkled/green: 1 9:3:3:1 phenotypic ratio RRYY RRYy RrYY RrYy RRyy Rryy rrYY rrYy rryy

Incomplete Dominance When the heterozygous condition is intermediate between dominant and recessive Happens because one allele is defective, example: R=red flowers, R’= white flowers so RR’ = pink flowers (R codes for enzyme to produce red pigment, R’=no enzyme)

RR x R’R’ Incomplete dominance R’R’ x R’R’ Red White F1 generation: 1 Red: 2 Pink: 1 white

Codominance 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 Key: B = black W = white BW = checkered W W BW BW Results: 100% checkered (BW) BW BW B

Now let’s cross a checkered hen with a checkered rooster. B W Results: 25% Black 50% Checkered 25% White (1:2:1) BB BW B BW WW W

Codominance Two alleles are expressed (multiple alleles) in heterozygous individuals. Example: blood type 1. type A = IAIA or IAi 2. type B = IBIB or IBi 3. type AB = IAIB 4. type O = ii

Blood 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

Surface molecule A is produced. Phenotype A Surface molecule A 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. Surface molecule A is produced.

Phenotype B The lB allele is also dominant to i. Surface molecule B The lB allele is also dominant to i. 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.

The lA and lB alleles are codominant. Phenotype AB Surface molecule B The lA and lB alleles are codominant. 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 A

Phenotype O No antigens produced

A B A or O A or AB B or O B or AB AB A and B None A, B, AB, O O both Blood Group Antigens Antibodies Can receive from Can give to A B A or O A or AB B or O B or AB AB A and B None A, B, AB, O O both

Codominance Problem Example:homozygous male Type B (IBIB) x heterozygous female Type A (IAi) IB IA i IAIB IBi 1/2 = IAIB 1/2 = IBi

Another Codominance Problem Example: male Type O (ii) x female type AB (IAIB) i IA IB IAi IBi 1/2 = IAi 1/2 = IBi