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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 Cross.
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DihybridPunnett 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 Cross • Traits: Seed shape & Seed color • Alleles:R round r wrinkled Y yellow y green RrYy x RrYy RY Ry rY ry RY RyrYry All possible gamete combinations
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 RY RRYY RRYy RrYY RrYy Ry RRYy RRyy RrYy Rryy rY RrYY RrYy rrYY rrYy ry RrYy Rryy rrYy rryy 9:3:3:1 phenotypic ratio Dihybrid Cross
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 B 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
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 Ais produced.
Phenotype B 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.
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
IA i IAIB IBi IB 1/2 = IAIB 1/2 = IBi IB IAIB IBi Codominance Problem Example:homozygous male Type B (IBIB) x heterozygous female Type A (IAi)
IA IB IAi IBi i 1/2 = IAi 1/2 = IBi i IAi IBi Another Codominance Problem Example:male Type O (ii) x female type AB (IAIB)
