Dihybrid Crosses Incomplete Dominance Codominance Multiple 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. PpPp P p pp Pp PP - PP (1); Pp (2); pp (1) - ratio 1:2:1 - purple (3); white (1) - ratio 3:1 GENOTYPES: PHENOTYPES:

Dihybrid Cross: a cross that shows the possible offspring for two traits Fur Color: B: Black b: White Coat Texture: R: Rough r: 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 Possible gametes: BR Br bR br Parent 2 Possible gametes: BR Br bR br FOILFOIL

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: Rough r: 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: Rough r: Smooth BbRr x BbRr BR bR br bR Br BR br Br BBRRBbRRBbRr BBRrBBrrBbRrBbrr BbRRBbRrbbRRbbRr BbRrBbrrbbRrbbrr BBRr

BR bR br bR Br BR br Br BBRRBbRRBbRr BBRrBBrrBbRrBbrr BbRRBbRrbbRRbbRr BbRrBbrrbbRrbbrr 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: Rough r: Smooth

BR bR br bR Br BR br Br BBRRBbRRBbRr BBRrBBrrBbRrBbrr BbRRBbRrbbRRbbRr BbRrBbrrbbRrbbrr 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: Rough r: 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 YyRryyRryyrr YyRrYyrryyRryyrr YyRrYyrryyRryyrr YyRrYyrryyRryyrr Yyrr

Incomplete Dominance A third (new) phenotype appears in the heterozygous condition. Flower Color in 4 O’clocks RR = redrr = whiteRr = 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 rrrr rrRr rr Rr GENOTYPES: PHENOTYPES:

Humans have 23 pairs of chromosomes. 22 of the pairs are called autosomes. One pair of chromosomes is related to the sex of an individual, these chromosomes are called sex chromosomes XX = Female XY = Male

Genes that are located on the X chromosome are called sex-linked genes. Traits determined by sex-linked genes are called sex-linked traits – Ex: Color Blindness (c = colorblind, C = normal)

Color Blindness is recessive. FEMALES: both X chromosomes must have the gene in order for the trait to be expressed. MALES: only have one X chromosome. If that X chromosome has the colorblind allele, then the trait will be expressed. Colorblindness affects 1 in 100 females; 1 in 10 males

X-linked Recessive Inheritance One copy of an altered gene on the X chromosome causes the disease in a male. = Hemizygote Male An altered copy on one of the X chromosome pair causes carrier status in a female. = Heterozygote XX XY Female

X C X C – Normal Female X C X c – Carrier Female X c X c – Color Blind Female X C Y– Normal Male X c Y– Colorblind Male

1. A colorblind male marries a normal female. What are the offspring genotypes and phenotypes? (C = normal, c = colorblind) XcYXcY XCXC XCXC

1. A colorblind male marries a normal female. What are the offspring genotypes and phenotypes? (C = normal, c = colorblind) X c Y X C X C X c X C Y Genotypic Ratio: __________ Phenotypic Ratio: __________ What percentage of sons with colorblindness? _______ daughters? ______

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 – I A, I B, I)

Rules for Blood Type A and B are codominant A and B are dominant over 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) -ratio 2:2:0:0:0:0 - type A (2); type B (2) - ratio 2:2:0:0 GENOTYPES: PHENOTYPES: O O ABAB 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 1:1:1:1:0:0 -type AB (1); type B (1) type A (1); type O (1) - ratio 1:1:1:1 GENOTYPES: PHENOTYPES: A O BOBO AB OO BO AO