Heredity and Genetics Dihybrid Crosses By: Mr. Powell

Bellwork – April 23rd In cats, grey fur is dominant over white fur. Your cat, Sir Whiskers, has white fur. Your cousin’s cat, Tabbytha, is homozygous dominant for grey fur. If Tabbytha and Sir Whiskers have babies (after getting married), what are the chances of them having kittens with white fur? Chance of kittens with white fur ? 0%

Review of Monohybrid Crosses Remember, monohybrid crosses only involve 1 trait Eye color, hair color, fur color, height, etc. Practice – In fruit flies, red eyes (R) are dominant over white eyes (r) Using the Punnett square below, cross 2 heterozygous red-eyed fly. Determine the genotypic and phenotypic ratios for the cross. R r 1:2:1 R Genotypic Ratio __________________ Phenotypic Ratio__________________ R R R r 3:1 r R r R r r r

More Review You come across a red-eyed fly in the wild. Can you tell, just by looking at it, if it is homozygous dominant or heterozygous? ______ To check, you can perform a ___________ with a white-eyed fly. If the red-eyed fly’s genotype was RR, how would the offspring of the cross look? _______________ If the red-eyed fly’s genotype was Rr, how would the offspring of the cross look?____________ NO Test Cross All red eyes 50% red-eyes, 50% white-eye

Dihybrid Cross Fur Color Fur Texture Black: B Rough: R White: b A dihybrid cross is a cross that shows the possible offspring for TWO traits. In this example, we will cross a heterozygous individual with another heterozygous individual for both traits. Their genotypes will be BbRr x BbRr Fur Color Black: B White: b Fur Texture Rough: R Smooth: r

Dihybrid Cross: BbRr x BbRr The first step is to find _____ possible gametes that can be made from each parent. ALL Possible gametes for BbRr ____ Possible gametes for BbRr ____ BR BR Br Br bR bR br br

Step 2 – Find the possible genotypes Possible Gametes for BbRr BR bR br Br Fur Color: B: Black b: White BBRR BBRr BbRR BbRr BBRr BBrr BbRr Bbrr Fur Texture: R: Rough r: Smooth Possible Gametes for BbRr BbRR BbRr bbRR bbRr BbRr Bbrr bbRr bbrr

9 3 3 1 BR bR Br br 9:3:3:1 Fur Color: B: Black b: White Fur Texture: R: Rough r: Smooth BR bR br Br BBRR BbRR BbRr BBRr BBrr Bbrr bbRR bbRr bbrr How many offspring would have a black, rough coat? 9 _________ How many offspring would have a black, smooth coat? 3 _________ 3 How many offspring would have a white, rough coat? _________ 1 How many offspring would have a white, smooth coat? _________ 9:3:3:1 Phenotypic Ratio would be:____________________

Another Example YyRr yyrr In pea plants, yellow seeds (Y) are dominant over green seeds (y), and rounded peas (R) are dominant over wrinkled peas (r). Let’s cross a plant that is heterozygous for both traits with a plant that is homozygous recessive for both traits. Plant that is heterozygous for both traits _____________ Plant that is homozygous recessive for both traits ___________ YyRr yyrr

YyRr yyrr YyRr yyrr YR yr yr Yr yr yR yr yr Cross: ________ x _________ YyRr yyrr Possible Gametes from _________ ________ Possible Gametes from _________ ________ YR yr yr Yr yr yR yr yr

YyRr yyrr 4 4 4 4 Cross: ______ x _______ YR Yr yR yr yr Number of peas that are Yellow and Round 4 ________ yr Number of peas that are Yellow and Wrinkled YyRr Yyrr yyRr yyrr 4 ________ Number of peas that are Green and Round yr YyRr Yyrr yyRr yyrr 4 ________ Number of peas that are Green and Wrinkled yr YyRr Yyrr yyRr yyrr 4 ________

Thank You!