1. Heredity and Genetics Dihybrid Crosses
By: Mr. Powell

2. 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%

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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:____________________

10. 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

11. 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

12. 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
________

13. Thank You!