1. Genetics
YEAR 12 BIOLOGY
Originally created by Bob Perry, Malibu High School
Modified by John Cary, Malibu High School

2. Dihybrid
Crosses: or
predicting 2 traits
at a time

3. characteristics of garden peas.
Gregor Mendel studied 7
characteristics of garden peas.

4. Let’s see how to predict crosses with TWO traits at a time:
Gregor Mendel studied 7
characteristics of garden peas.

5. RECALL THAT ROUND & YELLOW ARE BOTH DOMINANT TRAITS.
1. Choose symbols:
RR = round
Rr = round
r r = wrinkled
SEED
SHAPE
YY = yellow
Yy = yellow
y y = green
SEED
COLOR
RECALL THAT ROUND & YELLOW
ARE BOTH DOMINANT TRAITS.

6. CROSS BETWEEN 2 ROUND & YELLOW
WE WILL WORK A DIHYBRID
CROSS BETWEEN 2 ROUND & YELLOW
HYBRID PEAS.
Rr Yy
(BOTH PARENTS WILL
HAVE THE SAME GENOTYPE
IN THIS PROBLEM)

7. Notice there are now FOUR different genetic possibilities for a gamete
1. Write gamete genotypes using
Mendel’s Principle of Independent
Assortment:
Parental
Genotype
Both parents
Rr Yy Ry rY ry RY
Notice there are now FOUR different genetic possibilities for a gamete
possible
Gamete
genes

8. 2. Make a 16 square Punnett Square:
female genotype
and gametes
Rr Yy RY Ry rY ry
male genotype
and gametes RY Ry
Rr Yy rY ry

9. 3. Match up genes from both parents:
Rr Yy RY Ry rY ry
RRYy
RrYY
RrYy RY
RRYY Ry
RRYy
RRyy
RrYy
Rryy
Rr Yy rY
RrYY
RrYy
rrYY
rrYy ry
RrYy
Rryy
rrYy
rryy

10. 4. Match up genes from both parents:
Rr Yy RY Ry rY ry
RRYy
RrYY
RrYy RY
RRYY
RRyy Ry
RRYy
RrYy
Rryy
Rr Yy rY
RrYY
RrYy
rrYY
rrYy ry
RrYy
Rryy
rrYy
rryy

11. 5. Count and analyze the offspring:
Rr Yy RY Ry rY ry RY Ry
Rr Yy rY ry

12. 5. Count and analyze the offspring:9
9 smooth + yellow 1 2 3 4 3
3 smooth + green 6 5 1 2 3 7 1 2 8
3 wrinkled + yellow 1 9 3 3 1
1 wrinkled + green

13. 9 : 3 : 3 : 1 RATIO A DIHYBRID CROSS RESULTS IN A
5. Count and analyze the offspring: 9
A DIHYBRID CROSS
RESULTS IN A
9 smooth + yellow 3
3 smooth + green
9 : 3 : 3 : 1
RATIO 3
3 wrinkled + yellow
(If there is complete
dominance) 1
1 wrinkled + green

14. Review Checkpoint: 1. What is meant by a “dihybrid” ?
2. How many different gamete genotypes are possible in a dihybrid cross ?
3. How many boxes are needed in a dihybrid Punnett Square ?
4. What are the ratios of possible offspring from a dihybrid cross ?
5. Show a cross between a pure wrinkled green pea plant and a pure yellow and round pea plant. Give ratios for offspring.

15. Review Checkpoint:
6. Big feet are dominant over small feet in kangaroos. Similarly, tall ears are dominant over short ears. Show a cross between two hybrid kangaroos and give the ratios of expected baby kanga’s.
7. In buffalo, a cross between a black and a white give brown coat baby buffalo. And long horn buffalos crossed with shorties give medium horns. Give the ratios of expected offspring from two brown & medium horny buff’s.