1. Mendelian Genetics
12/8/2018
Dihybrid Cross
A breeding experiment that tracks the inheritance of two traits: 2 genes = 4 alleles
Mendel’s “Law of Independent Assortment”
Each pair of alleles aligns during meiosis and separates independently during gamete formation
Ss Bb SB Sb sB sb
Alleles independently assort during meiosis

2. Mendel’s next question…
Will I see the same inheritance patterns and predictability in crosses involving more than 1 gene?

3. Parents are homozygous for both traits
Dihybrid crosses f1
Traits: Seed shape & Seed color
Alleles: R =round r = wrinkled
Y = yellow y = green
Parents are homozygous for both traits
RRYY x rryy
How will the alleles independently assort for each parent?
To help determine how the alleles assort use the math
order of operations F O I L First-Outside-Inside-Last
RY RY RY RY and ry ry ry ry
What would a Punnett square look like for a dihybrid cross?

4. Dihybrid punnett F1 ry RY RrYy RrYy RrYy RrYy RrYy RrYy RrYy RrYy
Mendelian Genetics
12/8/2018
Dihybrid punnett F1
Each parent can produce 4 combinations of alleles( 24 = 16)
The Punnett square in a dihybrid cross will have 16 boxes
Parent alleles
are placed in the
same way… ry
Then the
alleles are
combined to
produce the
offspring; making
sure to keep
the alleles in
the same order
as the parent’s RY
RrYy RrYy RrYy RrYy
Genotypic ratio
16:16
RrYy
Phenotypic ratio
round/yellow
RrYy RrYy RrYy RrYy
RrYy RrYy RrYy RrYy
RrYy RrYy RrYy RrYy
Just like with a monohybrid cross ; all genotypes are heterozygous (RrYy)
and all phenotypes are dominant (Round/Yellow) for both traits

5. Dihybrid Cross RrYy x RrYy
Mendelian Genetics
12/8/2018
Dihybrid Cross
Traits: Seed shape & Seed color
Alleles: R =round r = wrinkled
Y = yellow y = green
To help determine allele combinations use the math order of operations FOIL
First-Outside-Inside-Last
RrYy x RrYy RY Ry rY ry RY Ry rY ry
All possible allele combinations when parents are heterozygous for both

6. Dihybrid punnett F2 RY Ry rY ry RY Ry rY ry RRYY RRYy RrYY RrYy
Mendelian Genetics
12/8/2018
Dihybrid punnett F2
Parent alleles
are placed in the
same way… RY Ry rY ry
Then the
alleles are
combined to
produce the
offspring; making
sure to keep
the alleles in
the same order
as the parent’s RY Ry rY ry
RRYY RRYy RrYY RrYy
RRYy RRyy RrYy Rryy
RrYY RrYy rrYY rrYy
RrYy Rryy rrYy rryy
Genotypic and phenotypic ratios are a bit trickier, however
there is a pattern if you look closely…

7. Dihybrid F2 genotypic and phenotypic ratios
Mendelian Genetics
12/8/2018
Dihybrid F2 genotypic and phenotypic ratios
Round/Yellow: 9
RRYY 1
RRYy 2
RrYY 2
RrYy 4
Round/green: 3
RRyy 1
Rryy 2
wrinkled/Yellow:3
rrYY 1
rrYy 2
wrinkled/green: 1
rryy 1 RY Ry rY ry
RRYY
RRYy
RrYY
RrYy
RRyy
Rryy
rrYY
rrYy
rryy
9:3:3:1 PHENOTYPIC RATIO

8. Crosses of two genes representing four alleles
Dihybrid crosses

9. Guinea pig dihybrid crosses
BBLl BbLL BbLl
BBLl BBll BbLl Bbll
BbLL BbLl bbLL bbLl
BbLl Bbll bbLl bbll
Phenotypic ratios: How many out of 16 are… 9 3 3 1
Black/Short____ Black/long____ white/Short_____ white/long_____

10. bL bl bL bl BL Bl bL bl bL bl bL bl How many of the offspring are:
Black/Short:______
Black/long: ______
white/Short:______
white/long: ______
bL bl bL bl 6 2 BL Bl bL bl
BbLL BbLl BbLL BbLl
BbLl Bbll BbLl Bbll
bbLL bbLl bbLL bbLl
bbLl bbll bbLl bbll

11. bL bL bL bL Bl Bl bl bl bL bL bL bL How many of the offspring are:
Black/Short:______
Black/long: ______
white/Short:______
white/long: ______
bL bL bL bL 8 Bl bl
BbLl BbLl BbLl BbLl
BbLl BbLl BbLl BbLl
bbLl bbLl bbLl bbLl
bbLl bbLl bbLl bbLl

12. Dominant alleles determine phenotype
If you pay attention to the dominant alleles in the parent genotypes…you can determine phenotypic ratios with out having to do the squares!
R = purple r = white
T = tall t = dwarf
RrTT x rrTt
50% or 8/16 will be purple :100% or 16/16 will be tall
rrTt x RRtt
100% or 16/16 will be purple: 50% or 8/16 will be tall
RrTt x rrTt
50% or 8/16 will be purple: 75% or 12/16 will be tall
Rrtt x RRtt
100% or 16/16 will be purple: none Tall, 100% or 16/16 short

13. Let’s look at a trihybrid…
Can you tell what the parent genotypes were?
This is an F2 so both parents are heterozygous for the 3 traits: AaBbCc X AaBbCc

14. Mendelian Genetics
12/8/2018
Dihybrid Cross

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18. b. Formula: 2n (n = # of heterozygotes)
Mendelian Genetics
12/8/2018
copyright cmassengale
Question: How many gametes will be produced for the following allele arrangements?
Remember: 2n (n = # of heterozygotes)
1. RrYy
2. AaBbCCDd
3. MmNnOoPPQQRrssTtQq
b. Formula: 2n (n = # of heterozygotes)

19. Answer: 1. RrYy: 2n = 22 = 4 gametes RY Ry rY ry
Mendelian Genetics
12/8/2018
Answer:
1. RrYy: 2n = 22 = 4 gametes
RY Ry rY ry
2. AaBbCCDd: 2n = 23 = 8 gametes
ABCD ABCd AbCD AbCd
aBCD aBCd abCD abCD
3. MmNnOoPPQQRrssTtQq: 2n = 26 = 64 gametes