1. Dihybrid Crosses
General Biology

2. Monohybrid cross
Some of Mendel’s experiments followed the inheritance of single traits
monohybrid crosses
flower color
seed color
height
General Biology

3. Monohybrid Review
Tongue rolling gene is dominant to non-rolling gene. Cross a heterozygous roller with a non-roller.
1st – Identify the dominant and recessive genes
2nd – Identify parent genotypes
3rd – Now use the Punnett Square
General Biology

4. Monohybrid Examples Rr x rr R r Rr rr 50% chance of being Rollers
Kinds of sperm
Kinds of eggs R r Rr rr
50% chance of being Rollers
50% chance of being non-rollers
**This does not mean that if you have four children, 2 will automatically be able to roll their tongues and 2 will not. Each Punnett square probability is for a single event.
General Biology

5. Dihybrid cross
Other experiments by Mendel followed the inheritance of 2 different characters
“Are genes inherited as
a package?”
Dihybrid crosses
seed color (green vs. yellow)
seed shape (round vs. wrinkled)
General Biology

6. Mendel’s Dihybrid Experiment
Round is dominant to wrinkled. Yellow is dominant to green.
Cross a homozygous round, homozygous yellow plant with a homozygous wrinkled, homozygous green plant.
Parents?
RRYY x rryy
General Biology

7. Dihybrid cross P YYRR yyrr 100% F1 YyRr 9:3:3:1 F2 x true-breeding
yellow, round peas
true-breeding
green, wrinkled peas x
YYRR
yyrr
Y = yellow
R = round
y = green
r = wrinkled
100% F1
generation
(hybrids)
yellow, round peas
YyRr
Wrinkled seeds in pea plants with two copies of the recessive allele are due to the accumulation of monosaccharides and excess water in seeds because of the lack of a key enzyme. The seeds wrinkle when they dry.
Both homozygous dominants and heterozygotes produce enough enzyme to convert all the monosaccharides into starch and form smooth seeds when they dry.
self-pollinate
9:3:3:1 F2
generation
9/16
yellow
round
peas
3/16
green
round
peas
3/16
yellow
wrinkled
peas
1/16
green
wrinkled
peas
General Biology

8. Which system explains the data?
What’s going on here?
If genes are on different chromosomes…
how do they assort in the gametes?
together or independently?
YyRr
Is it this?
Or this?
YyRr YR yr YR Yr yR yr
Which system explains the data?
General Biology

9.  Is this the way it works? YyRr x YyRr YR yr YR YYRR YyRr yr YyRr
9/16
yellow
round  YR yr
3/16
green
round
Well, that’s NOT right! YR
YYRR
YyRr
3/16
yellow
wrinkled yr
YyRr
yyrr
1/16
green
wrinkled
General Biology

10.  Dihybrid cross YyRr x YyRr YR Yr yR yr YR Yr yR yr YYRR YYRr YyRRor
Dihybrid cross
YyRr x
YyRr
9/16
yellow
round YR Yr yR yr YR Yr yR yr 
3/16
green
round
YYRR
YYRr
YyRR
YyRr
BINGO!
YYRr
YYrr
YyRr
Yyrr
3/16
yellow
wrinkled
YyRR
YyRr
yyRR
yyRr
1/16
green
wrinkled
YyRr
Yyrr
yyRr
yyrr
General Biology

11. Mendel’s 2nd law of heredity
Can you think of an exception to this?
Mendel’s 2nd law of heredity
Law of independent assortment
different genes separate into gametes independently
non-homologous chromosomes align independently
classes of gametes produced in equal amounts
YR = Yr = yR = yr
yellow
green
round
wrinkled
YyRr Yr Yr yR yR YR YR yr yr 1 : 1 : 1 : 1
General Biology

12. Law of Independent Assortment
Which stage of meiosis creates the law of independent assortment?
Metaphase 1
Remember Mendel didn’t even know DNA —or genes— existed!
EXCEPTION
If genes are on same chromosome & close together
will usually be inherited together
rarely crossover separately
“linked”
General Biology

13. Review: Mendel’s laws of heredity
Law of segregation
monohybrid cross
single trait
each allele for a gene segregates into separate gametes
established at Metaphase 1
Law of independent assortment
dihybrid (or more) cross
2 or more traits
genes on separate chromosomes assort into gametes independently
established by Metaphase 1
EXCEPTION
linked genes
metaphase1
General Biology

14. Rules of a Dihybrid Cross
Parents now have 4 letters! (2 traits)
Gametes have 2 letters (1/2 of parent!)
Gametes must have 1 gene from EACH trait. (1 of each letter on the outside)
Offspring (inside boxes) have 4 letters
Always write the dominant letter first, if present.
Don’t split your genes!
General Biology

15. Example: YyRr x YyRr
First! Find the possible gamete combinations (FOIL). R r Y y YR yR yr Yr
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16. Example: YyRr x YyRr YR yR Yr yr YYRR YyRR YYRr YyRr yyRR yyRr YYrr
Phenotypes: 9/16 yellow, round 3/16 yellow, wrinkled
3/16 green, round 1/16 green, wrinkled
General Biology

17. Dihybrid Problem 1
In humans, red hair is recessive to brown hair and brown eyes are dominant to blue eyes.
Cross a red-haired, blue-eyed man with a woman heterozygous for both brown hair and eyes. What are the genotypes of the offspring?
General Biology

18. Dihybrid Problem 1 Cont’d.
1st – Assign letters.
2nd – Figure out the parents.
3rd – Figure out the possible gametes (FOIL)
4th – Work out using a large Punnett Square
2 of each letter on the outside
4 letters on the inside
Capitol letter first
Don’t split your genes!
B,b and E,e
bbee x BbEe
be x BE, Be, bE, be
General Biology

19. Dihybrid Problem 1 Cont’dBE Be bE be
BbEe
Bbee
bbEe
bbee be be be be
Genotypes: BbEe, 4/16 Bbee, 4/16
bbEe, 4/16 bbee, 4/16
General Biology

20. Remember Probability? x x x ?%
Use rule of multiplication to predict crosses
YyRr
YyRr x Yy x Rr x
yyrr
1/16 ?% yy rr
1/4
1/4 x
General Biology

21. Dihybrid Problem 2
In summer squash, white fruit color is dominant over yellow fruit color.
Disk-shaped fruit is dominant over sphere-shaped fruit.
Cross a heterozygous white, homozygous disk-shaped plant with a yellow, sphere-shaped fruit plant.
What are the genotype and phenotype ratios of the offspring?
General Biology