Transcript Dihybrid Crosses

Dihybrid Crosses
Let’s review
• So far, we have looked only at different
types of monohybrid crosses
– the inheritance of only ONE trait at a time
– Ex:
• What’s the likelihood of a pea plant producing
round seeds?
• What’s the likelihood of a pea plant producing
green seeds?
Let’s review
• We use a basic Punnett square and the
Law of Segregation to help determine the
likely outcomes of a monohybrid cross
– Law of Segregation: An individual’s alleles
for a trait segregate into different gametes
during meiosis so that each gamete
receives only 1 allele for the trait
Segregation during
meiosis
Dihybrid crosses
• Dihybrid crosses:
–predict and/or observe the
inheritance of TWO traits at the
same time.
–Ex: What’s the likelihood of a
pea plant making seeds that are
round AND green?
Mendel’s Peas (again!)
Mendel had observed that the seeds of his
pea plants varied in several ways – among
them, whether the peas were round or
wrinkled and whether they were green or
yellow.
He did monohybrid crosses on both
characteristics, but then decided to look at
how they were inherited together….
• There are four possible combinations of
the two characteristics. Peas can be:
• 1) round and yellow
• 2) round and green
• 3) wrinkled and yellow
• 4) wrinkled and green
• From his monohybrid trials, Mendel had
discovered that the allele for yellow color
was dominant to that for green.
• He had also found that the allele for round
peas was dominant to the allele for
wrinkled peas.
• Starting with individuals that were
homozygous, either for both dominant
characteristics or both recessive
characteristics, he set off breeding again…
Phenotype
Round, yellow
Genotype
RRYY
Gametes
all
F1
X
RY
all
Wrinkled, green
rryy
all
RrYy
ry
Round, yellow
All of the F1 generation would be heterozygous for both
characteristics, meaning that they would all be round and
yellow. As with monohybrid crosses, he then crossed two
of the F1 generation together…
F1 Phenotype
Round, yellow
Genotype
RrYy
Gametes
RY, Ry, rY, ry
X
Round, yellow
RrYy
RY, Ry, rY, ry
© 2007 Paul Billiet ODWS
How does a dihybrid Punnett
square look?
• With a monohybrid, two different gametes
could form from one parent. A or a for
example. So the Punnett square had
room for 2 gametes for each parent.
• With a dihybrid cross, four different
gametes are possible. So the Punnett
square need to have 16 boxes (4 to a
side)
RY
Ry
rY
ry
RY
RY
Ry
rY
ry
Ry
rY
ry
RY
RY
Ry
rY
ry
Result of
fertilization
Ry
rY
ry
RY
Ry
rY
ry
RY
Ry
rY
ry
RRYY
RRYy
RrYY
RrYy
RY
Ry
rY
ry
RRYY
RRYy
RrYY
RrYy
RRYy
RRyy
RrYy
Rryy
RrYY
RrYy
rrYY
rrYy
RrYy
Rryy
rrYy
rryy
RY
Ry
rY
ry
Phenotypic Ratio for F2 generation
•
•
•
•
# of round, yellow:
# of round, green:
# of wrinkled, yellow:
# of wrinkled, green:
______
______
______
______
– Hint: use a different symbol to check off or
shade in each box.
RY
Ry
rY
ry
RRYY
RRYy
RrYY
RrYy
RRYy
RRyy
RrYy
Rryy
RrYY
RrYy
rrYY
rrYy
RrYy
Rryy
rrYy
rryy
RY
Ry
rY
ry
RY
Ry
rY
ry
RRYY
RRYy
RrYY
RrYy
RRYy
RRyy
RrYy
Rryy
RrYY
RrYy
rrYY
rrYy
RrYy
Rryy
rrYy
rryy
RY
Ry
rY
ry
F2
9 : 3 : 3 : 1
Round
Round Wrinkled Wrinkled
Yellow
Green Yellow
Green
Mendel’s Conclusion
• Mendel’s results showed that inheritance
of seed shape did NOT influence the
inheritance of seed color.
• The two traits are inherited
INDEPENDENTLY
• This led Mendel to develop the Law of
Independent Assortment
Law of Independent Assortment
• Genes for different traits are sorted and
inherited independently of each other
• The pairs of chromosomes could orientate in
different ways at Anaphase 1
• Of course, if the genes (alleles) are located on
the same chromosome then they will not sort
independently. They are “linked” and inherited
together.
– Since humans have 46 chromosomes but about
30,000 genes, many genes are linked.
© 2007 Paul Billiet ODWS
F2 generation
You can use monohybrid Punnett squares and multiply the results.
Ex: What is the probability of round, wrinkled seeds?
¾ probability of round x ¼ probability of wrinkled = 3/16 probability of
round and wrinkled.
Yellow vs. Green
Round vs. Wrinkled
R
r
R
RR
Rr
r
Rr
rr
Y
y
Y
YY
Yy
y
Yy
yy