Download Dihybrid Crosses - Mercer Island School District

Dihybrid Crosses:
Punnett squares
for two traits
Genes on Different Chromosomes
If two genes are on different chromosomes, all four
possible alleles combinations for two different genes in
a heterozygous cross are _____________ due to
independent assortment.
If parents are RrYy (heterozygous for both traits)
R
r
y
Y
OR
R
r
Y
y
Equally
likely to
line up
either way
when
dividing
Ry
rY
RY
ry
Gametes: ___________________________________
Setting up a Dihybrid Cross: RrYy x RrYy
 Each side of a Punnett Square represents all the
possible allele combinations in a gamete from a
parent.
 Parent gametes always contain one allele for___
 _______ .(____________-R or r & Y or y in this case).
 Four possible combinations of the alleles for the
two genes are possible if heterozygous for both traits.
(For example: ___________________)
 Due to independent assortment, each possible
combination is equally likely if genes are on separate
chromosomes. Therefore Punnett squares indicate
probabilities for each outcome.
Discuss with your table partner:
A.
Which is correct for a
R
dihybrid cross of two
r
heterozygous parents
Y
RrYy x RrYy?
y
R
r
Y
y
C. RY Ry rY ry
B. RR Rr rr rR
RY
YY
Ry
Yy
yy
rY
yY
ry
Explain how the correct answer relates to the genes
passed down in each gamete (egg or sperm)
Dihybrid Cross of 2 Heterozygotes
9
3
3
1
Heterozygous Dihybrid Cross
Dominant
for both
traits
Dominant
1st trait
Recessive
2nd trait
Round
Yellow
__
______ =
___
Round
Green
__
______ =
____
Recessive
1st trait
Dominant
2nd trait
Wrinkled
Yellow
__
______ =
____
Recessive
for both
traits
Wrinkled
Green
__
_______ =
_____
Heterozygous cross __________ratio if independent assortment
Mendel developed the Law of Independent
Assortment because he realized that the results for
his dihybrid crosses matched the probability of the
two genes being inherited independently.
Exceptions to the Law of Independent
Assortment: Linked Genes
Studying fruit flies, Thomas Hunt Morgan identified
dihybrid crosses that did not result in the phenotype
ratios expected through independent assortment.
Review of Recombinants
Recombinants have new combinations of the genes.
If the parental generation of cats are white furred
with short tails (SS bb) and brown fur with long tails
(ss BB),
Parental Genotypes
the F1 will be SsBb. In the gametes for the next
generation, ________________ are the parental
combinations for the gametes (egg or sperm) and
________________ are the recombinant gametes.
___________
Combinations
____________
gametes
Linked Genes
When two genes are linked
(relatively close together on the
same chromosome), it ________
___________________________
__________________________.
Recombinants do occur due to
crossing over.
The further apart the genes are
the more likely recombinants are
because crossing over is more
likely to occur.
Four possible gametes from heterzygote.
Equally likely if independently assorting.
SsBb
x ssbb
One possible gamete from
homozygous recessive
parent.
sb
SB
SsBb
Short, Brown
Sb
Ssbb
Short, White
sB
ssBb
Long, Brown
sb
ssbb
Long, White
Test cross (heterozygote crossed with a recessive)
results if independently assorting: ________
(_____ of each of 4 possible combinations.)
Parental combinations:
Gray with normal wings
and
Black with vestigal wings
Cross of the
heterozygous offspring
with a double recessive.
Expect 1:1:1:1 if
independently assorting.
Results show more of
____________________
because genes are on
the same chromosome.
Discuss with your table partner: Mythbuster
Many people assume that dominant traits are always more
common.
It is true that if both parents are heterozygous, it is more
likely that the offspring will have the dominant trait.
However, a recessive trait can be more common in the
population.
For example, Achrondoplasia dwarfism is caused by a
dominant allele, and yet the trait is very rare. Explain how
and why this is possible.
__________________________________
__________________________________
__________________________________
Genetics and ______________
_________________can change the allelic frequency.
(Allelic frequency = how common an allele is in the
gene pool.)
Bb
BB
bb
bb
Bb
Bb
bb
BB
Bb
Bb
BB
Bb
Initial allelic frequency of b = 50% (12/24 alleles)
If the white rabbits are more likely to _________ , the
frequency of b allele will increase in the gene pool.
bb
bb
bb
bb
bb
Bb
bb
bb
bb
bb
bb
bb
bb
bb
Allelic frequency of b after natural selection= 96% (23/24)
Discuss with your table partner:
Review the meaning of the words genotype and
phenotype.
Do you think natural selection acts upon the
genotype or phenotype of an organism? Explain
your reasoning.
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