dihybrid crosses - mercer island school district · setting up a dihybrid cross: rryy x rryy ....
TRANSCRIPT
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) Ry rY RY ry Gametes: ___________________________________
R
y
r
Y
R r
Y y
OR
Equally likely to line up either way when dividing
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.
Setting up a Dihybrid Cross: RrYy x RrYy
Which is correct for a dihybrid cross of two heterozygous parents RrYy x RrYy?
A. R r Y y R r
Y y
B. RR Rr rr rR YY Yy yy yY
C. RY Ry rY ry RY Ry rY ry
Explain how the correct answer relates to the genes passed down in each gamete (egg or sperm)
Discuss with your table partner:
Dihybrid Cross of 2 Heterozygotes
9
3
3
1
Heterozygous Dihybrid Cross Dominant for both
traits
Dominant 1st trait
Recessive 2nd trait
Recessive 1st trait
Dominant 2nd trait
Recessive for both
traits
Round Yellow
Round Green
Wrinkled Yellow
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), 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.
Parental Genotypes
___________ 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.
sb
SB SsBb Short, Brown
Sb Ssbb Short, White
sB ssBb Long, Brown
sb ssbb Long, White
SsBb x ssbb One possible gamete from homozygous recessive parent.
Four
pos
sible
gam
etes
from
het
erzy
gote
. Eq
ually
like
ly if
inde
pend
ently
ass
ortin
g.
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.)
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.
Allelic frequency of b after natural selection= 96% (23/24)
Bb BB bb bb Bb Bb bb BB Bb Bb BB Bb
bb bb bb bb bb Bb bb bb bb bb bb bb bb bb
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.