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The concept of dihybrid crosses, focusing on Punnett squares and the independent assortment of genes located on different chromosomes. It covers the formation of gametes, the probability of different combinations, and exceptions to independent assortment due to linked genes. Students will learn about the significance of independent assortment in genetics and its impact on the frequency of dominant and recessive traits in a population.
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R
y
r
Y
R (^) r
Y (^) y
Equally likely to line up either way when dividing
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
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.
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 possible gametes from heterzygote. Equally likely if independently assorting.
Test cross (heterozygote crossed with a recessive) results if independently assorting: ________ (_____ of each of 4 possible combinations.)
Discuss with your table partner: Mythbuster
_________________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.
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