# BIO152H5 Lecture Notes - Lecture 21: Dihybrid Cross, Pea, Housefly

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23 Jan 2016
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Biology Review Session Lecture
Wednesday, December 2, 2015
9:50 AM
Exam:
o115 questions
Application questions mostly
Study for comprehension, know concepts not
memorizing examples
oSame format as midterm tests
oCan have a calculator (non-programmable) but it is not needed
Genetic problems will have simple math and be more
Practice Problem:
oA black hamster is crossed with an albino hamster, and a litter
of 10 black hamster is born. When an albino hamster is crossed with
a 2nd generation black hamster, the offspring are black and albino
hamsters. What are the genotypes for the parents and offspring of the
2 crosses? (Let Black=B and Albino=b)
Frist Cross: Parents=BBxbb, Offspring=Bb
B B
b Bb Bb
b Bb Bb
Second Cross: Parents=Bbxbb, Offspring=1/2 Bb and 1/2
bb
B b
b Bb bb
b Bb bb
oIn tigers, a recessive allele results in lack of pigmentation
(white tiger) and a cross-eyed condition. At Toronto Zoo, two
phenotypically normal tigers, that are heterozygous, mated. What
fraction of their offspring will be white and cross-eyed?
rr=recessive=white&cross-eyed
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R r
R RR Rr
r Rr rr
RrxRr = 1/4 RR, 1/2 Rr, 1/4 rr
Therefore 1/4 of the offspring will be white and cross-
eyed
oIn humans, male pattern baldness is controlled by an autosomal
gene that occurs in 2 allelic forms. Allele Hn determines non-
baldness and allele Hb determines pattern baldness. In males, because
of the testosterone, allele Hb is domiant over Hn. If a man and
woman both with genotype HnHb have a son what is the chance that
he will eventually be bald?
Hb=bald, dominant
D Hn Hb
Hn HnHn HnHb
Hb HnHb HbHb
3/4 of a chance (or 75%)
oIn a dihybrid cross between green and inflated pea pod (GGII)
and yellow and constricted (ggii), the F1 generation yeilds GgIi
G=green, g=yellow, I=inflated, i=constricted
GGIIxggii=GgIi
oWhat is the probability of obtaining a pea pod, green and
inflatted, homogenous for both traits?
D GI gI Gi gi
GI GGII GgII GGIi GgIi
gI GgII ggII GgIi ggIi
Gi GGIi GgIi GGii Ggii
gi GgIi ggIi Ggii ggii
In a population of 200 cats, 72 are homozygous recessive for the
character of eye color (cc). One hundred indivisuals from this
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