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GENETIC CROSSES
REVIEWDo Now: Do the dihybrid cross on
your notes sheet.
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DIHYBRID CROSS: Phenotype
Ratio: 9:3:3:1
RR WW RR Ww Rr WW Rr Ww
RR Ww RR ww Rr Ww Rr ww
Rr WW Rr Ww rr WW rr Ww
Rr Ww Rr ww Rr Ww Rr ww
RW Rw rW rw
RW
Rw
rW
rw
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5 TYPES OF GENETIC CROSSES The dihybrid cross is one of the 5 types
of genetic crosses we have studied. In a dihybrid cross, the inheritance of 2
traits is predicted at the same timeSince each parent has 4 possible
combinations of alleles, dihybrid crosses are 4 x 4 squares.
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COMPLETE DOMINANCE Complete dominance is the first type we looked
at. One allele hides the effect of the other.
AA = dominant phenotype Aa = dominant phenotype aa = recessive phenotype
Possible crosses include: Heterozygous x heterozygous (Aa x Aa)
3:1 phenotype ratio, 1:2:1 genotype ratio Homozygous x heterozygous (aa x Aa and AA x aa)
1:1 phenotype and genotype ratios Homozygous x homozygous (AA x AA, aa x aa, and
AA x aa) 100% have the same genotype and phenotype
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COMPLETE DOMINANCE EXAMPLE In humans, albinism
(a) is recessive to normal pigmentation (A). If an albino man has a child with a woman who is heterozygous for the trait, what is the probability their child will be an albino?
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CODOMINANCE For codominant traits, BOTH alleles show up
in the phenotypeAA = dominant phenotypeAa = both phenotypes (i.e. red + white fur)aa = recessive phenotype
Possible crosses include:Heterozygous x heterozygous (Aa x Aa)
1:2:1 phenotype ratio, 1:2:1 genotype ratioHomozygous x heterozygous (aa x Aa and AA x
aa) 1:1 phenotype and genotype ratios
Homozygous x homozygous (AA x AA, aa x aa, and AA x aa) 100% have the same genotype and phenotype
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CODOMINANCE EXAMPLE In a species of
tropical frog, there are frogs that have blue skin, frogs that have black skin, and frogs that have both black and blue skin.
If two black and blue frogs mate, what will the phenotype ratio of their offspring be?
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INCOMPLETE DOMINANCE Traits with incomplete dominance produce a
heterozygous phenotype that is a blend of the two allele’s effect AA = dominant phenotype Aa = blended phenotype (pink flowers aa = recessive phenotype
Possible crosses include: Heterozygous x heterozygous (Aa x Aa)
1:2:1 phenotype ratio, 1:2:1 genotype ratio Homozygous x heterozygous (aa x Aa and AA x aa)
1:1 phenotype and genotype ratios Homozygous x homozygous (AA x AA, aa x aa, and
AA x aa) 100% have the same genotype and phenotype
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INCOMPLETE DOMINANCE EXAMPLE A type of pepper
comes in three colors, red, orange, and yellow. If a gardener crosses two orange pepper plants, what will the phenotype ratio of the offspring be?
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SEX-LINKED TRAITS The alleles for sex linked traits are
located on the sex chromosomes (X chromosome in humans)
Recessive sex-linked traits are much more common in males than females, since they have only 1 copy
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SEX-LINKED TRAITS
In a Punnett square for a sex-linked trait, the sex chromosomes are used instead of letters.
X+X+ normal f X+Xo carrier f XoXo color-blind f X+Y normal m XoY color-blind m
X+ Xo
X+ X+X+
X+Xo
Y X+Y XoY
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SEX-LINKED EXAMPLE Because she has a
brother with hemophilia, a biology student wanted to figure out the probability that she was a carrier for the X-linked disorder. Neither of her parents has hemophila. What is the probability she is a carrier of the trait?
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QUIZ MONDAY! There will be a quiz on genetic crosses
this Monday! Each of the 5 types will show up on it.
Do your homework for practice!