General Genetics
Ayesha M. Khan
Spring 2013
Test Cross
Suppose you were given a tall pea plant with no information about
its parents. Because tallness is a dominant trait in peas, your plant
could be either homozygous (TT) or heterozygous (Tt), but you
would not know which. You could determine its genotype by
performing a testcross.
One individual of unknown genotype is crossed with
another individual with a homozygous recessive genotype
for the trait in question.
If the plant were homozygous (TT), a testcross would produce all
tall progeny (TTxtt : all Tt); if the plant were heterozygous (Tt), the
testcross would produce half tall progeny and half short progeny
(Ttxtt :Tt and tt).
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Incomplete Dominance
When the heterozygote has a
phenotype intermediate between
the phenotypes of the two
homozygotes, the trait is said to
display incomplete dominance.
When a trait displays incomplete
dominance, the genotypic ratios
and phenotypic ratios of the
offspring are the same, because
each genotype has its own
phenotype.
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Example
of
incomplete
dominance
examples:
1. Feather color in chickens. A cross
between a homozygous black chicken and a
homozygous white chicken produces F1
chickens that are gray. If these gray F1 are
intercrossed, they produce F2 birds in a
ratio of 1 black: 2 gray: 1 white.
2. Leopard white spotting in horses is
incompletely dominant over unspotted
horses: LL horses are white with numerous
dark spots, heterozygous Ll horses have
fewer spots, and ll horses have no spots
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Ratios in Simple Crosses
Four phenotypic ratios:
1.The 3:1 ratio: Both of the parents are heterozygous for a dominant trait (AaxAa).
2. The 1:2:1 ratio: Progeny of crosses between two parents heterozygous for a character that exhibits
incomplete dominance (AaxAa).
3. The 1:1 ratio: Homozygous parent and a heterozygous parent. If the character exhibits dominance, the
homozygous parent in this cross must carry two recessive alleles (Aaxaa) to obtain a 1:1 ratio, because a
cross between a homozygous dominant parent and a heterozygous parent (AAxAa) produces only
offspring displaying the dominant trait. For a character with incomplete dominance, a 1:1 ratio results
from a cross between the heterozygote and either homozygote (Aaxaa or AaxAA).
4. The fourth phenotypic ratio is not really a ratio- all the offspring have the same phenotype. Several
combinations of parents can produce this outcome. A cross between any two homozygous parentseither between two of the same homozygotes (AAxAA and aaxaa) or between two different
homozygotes (AAxaa)- produces progeny all having the same phenotype. Progeny of a single phenotype
can also result from a cross between a homozygous dominant parent and a heterozygote (AAxAa).
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Multiple-Loci Crosses
Dihybrid Crosses
In addition to his work on monohybrid crosses, Mendel also crossed
varieties of peas that differed in two characteristics (dihybrid crosses).
For example, he had one homozygous variety of pea that produced round seeds
and yellow endosperm; another homozygous variety produced wrinkled seeds
and green endosperm.
When he crossed the two, all the F1 progeny had round seeds and yellow
endosperm. He then self-fertilized the F1 and obtained the following progeny in
the F2: 315 round, yellow seeds; 101 wrinkled, yellow seeds; 108 round, green
seeds; and 32 wrinkled, green seeds.
Mendel recognized that these traits appeared approximately in a 9:3:3:1 ratio;
that is, of the progeny were round and yellow, were wrinkled and yellow, were
round and green, and were wrinkled and green.
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Dihybrid Cross
9:3:3:1 ratio
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Another example
Dihybrid cross: flower color and stem length
TT PP  tt pp
(tall, purple)
Possible Gametes for parents
TP
and t p
F1 Generation: All tall, purple flowers
(Tt Pp)
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(short, white)
Dihybrid cross: flower color and stem length
(shortcut)
TT PP  tt pp
(tall, purple)
(short, white)
Possible Gametes for parents
T P
TP
tp
t p
F1 Generation: All tall, purple flowers (Tt Pp)
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Tt Pp
Dihybrid cross F2
If F1 generation is allowed to self pollinate, Mendel observed 4
phenotypes:
Tt Pp  Tt Pp
(tall, purple)
Possible gametes:
TP Tp tP tp
TP
TP
Tp
tP
tp
Four phenotypes observed
Tall, purple (9); Tall, white (3);
Short, purple (3); Short white (1)
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(tall, purple)
Tp
tP
tp
TTPP TTPp TtPP
TTPp TTpp TtPp
TtPp
Ttpp
TtPP
TtPp
ttPP
ttPp
TtPp
Ttpp
ttPp
ttpp
Dihybrid cross
9 Tall
purple
TP
TP
3 Tall
white
Tp
tP
tp
3 Short
1 Short
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purple
white
Tp
tP
tp
TTPP TTPp TtPP
TTPp TTpp TtPp
TtPp
Ttpp
TtPP
TtPp
ttPP
ttPp
TtPp
Ttpp
ttPp
ttpp
Phenotype Ratio = 9:3:3:1
Dihybrid cross: 9 genotypes
Genotype ratios (9):
1
TTPP
2
TTPp
2
TtPP
4
TtPp
1
TTpp
2
Ttpp
1
ttPP
2
ttPp
1
ttpp
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Four Phenotypes:
Tall, purple (9)
Tall, white (3)
Short, purple (3)
Short, white (1)
Relation of gene segregation to meiosis

There’s a correlation between the movement of
chromosomes in meiosis and the segregation of alleles
that occurs in meiosis
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The Principle of Independent Assortment
The principle of independent assortment (Mendel’s
second law):
This principle states that alleles at different loci
separate independently of one another.
The principle of segregation states that the two alleles of a
locus separate when gametes are formed.
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Branch diagram
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Dihybrid Test Cross
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Trihybrid crosses
Crosses including three characters:
 In one trihybrid cross, Mendel crossed a pure-breeding
variety that possessed round seeds, yellow endosperm,
and gray seed coats with another pure-breeding variety
that possessed wrinkled seeds, green endosperm, and
white seed coats.
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Trihybrid cross:
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Observed and Expected ratios


When two individuals of known genotype are crossed, we
expect certain ratios of genotypes and phenotypes in the
progeny; these expected ratios are based on the
Mendelian principles of segregation, independent
assortment, and dominance.
The ratios of genotypes and phenotypes actually observed
among the progeny, however, may deviate from these
expectations.
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The Goodness-of-Fit Chi-Square Test



Differences between observed and expected ratios can arise by
chance. The goodness-of-fit chi-square test can be used to evaluate
whether deviations between observed and expected numbers are
likely to be due to chance or to some other significant factor.
The chi-square test cannot tell us whether a genetic cross has been
correctly carried out, whether the results are correct, or whether
we have chosen the correct genetic explanation for the results.
What it does indicate is the probability that the difference between
the observed and the expected values is due to chance.
At the end of the chi square, we will retain or reject the null
hypothesis. If we retain the null hypothesis, we can say these results
could have been caused by chance (but most scientists prefer the
phrase “inconclusive result”). If we reject the null hypothesis, we can
say that it is unlikely that chance alone could not have resulted in
these numbers.
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Degrees of freedom (df) = n-1 where n is
the number of classes
Cutoff value= .05 probability level.
If the probability of chance being responsible
for the deviation is greater than or equal
to .05, it is accepted that chance may be
responsible for the deviation between the
observed and the expected values.
When the probability is less than .05, it is
assumed that chance is not responsible and a
significant difference exists.
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By statistical convention, we use the 0.05 probability level as our critical value. If the
calculated chi-square value is less than the 0 .05 value, we accept the hypothesis. If the
value is greater than the value, we reject the hypothesis.
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Penetrance and Expressivity
Penetrance is defined as the percentage of individuals
having a particular genotype that express the expected
phenotype.
For example, if we examined 42 people having an allele for
polydactyly and found that only 38 of them were
polydactylous, the penetrance would be 38/42 0.90 (90%).
Expressivity is the degree to which a character is
expressed.
Some polydactylous persons possess extra fingers and toes
that are fully functional, whereas others possess only a small
tag of extra skin.
Read the following article:
http://www.nature.com/scitable/topicpage/phenotype-variability-penetrance-andexpressivity-573
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