Mendelian Inheritance I 21 October, 2002 Text Chapter 14

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Mendelian Inheritance I21 October, 2002Text
Chapter 14
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Mendels Experiments
Gregor Mendels experiments used pea plants as a
model system. He examined the inheritance of
characters like flower color and seed shape by
mating plants and observing the offspring.
character a heritable feature, like flower
color.
trait a variant of a character, like purple or
white flowers.
true-breeding plants that, when self pollinated,
produce offspring of the same variety
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Mendel followed heritable characters for three
generations.
Mendels results refuted the blending hypothesis.
He proposed a particulate theory of inheritance
where characters are determined by genes (recipes
for a character) that come in different versions
(alleles).
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Alleles are different versions of a gene.
Diploid organisms have two copies of each gene.
These copies can be the same or different. One
copy was inherited from each parent. If the two
alleles differ, then one, the dominant allele
determines the appearance of the
organism. During gamete formation, the two
alleles segregate into gametes.
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Mendels Rules of Inheritance

• Each parent has two alleles.

• Gametes contain only one allele.

• Offspring have two alleles - one allele from each
  parent.

• When both alleles are present, the dominant
  allele determines appearance.

• Gametes contain only one allele.

• Offspring have two alleles - one allele from each
  parent.

• When both alleles are present, the dominant
  allele determines appearance.

• This leads to a 31 ratio of offspring.

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Important terms

• homozygous a diploid organism that has two
  copies of the same allele for a given gene.

• heterozygous a diploid organism that has two
  different alleles for a given gene.

• phenotype an organisms appearance.

• genotype an organisms genetic makeup, its
  collection of alleles.

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Testcross
We cannot be sure of the genotype of an
individual with a dominant phenotype. That
individual could be homozygous or heterozygous.
A testcross can reveal the genotype of the
individual in question. A homozygous dominant
individual will produce all dominant phenotype
offspring in a testcross. A heterozygote will
produce a 11 ratio of offspring (dominant to
recessive phenotype).
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Independent Assortment
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Mendelian inheritance follows the rules of
probability (chance).

• Probability of an event is measured from 0
  (impossible) to 1 (certain).

What is the probability of flipping a coin and
getting tails?
1/2 or 0.5
What is the probability of rolling a 3 on a
six-sided die?
1/6 or 0.167
When gametes are produced by a heterozygote, what
is the probability of any given gamete having a
recessive allele?
1/2 or 0.5
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Mendelian inheritance follows the rules of
probability (chance).continued...

• Probability of an event that requires multiple
  steps is determined by multiplying the
  probabilities of the individual steps. This
  makes the probability get smaller.

What is the probability of flipping a coin twice,
and getting tails both times?
(1/2)(1/2) (1/4) or 0.25
When gametes are produced by an individual
heterozygous at two loci (genes), what is the
probability of any given gamete having recessive
allele for both loci?
(1/2)(1/2) (1/4) or 0.25
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Mendelian inheritance follows the rules of
probability (chance).continued...

• Probability of an event that has more than one
  way of occurring is determined by adding the
  probabilities of the individual ways. This makes
  the probability get larger.

What is the probability of flipping a coin twice,
and getting one head and one tail?
Two ways HT or TH, each (1/2)(1/2) (1/4) or
0.25 0.25 0.25 0.5 or 1/2
In the self-cross of a heterozygote (Aa), what is
the probability of any individual offspring
being heterozygous?
Two ways aA or Aa, each (1/2)(1/2) (1/4) or
0.25 0.25 0.25 0.5 or 1/2
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Problem 1
In the self-cross of a heterozygote (Pp), what is
the probability of any individual offspring
being heterozygous?
Two ways pP or Pp, each (1/2)(1/2) (1/4) or
0.25 0.25 0.25 0.5 or 1/2
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Problem 2
In the three-factor cross where all three genes
assort independently, PpYyRr x Ppyyrr, what is
the probability of an offspring showing at least
two recessive traits?
1. How many ways can this happen?
PPyyrr, Ppyyrr, ppYyrr, ppyyRr, ppyyrr
Ppyyrr (1/2)(1/2)(1/2) 1/8
2/16 PPyyrr (1/4)(1/2)(1/2) 1/16 ppYyrr (1/4)(
1/2)(1/2) 1/16 ppyyRr (1/4)(1/2)(1/2)
1/16 ppyyrr (1/4)(1/2)(1/2) 1/16
2. What is the probability of each way?
3. Add the probabilities of the ways.
2/16 1/16 1/16 1/16 1/16 6/16
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Problem 3
How many unique gametes could be produced through
independent assortment by an individual with the
genotype Aa Bb CC Dd EE?
(2)(2)(1)(2)(1) 8
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Problem 4
You have been trying to train Great Northwestern
rabbits to do a pole-vaulting routine whenever
they hear a dog whistle. After much
experimentation, you realize that the ability to
perform pole-vaulting routines is determined by
an autosomal gene in this species of rabbit.
When you blow the whistle, the rabbits (depending
on their genotype) either do the pole-vault or
just ignore you and continue to chew thoughtfully
on their carrots. The results of a number of
crosses with vaulters and non-vaulters are shown
Cross F1 progeny (i) vaulter x
vaulter 12 vaulters, 0 non-vaulters (ii)
vaulter x non-vaulter 8 vaulters, 7
non-vaulters (iii) non-vaulter x non-vaulter 4
vaulters, 13 non-vaulters Which phenotype is
dominant ? Explain your logic briefly (only one
or two sentences needed!).
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Problem 5
In rabbits, the homozygous CC is normal, Cc
results in rabbits with deformed legs, and cc is
lethal. For a gene for coat color, the genotype
BB produces black, Bb brown, and bb a white coat.
Give the phenotypic proportions of offspring
from a cross of a deformed-leg, brown rabbit with
a deformed-leg, white rabbit.
1. Write genotypes. CcBb x Ccbb
2. Determine gametes, probability of each. CB,
Cb, cB, cb x Cb, cb Probabilities are 1/4, 1/4,
1/4, 1/4 x 1/2, 1/2
3. Fertilization CCBb (1/8) brown, normal.
CcBb (1/8) brown, deformed. CCbb (1/8) white,
normal. Ccbb (1/8) white, deformed. CcBb (1/8)
brown, deformed. ccBb (1/8) lethal. Ccbb
(1/8) white, deformed. ccbb (1/8) lethal.
4. 1/3 white, deformed. 1/3 brown, deformed.
1/6 white, normal. 1/6 brown, normal.
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