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Mendelian
Inheritance I
21 October, 2002
Text Chapter 14
Gregor Mendel’s 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
Mendel followed heritable characters for three generations.
Mendel’s 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).
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.
Mendel’s 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 3:1 ratio of offspring.
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 organism’s appearance.
•genotype: an organism’s genetic makeup, its collection of alleles.
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 1:1 ratio of offspring
(dominant to recessive phenotype).
Independent Assortment
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
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?
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
(1/2)(1/2) = (1/4) or 0.25
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
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
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
2. What is the
probability of each way?
Ppyyrr
PPyyrr
ppYyrr
ppyyRr
ppyyrr
3. Add the probabilities
of the ways.
2/16 + 1/16 + 1/16 + 1/16 + 1/16 = 6/16
(1/2)(1/2)(1/2) = 1/8 = 2/16
(1/4)(1/2)(1/2) = 1/16
(1/4)(1/2)(1/2) = 1/16
(1/4)(1/2)(1/2) = 1/16
(1/4)(1/2)(1/2) = 1/16
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
Problem 4
You have been trying to train Great Northwestern rabbits to do a polevaulting 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
(i) vaulter x vaulter
(ii) vaulter x non-vaulter
(iii) non-vaulter x non-vaulter
F1 progeny
12 vaulters, 0 non-vaulters
8 vaulters, 7 non-vaulters
4 vaulters, 13 non-vaulters
Which phenotype is dominant ? Explain your logic briefly (only one or
two sentences needed!).
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 deformedleg, 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) white, normal.
CcBb (1/8) brown, deformed.
Ccbb (1/8) white, deformed.
CcBb (1/8) brown, deformed.
Ccbb (1/8) white, deformed.
ccBb (1/8) lethal.
ccbb (1/8) lethal.
4. 1/3 white, deformed. 1/3 brown, deformed. 1/6 white, normal.
1/6 brown, normal.