Mendelian
Genetics
General Biology
SUNY Orange at S. S. Seward
Institute
Puzzler
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HOW??
Puzzler
 Why
are so many famous
painters Dutch?
Gregor Mendel
Painting of Mendel
A genetic cross
Mendel tracked heritable characters for three generations
Sweet pea flowers
Alleles, alternative versions of a gene
The Results of Mendel’s F1 Crosses for Seven Characters in Pea Plants
Round and wrinkled peas
Mendel’s law of segregation
Mendel’s law of segregation
Genotype versus phenotype
A testcross
Segregation of alleles and fertilization as chance events
Testing two hypotheses for segregation in a dihybrid cross
Some Rules
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Multiplication rule – multiply the
probability of one event by the
probability of the second event
to get the probability of both
happening
1/4 of getting a heart
 1/13 of getting a 3
 1/4 * 1/13 = 1/52 of getting a 3 of
hearts
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Some Rules
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Addition rule – probability that
any one of 2 or more mutually
exclusive events will occur is
calculated by adding together
their individual probabilities
1/52 chance of jack of clubs
1/52 chance of jack of spades
= 2/52 = 1/26 chance of a black
jack
Example
An organism with the genotype
BbDD is mated to one with the
genotype BBDd. Assuming
independent assortment of these
two genes, write the genotypes
of all possible offspring from this
cross and calculate the chance of
each genotype occurring using
the rules of probability.
BbDD X BBDd
What is the probability that an offspring will
exhibit either of the two recessive traits?
Explain.
The genotype of F1 individuals in a
tetrahybrid cross is AaBbCcDd. Assuming
independent assortment, what are the
probabilities that F2 will have:
aabbccdd
AaBbCcDd
AABBCCDD
AaBBccDd
AabbccDd
What is the probability that each of the
following pairs will produce the indicated
offspring?
AABBCC X aabbcc  AaBbCc
AABbCc X AaBbCc  AAbbCC
AaBbCc X AaBbCc  AaBbCc
aaBbCC X AABbcc  AaBbCc
Phenylketonuria (PKU) is an inherited disease
caused by a recessive allele. If a woman and
her husband, who are both carriers, have
three children, what is the probability of each
of the following?
a) All three children are of normal phenotype
b) One or more of the three children have the
disease
c) All three children have the disease.
d) At least on child is phenotypically normal.
Incomplete dominance in carnations
Incomplete dominance in snapdragon color
Incomplete dominance in carnations
Multiple alleles for the ABO blood groups
ABO blood types
Mrs. Doe and Mrs. Smith had babies at
the same hospital at the same time.
Mrs. Doe brought home a baby girl and
named her Nancy. Mrs. Smith received
a baby boy and named him Richard.
However, she was sure she had had a
girl and brought legal action against
the hospital. Blood tests showed that
Mr. Doe was type O, Mrs. Doe was type
AB and Mr. and Mrs. Smith were both
type B. Nancy was type A and Richard
type O. Had an exchange occurred?
Suppose you learned that “shmoos”
may have long, oval or round bodies
and that matings of shmoos resulted in
the following:
a) Long x oval gave 52 long: 48 oval
b) Long x round gave 99 oval
c) Oval x oval gave 24 long: 53 oval :
27 round
What hypothesis about the inheritance
of shmoo shape would be consistent
with these results? Assume that
shmoos are diploid.
A simplified model for polygenic inheritance of skin color
A mixed-race British mom gave birth to twins recently —
one of each. No, not a boy and a girl. Two girls — one
black, the other white. The odds of such a birth are
about a million to one, experts said.
"It was a shock when I realized that my twins were two
different colors," Kylie Hodgson, 19, told London's Daily
Mail. "But it doesn't matter to us — they are just our two
gorgeous little girls."
Hodgson and her partner, Remi Horder, 17, were both
born to mixed-race parents. Little Kian and Remee share
a love of apples and the Teletubbies, their proud mom
says. Fertility experts speculate that a sperm containing
all-white genes fused with an egg with all-white genes,
and a sperm with all-black genes fused with an all-black
gene egg to produce the fraternal twins.
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Epistasis – a type of gene
interaction in which one gene
alters the phenotypic effects of
another gene that is
independently inherited.
An example of epistasis
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In the summer squash
(Cucurbita pepo) spherical fruit
is recessive to disk, Spherical
fruit from different geographic
regions were crossed. The F1's
were disk, and the F2's
segregated 35 disk, 25 spherical
and 4 elongate. Explain these
results.
Start with determining the phenotypic ratios of
the individual phenotypes to the total
individuals and the relative frequency of the
individual phenotypes to each other. There are
a total of 35 + 25 + 4 = 64 individuals in this
data set. The smallest phenotypic frequency
(the elongate fruit) make up 4/46 = 0.0625 =
1/16 of the individuals. This suggests a classic
dihybrid cross which should result in a 9:3:3:1
ratio; however, there are only three phenotypes
which occur in a ratio of (35/4 = )8.75: (25/4 =
) 6.25 : 4/4 = 1 (suspiciously close to an
epistatic 9:6:1 ratio).
Pedigree analysis
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a) How many
children do the
parents A and B
have?
b) Indicate the
genotypes of the
parents.
c) Give the genotypes
of M and N.
Large families provide excellent case studies of human genetics
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http://gean.wwco.com/grandpa/
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On your first day interning in the office of
a human geneticist, a man with purple
ears walks in. You questioned the man
and wrote down the following family
history.
The man's mother and one of his sisters
also had purple ears, but his father, his
brother, and two other sisters had normal
ears. The man and his normal-eared wife
had seven children, including four boys
and three girls. Two girls and two boys
had purple ears.
Draw the family pedigree and indicate
what form of inheritance that the purpleear trait most likely follows.
Sex Linked Traits
Usually associated with a mutation
on the X chromosome
- as a heterozygote, the
phenotype is normal but woman
is a carrier
- as a single chromosome ( as in
a male )
phenotype is abnormal
Examples:
hemophilia
red/green color blindness
1.
2.
3.
Hemophilia in humans is due to an Xchromosome mutation. What will be the
results of mating between a normal (noncarrier) female and a hemophilac male?
In a cross between a white-eyed female fruit
fly and red-eyed male, what percent of the
female offspring will have white eyes? (White
eyes are X-linked, recessive)
In a cross between a pure bred, red-eyed
female fruit fly and a white-eyed male, what
percent of the male offspring will have white
eyes? (white eyes are X-linked, recessive)
Clouded leopards are a medium
sized, endangered species of cat, living in the
very wet cloud forests of Central America.
Assume that the normal spots (Xⁿ) are a
dominant, sex-linked trait and that dark spots
are the recessive counterpart. Suppose you
are involved in a clouded leopard breeding
program. One year you cross a male with dark
spots and a female with normal spots. She
has four cubs, two are male and two female.
One each of the male and female cubs have
normal spots and one each have dark spots.
What is the genotype of the mother?
Pleiotropic effects of the sickle-cell allele in a homozygote
Testing a fetus for genetic disorders
The effect of environment of phenotype
Epigenetics
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is the study of heritable changes in
gene expression or cellular
phenotype caused by mechanisms
other than changes in the underlying
DNA sequence
refers to functionally relevant
modifications to the genome that do
not involve a change in the
nucleotide sequence
PBS video