Download Introduction to Genetics

Introduction to Genetics
Honors Biology
Chapter 9
Short History of Genetics
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1.
1800’s: Blending……wrong!
Modern Genetics: Particulate
Monogenic Inheritance
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2.
Cytogenetics
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3.
Dominant, recessive, x-linked
Easy to understand and solved with a Punnett Square, but very
rare
Karyotype used to visualize, easy to understand, but rare
Multifactorial
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Common conditions but complex genetic traits
Must understand entire genome and look for variants
People are 99.6% the same
Common variants = common diseases
Quantatative Traits
Who is Gregor Mendel
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His experiments
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7 traits in pea plants
Why pea plants?
Dominant
Expression:
Trait:
Recessive
Expression:
1.
Form of ripe seed
Smooth
Wrinkled
2.
Color of seed
albumen
Yellow
Green
3.
Color of seed coat
Grey
White
4.
Form of ripe pods
Inflated
Constricted
5.
Color of unripe
pods
Green
Yellow
6.
Position of flowers
Axial
Terminal
7.
Length of stem
Tall
Dwarf
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Genetics Vocab
Genetics
F1 and F2 Generations
Reginald Punnett (Mendel  Statistics)
Chromosome vs. Gene
More Genetics Vocab
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Alleles
Dominant vs. Recessive
Homozygous vs. Heterozygous
Genotype vs. Phenotype
From gene to trait
Karyotypes
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What can this tell us? What can it
not tell us?
Genetics Problem Rules
1.
Always “assign” letters to represent the alleles. For
“complete” dominance problems, use the same
letter:
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2.
3.
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Capital Letter = Dominant Allele
Lower Case Letter = Recessive Allele
Read the problem thoroughly to determine the
parent’s genotypes. If heterozygous, the capital
letter is always first!
Solve the problem (predict the genotypes of the
offspring) using a Punnett Square.
You must always:
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Have a key for the letters
Show the genotypes of the parents
Show your work in a Punnett Square
Answer what the question is asking
Monohybrid Crosses
Involves a single gene where one allele is
dominant over another
 Use an upper and lower case letter
T=Tall
t=short
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tt
TT or Tt
The Monohybrid Cross
In dogs, wire hair (A) is dominant to smooth (a). In a cross of a
homozygous wire-haired dog with a smooth-haired dog, what will
be the genotype and phenotype of the F1 generation?
The Monohybrid Cross
Wood rats are medium sized rodents with lots of interesting behaviors.
You may know of them as packrats. Let’s assume that the trait of
bringing home shiny objects (B) is dominant to the trait of carrying
home only dull objects (b). Suppose two heterozygous individuals
are crossed. How many of each genotype would be expected if only
4 offspring were produced?
The Monohybrid Cross
In mice, black fur is dominant over white fur. If two mice are crossed
and produce all black offspring, list the possible genotypes and
phenotypes the parents could have.
The Monohybrid Cross
In humans, earlobes are either free or attached. The free earlobe is
completely dominant over attached earlobes. A man with free
earlobes marries a woman with attached earlobes. Of their four
children, one has free earlobes and three have attached earlobes.
Give the genotype for the man, his wife and their children.
The Monohybrid Cross
In humans, having a widow’s peak is recessive to a normal hair line.
Cross a woman who has a straight hair line (whose mother had a
widow’s peak) with a man who is homozygous recessive for the
trait. What are the genotypes and phenotypes of their offspring?
Incomplete Dominance
Involves a single gene where neither trait is dominant over
another. Results in an intermediate expression of the trait
 Use two upper case letters
R=Red
W=White
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RR
WW
RW
The Incomplete Dominance Cross
In the four o’clock flower like a petunia, the allele for red flowers is
incompletely dominant over the allele for white flower color. When a
red flower is crossed with a white one, an intermediate pink flower
results. Show the genotypes and phenotypes of the parents and the
F1 generation of a cross between a red and a white four o’clock
flower.
The Incomplete Dominance Cross
In guinea pigs, color of coat is determined by at least two alleles. Yellow coat
is determined by the homozygous genotype YY, white by the homozygous
genotype WW, and the cream by the heterozygous genotype YW.
Determine the expected genotype and phenotype ratio of the F1 generation
which would result from a cross between.
a. two cream colored guinea pigs
b. a yellow coated and a cream coated animal
Dihybrid Crosses
Involves two genes rather than one
 Follow the rules given in the problem, may be complete or incomplete
T=pointed wings
P=pink wings
t=rounded wings
p=yellow wings
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TT or Tt
PP or Pp
TtPp
ttpp
The Dihybrid Cross
Dark hair is dominant over blonde hair, and a widow’s peak hairline is
dominant over a straight hairline. If two people who were both
heterozygous for both traits had children, what is the genotype and
phenotype ratio for the possible offspring.
The Dihybrid Cross
In rabbits, the allele for short hair is dominant over long hair and black
fur is dominant over brown. Cross a heterozygous short hair and
homozygous dominant black fur rabbit with a female rabbit that is
homozygous recessive for both traits. What are the phenotypes and
genotypes of their offspring?
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Sex-linked
Traits
Involves genes on the X sex chromosome
First discovered in the early 1900’s by Thomas Hunt Morgan who was
studying fruit flies.
Most sex-linked genes are recessive which means it only takes one gene from
the mother to give the male offspring (XY) the mutant trait. Females (XX)
need two genes, one from each parent to have the mutant trait.
Carrier Females: females who carry a recessive allele but do not have the
recessive phenotype
R=red
r=white
Females: XRXR or XRXr
Females: XrXr
Males: XRY
Males: XrY
Sex-linked Problems
A woman heterozygous for hemophilia (a sex-linked trait) marries a
normal male. What are these person’s genotypes? Can they have
male children with hemophila? How about female children with the
disease?
Sex-linked Problems
Baldness is a recessive sex-linked trait. Cross a carrier female with a
bald male.
a.
What percentage of their daughters will be bald?
b.
What percentage of children will be bald?
Multiple Alleles
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Involves a single trait that is controlled by 3 alleles (rather
than 2 which is what we are used to)
Blood types are the most common example, O is
recessive, but A and B are both dominant
Alleles: A, B, O
AA or AO
BB or BO
AB
OO
Multiple Allele Problems
What are the possible blood types in the children with parents having
type A and type B blood?
Multiple Allele Problems
Show how a father with blood type AB could have one daughter with
blood type A and another with blood type B.
a.
What must be the genotype of the mother?
b.
What must be the genotype of the two daughters?
Epistasis
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Involves a single trait, but two genes at different loci on a single
chromosome
Loci is the location of the gene on a chromosome
One gene controls the trait while the other turns the first gene on or off
depending on the genotype.
Gene 1: gene for the trait
R=purple
r=white
Gene 2: controlling gene
N= gene on
n=gene off
Purple Trait: RR
or Rr (Must have
NN or Nn)
White Trait: rr (Must
have NN or Nn
OR
RR/Rr with nn
Epistasis Problems
Two unlinked loci effect mouse hair color. AA or Aa mice are agouti
(brown). Mice with genotype aa are albino because all pigment
production is blocked, regardless of the phenotype at the second
locus. At the second locus, the B allele (agouti coat) is dominant to
the b allele (black coat). What would be the result of a cross
between two agouti mice of the genotype AaBb?
Epistasis Problems
In wheat kernels, the R allele confers red color and the r allele brown
kernel color. The B gene controls expression of the R gene. Corn of
genotype bb will have white kernels. If a red kernel wheat plant
with the genotype RrBb is fertilized by a brown wheat plant rrBb,
what type of wheat plants are produced?
Pedigress
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A chart used to track traits through generations
Females are circles, Males are squares
Shaded individuals show a trait (usually recessive)
Generations are given roman numerals
Individuals in generations are given numbers
Pedigree Problems
N= Normal Vision
n= Night Blindness
1.Write the genotypes of all
individual below their symbol
in the pedigree above.
2.If individuals II-2 and II-8
were to marry and have
children, what are the
possible genotypes of any
children they might have?
Pedigree Problems
Could this trait be inherited as a simple autosomal recessive?
Could this trait be inherited as a simple autosomal dominant?
Could this trait be inherited as a simple X-linked recessive?
Mutations, Evolution, Natural
Selection
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Mutations are random
changes in a genome that
occur during cell division
and/or reproduction
Evolution: random
process controlled by
mutations
Natural Selection: nonrandom process
controlling populations
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Population Genetics
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Hardy-Weinburg
Gene Pool
Gene Frequency
The Case of the Blind Fish
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Does natural selection always choose
against the recessive trait?