Genetics
Mendelian Genetics
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Genetics – The branch of Biology that
studies heredity.
Heredity – The passing on of characteristics
from parent to offspring.
Since early days of farming humans have
been involved with genetics and heredity.
The first to actively study inheritance was
Gregor Mendel
Mendel
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Mendel was an Austrian monk who studied pea
plants.
Why would you choose to study peas rather
than an elephant?
Traits Mendel studied:
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Seed shape
Seed color
Flower position
Pod color
Pod shape
Plant height
Pure breeding and inbreeding
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Pure breeding – Organisms that always
produce the same type of offspring.
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Tall plants always produce tall plants
Red flowers always produced red flowers
Mendel did all of his genetic work before the
discovery of DNA, chromosomes, genes,
evolution and natural selection.
Mendel used mathematics to analyze his data.
Monohybrid crosses
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Monohybrid cross – Is a cross between two parent
organisms that differ in only one trait.
Mendel first crossed a pure breeding tall plant to a
pure breeding short plant.
Parental (P1)
tall x short
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First generation (F1)
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Second generation (F2)
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tall
(crossed with another F1)
3 Tall : 1 short
Mendel concluded
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There are two “factors” for each trait
These factors or genes have different alleles
Allele – A gene form for each variation of an
organism.
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So a plant could have 2 alleles for tall or 2 alleles
for short, or one from each.
Mendel further concluded that “factors”
were either dominant or recessive.
Dominant and recessive
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Dominant alleles – Are traits that are visible.
Observable traits of organisms that masks a
recessive form (T)
Recessive alleles – Hidden trait of an organism
that is masked by a dominant trait. (t)
Possible combinations of traits in Mendelian
Genetics:
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TT, tt, or Tt
The dominant letter is always written first
Once you select a letter to represent a trait you must
not change it.
Mendel’s Law of Segregation
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A parent passes on at random only one
allele for each trait to each offspring
TT or
Tt or tt
T
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T
T
t
t
t
Alleles give an organism its individual
appearance.
Phenotype and Genotype
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Phenotype – the way an organism looks and
feels. Its PHYSICAL appearance.
Genotype – The allele (gene) combination an
organism contains.
Phenotype  tall, short, round
Genotype  TT, tt, Rr
Homozygous – Organism has two alleles for the
trait that are the same (TT or tt)
Heterozygous – Two alleles for a trait are
different (Tt)
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Always show the dominant trait
Mendelian genetics and Punnet
squares
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P = TT x tt  Tt
F1 cross = Tt x Tt
F2 generation = 3 tall:1 short
T
T
t
TT
tall
t
Tt
tall
Tt
tt
tall
short
Genotypic ratio  1TT:2Tt:1tt
Phenotypic ratio 3 tall:1 short
Practice problem
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Cross a homozygous blue flower with a heterozygous purple
flower.
What color is dominant?
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What color is recessive?
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Pp
What is the genotype of the blue flower?
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The heterozygote always shows the dominant trait
What is the genotype of the Purple flower?
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Blue
How do you know?
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Purple
pp
What are the genotypic and phenotypic ratios of the cross?
Seat work
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Cross the following: TT x Tt, tt x tt, Tt X tt
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T = tall, t = short, what are the genotypic and
phenotypic ratios?
Cross a homozygous red flower with a
heterozygous blue flower
Cross a homozygous recessive to a
homozygous dominant pink flower
Cross a heterozygous white flower to
another heterozygous white flower
Dihybrid Cross
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A dihybrid cross involves two different traits.
Mendel crossed true breeding round and
yellow seeds with green and wrinkled seeds.
The Genotypes are:
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RR = round
rr = wrinkled
YY = yellow
yy = green
The genotypes for this dihybrid cross
are: RRYY x rryy
When writing dihybrid crosses, we
always keep the alleles together RrYy,
rrYY, Rryy
Dihybrid cross
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True breeding P1 = RRYY x rryy
Resulting F1 = RrYy All are round and yellow
Mendel then crossed the F1 together and observed the
following:
9 round and yellow:3 round and green:3 wrinkled and
yellow: 1 wrinkled and green.
Law of independent assortment – Genes for different
traits are inherited independently of each other.
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That means in the above example, the seed color does not
affect the seed shape, and the seed shape does not affect
the seed color.
Dihybrid cross
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See side board for how to use a punnet
square for a dihybrid cross.
RRYY x rryy, RrYy x RrYy, Rryy x RrYy
R=round, r=wrinkled
Y=yellow, y=green
Non-Mendelian Genetics
The exceptions to the rule
Non-Mendelian Genetics
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Even though a trait is not simply dominant or
recessive, geneticists can still predict and
explain their appearance.
We will discuss 3 examples of non-mendelian
genetics:
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Incomplete dominance
Co-dominance
Sex-linked traits
Incomplete Dominance
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When the phenotype of the heterozygote is
an intermediate of the two homozygotes.
Sometimes called blending
Red flower x white flower  pink flower
RR – red
x
WW – white
W
W
R
R
RW
pink
RW
pink
RW
pink
RW
pink
Notice that I used a capital letter
for both and distinguished them by
using different letters
The only difference is in how we
express the genotype and the
appearance of the organism
Incomplete dominance
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Cross two pink flowers, what do you think
the phenotypes will be?
RW x RW
R
W
R
RR
red
W
RW
pink
RW
pink
WW
white
Co-dominance
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Both alleles are expressed equally.
black rabbit x white rabbit  spotted rabbit
With co-dominance since both alleles are
seen we use capital letters and the prime
symbol (‘)
Co-dominance
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BB (black) x B’B’
BB’ x BB’  B
B
(white)
BB
Black
B’
 BB’
B’
BB’
Black &
white spots
BB’
B’B’
Black &
white spots
White
(black and white)
Sex linked traits
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Also called x linked.
Sex linked traits are controlled by genes located on
sex chromosomes.
Sex chromosomes determine the gender of an
individual. In humans, the sex chromosomes are the
23rd pair.
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These are the only chromosomes that are not homologous!
Autosomes – pairs of matching homologous
chromosomes. (all the chromosomes except the sex
chromosomes
Sex linked traits
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In humans as well as fruit flies, males are
XY, females are XX.
Males produce gametes that will have
either the X or Y sex chromosome.
Females produce gametes that will just
carry the X sex chromosome.
This means that the gender of a child is
determined by the father.
Sex linked traits
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Color blindness in humans and eye color
in fruit flies are two examples of sex
linked traits.
See the side board for how to work on a
sex linked trait
Karyotypes
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A karyotype is a picture of an individual’s
chromosomes.
Karyotypes can be used to determine if there
have been mutations to a chromosome.
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Mutations like duplication or deletions
Human males and females have different
Karyotypes because of differences in the sex
chromosomes (X and Y chromosomes).
Pedigree
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Pedigrees are graphical representations used to
predict the likelihood of the appearance of a
trait.
Symbols of a pedigree:
Affected male
Unaffected male
Affected female
Unaffected female
Known heterozygote,
male
Known heterozygote,
female
I
A mating
offspring
II
1
2
3
4
Pedigrees
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Used to determine the possibility of a
genetic defect or probability of a specific
trait.
See side board for a pedigree
Odds and Ends
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Test cross is cross of an individual of
unkown genotype with an individual of
known genotype in order to determine the
unknown genotype
A carrier is another term for a heterozygous
individual.
A hybrid is an offspring produced when two
varieties of plants or animals are mated.
Hybrids often exhibit greater vigor and size
than their parents. A phenomena termed
hybrid vigor.
Chromosome structure and
allele locus
Allele for purple flowers
Locus for flower-color gene
Allele for white flowers
Homologous
pair of
chromosomes
Linked genes
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Linked genes are genes for two different
traits that are located close together on
the same chromosome.
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Linked genes tend to travel together during
crossing over
Closely linked
Not
Linked
Pleiotropy
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Most genes have multiple phenotypic
effects, a property called pleiotropy
For example, pleiotropic alleles are
responsible for the multiple symptoms
of certain hereditary diseases, such as
cystic fibrosis and sickle-cell disease
Epistasis
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In epistasis, a gene at one locus alters the
phenotypic expression of a gene at a
second locus
For example, in mice and many other
mammals, coat color depends on two
genes
One gene determines the pigment color
(with alleles B for black and b for brown)
The other gene (with alleles C for color and
c for no color) determines whether the
pigment will be deposited in the hair
Polygenic Inheritance
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Quantitative characters are those
that vary in the population along a
continuum
Quantitative variation usually indicates
polygenic inheritance, an additive
effect of two or more genes on a single
phenotype
Skin color in humans is an example of
polygenic inheritance
Multiple Allele Traits
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Most genes exist in populations in more than two
allelic forms
For example, the four phenotypes of the ABO
blood group in humans are determined by three
alleles for the enzyme (I) that attaches A or B
carbohydrates to red blood cells: IA, IB, and i.
The enzyme encoded by the IA allele adds the A
carbohydrate, whereas the enzyme encoded by
the IB allele adds the B carbohydrate; the enzyme
encoded by the i allele adds neither
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Any of a set of three or more alleles, or
alternative states of a gene, only two of
which can be present at any given time in a
diploid organism.
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Example: Physical characteristics such as eye color,
skin color, height and coat color in animals …these are
also POLYGENIC traits controlled by many genes
interacting together!
copyright cmassengale
35
Multiple Alleles and Codiminance BLOOD TYPING
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Blood is a complex, living tissue that contains many cell
types and proteins. Distinct molecules called
agglutinogens (a type of antigen) are attached to the
surface of red blood cells.
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There are two different types of antigen markers, type “A” and
type “B”. Each type has different properties.
The ABO blood type classification system uses the presence or
absence of these molecules to categorize blood into four types.
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type A
type B
type AB
type O
=
=
=
=
AA or AO
BB or BO
AB
OO (sometimes ii)
•A, B, AB, and O are the POSSIBLE
PHENOTYPES
•A and B are DOMINANT alleles
•i or type O is the RECESSIVE allele
Chart of Blood Types
WHEN BLOOD TYPES MIX
Blood plasma is packed with proteins called antibodies. The
body produces a wide variety of antibodies that will recognize
and attack foreign molecules that may enter from the outside
world. A person’s plasma does not contain any antibodies
that will bind to molecules that are part of his or her own body.
When conducting a blood transfusion, it is important to
carefully match the donor and recipient blood types. If the
donor blood cells have surface molecules that are different
from those of the recipient, antibodies in the recipient’s blood
recognize the donor blood as foreign. This triggers an immune
response resulting in blood clotting. If the donor blood cells
have surface molecules that are the same as those of the
recipient, the recipient’s body will not see them as foreign and
will not mount an immune response.
There are two special blood types when it comes to blood
transfusions. People with type O blood are universal donors
because there are no molecules on the surface of the red
blood cells that can trigger an immune response. People with
type AB blood are universal recipients because they do not
have any antibodies that will recognize type A or B surface
molecules.
Note: Blood cells are covered with a variety of surface
molecules. For simplicity, only type A and B surface
molecules are shown here.
Nature and Nurture: The
Environmental Impact on Phenotype
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Another departure from Mendelian genetics
arises when the phenotype for a character
depends on environment as well as genotype
The norm of reaction is the phenotypic
range of a genotype influenced by the
environment
For example, hydrangea flowers of the same
genotype range from blue-violet to pink,
depending on soil acidity
Fig. 14-14