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Genetics
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Gregor Mendel
Genotypes: Homozygous and Heterozygous,
Dominant, Recessive
Phenotypes: Traits you see
Law of Heredity
- law of segregation
- law of independent assortment
Probability: punnett squares
Pedigree charts: genes are sex-linked or
autosomal
Polygenetic traits, incomplete dominance,
codominance, multiple alleles.
Gregor Mendel: Father of Genetics
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Found the following using Pea plant
- Inheritance: passing of traits
- Heredity: transmission of traits from parents to offspring
Used pea plants for his experiments. WHY??
- The flower is either purple or white, no intermediate
colors such as pink
- You can control pollination. (able to control the mating)
- small, easily grown, matures quickly, produces many offspring
Mendel’s Experiments
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He did the experiment by both self-pollinating (pollen is not
transferred to another plant, the plant uses its pollen) and crosspollinating (cross-fertilization: transferring pollen from one plant to
the other)
His experiment
1. He crossed a purple flower with a purple flower producing
plants with purple flowers and a white flower with a white flower
producing plants with only white flowers. He referred to this as
“True-breeding” (display 1 particular trait). These plants
served as his parental generation or “P” generation
EX: White X White = All White
Purple X Purple = All Purple
Mendel’s Experiment Con’t
2. He then crossed a white P generation flower with a purple
P generation flower. He called the offspring of the P generation
the F1 Generation. All offspring were purple
EX: White X Purple == All Purple
3. He then enabled the F1 generation to self-pollinate. This produced
the F2 generation. 705 were purple and 224 flowers were white .
A ratio of 3:1
EX: Purple X purple == 3 Purple, 1 White
See the following website:
http://www2.edc.org/weblabs/Mendel/mendel.html
Mendel’s Conclusions
1. He concluded that purple was the dominant color
2. He concluded that purple was masking the white color.
3. He concluded that white was recessive because it
returned in the F2 generation
Mendel’s Hypotheses
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1. For each inherited trait, an individual has 2 copies of
the gene- one from the father one from the mother
2. Alleles: different versions of genes. Ex: the gene in
flower color for pea plants can exist in purple or white.
The gene for hair color can exist in brown, black, or
blonde.
Through the process of meiosis, parents can only
contribute one allele for an inherited trait.
When two different alleles occur together, only 1 may be
completely expressed. The other may not have an
observable effect on the organism’s appearance.
Other Important Info.
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Dominant: The gene (trait) that is expressed in the physical
appearance. Represented by a capitol letter (T, S, H)
Recessive: the gene (trait) that is not expressed, but you still have
the gene for the trait. It is masked by the dominant trait.
Represented by a lower case letter. (t, s, h)
Genotype: set of alleles or genes that an individual has
Phenotype: The physical appearance of an individual. (determined
by your alleles)
Alleles can be homozygous or heterozygous
-- homozygous: alleles are the same for a particular trait
ex: TT, SS, HH=== Homozygous dominant
tt, ss, hh=== Homozygous recessive
--- heterozygous: alleles are different for a particular trait
ex: Tt, Ss, Hh
Laws of Heredity
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Mendel’s findings led to the laws of heredity
2 laws
1. Law of segregation: two alleles for the same trait
separate when gametes are formed. (remember when
chromosomes separate during meiosis)
2. Law of independent assortment: alleles of different
genes (ex hair and eye color), separate independently of
one another during gamete formation. One gene does
not influence the inheritance of the other.
Studying Heredity
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Punnett Square: Finding the probability that a
trait will be passed from one generation to the
next.
-- monohybrid: 1 trait
-- dihybrid: 2 traits
Pedigree charts: family history chart that shows
how a trait has been inherited over several
generations.
Punnett Squares
Monohybrid crosses
A man homozygous dominant for blonde hair marries a women
who in homozygous recessive for black hair. What is the
likelihood that their children will have black hair??
b
b
B Bb
Bb
Genotypes: 4 or 100% Heterozygous
Phenotypes: 100% Blonde
B Bb
Bb
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No chance of having a child with black hair
Punnett Squares Continued
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Dihybrid crosses
-- Look at 2 traits. Two heterozygous black haired and blue eyed
people marry. Look at probability of having a child with black
hair and blue eyes. B- black, b-blonde, E-blue, e-brown
-- Genotypes Father: Bb, Ee
Mother: Bb, Ee
1. find all possible genotypes: Father BE, Be, bE, be
Mother: BE, Be, bE,be
Dihybrid cross
BE
Be
bE
be
BE
Be
BBEE BBEe
BBEe BBee
BbEE BbEe
BbEe Bbee
bE
BbEE
BbEe
bbEE
bbEe
Phenotypes
9 or 56.25% black haired and blue eyed
3 or 18.75% blonde haired and blue eyed
1 or 6.25% blonde haired and brown eyed
3 or 18.75% black haired and brown eyed
be
BbEe
Bbee
bbEe
bbee
Genotypes
4 Homo dominant
8 Heter
4 Homo Recessive
Ratio: 1:2:1
Pedigree Charts
 A family history that shows how a trait has been inherited over many
generations.
 Scientist can determine the following:
 Autosomal or sex-linked? If a trait is autosomal, traits will appear in both sexes
equally. If a trait is sex-linked, males will show the trait. A sex-linked trait is a
recessive trait whose allele is located on the X chromosome. Because males
only have one X chromosome, a male who carries this recessive allele on the x
chromosome will show the trait. The only way a female will exhibit the trait if both
of her X chromosomes carries the recessive trait.
 Dominant or Recessive? If the trait is autosomal dominant, every offspring that
has the trait will have a parent with the trait. If it is recessive, the individual will
have 1, 2, or neither parent exhibit the trait.
(depending on the genotypes). Also dominant traits will show up more
throughout the pedigree.
 Heterozygous or Homozygous? If the trait is dominant, they will have a genotype
of homozygous dominant or heterozygous and their phenotype will show the trait.
Two people heterozygous for a recessive trait will not show the trait but can pass
it on to their children.
Pedigree charts
 Horizontal lines: indicate matings
 Vertical lines: Offspring
Males
Male affected
females
female affected
Carrier
Complex Heredity
There is more to the patterns of heredity
than the simple dominant/recessive
patterns.
 Polygenic traits
 Incomplete dominance
 Codominance
 Multiple alleles
 Environmental influences
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Polygenic traits
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Traits influenced by several genes
Hard to determine effects because many different
combinations occur due to independent assortment and
crossing-over.
Examples: eye color, height, weight, hair and skin color.
Incomplete Dominance
An individual displays a trait that is
intermediate between the two parents.
 Ex: Red flower is crossed with a White
flower: Results in pink flowers
 Pink flowers because they show less
pigment than red but more than white.
 Ex: One parent with curly hair and one
parent has straight hair and their child has
wavy hair.
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Codominance
Traits with two forms displayed at once.
 2 dominant alleles displayed at the same
time
 EX: A homozygous white horse and a
homozygous red horse produces a
heterozygous offspring. The offspring will
show both red and white hair and in equal
numbers. (called a Roan horse)
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Multiple Alleles
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Genes with 3 or more alleles.
ABO blood system: IA, IB, i
A and B are carbohydrates located on red blood cells
i does not have this carbohydrates
Nor A or B are dominate over each other: they are codominant
They are dominant over i
3 alleles can produce 4 genotypes: A, B, AB, O
IA
IB
i
IA
IAIA
IAIB
IAi
IB
IBIA
IBIB
IBi
i
IAi
IBi
ii
Environmental Influences
EX: Hydrangea plants: color of flowers depends
on the acidity of the soil
 Hair Color of some animals: Artic fox; during
summer months it turns reddish brown to blend
in with its environment. In the winter it is white
 Humans: skin color- exposure to the sun,
behavior, height and weight controlled by
nutrition. Identical twins are genetically the
same but can be very different
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