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MENDELIAN
GENETICS
Gregor Mendel
Probability & Punnet Squares
Mendelian Principles
Vocabulary
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Dominant ≡ allele which masks the
expression of another; represented by capital
letters. (B, T)
Recessive ≡ allele which is expressed only if
both parents contribute it; represented by small
letters.(b, t)
Incomplete dominance ≡ phenotype of the
trait isn’t a total masking of trait. Example is
red tints in the hair; represented by capital
letters and subscripts. (B1, B2)
Vocabulary
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Polygenic ≡ several genes contribute to the
overall phenotype; example is skin color.
Sex–linked ≡ commonly applied to genes on
the X chromosome, the more current term is Xlinked; genes on the Y chromosome are
holandric genes.
Epistasis ≡ one gene masking the effects of
another; and example is hair color to red color
tints.
Vocabulary
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Phenotype ≡ the physical characteristics from
the expression of the genes.
Genotypes ≡ the actual genes that an
individual has inherited.
Haploid ≡ having half of the chromosome
pairs. (23 single chromosomes for humans)
Diploid ≡ having the complete pairs of
chromosomes (23 pairs of chromosomes for
humans)
Vocabulary
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Homozygous ≡ both genes for a particular
trait are the same. (BB or bb)
Heterozygous ≡ both genes for a particular
trait are opposite. (Bb)
Gametes ≡ sex cells, the sperm or egg.
Why do people,
even closely
related people,
look slightly
different from
each other?
The reason for
these differences
in their
phenotype is the
different
combination of
genes
possessed by
each individual
Genetics
Genetics
Each normal human
being has 46
chromosomes,
diploid, in each body
cell.
In forming the gametes
one chromosome
from each pair will be
given, so the gametes
are haploid.
Genetics
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In this way, you
contribute half of the
genetic information for
the child – your partner
will contribute the other
half.
In reality there are
thousands of different
gene pairs, and so
there are millions of
possible gene
combinations.
Gregor Mendel
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He was an Austrian monk who was
born in 1822.
He is known as the “Father of
Genetics”.
He carried out his work on
inheritance with ordinary garden
peas.
He spent years making sure the
pea plants were true breeding.
True Breeding ≡ if allowed to self pollinate, they
would produce offspring identical to themselves.
Pea Plant
Traits
Genes & Dominance
Fertilization ≡ A process during sexual
reproduction when the male and female
reproductive cells join.
Trait ≡ a specific inherited characteristic
(Gene)
Genes & Dominance
F1 ≡ “First filial” or
first generation.
Hybrid ≡ a cross
between two truebreeding parents.
Mendelian Genetics
Mendel made 2 conclusions from his first
experiments:
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First, biological inheritance is determined by
factors that are passed from one generation to
the next.
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Today we call those factors – Genes.
Second, this conclusion is called “The Principle
of Dominance”.
PRINCIPLE OF
DOMINANCE
Some alleles are dominant
and others are recessive.
Dominant & Recessive
Allele ≡ a form of a particular trait / gene.
Dominant ≡ an organism with a dominant allele
for a particular form of a trait will always
exhibit that form of the trait… it will be
‘expressed’.
Recessive ≡ An organism with a recessive
allele for a particular form of a trait will exhibit
that form ONLY when the dominant allele is
not present.
Segregation
During gamete
formation,
alleles
segregate
from each
other so that
each gamete
carries only a
single copy of
each gene.
Probability & Punnett Squares
Probability ≡ the likelihood that a particular
event will occur is called probability.
Flipping a coin:
The probability that a single coin flip will
come up heads is 1 chance in 2.
This is ½ or 50%.
The Probability of Coin Tosses
# of
Tosses
1
5
10
100
# landed
on Heads
% landed on Heads
(# of heads ÷ # of Tosses) x 100
Punnett Squares
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Punnett squares can be used to predict
and compare the genetic variations that
will result from a cross.
Cross ≡ When two organisms sexually
reproduce offspring.
Punnett Squares
Menellian Crosses
(1) BB x bb
(2) Bb x Bb
Di-Hybrid Cross
(1-3, 1-4, 2-3, 2-4)
(1) RRYY x rryy
Di-Hybrid Cross
(1-3, 1-4, 2-3, 2-4)
(2) RrYy x RrYy
Principle of
Independent Assortment
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Genes for different traits can segregate
independently during the formation of
gametes.
Independent assortment helps account for
many genetic variations observed in
plants, animals, and other organisms.
Incomplete Dominance
vs.
Codominance
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Incomplete Dominance ≡ where neither
allele is truly dominant and there is a blending
of the traits.
Codominance ≡ Both alleles contribute to the
phenotype because neighter allele is dominant.
However, unlike the blending you have in
Incomplete Dominance, in Codominance, you
see both traits showing up separately in the
phenotype.
Incomplete Dominance - Codominance
Codominance
Multiple Alleles
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Multiple Alleles ≡
When genes have
more than two alleles
that exist in a
population.
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EX: coat color in
rabbits, blood type
Polygenic Traits
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Polygenic Traits ≡ Traits controlled by two or more
genes. These traits are produced by the interaction of
several genes.
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EX: skin color in humans, height