Mendelian Genetics
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The idea that biological traits can be inherited has existed for over 6000 years (the time
of the Babylonians), where pedigrees were depicted in cave paintings.
Traits were initially thought to be a blending of parental characteristics
A trait is a particular version of a characteristic that is inherited , such as hair colour or
blood type (Characteristic = eye colour; trait =blue eyes)
Gregor Mendel (1822-1884):
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Pioneer of genetics
Austrian monk who worked with garden peas
o Pea plants reproduce quickly and can be either self-pollinated or cross-pollinated
o A true-breeding plant can be crossed over and over with itself or similarly looking
pea plants to produce offspring with the same traits
o A hybrid is the offspring of two true-breeding plants
o He cut the anthers off parent plant that he was pollinating, to eliminate self pollination
His experiments explained the mechanism of inheritance in plants and heredity in
general
Mendel examined 7 characteristics in pea plants and only crossed plants that only
differed at a single trait at one time (i.e. plants with purple or white flowers, while all
other traits were the same)
Characteristic
Flower colour
Flower position
Stem length
Seed shape
Seed colour
Pod shape
Pod colour
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Trait
Purple/white
Axial (along stems)/ terminal (at tips)
Tall/dwarf
Wrinkled/ round
Yellow/ green
Inflated/constricted
Yellow/green
According to prior belief if Mendel crossed a plant with wrinkled peas with one with
smooth seeds then it would have been expected to get a mixture of the two traits
Mendel proved this to be incorrect!!!!
o Wrinkled + smooth = smooth
With the above traits  one always dominates the other
Mendels Experiments:
Definitions:
Cross: The breeding of two organisms with different traits is called a cross.
P- generation: the parental generation. True-breeding.
F1 generation: the filial generation. The hybrid offspring are produced from crossing the Pgeneration. They are referred to as monohybrids or offspring of two true-breeding plants that
differ in only one characteristic.
F2 generation: the offspring of an F1 generation cross. Resulted when Mendel allowed the F1
generation to self-pollinate.
Monohybrid cross: a cross designed to study the inheritance of only one trait.
What he did:
1. Crossed a true-breeding pea plant with purple flowers, with one with white flowers.
Wondered if he would get individuals with pink (“blended”) flowers
2. Mendel observed that the F1 generation all had purple flowers
3. He initially believed that the white trait had disappeared, but when he crossed the F1
generation, he observed both purple and white flowers again
4. With repeated experimentation he was able to observe that not only did the white trait not
disappear, but that these traits in his F2-generation were always expressed in a 3:1 ratio (purple
to white)
Characterisitic
P
F1
F2
F2 Ratio
Flower colour
Purple x white
All purple
3:1
Seed shape
Round x wrinkled
All round
Seed colour
Yellow x green
All yellow
Pod shape
All inflated
Pod colour
Inflated x
constricted
Green x yellow
Flower position
Axial x terminal
All axial
Stem length
Tall x dwarf
All tall
Purple (705) and white
(224)
Round (5474) and wrinkled
(1850)
Yellow (6022) and green
(2001)
Inflated(882) and
constricted (299)
Green (428) and yellow
(152)
Axial (651) and terminal
(207)
Tall (787) and dwarf (277)
All green
3:1
3:1
3:1
3:1
3:1
3:1
Mendel’s Conclusions:
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Mendel concluded that traits must be passed in distinct heredity units, factors, which we
now call genes
Even if not expressed in the F1 generation they can still be passed on
Those that were expressed in all F1 generations were called “dominant factors” and those
that were hidden but expressed in the F2 generation “recessive factors”
From his conclusions he came up with the first law of inheritance; the law of segregation
Law of segregation:
A scientific law stating that;
1. organisms inherit two copies of genes, one from each parent
2. organisms only donate one copy of each gene to their gametes because the genes
separate during gamete formation (meiosis)
Alleles: Dominant or Recessive
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Mendel also realized that each characteristic had multiple traits. Today the different forms
of a gene are known as alleles.
One allele from your father and one from your mother
Which allele is passed is purely random. The combination of alleles can be the same as one
another or different (i.e. both code for a purple flower or one codes purple and one codes
white).
If two alleles are the same this condition is called homozygous.
If the two alleles are different this condition is called heterozygous.
An individual’s genotype includes all forms of an individual’s genes, even if “hidden”.
An individual phenotype includes the allele that has been expressed or the appearance of
that individual.
A dominant allele is one that does not stay hidden, and is always expressed when the
recessive gene is present
For Mendel’s pea plant we assign letters to the different alleles. Upper case for dominant
and lower case for recessive.
If the dominant allele is present then the plant will look like the dominant gene.
Re-cap:
Mendel concluded that sex cells contain 1 allele and today we know this because of _________
Factors=______________ and different forms of a gene =__________________
Trait
Homozygous
Recessive
pp (white phenotype)
Seed colour (Y, y)
Homozygous
Heterozygous
Dominant
Dominant
PP (purple phenotype) Pp (Purple phenotype
but purple and white
genotype)
YY
Yy
Seed shape ( R, r)
RR
rr
Flower colour (P, p)
Rr
yy
Pod shape (G, g)
Flower Position
(A,a)
Stem Length (T, t)
Law of Independent Assortment:
A scientific law stating that;
1. Segregation for different pairs of alleles occurs independently (a plant that is TtRr forms
gametes that are TR, Tr, tR, tr)
Punnet Squares and Predicting Inheritance
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A Punnett square is a diagram used to predict the proportions of genotypes in the
offspring resulting from a cross between two individuals
P
Cross between a
true-breeding white
plant pp and truebreeding purple
plant, PP
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p
P
Possible
Genotypes
p
It can help us determine the probability, or likelihood that an outcome will occur.
Ex. If we flip a coin 10 times there is a 50% it will be heads or tail; a 50:50 ratio. This
does not mean you will get head 5 times and tails 5 times.
Each flip is an independent event. The same is true for the inheritance of alleles.
Human Traits
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During meiosis we observed that during gamete formation only 1 set of chromosomes
(n=23) from each parent is used in the process of fertilization
After fertilization, our genes are made up of a set of homologous chromosomes (2n=46)
Therefore, half of our genes come from our mother and half come from our father
Recall that each homologous chromosome codes for similar traits ( ex. Eye colour)
Every individual is made up of many different traits, some of which you are already familiar
with like baldness, hair colour, colour blindness and blood types.
Others include position of eyes, shape of fingers, body size and proportion (these traits can
be influenced by our environment as well)
The following traits are known as single-gene traits, expressions of two alleles at a single
gene locus.
Trait (alleles)
Bent pinky (B, b)
PTC taster (T, t)
Blue Eyes (E, e)
Mid-digital hair (M, m)
Tongue rolling (R, r)
Widow’s Peak (W, w)
Thumb crossing (C, c)
Ear lobes (A, a)
Hitchhiker’s thumb (H, h)
Expression
Dominant allele causes the distal segment of the fifth finger to bend
distinctly toward the fourth finger
Phenylthiocarbamide (PTC) tastes bitter to heterozygous or
homozygous dominant, but tasteless to homozygous recessive.
Several related compounds occur naturally in foods and if you are a
PTC taster there is a good chance you are a non-smoker, do not like
Brussels sprouts, grapefruit juice or green tea.
Blue-eyed individuals are homozygous recessive and lack pigment in
their iris. Homozygous and heterozygous dominant individuals have
pigment and the colour is determined by another gene
People lacking hair in the middle segments of their fingers are
homozygous recessive. The presence of hair on one or more of the
middle segments of the fingers is governed by a series of alleles all
dominant to the recessive.
Persons with a dominant allele in heterozygous or homozygous
condition can roll their tongue into a tube-like shape. Homozygous
recessives are non-rollers and can never learn to roll their tongue.
Dominant allele in heterozygous or homozygous individuals results
in a V-shaped front hairline; homozygous have a straight hairline.
In relaxed interlocking fingers, left over right indicates the dominate
allele in either homozygous of heterozygous individuals and right
over left indicated the homozygous recessive condition
Homozygous recessives have attached earlobes and the
heterozygous or homozygous dominant individuals have detached
earlobes
Homozygous recessives can bend the distal joint o the thumb
backward to nearly a 90 degree angle; heterozygous or homozygous
dominant individual cannot bend further than approximately 30
degrees