Mendelian Genetics
We know what genotype and phenotype are
We know what genes are
What do genes do?
Genes provide the instructions for an
organism’s potential development
Why potential ?
What affects the phenotype?
Mendelian Genetics
Phenotype is affected by genotype, environment
chemicals & other genes
Example: 1
Why is this horse black &
white?
His genotype is either EE or
Ee for black color
Why the white markings?
Mendelian Genetics
Remember the phenotype is affected by other
genes as well
The “other” gene is called the Tobiano gene, which
overrides and cancels any color production at all in
those locations
Why the white markings?
The gene for black hair production has been
turned “off” or overridden at those locations
Mendelian Genetics
Example: 2 – Your height
The genes you received from mom & dad
determine your genotype
However, environmental factors, diet and hormone
involvement affect the phenotype
Can different genotypes result in the same
phenotype?
Can the same genotypes result in different
phenotypes?
Mendelian Genetics
So someone who is “supposed” to be 6’3” but doesn’t get
the nutrition probably won’t reach that height
The opposite could be true for someone who is “supposed”
to be 5’8” and gets too much hormone, they could be taller
Gregor Mendel (1822-1884)
1843 – admitted into Augustinian Monastery
1854 – began series of breeding experiments with
pea plants
- no knowledge of mitosis or meiosis
Mendelian Genetics
1865 – reports conclusions of experiments
KEY POINTS:
- began studying the inheritance of only 1 trait at
a time
- controlled matings
- kept accurate records of outcomes
Why pea plants?
Easy to handle, produce lots of offspring, short
life cycle, variation existed
Mendelian Genetics
Mendel allowed the plants he had to
Self-pollinate (selfing) – pollen fertilizes an
egg from the same flower
For many generations to attain true-breeders
There were 7 traits he studied in his experiments
1. Flower & seed coat color
5. Pod shape
2. Seed color
6. Stem height
3. Seed shape
4. Pod color
7. Flower position
Mendelian Genetics
Mendelian Genetics
Mendel had to be sure these plants didn’t
Self-pollinate
To manage this he removed the male parts
(anthers) and used them where he desired to
Cross-pollinate – pollen fertilizes an egg from a
different flower
Phenotypes of the resulting seeds (peas) were
analyzed and then planted to produce the next gen
Mendelian Genetics
P-generation
P X P = F1
F1 X F1 = F2 (through self-fertilization)
Monohybrid crosses
Reciprocal crosses – done in both directions
Ex. Wrinkled female x smooth male AND
Smooth female x wrinkled male
If the results are the same…
If the results are different…
Mendelian Genetics
F1 generation always showed traits of one parent,
not both (dominant vs recessive)
F2 generation showed traits of both parents
(3:1) (1:2:1)
Mendel reasoned that there were “factors” (genes)
that were passed from parent to offspring
Since the 2 traits he was examining replaced each
other they were assumed to be alternative forms
of the same trait (alleles)
Homozygous / heterozygous
Punnett squares
Mendelian Genetics
Medel’s principle of segregation The two members of a gene pair (alleles) segregate
from each other during gamete formation
This means that all offspring carry one allele from
each parent – the combination of alleles in the
offspring is completely random
T1T2 x T3T4
T1T3 / T1T4 / T2T3 / T2T4
Branch or Fork Diagram
Mendelian Genetics
Branch or Fork Diagram
Mendelian Genetics
What do punnett squares or the branch diagram
actually show us?
Possible outcomes, not actual – the percentages
are for each offspring produced
Wild-type allele – the allele of a gene that is
present in the highest frequency in a wild
population
*mutations to these genes could produce
nonfunctional, partially functional or totally
absent proteins
Mendelian Genetics
*If the function of the protein is lost due to the
mutation it is called a loss-of-function mutation
(usually recessive)
Mendelian Genetics
Mendel’s Principle of Independent Assortment
–
genes on different chromosomes behave
independently in gamete production
This means that the passing of one gene has no
correlation with the passing of a second gene (TtGg) –
the passing of the ‘T’ has no correlation with the passing
of the ‘G’.
Complete a punnett square for the cross TtGg x ttGg
TG
Tg
tG
tg
TG
tG
tg
tG
tg
Tg
tG
tg
tG
tg
tG
tg
TG
Tg
tG
tg
TtGG
TtGg
ttGG
ttGg
tG
tg
tG
tg
TG
Tg
tG
tg
TtGG
TtGg
ttGG
ttGg
TtGg
Ttgg
ttGg
ttgg
tG
tg
tG
tg
TG
Tg
tG
tg
TtGG
TtGg
ttGG
ttGg
TtGg
Ttgg
ttGg
ttgg
TtGG
TtGg
ttGG
ttGg
TG
Tg
tG
tg
tG
TtGG
TtGg
ttGG
ttGg
tg
TtGg
Ttgg
ttGg
ttgg
tG
TtGG
TtGg
ttGG
ttGg
tg
TtGg
Ttgg
ttGg
ttgg
Mendelian Genetics
Dihybrid cross – cross between 2 individuals that
are ‘dihybrid’, meaning they are both hybrid for 2
traits (TtGg x TtGg – 9:3:3:1)
If you were to test 2 traits at the same time…
P generation: TTGG x ttgg both are ‘true-breeders’
therefore the F1 would be TtGg, completely hybrid
Trihybrid cross – cross between 2 individuals that
are hybrid for 3 traits
P generation: TTGGBB x ttggbb both are ‘truebreeders’ therefore the F1 would be TtGgBb,
completely hybrid
Mendelian Genetics
Monohybrid cross produces ____
2 phenotypes
Dihybrid cross produces ____
4 phenotypes
Trihybrid cross produces ____
8 phenotypes
Can you come up with a mathematical formula to
be able to determine the number of phenotypes
produced in a genetic cross?
n
2 n = number of independently assorting,
heterozygous gene pairs
Mendelian Genetics
Monohybrid cross produces ____
3 genotypes
Dihybrid cross produces ____
9 genotypes
Trihybrid cross produces ____
27 genotypes
Can you come up with a mathematical formula to
be able to determine the number of phenotypes
produced in a genetic cross?
n
3 n = number of independently assorting,
heterozygous gene pairs
Pedigree Analysis
Genetic evaluation of human inheritance is
difficult because it is not ethically possible to
control the matings
Therefore we often rely on pedigree analysis to
determine patterns of inheritance (how it is passed
from gen to gen)
How do we know if genes are passed? We rely
strictly on phenotypes over several generations
Proband – affected individual in which the
pedigree is discovered
Pedigree Symbols
Male
Female
Affected / Unaffected
Carrier / Heterozygous
Mating
Parents with 1 boy and
1 girl (birth order)
Carrier of sex-linked
Stillbirth
Twins
Marriage of blood
relatives
Pedigree Analysis
Generations – numbered with Roman numerals (II)
Individuals – numbered with Arabic numerals (2)
Refer to a particular person as II - 2
*If affected individual is born to unaffected
parents: probably caused by recessive trait
*If affected individual is born to affected parents:
May be caused by dominant or recessive trait
Recessive Traits
Require homozygosity
May have originated from a mutation
Recessive Traits
Ex. albinism
In U.S. – 1 in 17,000 of the white population
1 in 28,000 of the African American pop
1 in 10,000 of the Irish population
Of those affected by rare recessive traits…
1. Most have “normal” parents (heterozygous)
2. Matings between heterozygous individuals
should produce a 3:1 ratio of “normal” progeny
3. When both parents are affected, homozygous,
their offspring will usually exhibit the trait
Dominant Traits
Expressed when heterozygous or homozygous
Dominant mutant alleles produce phenotypes due
to gain-of-function mutations they produce new
genes with new functions
Ex. Woolly hair, Achondroplasi,
Brachydactyly, Marfan syndrome
Because mutant dominant alleles are rare it is rare
to find an individual homozygous for the mutant
dominant allele
Dominant Traits
Characteristics of dominant inheritance
1. An affected individual must have an affected
parent
2. Usually does not skip generations
3. Heterozygous individual will transmit the
mutant gene to half their progeny
Mendelian Genetics
How can one determine the genotype of an
individual exhibiting the dominant phenotype?
Test cross – cross of an individual of unknown
genotype, usually dominant, with a homozygous
recessive individual to determine the unknown
Data resulting from genetic crosses rarely match
the “expected” ratios
It is the job of the geneticist to do statistical
analysis to understand the significance of the
deviation from the predicted results
Questions
1. A purple-flowered pea plant is crossed with a
white-flowered pea plant. All the F1 plants
produce purple flowers. When the F1 plants are
allowed to self-pollinate, 401 of the F2 have
purple flowers and 131 have white flowers. What
are the genotypes of the parental and F1
generation plants?
ANSWER: P – PP x pp
F1 – Pp x Pp
F2 – probably deduce a 1:2:1ratio
Questions
2. Consider 3 gene pairs Aa, Bb, and Cc, each of
which affects a different character. In each case
the uppercase latter signifies the dominant allele
and the lowercase letter the recessive allele.
These 3 gene pairs assort independently of each
other. Calculate the probability of obtaining a the
following:
a. an AaBBCc zygote from a cross of
individuals that are AaBbCc x AaBbCc
Questions
b. an AaBBcc zygote from a cross of
individuals that are aaBBcc x AAbbCC
c. an A_B_C_ phenotype from a cross
of individuals that are AaBbCC x
AaBbcc
d. an aabbcc phenotype from a cross
of individuals that are AaBbCc x
aaBbcc
Questions
3. In chickens, the white plumage of the leghorn
breed is dominant over colored plumage,
feathered shanks are dominant over clean shanks,
and pea comb is dominant over single comb.
Each of the gene pairs segregates independently.
If a homozygous white, feathered, pea-combed
chicken is crossed with a homozygous colored,
clean, single-combed chicken and the F1 are
allowed to interbreed, what proportion of the
birds in F2 the will produce only white, feathered,
pea-combed progeny if mated to a colored, cleanshanked, single combed birds?
Questions
4. In tomatoes, red fruit color is dominant to
yellow. Suppose a tomato plant homozygous for
red is crossed with one homozygous for yellow.
Determine the appearance of:
a. the F1 tomatoes
b. the F2 tomatoes
c. the offspring of a cross of the F1 tomatoes
back to the red parent
d. the offspring of a cross of the F1 tomatoes
back to the yellow parent
Questions
5. In maize, a dominant allele A is necessary for
seed color, as opposed to colorless (a). Another
gene has a recessive allele wx that results in waxy
starch, as opposed to normal starch (Wx). The
two genes segregate independently. An AaWxWx
plant is testcrossed. What are the phenotypes and
relative frequencies of offspring?
Questions
6. In guinea pigs, rough coat (R) is dominant over
smooth coat (r). A rough-coated guinea pig is
bred to a smooth one, giving eight rough and
seven smooth progeny in the F1 generation.
a. What are the genotypes of the parents and
their offspring?
b. If one of the rough F animals is mated to
its rough parent, what progeny would you
expect?
Questions
7. In cattle, the polled (hornless) condition (P) is
dominant over the horned (p) phenotype. A
particular polled bull is bred to three cows. Cow
A, which is horned, produces a horned calf;
polled cow B produces a horned calf; and horned
cow C produces a polled calf. What are the
genotypes of the bull and the three cows, and
what phenotypic ratios do you expect in the
offspring of these three matings?
Questions
8. Consider the following pedigree, in which the
allele responsible for the trait (a) is recessive to
the normal allele (A):
a. What is the genotype of the mother?
b. What is the genotype of the father?
c. What are the genotypes of the children?
d. Given the mechanism of inheritance
involved, does the ratio of children with the trait
to those without match what would be expected?