GENETICS
GENETICS II
(BIO 301)
ABAYOMI E ADELEKE
DEPT. OF ZOOLOGY, UNIVERSITY OF JOS, NIGERIA
OUTLINE

EVOLUTION OF DOMINANCE

Definitions of commonly used terms
i. Chromosomes/Chromatids
ii. Genes
iii. Traits
iv. Alleles
v. Locus
vi. Homologous
vii. Heterozygous
viii. Homozygous
ix. Hybrid
x. Genotype
xi. Phenotype
xii. Gametes
xiii. Heredity (Hypothesis)
xiv. Self pollination
OUTLINE

Cycles in a cell
i. Mitosis
ii. Meoisis
cont…..
OUTLINE cont…..
Mendelian genetics-qualitative
i. 1st law- Segregation
a. Dominance
b. Monohybrid cross
c. Alternatives of inheritance
i. Incomplete dominance
ii. Codominance
iii. Multiple alleles
ii. 2nd law- Independent assortment
a. Dihybrid cross
i. two-factor cross F1
ii. two-factor cross F2
OUTLINE
cont…..
 Quantitative genetics
i. Additive inheritance
a. polygenic traits
b. epistasis
c. pleitropy
ii. Environmental influence
DEFINITION OF COMMON TERMS
CHROMOSOME
Thread-like, gene-carrying bodies in the nucleus of a
cell. Chromosomes are composed primarily of DNA and
protein. They are visible only under magnification during certain
stages of cell division. Humans have 46 chromosomes in each
somatic cell and 23 in each sex cell.(Rod-shaped structures within
the cell nucleus that carry genes encoded by DNA).
CHROMATID
One of the two side-by-side replicas produced by chromosome
duplication.
GENE
a unit of heredity which is transferred from a parent to
offspring and is held to determine some characteristic of the
offspring.units of inheritance usually occurring at specific
locations, or loci, on a chromosome.
A gene may be made up of hundreds of thousands of DNA
bases. Genes are responsible for the hereditary traits in plants
and animals.
ALLELIC GENE
Some genes have a variety of different forms, which are
located at the same position, or genetic locus, on a chromosome.
NON-ALLELIC GENE
is alleles at different position of chromosome loci but can affect
one gene over the other in different way of intereaction.
ALLELE
An alternative form of a gene that occurs at the same locus
on homologous chromosomes, e.g., A, B, and O genes are
alleles. One of the different forms of a gene or DNA
sequence that can exist at a single locus.
An alternate forms or varieties of a gene. The alleles for a
trait occupy the same locus or position on homologous
chromosomes and thus govern the same trait. However,
because they are different, their action may result in
different expressions of that trait.
GAMETE
A reproductive sex cell (ovum or sperm) with the haploid
number (23) of chromosomes that results from meiosis.
HOMOLOGOUS CHROMOSOMES
Chromosomes that are paired during the production of
of sex cells in meiosis. Such chromosomes are alike with regard
to size and also position of the centromere. They also have the
same genes, but not necessarily the same alleles, at the same
locus or location.
HOMOZYGOUS
having the same allele at the same locus on both members of
a pair of homologous chromosomes Homozygous also refers
to a genotype consisting of two identical alleles of a gene for a
particular trait. An individual may be homozygous dominant
(AA) or homozygous recessive (aa). Individuals who are
homozygous for a trait are referred to as homozygotes.
The situation in which allelic genes are identical, e.g., the KK
genotype or the Fya Fya genotype.
HETEROZYGOUS
A genotype consisting of two different alleles of a
gene for a particular trait (Aa). Individuals who are
heterozygous for a trait are referred to as heterozygotes or the
situation in which allelic genes are different that is having two
different alleles at a given locus on a pair of homologous
chromosomes.
LOCUS
The location of allelic genes on the chromosome, a specific
location on a chromosome. e.g., A, B, and O genes occur at the
ABO locus. (Plural = loci)
DOMINANT ALLELE
An allele that masks the presence of a recessive allele in the
phenotype. Dominant alleles for a trait are usually expressed if an
individual is homozygous dominant or heterozygous.
RCESSIVE ALLELE
An allele that is masked in the phenotype by the presence of a
dominant allele. Recessive alleles are expressed in the phenotype
when the genotype is homozygous recessive (aa).
PHENOTYPE
The observable or detectable characteristics of an
individual organism—the detectable expression of a
genotype. Observable characterisics of an organism.
GENOTYPE
The genetic makeup of an individual. Genotype can
refer to an organism's entire genetic makeup or the alleles at a
particular locus. The actual alleles present in an individual.
Phenotype vs Genotype
2 organisms can have the same phenotype but have
different genotypes
purple
PP
homozygous dominant
purple
Pp
heterozygous
1. phenotype is the actual appearance or characteristic,
and is determined by genotype; knowing the
phenotype will not always directly reveal the
genotype (recessive traits can be masked)
2. genotype is the listing of the actual alleles present; if
you know the genotype, you should be able to
predict the phenotype
genotypes are either homozygous or heterozygous
TRAIT

Any detectable phenotypic variation of a particular
inherited character.
F1 GENERATION
The first offspring (or filial) generation. The next and
subsequent
generations are referred to as F2, F3, etc.
Traits:
I’ll speak for
both of us!
Dominant allele leads to
production of
purple pigment
mutant allele
leads to no pigment
homologous
chromosomes
CODOMINANCE
The situation in which two different alleles for a trait are
expressed unblended in the phenotype of heterozygous
individuals. Neither allele is dominant or recessive, so that both
appear in the phenotype or influence it. Type AB blood is an
example. Such traits are said to be codominant.
MULTIPLE ALLELE SERIES
A situation in which a gene has more than two alleles. The ABO
blood type system is an example. Multiple-allele series only partly
follow simple Mendelian genetics.
HEREDITY (HYPOTHESIS)
THE BLENDED INHERITANCE HYPOTHESIS
Suggests that physical traits (or phenotypes) of offspring are an
intermediate of the parents. For example if a tall man and a short
woman have a child, this hypothesis predicts their child would have a
height intermediate relative to her parents.
PARTICULATE INHERITANCE
Is a pattern of inheritance discovered by Mendelian genetics theorists,
such as William Bateson, Ronald Fisher or Gregor Mendel himself.
This states that phenotypic traits can be passed from generation to
generation through "discrete particles" known as genes, which can
keep their ability to be expressed while not always appearing in a
descending generation
Gregor Mendel: Provided evidence for the particulate hypothesis.
Mendel formulated some of the basic laws of genetics.
MENDELIAN GENETICS
 KEY IDEA
Mendel’s research showed that traits are inherited as discrete
units.
 Traits are distinguishing characteristics that are inherited.
 Many in Mendel’s day thought traits were blended.
 Mendel made three key decisions in his experiments.
 use of purebred plants
 control over breeding
 observation of seven “either-or” traits







Flower colour: Violet/white
Flower position: Axial/terminal
Pod colour: Green/yellow
Pod shape: Inflated/constricted
Seed colour: Yellow/green
Seed shape: Round/wrinkled
Stem height: Tall/dwarf
Mendel drew three important conclusions.
 Traits are inherited as discrete units.
 Organisms inherit two copies of each gene, one from each parent.
 The two copies segregate during gamete formation.
LAW OF SEGREGATION
States that
“ The two alleles for each trait segregate, or
separate, during the formation of new
zygotes, the alleles will combine at random
with other alleles”.
This law ensures that a parent, with two copies of each gene can
pass on either allele.

A. MENDEL’S LAW OF SEGREGATION
(Monohybrid Crossing)
1. when Mendel crossed pure lines of different, competing
phenotypes, he found that the F1 generation was uniform
and matched one of the parents’ phenotypes
example: P1 yellow seed X green seed = all F1 yellow seed
2. when F1 plants were crossed or selfed, the F2 plants had
both P1 phenotypes in a ratio of roughly 3:1
3. using offspring from above F1 X F1 = F2 3 yellow seed: 1
green seed
 3.
thus, contrary to the popular belief of the time, recessive traits
are not lost in a mixing of parental phenotypes – they are
merely hidden in some “carrier” individuals
 4.
Mendel explained these ratios with what we now call his law
of segregation; stated in modern terms: individuals normally
carry two alleles for each gene, these alleles must segregate in
production of sex cells
 5.
later investigations of cell division revealed the mechanism
for segregation: the pairing and subsequent separation of
homologous chromosomes during meiosis
Crossing of a purple flower with a white flower
Crossing of the F1 individuals
DOMINANCE
In genetics, dominance is the phenomenon of one variant
(allele) of a gene on a chromosome masking or overriding the
effect of a different variant of the same gene on the other copy of
the chromosome. The first variant is termed dominant and the
second recessive. This state of having two different variants of the
same gene on each chromosome is originally caused by a mutation
in one of the genes, either new (de novo) or inherited.
The terms autosomal dominant or autosomal recessive
are used to describe gene variants on non-sex chromosomes
(autosomes) and their associated traits, while those on sex
chromosomes (allosomes) are termed X-linked dominant, Xlinked recessive or Y-linked; these have an inheritance and
presentation pattern that depends on the sex of both the parent
and the child (see Sex linkage). Since there is only one copy
of the Y chromosome, Y-linked traits cannot be dominant nor
recessive.
Dominance is not inherent to an allele or its traits
(phenotype). It is a strictly relative effect between two alleles of
a given gene of any function; one allele can be dominant over a
second allele of the same gene, recessive to a third and
co-dominant with a fourth. Additionally, one allele may be
dominant for one trait but not others.
Dominance is a key concept in Mendelian inheritance and
classical genetics. Letters and Punnett squares are used to
demonstrate the principles of dominance in teaching, and the use of
upper case letters for dominant alleles and lower case letters for
recessive alleles is a widely followed convention.
A classic example of dominance is the inheritance of seed
shape in peas. Peas may be round, associated with allele R, or
wrinkled, associated with allele r. In this case, three combinations
of alleles (genotypes) are possible: RR, Rr, and rr. The RR
(homozygous) individuals have round peas, and the rr
(homozygous) individuals have wrinkled peas. In Rr
(heterozygous) individuals, the R allele masks the presence of the
r allele, so these individuals also have round peas. Thus, allele R
is dominant over allele r, and allele r is recessive to allele R.
LAW OF INDEPENDENT
ASSORTMENT
 States that
“The alleles of two (or more) different
genes get sorted into gametes independently
of one another”.
In other words, the allele a gamete receives for one gene
does not influence the allele received for another gene.
MENDEL LAW OF INDEPENDENT ASSORTMENT
(Dihybrid Cross)
There are two types of breeding processes to know the
mechanism of genes and examine the inheritance of traits
from parents and grandparents, one is monohybrid cross
and the other is dihybrid cross. The latter occurs when the
F1 generation offspring differ in two traits. It is a cross
between two entities that are heterozygous for two
different traits. Mendel carried out the following
experiment for this cross:
 For crossing, he took a pair of contradicting characteristics
or traits
Mendel crossed round-yellow seed and wrinkled-green
seed
In the F1 generation, the outcome was seeds that were
round and yellow
The F1 generation indicated that the round and yellow
traits are dominant while the green colour and the wrinkled
shape were recessive traits.
Self-pollination of F1 progeny resulted in four varying
combinations of seeds in the subsequent generation, the F2
generation.
The outcome and the dihybrid cross-ratio were – roundyellow, wrinkled-yellow, wrinkled-green, round-green and
the ratio was – 9:3:3:1.
EXPLANATIONS
 Dihybrid cross illustrates the inheritance of two
characters
Produces four phenotypes in the F2 generation
 When the F1 dihybrid progeny self-pollinate.
If the two characters segregate together, the F1 hybrids
can only produce the same two classes of gametes (RY
and ry) that they received from the parents, and the F2
progeny will show a 3:1 phenotypic ratio.
If the two characters segregate independently, the F1
hybrids will produce four classes of gametes (RY, Ry, rY,
ry), and the F2 progeny will show a 9:3:3:1 phenotypic
ratio.
EXAMPLE:
Two true-breeding pea plants—one with yellow-round
seeds and the other with green-wrinkled seeds—were crossed,
producing dihybrid F1 plants. Self-pollination of the F1
dihybrids, which are heterozygous for both characters,
produced the F2 generation. The two hypotheses predict
different phenotypic ratios. Note that yellow color (Y) and
round shape (R) are dominant.
YRYR
P Generation
yryr
YR
Gametes

yr
YRyr
F1 Generation
Hypothesis of
dependent
assortment
Hypothesis of
independent
assortment
1⁄
Sperm
1⁄
1⁄
F2 Generation
(predicted
offspring)
2
2
YR
1⁄
yr
2
Eggs
1⁄
4
YR
4
1⁄
YR
2
1
YRYR
YR
Eggs
YRyr
1⁄
4
2
yr
yryr
YRyr
1⁄
4
1⁄
1⁄
4
7
YRyr
YRyr
14
yRyR
15
YRyr
16
Yryr
3⁄
16
yRyr
3⁄
16
Phenotypic ratio 9:3:3:1
101
32
yRyr
16
yr
9⁄
108
Yryr
12
yR
Phenotypic ratio 3:1
315
YRyr
8
11
10
13
4
4
YrYr
YRyR
3⁄
4
YRyR
6
9
4
Sperm
yr
1⁄
Yr
YRYr
1⁄
4
yR
3
YRYr
5
YRYR
4
1⁄
Yr
Phenotypic ratio approximately 9:3:3:1
yryr
1⁄
16
PHENOTYPE & GENOTYPIC RATIO EXPLANATION

PHYNOTYPIC RATIO 9 : 3 : 3 :1
PHYNOTYPE:
9 = YELLOW & ROUND
3 = YELLOW & WRINKLE
3 = GREEN & ROUND
1 = GREEN & WRINKLE
GENOTYPIC RATIO 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1
GENOTYPE:
1 = HOMOZYGOUS YELLOW AND ROUND (1-YRYR)
2 = HOMOZYGOUS YELLOW, HETEROZYGOUS ROUND (2,5YRyr)
1 = HOMOZYGOUS YELLOW, HOMOZYGOUS WRINKLE ( 6-YrYr)
2 = HETEROZYGOUS YELLOW, HOMOZYGOUS ROUND
(3,9- YRyR)
4 = HETEROZYGOUS YELLOW, HETEROZYGOUS ROUND
(4,7,10,13-YRyr)
2 = HETEROZYGOUS YELLOW, HOMOZYGOUS WRINKLE
(8,14-Yryr)
1 = HOMOZYGOUS GREEN, HOMOZYGOUS ROUND (11-yRyR)
2 = HOMOZYGOUS GREEN, HETEROZYGOUS ROUND (12,15-yRyr)
1 = HOMOZYGOUS GREEN, HOMOZYGOUS WRINKLE (16- yryr)
ALTERNATIVES OF INHERITANCE
Additionally, there are other forms of dominance such
as
a) Incomplete dominance
b) Co-dominance
c) Multiple allele
INCOMPLETE DOMINANCE
Incomplete dominance (also called partial dominance, semidominance or intermediate inheritance) occurs when the
phenotype of the heterozygous genotype is distinct from and often
intermediate to the phenotypes of the homozygous genotypes.
For example, the snapdragon flower color is homozygous for
either red or white. When the red homozygous flower is paired
with the white homozygous flower, the result yields a pink
snapdragon flower. The pink snapdragon is the result of
incomplete dominance.
The plant incompletely expresses the dominant trait (R)
causing plants with the Rr genotype to express flowers with less
red pigment resulting in pink flowers. The colors are not blended
together, the dominant trait is just expressed less strongly.
CO-DOMINANCE
This occurs when the contributions of both alleles
are visible in the phenotype.
For example, in the ABO blood group system,
chemical modifications to a glycoprotein (the H antigen)
on the surfaces of blood cells are controlled by three
alleles, two of which are co-dominant to each other
(IA, IB) and dominant over the recessive i at the ABO
locus.
The IA and IB alleles produce different
modifications. The enzyme coded for by IA adds
an N-acetylgalactosamine to a membrane-bound
H antigen. The IB enzyme adds a galactose.
The i allele produces no modification. Thus
the IA and IB alleles are each dominant to i
(i.e. IAIA and IAiA individuals both have type A
blood, and IBIB and IBiB individuals both have
type B blood), but IAIB individuals have both
modifications on their blood cells and thus
have type AB blood, so the IA and IB alleles are
said to be co-dominant.
Another example occurs at the locus for the betaglobin component of hemoglobin, where the three
molecular phenotypes of HbA/HbA, HbA/HbS, and
HbS/HbS are all distinguishable by protein
electrophoresis. (The medical condition produced by the
heterozygous genotype is called sickle-cell trait and is a
milder condition distinguishable from sickle-cell anemia,
thus the alleles show incomplete dominance with respect
to anemia.
This Punnett square shows co-dominance. In this example a white
bull (WW) mates with a red cow (RR), and their offspring exhibit codominance expressing both white and red hairs.
Co-dominance, where allelic products co-exist in the
phenotype, is different from incomplete dominance, where the
quantitative interaction of allele products produces an
intermediate phenotype. For example, in co-dominance, a red
homozygous flower and a white homozygous flower will
produce offspring that have red and white spots.
MULTIPLE ALLELES
Multiple alleles exist in a population when there are
many variations of a gene present. In organisms with two
copies of every gene, also known as diploid organisms, each
organism has the ability to express two alleles at the same
time. They can be the same allele, which is called a
homozygous genotype. Alternatively, the genotype can
consist of alleles of different types, known as a heterozygous
genotype. Haploid organisms and cells only have one copy of
a gene, but the population can still have many alleles.
Three or more kinds of genes occupying the same locus
in individual chromosome are referred to as multiple alleles.
When more than two possible alleles exist in a population
In short many alleles of a
single gene are called multiple
alleles.
Example of Multiple Allele
1. COAT COLOUR IN CATS
2. COAT COLOUR IN RABBITS
Here, four alleles exist for the c gene.
a) The wild-type version, C+C+, is expressed as brown fur.
b) The chinchilla phenotype, cchcch, is expressed as blacktipped white fur.
c) The Himalayan phenotype, chch, has black fur on the
extremities and white fur elsewhere.
d) The albino, or “colorless” phenotype, cc, is expressed as
white fur.
In cases of multiple alleles, dominance hierarchies can
exist. In this case,
1. The wild-type allele is dominant over all the others.
2. Chinchilla is incompletely dominant over Himalayan and
albino
3. Himalayan is dominant over albino.
This hierarchy, or allelic series, was revealed by observing the
phenotypes of each possible heterozygote offspring.
OTHER EXAMPLES INCLUDE
3. Wings of Drosophila
4. Self-Sterility in Plants
5. Blood Groups in Man
6.The ‘Rhesus’ Blood Group in Man
PROBABILITY
Segregation, independent assortment and
fertilization are random events and
Reflect the rules of probability
From the genotypes of parents, we can
predict the most likely genotypes of their
offspring using simple laws of probability.
PROBABILITY SCALE
The probability scale: ranges from 0 to 1; an event that is
certain to occur has a probability of 1, and an event that is certain
not to occur has a probability of 0.
The probabilities of all possible outcomes for an event must add
up to 1.
Random events are independent of one another.
The outcome of a random event is unaffected by the
outcome of previous such events.
Example: it is possible that five successive tosses of a normal
coin will produce five heads; however, the
probability of heads on the sixth toss is still 1/2.
RULES OF PROBABILITY
1.
Multiplication Rule
states that the probability that independent events will occur
simultaneously is the product of their individual probabilities.
 Question: In a monohybrid cross
between pea plants (Rr), what is
the probability that the offspring
will be homozygous recessive?
Rr
Segregation of
alleles into eggs
Segregation of
alleles into sperm
Sperm
 Answer:
 Probability that an egg from the F1
(Rr) will receive an r allele = 1/2.
 Probability that a sperm from the
F1 will receive an r allele = 1/2.
 The overall probability that two
recessive alleles will unite at
fertilization: 1/2 x 1/2 = 1/4.
Rr

1⁄
R
2
1⁄
Eggs
1⁄
r
2
r
r
R
R
2
r
2
R
R
1⁄
1⁄
1⁄
4
R
1⁄
4
r
4
r
1⁄
4
Multiplication rule also applies to dihybrid
crosses
 Question: For a dihybrid cross, YRyr x YRyr, what is the
probability of an F2 plant having the genotype YRYR?
 Answer:
Probability that an egg from a YRyr parent will receive the Y
and R alleles = 1/2 x 1/2 = 1/4.
Probability that a sperm from a YRyr parent will receive the Y
and R alleles = 1/2 x 1/2 = 1/4.
The overall probability of an F2 plant with the genotype
YYRR: 1/4 x 1/4 = 1/16.
2. Addition Rule
states that the probability of an event that can occur in two or
more independent ways = sum of the separate probabilities of the different
ways.
 Question: In this cross between pea plants, Pp x Pp, what is the probability of the
offspring being heterozygous?
 Answer: There are two ways a heterozygote may be produced: the
dominant allele (P) may be in the egg and the recessive allele (p)
in the sperm, or vice versa.
 So, the probability that the offspring will be heterozygous is the sum
of the probabilities of those two possible ways:
Probability that the dominant allele will be in the egg with the
recessive in the sperm is 1/2 x 1/2 = 1/4.
Probability that the dominant allele will be in the sperm and the
recessive in the egg is 1/2 x 1/2 = 1/4.
 So, the probability that a heterozygous offspring will be produced is
1/4 + 1/4 = 1/2.
Complex Genetics Problems
 A dihybrid or other multicharacter cross
 Is equivalent to two or more independent monohybrid crosses occurring
simultaneously
 In calculating the chances for various genotypes from such crosses
 Each character first is considered separately and then the individual
probabilities are multiplied together
Multiple Locus Problem
 Question: What is the probability that a trihybrid cross between
organisms with genotypes AaBbCc and AaBbCc will produce an offspring
with genotype aabbcc?
 Answer: Segregation of each allele pair is an independent event, we
can treat this as three separate monohybrid crosses:
Aa x Aa: probability for aa offspring = 1/4
Bb x Bb: probability for bb offspring = 1/4
Cc x Cc: probability for cc offspring = 1/4
 The probability that these independent events will occur simultaneously is the
product of their independent probabilities (rule of multiplication).
 The probability that the offspring will be aabbcc is: 1/4 aa x 1/4 bb x 1/4 cc =
1/64
Problem 2
 Question: Using garden peas, where and assuming the cross is PpYyRr x
Ppyyrr: what is the probability of obtaining offspring with homozygous
recessive genotypes for at least two of the three traits?
 Answer:
Write the genotypes that are homozygous recessive for at least two
characters, (note that this includes the homozygous recessive for all three). Use the rule
of multiplication to calculate the probability that offspring would be one of these
genotypes. Then use the rule of addition to calculate the probability of offspring in which
at least two of the three traits would be homozygous recessive.
 Genotypes with at least two homozygous recessives
 PpyyRr - 1/4 x 1/2 x 1/2 = 1/16
 ppYyrr - 1/4 x 1/2 x 1/2 = 1/16
 Ppyyrr - 1/2 x 1/2 x 1/2 = 2/16
 PPyyrr - 1/4 x 1/2 x 1/2 = 1/16
 ppyyrr - 1/4 x 1/2 x 1/2 = 1/16
= 6/16 or 3/8 chance of two recessive traits
Quantitative genetics
i. Additive inheritance
a. polygenic traits
b. epistasis
c. pleitropy
ii. Environmental influence
POLYGENIC TRAITS
Definition
Polygenic traits are traits that are controlled by multiple genes
instead of just one. The genes that control them may be located near
each other or even on separate chromosomes.
Because multiple genes are involved, polygenic traits do not
follow Mendel’s pattern of inheritance.
Instead of being measured discretely, they are often represented
as a range of continuous variation.
Some examples of polygenic traits are height, skin color, eye
color, and hair color.

Polygenic traits are complex and unable to be explained
by simple Mendelian inheritance alone. Mendelian
inheritance is involved when one particular gene controls for a
trait, and the traits are discrete.
Polygenic traits also have dominant and recessive alleles,
but so many genes play a role in an organism’s phenotype for these
traits that the final result is the sum of many complex interactions.
It can be hard or impossible to figure out one gene’s effect on a
polygenic trait. Instead of being expressed in a ratio as single-gene
traits are, polygenic traits are expressed continuously and usually
form a bell curve when charted.
For example, human skin color varies on a continuous gradient
from light to dark, and it is not quantifiable; one’s skin color
can only be compared to others for a sense of how light or dark his or
her skin tone is. Some people have extremely light or extremely dark
skin, but the majority of the world’s people do not, and fall somewhere
in the middle.
This figure depicts a bell curve. For a trait like skin color, shade
(light to dark) would be on the X (horizontal) axis, and proportion of
population would be on the Y (vertical) axis. When data form a bell
curve, they are said to show a normal distribution.
A bell curve depicting skin color trait.
When data form a bell curve, they are said to show a normal
distribution.
HEIGHT
Human height is controlled by many genes; in fact, there
are over 400 genes related to height, and all of these genes
interact to make up a person’s phenotype.
This is a very large number, but it makes sense because height is a
compilation of the lengths of many different body parts, such as
leg bones, the torso, and even the neck.
Polygenic traits can also be influenced by an organism’s
environment. If a person gets inadequate nutrition during
childhood, they can have stunted growth and end up smaller and
shorter than they would otherwise. It is estimated that 90% of a
person’s adult height is controlled by genetics, and 10% is
affected by the environment.
SKIN COLOR
In humans, skin color is influenced by many things, but the
pigment melanin influences most of a person’s phenotype. In
general, the more melanin a person has, the darker their skin is.
Albino people produce no melanin at all. The body creates more
melanin to protect against the sun’s UV rays, which is why skin
darkens after prolonged sun exposure. The amount and type of
melanin that a person produces, such as eumelanin,
pheomelanin, and neuromelanin, is controlled by multiple genes,
and the different types of melanin interact to form the final
phenotype. For example, people with red hair have more
pheomelanin and often have a pinkish skin tone.
EYE COLOR
There are 2 major human eye color genes, OCA2 and HERC2,
but at least 13 other genes also play a role. The colored part of a
person’s eye is the iris. It is a muscle that changes the size of the
pupil in order to change the amount of light that is absorbed by the
retina.
A person’s eye color is determined by the
pigmentation of their irises, but also by the way the cells in their
irises scatter light. As with skin color, eye color is affected by the
presence of melanin.
People with brown eyes have a lot of melanin, while people with
blue eyes have low melanin in the front part of the iris that is
visible. Green eyes are caused by multiple factors; they are the
result of a light brown iris combined with a blue tone given by
light scattering.
EPISTASIS
Epistasis is the interaction between genes that influences a
phenotype. Genes can either mask each other so that one is
considered “dominant” or they can combine to produce a new trait.
It is the conditional relationship between two genes that can
determine a single phenotype of some traits.
At each locus are two alleles that dictate phenotypes. They can
affect one another in such a way that, regardless of the allele of one
gene, it is recessive to one dominant allele
A gene that masks another gene’s expression is said to be
epistatic and the gene whose expression is masked by a nonallelic
gene is said to be hypostatic.
TYPES OF EPISTASIS
There are six (6) common types of epistasis gene interactions:
 Dominant Epiatasis
 Dominant Inhibitory Epistasis,
 Duplicate Dominant Epistasis,
 Duplicate Recessive Epistasis,
 Polymeric Gene Interaction, and
 Recessive Epistasis
 When a dominant allele masks the expression of both dominant
and recessive alleles at another locus, it is referred to as
dominant epistasis or simple epistasis.
 When it is a recessive allele that masks the expression, it is
called recessive epistasis.
 Some genes can also mask other genes by suppression. This is
referred to as dominant inhibitory or suppression epistasis
because the gene is acting as a suppressor, or a factor that
prevents the expression of another allele.
 Duplicate types of epistasis depend on two loci. When there is a
dominant allele masking the expression of recessive alleles at two
loci, this is known as duplicate dominant epistasis or duplicate
gene action.
 When there is a recessive allele masking the expression of
dominant alleles at two loci, this is known as duplicate recessive
epistasis. It is also known as complementary gene action because
both genes are required in order for the correct phenotype to be
present.
 Polymeric gene interaction is the combination of two dominant
alleles that intensifies the phenotype or creates a median
variation. Alone, each dominant allele produces a physical trait
different from the combined dominant alleles. Therefore, this
creates three phenotypes for only two dominant alleles. This
shows that neither dominant allele is prevailing over the other
dominant allele.
PLEIOTROPY
Pleiotropy refers to the expression of multiple traits by a single gene.
These expressed traits may or may not be related.
In pleiotropy, one gene controls the expression of several phenotypic
traits
In several breeds of cattle, a single gene responsible for double
muscling is also associated with reduced fertility, lower calf survival
and sometimes increased stress susceptibility.
Pleitropy can be spoken of in various forms:
 Gene Pleiotropy,
 Developmental Pleiotropy,
 Selectional Pleiotropy, and
 Antagonistic Pleiotropy.
Gene pleiotropy
is focused on the number of traits and biochemical factors
impacted by a gene.
Developmental pleiotropy
is focused on mutations and their influence on multiple traits.
Selectional pleiotropy
is focused on the number of separate fitness components
affected by a gene mutation.
Antagonistic pleiotropy
is focused on the prevalence of gene mutations that have
advantages early in life and disadvantages later in life.