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Genetics
The study of heredity
Genetics

Genetics is the scientific study of heredity
- how traits are passed from generation to
generation.

The characteristics that are inherited are
called traits.
Genes


Humans have 23
homologous pairs of
chromosomes.
On each
chromosomes, there
are sections called
genes that code for
traits.
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Genes
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Alleles


An allele is a distinct form of a gene.
Every person has 2 alleles for a gene 
1 from the father and 1 from the mother
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Alleles

Letters of the alphabet are used to represent
an allele of interest.
T
A

P
Y
R
Every person has two copies of an allele, so
they will have two letters.
Alleles

Dominant alleles are symbolized with a
capital letter. Dominant alleles will mask a
recessive alleles in cases of simple
A
dominance/recessiveness.

Recessive alleles are symbolized with lower
case letters.
a
Homozygous Alleles

If an organism has two like copies of an
allele, it is homozygous (homo = same).

If the two alleles are dominant, the
organism is homozygous dominant. AA
If the two alleles are recessive, the
organism is homozygous recessive. aa

Heterozygous Alleles

If an organism has two different copies of
an allele, it is heterozygous (hetero =
different).
Aa
Genotype and Phenotype

The letters an organism has represent the
organism’s genotype - what alleles the
organism has.

As a result of the alleles present, a trait is
expressed. The phenotype is the
expressed trait.
Genotype and Phenotype

Example: In a plant
species, there are two
alleles for flower color:
R and r.
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

R is dominant, and
codes for red flowers
r is recessive and
codes for white flowers
Genotype and Phenotype

The genotype is the combination of
alleles: either RR, Rr, or rr.

The phenotype is what is expressed:
either red or white flowers.
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Genotype and Phenotype



RR  Homozygous dominant
 Red flowers
Rr  Heterozygous dominant
 Red flowers
rr  Homozygous recessive
 White flowers
In cases of simple
dominance, an
organism must have
two copies of the
recessive alleles to
express the
recessive trait.
Purebreds and Hybrids

Purebred - an organism that receives the
same genetic traits from both of its parents

Hybrid - an organism that receives different
forms of a genetic trait (different alleles) from
each parent
Mendel’s Laws
Contributions of Gregor Mendel
Law of Dominance

The dominant alleles is expressed and may
mask a recessive allele. The recessive form
of a trait is only shown in a homozygous
recessive organism.

Ex. R is allele for round, r is allele for square.



RR - round
Rr - round
rr - square
Law of Segregation

Gene pairs separate when gametes are
formed.
Parent:
Dd
D
d
Gametes
Parent:
dd
d
d
Gametes
Law of Independent Assortment

Genes segregate randomly and
independently. This means that if there are 2
or more traits, every combination of those
traits is possible.
AabbCc
AbC
Abc
abC
abc
Probability and
Punnett Squares
Predicting the genotypes and
phenotypes of offspring
Probability

Probability - the likelihood that a
particular event will occur (what are the
odds?)

What is the probability that a single coin
flip comes up heads?
 50% or 1/2
Probability

True or False? The past outcomes of
coin flips greatly affects the outcomes of
future coin flips.

False.
 There’s still a 50% chance of heads and
50% chance of tails!
Probability

The way in which alleles separate is random,
like a coin flip. (Mendel’s Law of Segregation)

From a mother who is heterozygous for an
allele, there is a 50% chance she passes on
the dominant allele and a 50% chance she
passes on the recessive allele.
Punnett Squares

Punnett squares show probabilities for
genotypes and phenotypes of offspring of
two parent organisms.

Example:
 In Mendel’s pea plants, the plants had
either purple (P) or
white (p) flowers.
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Punnett Squares

Step 1. Make the grid.
 If there is 1 trait, it is a 2x2 grid.
 If there are 2 traits, it is a 4x4 grid.

Because we are only looking at 1 trait (flower
color), a 2x2 grid is needed.
Punnett Squares
Pp

Step 2: Determine the
parents’ genotypes and
possible gametes.
P
P
PP

Example: a
heterozygous pea plant
and a homozygous
dominant pea plant.
P
p
Punnett Squares
Pp

Step 3: Fill in the
squares by combining
what is on top of the
column and to the left
PP
of the row.
P
p
P
PP
Pp
P
PP
Pp
Punnett Squares
Pp

Step 4: Use the
Punnett square to
determine probabilities
and ratios.
PP
P
p
P
PP
Pp
P
PP
Pp
Punnett Squares


What is the probability of
an offspring plant having
purple flowers?
 100%
What is the probability of
an offpsring plant being
heterozygous?
 2/4 = 1/2 = 50%
PP
Pp
PP
Pp
Punnett Squares

If there are 2 traits, the Punnett square will
be a 4x4 grid.
P - purple; p - white
R - round, r - wrinkled

Example: Cross a pea plant that is
heterozygous for both flower color and seed
shape with a plant that has white flowers and
is heterozygous for seed shape
Punnett Squares

Cross a pea plant that is heterozygous for
both flower color and seed shape with a plant
that has white flowers and is heterozygous for
seed shape
PpRr
PR, Pr, pR, pr
ppRr
pR, pr, pR, pr
Punnett Squares
ppRr
pR
PR
Pr
PpRr
pR
pr
pR
pr
pr
Punnett Squares
ppRr
pR
PR PpRR
Pr PpRr
PpRr
pR ppRR
pr ppRr
pR
pr
pr
Punnett Squares
ppRr
pR
pR
pr
pr
PR PpRR PpRR PpRr PpRr
Pr PpRr PpRr Pprr
PpRr
Pprr
pR ppRR ppRR ppRr ppRr
pr ppRr ppRr pprr
pprr
Punnett Squares
pR
pR
pr
pr
PR PpRR PpRR PpRr PpRr
2 / 16 = 1 / 8
Pr PpRr PpRr Pprr Pprr
or
pR ppRR ppRR ppRr ppRr
12.5%
pr ppRr ppRr pprr

pprr
What is the probability of an offspring having
white flowers and wrinkled seeds?
Punnett Squares
pR
pR
pr
pr
PR PpRR PpRR PpRr PpRr
6 / 16 = 3 / 8
Pr PpRr PpRr Pprr Pprr
or
pR ppRR ppRR ppRr ppRr
37.5%
pr ppRr ppRr pprr

pprr
What is the probability of an offspring having
purple flowers and round seeds?
Punnett Squares
PpRR PpRR PpRr PpRr
PpRr PpRr
Pprr
Pprr
ppRR ppRR ppRr ppRr
ppRr ppRr
pprr
pprr

Write the probable genotypic ratio.

2 PpRR : 4 PpRr : 2 Pprr : 2 ppRR : 4 ppRr : 2 pprr
1 PpRR : 2 PpRr : 1 Pprr : 1 ppRR : 2 ppRr : 1 pprr

Intermediate
Inheritance
Beyond Simple Dominance
Intermediate Inheritance

There are 3 types of intermediate
inheritance, genetic patterns that don’t
follow the simple dominant-recessive
rules.



Incomplete dominance
Codominance
Multiple alleles
Incomplete Dominance

Incomplete dominance - neither allele is
completely dominant over the other

The heterozygous form is a “blended” form
of the two alleles.
Incomplete Dominance

Example: In snapdragon flowers, there is
an allele that codes for red (r), and allele
that codes for white (w).

rr - red
ww - white
rw - pink


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Incomplete Dominance

Ex. Cross a red and a pink snapdragon.
r
w
r
rr
rw
r
rr
rw
Incomplete Dominance

Sometimes two like capital letters are
used, but one gets a prime sign (‘).

Ex: Human hair
 Curly hair HH
 Straight hair H’H’
 Wavy hair HH’
Codominance

Codominance - both alleles are
dominant and get expressed equally

In the heterozygous has some of
each phenotype, but they are not
blended.
Codominance

Example - in a type of cattle, red hair (R)
and white hair (W) are codominant.

RR - red
WW - white
RW - roan
 Some red, some white, but not pink!


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Codominance

Ex. Cross a red parent and a white
parent.
R
R
W
RW
RW
W
RW
RW
Multiple Alleles

Multiple alleles - there are more than 2 alleles
for a trait.

Ex. Fur color - gray, black, striped

Ex. Human blood types
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Sex Linkage
Sex-linked, sex-limited,
and sex-influenced traits
Human Chromosomes

Humans have 23 homologous pairs of
chromosomes, for a total of 46.

22 pairs are called autosomes , which are
all of the non-sex chromosomes

The 23rd pair is the sex chromosomes X and Y.
Sex Chromosomes

X and Y


Females - XX
 All eggs have an X
Males - XY
 Sperm have either an X or Y
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Sex-Linked Traits

Traits controlled by genes on the sex
chromosomes are sex-linked traits.

Examples of sex-linked traits: hemophilia,
color blindness, male pattern baldness

Most are “attached” to the X chromosome.
 Therefore, females have 2 copies of
these alleles and males only have one
Example - Hemophilia

Hemophilia - a blood clotting disorder

Hemophilia is X-linked.


XH = normal
Xh = hemophilia

Y is still just a Y
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Example - Hemophilia

Females could be:

XHXH - don’t have hemophilia, not a
carrier

XHXh - don’t have hemophilia, is a carrier

XhXh - have hemophilia
Example - Hemophilia

Males can be:
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
XHY - does not have hemophilia
XhY - has hemophilia
 Males
cannot be carriers - they either
have it or they don’t!
Example - Hemophilia


Draw a Punnett square for cross between a carrier
female and an unaffected male.
Female: XHXh Male: XHY
XH
Y
XH XHXH
XHY
Xh XHXh
XhY
Example - Hemophilia

What is the percent chance that a child of theirs
will have the disorder?
 25%
XH
Y
XH XHXH
XHY
Xh XHXh
XhY
Example - Hemophilia

What is the percent chance that a child of theirs
will have the disorder?
 25%
XH
Y
XH XHXH
XHY
Xh XHXh
XhY
Example - Hemophilia

What is the percent chance that a a son would
have the disorder?
 50%
XH
Y
XH XHXH
XHY
Xh XHXh
XhY
Example - Hemophilia

What is the percent chance that a daughter
would be a carrier?
 50%
XH
Y
XH XHXH
XHY
Xh XHXh
XhY
Example - Colorblindness

Color blindness is also X-linked.

X = normal
Xc = colorblind

Example - Colorblindness

Cross a colorblind male and a carrier
female.
Xc
Y
XXc
XY
Xc XcXc
XcY
X
Sex-limited traits

Sex-limited traits are only expressed in the
presence of sex hormones, or are only
observed in one sex or the other.

Ex. Beard growth
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Sex-influenced traits

Sex-influenced traits are expressed in both
sexes, but they are expressed differently.

Ex. Baldness is dominant in men,
recessive in women
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Pedigrees
Pedigrees





Males
Females
Affected - shaded
Unaffected - not shaded
Carrier - half shaded
Pedigrees

A pedigree is a diagram showing family
history and tracing a genetic trait.
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