Biology
Genetics Part 2
 Objectives: a) Review Mendelian Genetics
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b) Types of cells/Types of
Chromosomes
c) Sex linked traits
d) Sex Influenced traits
e) Polygenics
f) Epistasis
Cells
 We have 2 types of cells
 1. Somatic Cells (2n) – body cells
 2. Sex Cells (n) – sperm and egg
Chromosomes
 We have 2 types of Chromosomes:
 1. Autosomal chromosomes – pairs 1
through 22 in humans (non-sex
chromososmes)
 2. Sex chromosomes – xx or xy
 What about a fruit fly with a 2n = 8?
Sex-linked Traits
Definition
 Sex-linked traits are traits that are
controlled by genes on the sex
chromosomes
• The X and Y chromosomes
Are they controlled by both sex
chromosomes?

Most sex-linked traits are controlled by genes on
the X chromosome.
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This is because an X chromosome is much larger
than a Y chromosome.
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A few traits are suspected to be controlled by
genes on the Y chromosome, however there is
less research about Y-linked traits.
Different Forms of Sex-linked Inheritance
 There are three different forms of sex-
linked inheritance that we will be
examining:
• X-linked recessive inheritance
• X-linked dominant inheritance
• Y-linked inheritance
X-linked Recessive Inheritance

X-linked recessive traits are traits resulting from
a recessive allele on the X chromosome.
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There are over 100 different human conditions
that are caused by recessive alleles found on the
X chromosomes.
X-linked recessive alleles are represented by a X ,
superscript lower case letter or a plus/minus
 (+/-)

X-linked Recessive Inheritance
 These traits tend to show up in males more
than females.
• Why?
Because males only carry one X chromosome!
( they have one x chromosome, and one Y
chromosome that make up their sex chromosomes).
X-linked Recessive Inheritance
 Example #1: Colorblindness
 The allele that controls colorblindness is
found on the X chromosome and is
recessive .
 What genotype must a female have to be
diagnosed as colorblind?
Homozygous recessive (X-X-)
Are you colorblind?
Normal Color Vision:
A: 29, B: 45, C: --, D: 26
Red-Green Color-Blind:
A: 70, B: --, C: 5, D: -- 3.
Red Color-blind:
A: 70, B: --, C: 5, D: 6 4.
Green Color-Blind:
A: 70, B: --, C: 5, D: 2
Try this problem!
 What is the probability that the sons of a
homozygous recessive mother would be
colorblind?
 Represent colorblindness with a “-”
Mothers Genotype X- X Each son would receive an X chromosome
from their mother which means…
Each son would have an X chromosome
with a recessive allele for colorblindness
and would express the trait!
X-linked Recessive Inheritance

Example #2: Hemophilia Hemophilia

Hemophilia is a serious ailment in which the
blood lacks a clotting factor, and therefore when
an individual is injured, they cannot stop
bleeding.
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Hemophilia is also controlled by recessive alleles
on the X chromosome.
Try this problem
A woman who is heterozygous (a carrier) for
hemophilia marries a normal man:
 a. What are the genotypes of the parents?
 b. Make a Punnett square for the above cross.
 c. What is the probability that a male offspring
will have hemophilia? __________
 d. What is the probability of having a hemophiliac
female offspring? _________

X-linked Dominant Inheritance

X-linked dominant traits are traits that result from
the presence of a dominant allele on the X
chromosome.

Unlike X-linked recessive traits, females and
males both require only ONE dominant allele in
order to express the trait.

X-linked dominant traits are represented by an X,
superscript capital letter or a (+)
X-linked Dominant Inheritance
 Example: Faulty Tooth Enamel and Dental
Discoloration
 Individuals who have an X chromosome
that carries a dominant allele for this trait
will have dental discoloration.
Try these problems!
 Would a heterozygous woman for dental
discoloration display the trait?
YES! Because this is a X-linked dominant
trait, you only need to carry one dominant
allele in order to express the trait.
Try these problems!
 What percentage of the children from a
heterozygous mother and an affected
father would have dental discoloration?
 Represent dental discoloration with a “D”
D
D
X
Y
d
X
X
D D
X X
D d
X X
D
d
X Y
X Y
All of the daughters will
display the trait as they each
carry at least one dominant
allele.
One son will demonstrate the
trait while the other won’t.
Y-linked Inheritance
 Y-linked traits are controlled by alleles on
the Y chromosome
 Another word for Y-linked traits is
holandric traits as they are “wholly male”.
Y-linked Inheritance
 Do the words homozygous or heterozygous
apply to Y-linked traits?
No, because there is only ever one Y chromosome
present at a time!
 Are females affected by Y-linked traits?
No, because females don’t carry Y chromosomes!
What is this?
Y-linked Inheritance
 An example of a Y-linked trait is . . . . .
HAIRY EARS!
Y-linked Inheritance
 All of the sons of an affected male will
display this Y-linked trait.
A final example of Sex-linked
Traits
Eye color in fruit flies
( Drosophilia melanogaster )
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Eye color is controlled by the X chromosome.
Red eyes are dominant to white.
White eyes are most common in males.
Females only display white eyes if they are homozygous
recessive for the trait.
Fruit Flies Continued
 What type of X-linked inheritance is this?
X-linked recessive Inheritance because the males
show the trait more often that the females. Also,
the females must have two X chromosomes, both of
which carry the recessive alleles for white eye color
in order to have white eyes!
 Try your new knowledge out on the handout
you are about to receive!
Let’s Try These
 Problems
 Practice Problems