Transcript X-linked Genes

Human Genetics:
Patterns of Inheritance for
Human Traits
What is the difference between
heterozygous and homozygous
alleles?
What is the difference between
dominant and recessive traits?
We will be discussing 5 Patterns of
Inheritance for Human Traits
1. Single Allele Dominant
2. Single Allele Recessive
3. Sex Linked (X-Linked)
4. Multiple Alleles
5. Polygenic Traits
Single Allele Genes
Regular traits that are either determined by a
dominant or recessive allele on an autosome
1.
Autosomal Dominant examples:
a. Huntington’s Disease
b. Achondroplasia (dwarfisim)
c. Polydactyly (extra fingers and toes)
2. Autosomal Recessive examples:
a. Albinism
b. Cystic Fibrosis
c. Sickle Cell Anemia
1. Dominant Allele Disorders
Huntington’s Disease (HD)
 Results in loss of muscle
control and mental
deterioration
 No signs are shown until
30’s
 Brain degeneration
 Treatment: No cure, but
drug treatments are
available to help manage
symptoms.
1. Dominant Allele Disorders
Achondroplasia


Dwarfism
Person grows no
taller than 4’4
1. Dominant Allele Disorders
Polydactyly
 The presence of
more than the
normal number of
fingers or toes.
 Can usually be
corrected by surgery.
2. Recessive Allele Disorders
Albinism
 Lack of pigment in
skin, hair, and eyes
 Mutation in one of
several genes which
provide the
instructions for
producing one of
several proteins in
charge of making
melanin.
2. Recessive Allele Disorders
Cystic Fibrosis (CF)
 Caused by recessive
allele on
chromosome 7

Small genetic change
(removes one Amino
Acid)  changes
protein

Results in: Excess
mucus in the lungs,
liver and digestive
tract, gets infection
easily, and early
death unless treated.
2. Recessive Allele Disorders
Sickle Cell Disease
 Red blood cells
are bent and
twisted
 Get stuck in
capillaries 
damage tissues
 Results in
weakness, damage
to brain and heart
What is genotype?
What is phenotype?
3. Sex Link Traits:
Genes on the X and Y
chromosomes
Human Chromosomes: 2 Sex
Chromosomes
Human Chromosomes:
44 Autosomes
Gender

We determine the gender
of an individual through sex
chromosomes

Sex Chromosomes: X and Y
Female: XX
Male: XY

The father gives an X or Y
to the gametes. The mother
only gives an X to the
gamete
The X chromosome is
larger than the Y
chromosome.

What are sex-linked genes?
 genes found on a sex chromosome

X-linked genes are genes
found on the X
chromosome, symbolized
by Xr, XR,Y0.

Y-linked genes are found
on the Y chromosome,
symbolized by X0,YR,Yr
Sex-Linked Traits


It is possible for a
female to be a
carrier of an Xlinked trait, but
not express it
Men will express
all X-linked traits
they inherit
because they have
one X
chromosome
X-linked recessive, carrier mother
Unaffected
father
Carrier
mother
Unaffected
Affected
Carrier
Unaffected
son
Unaffected
daughter
Carrier
daughter
Affected
son
X-linked Genes

Let’s do a punnett square for a female carrier
of an x-linked gene and a recessive male

What will the genotypes be?
 XRXr and XRY
Here’s the results
For girls: 0% have
the trait
 For boys 50% have
it.
 Probability is higher
for boys because
whatever X they
get determines the
trait, for girls they
have to get 2
recessive X’s.

R
X
r
X
X R XR
XR
XR
Xr
Y 0 XR Y0 Xr
0
Y
Examples of X-linked traits:
1. Color Blindness
2. Hemophilia
3. Muscular Dystrophy
4. Icthyosis simplex (scaly skin)
Colorblindness

A person with normal color vision sees a number
seven in the circle above.

Those who are color blind usually do not see any
number at all.
Colorblindness

RED-GREEN
COLORBLINDNESS:

People with red-green color
blindness see either a three
or nothing at all.

Those with normal color
vision see an 8.
Hemophilia
Hemophilia- Lacking in
the ability to clot blood
◦ There is a gene on the
“X” chromosome that
control blood clotting
◦ People who have
hemophilia are missing
the protein to clot
blood
◦ They can bleed to
death by minor cut.
Muscular Dystrophy
- Results in
weakening/
loss of
muscles
- Caused by
defective
version of
gene that
codes for
muscle
Sex-influenced Traits

Sex-Influenced traits are those that are on
autosomes, but occur because of the sex
hormones in male and female bodies.

Examples: Facial hair
Baldness
4. Multiple Alleles
Multiple Alleles – any gene that has 3 or
more alleles (not just 1 dominant and 1
recessive)
Example: Blood type has 3 alleles:
IA= Type A blood (dominant)
IB= Type B blood (dominant)
i = Type O blood (recessive)
Check out the possible genotypes and
phenotypes of blood below:
Genotypes
Phenotypes
IAIA or IAi
Type A
IBIB or IBi
Type B
IA IB
Type AB
ii
Type O
Blood has both Multiple Alleles
and is CoDominant

If you have IAIB as your genotype, you
have both Type A and Type B blood,
also known as Type AB

If you have IAi, i is recessive
to IA, so you have type A blood
Q. When would you have Type O
blood?
 A. When you have ii as your genotype.

5. Polygenic Traits – traits controlled by 2 or
more genes that interact, forming the trait
Usually show a wide range of phenotypes
 Ex: Skin color, eye color, foot size, height

 Wide range of skin colors because there are more than 4 genes
that control this trait.

These may also be influenced by the environment, for
example height. If not given the proper nutrition as a child,
they might not be as tall as their genes dictate.