Transcript Chapter 12 sec. 12.1 Sex Linked Traits

Human Genetics:
Patterns of Inheritance for
Human 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
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 X-linked 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/l
oss 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.