Disorders & Sex Linked Traits

Genetic Disorders
Sex Linked Traits
Honors Biology
Unit 5
Recessive Disorders
 Disorders that are only expressed in
the phenotype when 2 recessive alleles
are present.
 DD = Normal
 Dd = Carrier
 dd = Affected by disorder
2 Examples
of Recessive Disorders
Tay-Sachs Disease
 A genetically caused disease in which the
gene to make the enzyme Hex-A
(Hexosaminidase A) is not working.
 Hex-A is an enzyme that breaks down the
lipid GM2 ganglioside.
 Without Hex-A this lipid accumulates on
nerve cells, specifically in the brain causing
severe brain damage.
 Victims of this disease to not live past age 5
Common in Eastern European Ashkenazi Jews
 This is a group of
people descendent of
medieval Jews from
the Rhineland area.
(Rhineland: near the
river Rhine in
 Common in this
population (1 in 30)
Tay-Sachs Brain
How is Tay-Sachs disease passed?
Each parent must be a Carrier
25% Normal
50% Carriers
25% Tay-Sachs
Cystic Fibrosis
 Thick mucus is produced by the body
 Mucus fills lungs causing lung infections
 Mucus blocks pancreas which causes digestive
 Mucus can block bile ducts in liver causing liver
Cystic Fibrosis
Cystic Fibrosis
Most common in Caucasian
(white) populations
(1 in 2500 to 3500)
1 in 17,000 African Americans
1 in 31,000 Asian Americans
Carriers of Cystic Fibrosis
25% Normal
50% Carriers
25% Cystic Fibrosis
Dominant Disorders
Disorders that are expressed in the
phenotype of heterozygous and
homozygous dominant individuals
 DD = Affected
 Dd = Affected
 dd = Normal
2 Examples
1) Huntington’s Disease
 Brain cells degenerate over time
 Mood swings, loss of muscle control, loss of memory and
inability to learn, death.
 Usually adult-onset, appears around ages 40-50
 Outlook is 10-15 years of survivablity
 hh = Normal
 HH or Hh = will get and die of this
Person with Huntington’s and a person
without Huntington’s
Hh x hh
50% Huntington’s
50% Normal
Huntington’s is most common in certain parts of
Venezuela (700 in 100,000)
Generally affect 3-7 in 100,000 of European ancestry
Less common in African-American & Asian American
2) Marfan’s Syndrome
 Defective gene for fibrillin-1 that results in
abnormal connective tissue
 Aorta may stretch or become weak, causing aortic
rupture, the leading cause of death
 Eye/lens problems
 Excessive long bone growth (long arms & fingers)
 Hypermobile joints (too flexible)
Chromosome Structure
 Chromosomes can be broken by X-rays and by certain
chemicals. The broken ends spontaneously rejoin, but if
there are multiple breaks, the ends join at random. This
leads to alterations in chromosome structure.
 Breaking the chromosome often means breaking a gene.
Since most genes are necessary for life, many
chromosome breaks are lethal or cause serious defects.
 Also, chromosomes with structural variations often have
trouble going through meiosis, giving embryos with
missing or extra large regions of the chromosomes.
Chromosome Structure
 The major categories:
duplication (an extra copy of a region of chromosome)
deletion (missing a region of chromosome)
inversion (part of the chromosome is inserted backwards)
translocation (two different chromosomes switch pieces).
Chromosome Variations
 Changes in number and structure are possible: first look at
number variations.
 Aneuploidy: having an extra or missing chromosome– is fairly
common in sperm and eggs. Errors in meiosis causes
chromosomes to not separate equally into the gametes.
The rate of aneuploidy
in males is constant:
1-2% of sperm have
an extra or missing
Chromosome Variations
 In females, the rate increases
with age. This is illustrated by
the frequency of Down
syndrome births at different
ages of mother. Down
syndrome is the most frequent
result of aneuploidy.
Chromosome Number Variations:
 Except for the X and Y, humans don’t survive with
only 1 copy of any chromosome. Also, 3 copies is
lethal in most cases. Aneuploidy is a major cause of
spontaneous abortion in early pregnancy.
 Down Syndrome is the most common human
aneuploidy. It is also called trisomy-21, meaning 3
copies of chromosome number 21.
*Having 3 chromosomes of each kind
instead of 2
*Normally trisomy results in death.
1 Example
Down Syndrome:
 a genetic condition in which the individual has 3
copies of the 21st chromosome.
 Genotype: 3 copies of 21st chromosome
Down Syndrome:
 Phenotype: People with Down’s have a characteristic appearance:
flattened face, turned up nose, epicanthal folds at the outer corners of
the eyes. In most cases the diagnosis is made immediately at birth.
Heart defects, protruding tongue, and mental retardation are also found
in most people with Down’s. Occurs about 1 in 1000 births.
Sex Linked Traits
Sex Linked Traits
 Traits that occur on the X or Y chromosome
 X-linked traits are traits found on the X Chromosome
Sex-linked Genes
 Genes on the X chromosome are called “sex-
linked”, because they expressed more often in
males than in females
 There are very few genes on the Y chromosome.
 Since males only have one X chromosome, all
genes on it, whether dominant or recessive, are
Sex-linked Genes
 In contrast, a mutant gene on an X chromosome in a
female is usually covered up by the normal allele on the
other X. Most mutations are recessive. So, most
people with sex-linked genetic conditions are
 Another fact about sex-linked genes. Males produce ½
their sperm with their X chromosome, and half with
their Y chromosome. The X-bearing sperm lead to
daughters and the Y-bearing sperm lead to sons. So, sons
get their only X from their mothers, and the father’s X
goes only to daughters.
 The Y chromosome is passed from father to son.
Why can females have 2 copies of the X
chromosome, when 2 copies of most
chromosomes is deadly?
 Answer:
In each cell one of the X chromosomes ‘turns off’.
This turned off chromosome is known as a Barr
The effect of Barr bodies can be seen in Calico
colored cats.
Why can females have 2 copies of the X chromosome, when 2
copies of most chromosomes is deadly?
 Example: Calico Colored cats.
A calico cat has patches of orange and patches of black
X = orange X1 = black
Males: XY = orange / X1Y = black
Females: XX = orange / X1X1 = black / XX1 = calico
Because individual cells determine which X chromosome
to deactivate, the patches of color arise
 We have 3 color receptors in the retinas of our eyes.
They respond best to red, green, and blue light.
 Each receptor is made by a gene. The blue receptor is
on an autosome, while the red and green receptors are
on the X chromosome (sex-linked).
 Most colorblind people are males, who have
mutated, inactive versions of either the red or
the green (sometimes both) color receptors.
Most females with a mutant receptor gene are
heterozygous: the normal version of the receptor
genes gives them normal color vision.
Colorblind Test!
 You will see circles with many colors of
 The dot pattern makes up a number
 What number do you see?
With Color Vision:
This one you can even see in black and
Color Blind Test
do you
Color Blind Test
do you
This what you would see if you were
color blind
do you
Color Blind Test
do you
Color Blind Test
do you
Color Blind Test
do you
Color Blind Test
do you
With color vision you see this:
But if you were red-green colorblind….
You would see the #:
What do the colorblind see?
Types of Colorblindness
Red Blind
Green Blind
Blue Blind
Types of Colorblindness –
Protanopia: no red
No color vision
Deuteranopia: no green
Tritanopia: no blue
How to write Alleles for X-Linked Traits
 Women:
Normal: XBXB
Carrier: XBXb
Colorblind: XbXb
 Men:
Normal: XBY
Colorblind: XbY
 Hemophilia is a disease in which the blood does not clot when
exposed to air. People with hemophilia can easily bleed to death
from very minor wounds. Hemophilia is another sex-linked
 Hemophilia is treated by injecting the proper clotting proteins,
isolated from the blood of normal people. In the early 1980’s,
the blood supply was contaminated by HIV, the AIDS virus, and
many hemophiliacs contracted AIDS at that time.
 Small cuts, scrapes and bruises can be life threatening
 1 in 10, 000 males
 1 in 100,000,000 females
Common amongst royalty in Europe
Queen Victoria
= Carrier
Turner Syndrome
 Also called XO, because people with
Turner’s have only 1 X chromosome
 No Y means Turner’s people are
female. However, no ovaries
develop, so they don’t undergo the
body changes of puberty and they
are sterile.
 Hormone treatment cures all but the
 Other symptoms: short stature,
webbed skin and low hairline at the
neck, some oddities of spatial
perception. Not retarded.
Klinefelter Syndrome
 Non-disjunction can also result in a
person with 2 X’s and a Y: XXY. This
is called Klinefelter Syndrome.
The Y chromosome makes a person
with Klinefelter’s male: possessing
Symptoms: female body hair pattern,
breast development, sterile, can be
some developmental delay or
retardation, especially for verbal
Often not diagnosed, or diagnosed
only accidentally.
Most symptoms are helped by
testosterone treatment.
Other Number Variations
 Triplo-X, having 3 X chromosomes. No Y chromosome
means female. Many with this syndrome are undiagnosed
because they have no symptoms. Some have slight social and
developmental problems, especially language-related.
Occasional fertility problems, but many have normal fertility.
Not well studied.
 XYY: having 2 Y chromosomes plus an X. Male because they
have a Y. Many are never diagnosed due to a lack of
symptoms. Tend to be taller, more physically active, slightly
retarded, prone to acne.
A few oddities
 It is possible to be XY and female. Two ways this can happen:
1. the SRY gene can be inactivated by a mutation. If SRY
doesn’t work, testes don’t develop and the embryo develops as
a normal female.
2. In a condition called “androgen insensitivity”, the person
is XY with a functional SRY gene, but her cells lack the
testosterone receptor protein, so the cells don’t ever get the
message that the testosterone is sending. Testes develop in the
abdominal cavity, and no ovaries, fallopian tubes, or uterus
develop. At puberty, the internal testes secrete testosterone,
which gets converted into estrogen and the body develops as a
normal (but sterile) adult female.
XY women show some “male-like”
features that make them good models
 Taller than most women
 Chiseled Jaw:
 Good Muscle definition:
 Don’t have as much body fat
Hermaphrodites ?!?
 Hermaphrodite: An individual that
has all female reproductive parts, and
all male reproductive parts
 No such thing in Humans
In some cases, androgen insensitivity is only partial: the cells
respond a little bit to testosterone produced by the testes. The
embryo develops with ambiguous genitalia, neither
completely male not completely female.
Another condition, congenital adrenal dysplasia, causes the
adrenal glands to produce an abnormally large amount of
testosterone in a female embryo, This can also cause
development of ambiguous genitalia.
 Another rare condition: a chimera occurs when two separate
embryos fuse together. This can result in a person with some
XX cells and some XY cells. This condition is extremely rare:
more people say they have it than actually do.
 2% of births
 Monozygotic (Identical) 30% of twins
A single zygote splits into two. This happens between 1 to 9 days after the zygote forms.
The twins share the same genome
 Dizygotic (Fraternal) 70% of twins
Two separate eggs are fertilized with two separate sperm. Two totally independent
zygotes are created.
The twins have different genomes
 Conjoined twins – very rare (1 in 200,000)
Identical twins who fail to completely separate after the 13th day after fertilization
This may be due to the fusion, or incomplete separation of zygotes
May be two fully formed individuals connected at various locations, or rarely, parasitic
twins, where one is much smaller and less formed, or even completely contained.
Sex-Influenced Traits
 Some traits appear to be specific to one
sex, but are not sex-linked: their genes
are not on the X chromosome. It is
 The best human example is male
pattern baldness.
 Baldness is dominant in males:
heterozygotes and homozygotes both
become bald. In females, baldness is
recessive: only homozygotes (which are
relatively rare) become bald. Also,
females tend to lose hair more evenly
than men, giving a sparse hair pattern
rather than completely baldness.

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