Transcript Sex Genes - mcdowellscience

-Sex (gender) Determination,
Sex-linked Diseases, and
Non-disjunction
Dr. Thomas Hunt Morgan
(1866-1945)
-worked with fruit flies
(Drosophila melanogaster)
-pioneer in the use of fruit flies
to study genetics and heredity
-Early in his work, he viewed
karyotypes of fruit fly chromosomes
and noticed that male and female
flies had slightly different
chromosomes.
Fruit flies have 4 pairs of
chromosomes, so the diploid
number is 8.
The Fruit Fly Karyotype
3 of the pairs are homologous, and the
same in male and female.
The 4th pair is different in male and
female.
The 3 pairs that are the same are called
autosomal, and are not involved in
gender determination.
Sex-Linked Traits
-In 1910, Dr. Morgan found that the
trait of white eyes was found mostly
in males (but does happen in females).
-He hypothesized that the recessive
gene for eye color was on the
X chromosome, and that the trait
was sex-linked.
He carried out 2 crosses to test his
hypothesis.
1) Cross a white-eyed male with a
red-eyed female.
Result: 1/2 red eyed females
1/2 red eyed males
So what will the
genotypes of
the parents look like?
Xr
Y
XR
XRXr XRY
XR
XRXr XRY
2) Cross one of the F1 red-eyed
females with a red-eyed male
Result: 1/2 red-eyed females
1/4 red-eyed males
1/4 white-eyed males
Start with the genotypes
of the parents.
XR
Y
XR
XRXR XRY
Xr
XRXr X r Y
He predicted that he could produce a
white-eyed female by crossing an F1
red-eye female with a white-eye male.
Parent Genotypes:
XRXr and XrY
Xr
Y
XR
XRXr XRY
Xr
XrXr
X rY
Many sex-linked abnormalities are
caused by a recessive gene on
the X chromosome.
Examples:
Red-green color blindness
Hemophilia
Nondisjunction
-failure of chromosomes to segregate
during meiosis, resulting in abnormal
chromosome numbers in future
generations.
-Nondisjunction may happen to any
chromosome, autosomal or not.
Nondisjunction in sex chromosomes
can result in many genotypes, such
as:
X
Y
XXX
XXY
XXXY
Downe’s Syndrome:
-caused by a nondisjunction in the
21st pair of chromosomes.
-symptoms include mental differences,
different facial traits, shorter arms and
legs, sometimes internal defects.
The degree with which Downe’s symptoms
appear vary WIDELY, and cannot be
predicted from a karyotype.
Downes Syndrome
Turner Syndrome:
-caused by a nondisjunction of sex
chromosomes.
-X genotype (therefore female)
Symptoms: sterile, usually short,
below average intelligence, usually
fail to develop normal female
characteristics.
Turner Syndrome
Kleinfelter’s Syndrome
-caused by a nondisjunction of sex
chromosomes.
-XXY genotype (therefore male)
Symptoms: sterile, usually very tall,
below average intelligence
Kleinfelter’s Syndrome
Cri-Du-Chat Syndrome
NOT caused by a non-disjunction, but is a
chromosomal disease.
Part of Chromosome #5 is missing (a small
part of one end of the chromosome)
Reduced intelligence, neural problems, a
constant crying, resembling a cats “meow”
Usually results in death by 2-3 years of age.
Cri-Du-Chat Syndrome
Tay-Sachs Disease
-Affects mostly Jewish people.
-caused by a homozygous recessive
gene.
Symptoms: nervous system does not
develop normally, causes inability
to move. Death by age 2 or 3.
Phenylketonuria (PKU)
-caused by a homozygous recessive
gene.
Symptoms: missing enzyme, can lead
to severe mental retardation.
This condition can be helped with a
special diet (no phenylalanine)
Huntington’s Disease
-due to a dominant gene, therefore
a person heterozygous can have the
condition.
Symptoms: deterioration of the brain,
leading to memory loss, and loss
of control of movement.
Symptoms don’t usually show up until
the mid-40’s.
Cystic Fibrosis
-caused by a recessive gene on the
7th chromosome pair.
Symptoms: lining of lungs does not
produce fluid, causing particles to
be retained in lungs. Chronic cough,
difficulty breathing. Death usually
by 20 years of age.
Methods of Disease Detection
1) Amniocentesis: Fluid that
surrounds the fetus is withdrawn with
a needle, and analyzed for chromosome
abnormalities
Examples:
Down Syndrome,Turner Syndrome
Kleinfelter Syndrome
Sex of fetus can also be determined
2) Ultrasound: High frequency sound
waves “echo” from the fetus, and give
a picture.
-can be used to see any physical
abnormalities.
EX: limb development, internal organs
3) Fetoscopy:fetus is viewed with a
small camera called an endoscope.
The endoscope is inserted through a
small incision in the mothers
abdomen.
Small samples of tissue or blood may
be taken, some surgical procedures
are now performed before birth.
EX: heart septum repairs, digestive
tract repairs.