-sex (gender) determination, sex-linked diseases, and non-disjunction -pioneer in the use of fruit...
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-Sex (gender) Determination,Sex-linked Diseases, and Non-disjunction
-pioneer in the use of fruit flies to study genetics and heredity
-worked with fruit flies (Drosophila melanogaster)
Dr. Thomas Hunt Morgan (1866-1945)
-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 thesame 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 mostlyin 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 hishypothesis.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
XR
XRXr
XRXr
XRY
XRY
2) Cross one of the F1 red-eyedfemales with a red-eyed maleResult: 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
Xr
XRXR
XRXr
XRY
XrY
He predicted that he could produce awhite-eyed female by crossing an F1
red-eye female with a white-eye male.
Parent Genotypes:XRXr and XrY
Xr Y
XR
Xr
XRXr
XrXr
XRY
XrY
Many sex-linked abnormalities arecaused by a recessive gene on the X chromosome.
Examples:Red-green color blindnessHemophilia
Nondisjunction-failure of chromosomes to segregateduring meiosis, resulting in abnormalchromosome numbers in future generations.
-Nondisjunction may happen to anychromosome, autosomal or not.
Nondisjunction in sex chromosomescan result in many genotypes, such as:
XYXXXXXYXXXY
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 symptomsappear vary WIDELY, and cannot be predicted from a karyotype.
Downes Syndrome
Turner Syndrome:-caused by a nondisjunction of sexchromosomes.
Symptoms: sterile, usually short, below average intelligence, usually fail to develop normal female characteristics.
-X genotype (therefore female)
Turner Syndrome
Kleinfelter’s Syndrome-caused by a nondisjunction of sexchromosomes.
Symptoms: sterile, usually very tall, below average intelligence
-XXY genotype (therefore male)
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, aconstant crying, resembling a cats “meow”
Usually results in death by 2-3 years of age.
Cri-Du-Chat Syndrome
-Affects mostly Jewish people.-caused by a homozygous recessivegene.
Symptoms: nervous system does notdevelop normally, causes inability to move. Death by age 2 or 3.
Tay-Sachs Disease
Phenylketonuria (PKU)
Symptoms: missing enzyme, can leadto severe mental retardation. This condition can be helped with a special diet (no phenylalanine)
-caused by a homozygous recessive gene.
Huntington’s Disease
Symptoms: deterioration of the brain,leading to memory loss, and lossof control of movement.
-due to a dominant gene, thereforea person heterozygous can have the condition.
Symptoms don’t usually show up until the mid-40’s.
Cystic Fibrosis
Symptoms: lining of lungs does notproduce fluid, causing particles tobe retained in lungs. Chronic cough,difficulty breathing. Death usuallyby 20 years of age.
-caused by a recessive gene on the 7th chromosome pair.
Methods of Disease Detection1) Amniocentesis: Fluid that surrounds the fetus is withdrawn with a needle, and analyzed for chromosomeabnormalities
Examples: Down Syndrome,Turner SyndromeKleinfelter Syndrome
Sex of fetus can also be determined
2) Ultrasound: High frequency soundwaves “echo” from the fetus, and givea picture.
EX: limb development, internal organs
-can be used to see any physical abnormalities.
3) Fetoscopy:fetus is viewed with a small camera called an endoscope.
Small samples of tissue or blood maybe taken, some surgical procedures are now performed before birth.
The endoscope is inserted through a small incision in the mothers abdomen.
EX: heart septum repairs, digestivetract repairs.