-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.