-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 ( )

-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?

XrXr Y XRXR XRXR XRXrXRXr XRXrXRXr XRYXRY XRYXRY

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.

XRXR Y XRXR XrXr XRXRXRXR XRXrXRXr XRYXRY XrYXrY

He predicted that he could produce a white-eyed female by crossing an F1 red-eye female with a white-eye male. Parent Genotypes: X R X r and X r Y

XrXr Y XRXR XrXr XRXrXRXr XrXrXrXr XRYXRY XrYXrY

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.

Turner Syndrome: - caused by a nondisjunction of sex chromosomes. 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 sex chromosomes. 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, a constant 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 recessive gene. Symptoms: nervous system does not develop normally, causes inability to move. Death by age 2 or 3. Tay-Sachs Disease

Phenylketonuria (PKU) Symptoms: missing enzyme, can lead to 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 loss of control of movement. -due to a dominant gene, therefore a person heterozygous can have the condition. Symptoms don’t usually show up until the mid-40’s.

Cystic Fibrosis 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. -caused by a recessive gene on the 7th chromosome pair.

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. 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 may be 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, digestive tract repairs.