-Sex (gender) Determination Dr. Thomas Hunt Morgan ( ) -worked with fruit flies (Drosophila melanogaster) -pioneer in the use of fruit flies to study genetics and heredity

Normal Fly White Eyes Short Wings Curly WingsYellow Body Ebony Body Fruit Fly (Drosophila melanogaster) Phenotypes

-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. X R X r x 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 (not viable) XXX (“super females”) XXY XXXY (not viable)

The following diseases are caused by a nondisjunction of the chromosomes.

Down’s Syndrome: -caused by a nondisjunction in the 21st pair of chromosomes. -symptoms include mental retardation, abnormal facial traits, short arms and legs, many internal defects.

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.

Klinefelter’s Syndrome -Individuals have an XXY genotype, so therefore are male. -Sterile, usually fail to develop normal male sex characteristics. -Very long arms and legs -Below average intelligence

Klinefelter’s Syndrome

Affected individuals rarely live past infancy because of the life-threatening medical problems associated with this condition. -skeletal abnormalities, including polydactyly and syndactyly -weak muscle tone (hypotonia) -a cleft lip -an opening in the roof of the mouth (a cleft palate) These abnormalities include: small eyes -a chromosomal condition that is associated with severe cognitive disability and certain physical abnormalities. Trisomy 13 (Patau Syndrome)

Patau Syndrome

Polydactyly

Syndactyly

Most cases of trisomy 13 result when each cell in the body has three copies of chromosome 13 instead of the usual two copies. A small percentage of cases occur when only some of the body's cells have an extra copy of chromosome 13, resulting in a mixed population of cells with a differing number of chromosomes. Such cases are sometimes called mosaic trisomy 13. Mosaic trisomy 13 results from a non-disjunction in a somatic cell during early development.

A young boy (7) suffering from Patau Syndrome. He is deaf, and legally blind.

The following diseases are not caused by a nondisjunction in any chromosomes.

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.

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) NO!- Diet sodas, nutra-sweet

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.

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.

Sickle-Cell Anemia -This disease affects mostly African Americans. -Homozygous recessive disease. -Causes irregular shaped red blood cells,hemoglobin that does not hold oxygen well. -Clotting due to cell shape, anemia due to low affinity of hemoglobin for oxygen.

In equatorial Africa, where the disease originated, it is actually beneficial to have sickle cell, because it protects you from having malaria, which is far more deadly.

Methods of Disease Detection 1) Amniocentesis: Fluid that surrounds the fetus is withdrawn with a needle, and analyzed for chromosome abnormalities Examples: Downs Syndrome,Turner Syndrome Klinefelter Syndrome

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 DNA needs to be in the form of chromosomes to be seen clearly. Sex of fetus can also be determined.

3) Fetoscopy:fetus is viewed with a small camera called an endoscope. The endoscope is inserted through a small incision in the mothers abdomen.

EX: heart septum repairs, digestive tract repairs. Small samples of tissue or blood may be taken, some surgical procedures are now performed.