Sex-Linked Traits (11/6-11/8)

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Sex-Linked Characteristic

Traits that are determined by genes on the sex chromosomes

Thomas Hunt Morgan (Hint: 5)

Worked with Drosophilia melanogaster

Exhibits XX-XY sex determination

Discovers a single white-eyed male fly

He set up crosses with true breeding with wild type females (red eyes)

The unusual results revealed X-linked inheritance

X-Linked Inheritance (Hint: 3)

Genes on the X chromosome

Give different results from reciprocal crosses

Male always inherits their X from the maternal line

Y-Linked Inheritance (Hint: 3)

Genes on the Y chromosome

All male offspring express the genotype of their fathers

Females unaffected

Dosage Compensation (Hint: 2)

Equalization of the amount of X-linked genes in males and females

Different ways (Double X expression in males; Halve X expression in females – X inactivation)

Barr Body (Hint: 4)

One X chromosomes in females is completely inactivated in all cells

Coated with Xist RNA

That X chromosome is compact and total inactivated

Forms a barr body

Xist

RNA transcribed off the X that will be inactivated

Hemizygous

Only 1 copy of the genes are present for both the X and Y chromosomes

Turner Syndrome (Hint: 4)

XO females

Females with only 1 X chromosome

1/3000 female births

Characteristics (Short statue; low hairline; broad chest; folds of skin on neck; normal intelligence)

Klinefelter Syndrome (Hint: 3)

Males with extra X chromosomes (XXY, XXXY, XXYY, XXXXY)

1/1000 male births

Characteristics (small testes; reduced facial and pubic hair; taller than normal; normal intelligence)

Poly-X Syndrome (Hint: 5)

Females with more than 2 X chromosomes

1/1000 females

Tall and thin

Triple-X Syndrome (XXX; Usually of normal intelligence)

Poly-X Syndrome (4-5 Xs; More Xs  = more likely to be intellectually disabled with a number of physical problems)

How many Barr bodies would you find in the nuclei of individuals with each of these conditions? (Turner, Kleinfelter, and Poly-X)

Turner: 0 barr bodies

Klinefelter: XXY = 1 barr bodies

Poly-X: XXXX = 3 barr bodies

Why is Red-Green colorblindness an X-linked trait? (Hint: 3)

Color is perceived by light-sensitive cone cells that line the retina of the eye

Each cone cell contains 1 of the 3 pigments that absorb a particular wavelength of light

X⁺ is normal vision, X^c  is colorblind, and Y is the Y chromosome