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Sex Determination and Sex-linked Genes

Genes located on X or Y chromosomesbehave differently than autosomal genes.

Reciprocal crosses distinguish betweensex-linked and autosomal genes.

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Sexual Differences - Human Chromosomes

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Sexual Differences - Drosophila Chromosomes

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Sexual Determination in Drosophila

Bridges - Dosage Effect

X:A ratio >1.0 = female <0.5 = male Y sperm motility

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Sexual Determination in Humans

Y determines maleness - SRY (sex determining region) TDF (testis determining factor)

XY females - non functional TDF ORandrogen insensitivity (AR defect)

CAIS, PAIS

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Sexual Determination in Various Species

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Sexual Determination in Various Species

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Sexual Determination in Various Species

Reptiles - Sex depends on temperature during development.

Caribbean bluehead wrasse -

Dominant supermale replaced by male or female

Brain chemical - arginine vasotocin (AVT)

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X-linked human genes - NCBI Map Viewer

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X-linked human genes - Examples

Many housekeeping (not sex related) genes

Recessive X-linked traits:Color blindness, hemophiliaDuschene muscular dystrophy,Lesch Nyhan syndrome, many others

Dominant X-linked traits:Hypophosphatemic rickets - growth retardation,

bone disease, renal defectsChondrodysplasia punctata - hydroxysteroid isomerase

skin and skeletal manifestations

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X-linked human genes - Examples

Generalized Hypertrichosis - X-linked dominant

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X-linked human genes - Examples

X-linked dominant

faulty enamel

All daughters with affectedfather will inherit andexpress thetrait.

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X-linked human genes - Examples

X-linked recessive

Hemophilia in descendents ofQueen Victoria - germline mutation

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Y-linked human genes - NCBI Map Viewer

Many in PseudoautosomalRegion - nonfunctional

SHOX - short stature

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Y-linked human genes - Examples

SRY, Y-blood type antigen, Hairy Pinnae?

Anthony Victor Guinness record

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Inheritance of X-linked traits

Crisscross pattern of inheritance: Son inherits only X from mother (hemizygous)Daughter inherits one X from each parent

(homozygous, heterozygous)

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Inheritance of X-linked traits

Reciprocal Crosses: Sons inherit and express trait from mother

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Inheritance of X-linked traitsF1 x F1 Crosses:

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Sex-Influenced Dominance

Horns in sheep

LILI X X LRLR X Y

LILR X X LILR X Y

X

F2 Ratio: 1 LILI : 2 LILR : 1 LRLR

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Sex-Limited Dominance

Milk production in cattle

Beard attributes in humans

Feather colors in birds

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Nondisjunction of Sex Chromosomes during Meiosis

Failure to Segregate - Aneuploidy in offspring

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Nondisjunction Example in Drosophila

XXX and Y die

XO male, XXY female

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Nondisjunction Example in Drosophila

Gametes withXX and XY

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Nondisjunction Examples in Humans

Turner Syndrome (XO)

1 in 10,000 females born90% die before birthShort, weblike necks, barrel chest, poor breast development, often infertileless ability spacial relationships

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Nondisjunction Examples in Humans

Klinefelter Syndrome (XXY) Also XXXY, XXYY, etc.

1 in 1,000 males born

Underdeveloped testes, taller than average,50% some breast development, some subnormal intelligence

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Nondisjunction Examples in Humans

XYY Syndrome

1 in 1,000 born

Often taller than average

Some reduced fertility

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Dosage Compensation in Mammals

X-linked genes - Most needed for both sexes

X inactivation - Barr body - Random in each cell of embryo 16 day, 500-1000 cells

normal female XX normal male XY

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Dosage Compensation in Mammals

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Dosage Compensation in Mammals

Heterozygous XAXa females - Mosaic

Calico Cat Sweat glands orange/black absent