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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