Sex Determination and Sex-linked Genes
Genes located on X or Y chromosomes
behave differently than autosomal genes.
Reciprocal crosses distinguish between
sex-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 OR
androgen 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, hemophilia
Duschene muscular dystrophy,
Lesch Nyhan syndrome, many others
Dominant X-linked traits:
Hypophosphatemic rickets - growth retardation,
bone disease, renal defects
Chondrodysplasia 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 affected
father will
inherit and
express the
trait.
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X-linked human genes - Examples
X-linked recessive
Hemophilia in descendents of
Queen Victoria - germline mutation
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Y-linked human genes - NCBI Map Viewer
Many in
Pseudoautosomal
Region 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 traits
F1 x F1 Crosses:
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Sex-Influenced Dominance
Horns in sheep
X
LILI X X
LRLR X Y
LILR X X
LILR X Y
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 with
XX and XY
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Nondisjunction Examples in Humans
Turner Syndrome (XO)
1 in 10,000 females born
90% die before birth
Short, weblike necks, barrel chest,
poor breast development, often infertile
less 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
orange/black
Sweat glands
absent
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