lec 3 Sex linkage_posted

lec 3 Sex linkage_posted - The chromosome theory of...

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The chromosome theory of inheritance - sex linkage Thomas Morgan Alfred Sturtevant Calvin Bridges Herman Muller In the early 1900s many researchers thought genes were in chromosomes, but proof was lacking. What was needed was: - a gene defined by a mutant allele - morphologically distinguishable chromosomes - association of gene and chromosome Drosophila melanogaster Easy to grow and maintain Short generation span numbers!
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Chromosomes in Drosophila II III IV X X II IV III X Y Sex chromosomes: X and Y Autosomes: II, III and IV XY Best understood example of sex-determining chromosomes is in humans: Region differential for X: contains X-specific genes Region differential for Y: contains degenerating remains of ancestral X chromosome. Contains few genes (primarily involved in male fertility) Pseudoautosomal region: contains same genes in both X and Y. In humans, it is essential for pairing of sex chromosomes in meiosis
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x All have red eyes Red-eyed female White-eyed male (rare finding) Cross males and females P F1 F2 X w Y X w+ X w X w+ Y X w+ X w+ X w+ X w+ X w+ X w X w+ Y X w Y X w Y X w+ X w X w+ X w X w+ Y X w+ Y X w+ X w+ When only one allele of a gene is present, as in the X in males or over deleted genomic regions, individuals are said to be hemizygous X w+ Y X w+ X w+ X w+ X w X w Y X w+ Y X w+ X w All females have red eyes, 1/2 males have white eyes
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x All females have red eyes F2 F3 F4 X w Y X w+ X w X w+ X w X w Y X w Y X w+ X w X w+ X w X w+ X w X w Y X w Y X w+ Y X w X w X w+ X w X w X w X w+ Y X w Y X w+ X w X w X w X w Y X w+ Y X w Y X w+ X w All males have white eyes 1/2 females have red eyes, 1/2 males have white eyes What phenotypic ratios are obtained from this F3 cross?
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x All females have red eyes X w+ X w X w Y X w+ X w X w Y X w+ Y X w X w X w X w+ Y All males have white eyes On rare occasion (1/2000) exceptional progeny were observed: X w X w 0 X w+ X w X w X w X w Y X w+ Y (sick) (lethal) Normal gametes exceptional In Drosophila: Two doses of X = female (XX, XXY) One dose of X = male (XY, X) (Y is required for fertility in the male) (XXX is subviable due to genetic imbalance) This genetic model was confirmed through cytology Full correlation between two rare types of events (gene transmission and chromosomal transmission) strong proof of chromosome theory Calvin Bridges (ca. 1915) through non-disjunction
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autosome X X X Y autosome Sex determination : pathway leading to male or female sexual forms Dosage compensation : pathway required to control in different sexes the relative level of expression of genes in sex chromosomes with respect to that in autosomal genes Note that in XX individuals the sex chromosome:autosome (X:A) ratio is 1 (two alleles for genes in the X to two alleles for genes in autosomes) but in XY individuals the sex chromosome:autosome ratio nears 0.5 (one allele for genes in the X to two alleles for genes in autosomes) i.e. without dosage compensations, XY cells would make 1/2 less product for genes in X (relative to product from genes in autosomes) as XX cells) Therefore, mechanisms must balance, or compensate, the level of expression of sex chromosome genes in different sexes.
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lec 3 Sex linkage_posted - The chromosome theory of...

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