t.h. morgan 1910 working with fruit fly, drosophila melanogaster discovered a mutant male fly with...
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Sex Chromosomes Sex determination in Drosophila is based on the number of X chromosomes –2 X chromosomes = female –1 X and 1 Y chromosome = male Sex determination in humans is based on the presence of a Y chromosome –2 X chromosomes = female –Having a Y chromosome (XY) = maleTRANSCRIPT
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• T.H. Morgan – 1910– Working with fruit fly, Drosophila melanogaster– Discovered a mutant male fly with white eyes instead
of red– Crossed the mutant male to a normal red-eyed female
• All F1 progeny red eyed = dominant trait
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• Morgan crossed F1 females x F1 males
• F2 generation contained red and white- eyed flies– But all white-eyed flies were male
• Testcross of a F1 female with a white-eyed male showed the viability of white-eyed females
• Morgan concluded that the eye color gene resides on the X chromosome
• Morgan crossed F1 females x F1 males
• F2 generation contained red and white- eyed flies– But all white-eyed flies were male
• Testcross of a F1 female with a white-eyed male showed the viability of white-eyed females
• Morgan concluded that the eye color gene resides on the X chromosome
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Sex Chromosomes
• Sex determination in Drosophila is based on the number of X chromosomes– 2 X chromosomes = female– 1 X and 1 Y chromosome = male
• Sex determination in humans is based on the presence of a Y chromosome– 2 X chromosomes = female– Having a Y chromosome (XY) = male
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Sex-Linked Inherited Traits
• In many organisms, the Y chromosome is greatly reduced or inactive.
• genes on the X chromosome are present in only 1 copy in males
• Sex-linked traits: controlled by genes present on the X chromosome
• Certain genetic diseases affect males to a greater degree than females– X-linked recessive alleles
• Red-green color blindness• Hemophilia
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• Humans have 46 total chromosomes– 22 pairs are autosomes– 1 pair of sex chromosomes– Y chromosome highly condensed
• Recessive alleles on male’s X have no active counterpart on Y
– “Default” for humans is female• Requires SRY gene on Y for “maleness”
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Hemophilia
• Disease that affects a single protein in a cascade of proteins involved in the formation of blood clots
• Form of hemophilia is caused by an X-linked recessive allele– heterozygous females are asymptomatic carriers
• Allele for hemophilia was introduced into a number of different European royal families by Queen Victoria of England
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Dosage Compensation
• Ensures an equal expression of genes from the sex chromosomes even though females have 2 X chromosomes and males have only 1
• In each cell of a female, 1 X chromosome is inactivated and is highly condensed into a Barr body– the other X chromosome provides phenotype
• Females heterozygous for genes on the X chromosome are genetic mosaics
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Genetic Mapping
• Early geneticists realized that they could obtain information about the distance between genes on a chromosome
• Based on genetic recombination (crossing over) between genes
• If crossover occurs, parental alleles are recombined producing recombinant gametes
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• Alfred Sturtevant– Undergraduate in T.H. Morgan’s lab– Put Morgan’s observation that recombinant progeny
reflected relevant location of genes in quantitative terms
– As physical distance on a chromosome increases, so does the probability of recombination (crossover) occurring between the gene loci
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Constructing Maps
• The distance between genes is proportional to the frequency of recombination events
recombination frequency recombinant progenytotal progeny
• 1% recombination = 1 map unit (m.u.)• 1 map unit = 1 centimorgan (cM)
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