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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 • Morgan crossed F1 females x F1 males • F2 generation contained red and white- eyed flies – But all white-eyed flies were male • Testcross Testcross of of aa F F11 female female with with aa white-eyed white-eyed male male showed the viability of white-eyed females • Morgan concluded that the eye color gene resides on the X chromosome 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 4 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 • 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” 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 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 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 • 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 Constructing Maps • The distance between genes is proportional to the frequency of recombination events recombination frequency = recombinant progeny total progeny • 1% recombination = 1 map unit (m.u.) • 1 map unit = 1 centimorgan (cM)