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8/26/10 Sex Linkage Reciprocal Cross • Organisms that have different sexes can be crossed in two different ways Phenotype A male X Phenotype B female Phenotype B male X Phenotype A female • Mendel concluded that reciprocal crosses are equal Exceptions • 1906 - Doncaster and G.H. Raynor – Working in magpie moths – Light winged females X dark winged males = all dark wings – Light winged males X dark winged females = dark winged males and light winged females • 1908 - W.M. Bateson – Working with chicken feather patterns – Barred male X nonbarred female = all barred – Nonbarred male X barred female = barred males and nonbarred females 1 8/26/10 Thomas Hunt Morgan - 1909 • Working with the fruit fly - Drosophila melanogaster • White-eyed male X red-eyed female – F1 all red-eyed offspring – F2 all red-eyed females and males are 1/2 white-eyed and 1/2 red eyed Heteromorphic Chromosomes • A chromosome pair in which the two members of the pair have some homology but differ in shape, size or staining properties 2 8/26/10 Terms • Homogametic - the sex that produces a single type of gamete • Heterogametic - the sex that produces two types of gametes Now remember: • 1906 - Doncaster and G.H. Raynor – Working in magpie moths – Light winged females X dark winged males = all dark wings – Light winged males X dark winged females = dark winged males and light winged females • 1908 - W.M. Bateson – Working with chicken feather patterns – Barred male X nonbarred female = all barred – Nonbarred male X barred female = barred males and nonbarred females 3 8/26/10 More Terms • Hemizygous - only one copy in a diploid organism • X linkage -genes located on X chromosome • Y linkage - genes located on Y chromosome • XY linkage - genes located in the common XY region Sex determination • Y chromosome mechanism - the Y chromosome actively determines sex • X chromosome - autosome balance mechanism -ratio between the number of X chromosomes and the number of sets of autosomes Turner Syndrome • Monosomic - Sex chromosome complement of 44 autosomes + 1 X chromosome • 1 in 5,000 female births 4 8/26/10 Turner Syndrome Kleinfelter Syndrome • Trisomic - 44 autosomes +XXY sex chromosomes Klinefelter Syndrome 5 8/26/10 X Chromosome Inactivation • One X chromosome condenses and becomes “inactive” in female cells early in development Barr body • Dosage Compensation Adjusts the dosage of Xlinked genes Lyon Hypothesis • Proposed by Mary Lyon and Lillian Russell -1961 • Barr body is highly condensed and mostly genetically inactive X chromosome is “lyonized” • Inactivation occurs on Day 16 of development • X chromosome to be inactivated is randomly chosen from the maternal and paternal X chromosome in an event that is independent from cell to cell. Mosaic • Females are a mixture of cells-some with one X chromosome inactivated and some with the other X chromosome inactivated • Forms sectors of phenotypically different cells. 6 8/26/10 Anhidrotic Ectodermal Dysplasia X-linked Inheritance • Dominant – Affected males pass the trait to all daughters and no sons – Affected heterozygous females pass the trait to 1/2 both sons and daughters 7 8/26/10 X-linked recessive • Males display trait more often than females • Offspring of affected males are not affected but his daughters are carriers • None of the sons of affected males display the trait nor do they pass it to their offspring Testicular feminization syndrome • X-linked recessive disorder – 1 in 65,000 male births • Affected males are phenotypically female with a chromosome complement of a male 8 8/26/10 Y-linked Inheritance • Traits transmitted from father to son • TDF - name given to hypothetical gene that encoded a hypothetical product called testisdetermining factor • Causes the gonadal primordia to differentiate into testes instead of ovaries • SRY - gene that has been isolated in humans that can induce testis determination Holandric traits • Traits associated with Y-chromosome “wholly male” traits Environmental Determination of Sex • A mechanism of sex determination that is not genetically determined at fertilization • Directed by environmental factors – related to association or lack of association with other members of species – temperature determination • turtle eggs incubated above 32 oC produce females, below 28 oC produce males 9 8/26/10 Sex-limited traits • Autosomal genes that determine a trait that appears in one sex but not the other – Hen feathering/cock feathering in birds – Milk yield in dairy cattle – Formation of breasts and ovaries in human females – Facial hair distribution – Ability to produce sperm and oocytes Sex-Influenced Traits • Controlled by autosomal genes • Traits appear in both sexes but the frequency of occurrence in the two sexes is different or relationship between genotype and phenotype is different – Pattern baldness - determined by alleles b and b+ • b/b is bald in both sexes • B/b in males = bald, females =nonbald 10