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This presentation explores the concepts of dihybrid crosses, sex-linked traits, and mutations through the lens of Thomas Hunt Morgan's research on fruit flies. It covers the basics of dihybrid crosses, the determination of sex-linked traits via X and Y chromosomes, and the inheritance patterns of eye color in fruit flies, highlighting red and white eye variations. By employing Punnett squares and exploring linkage groups, we will also delve into the genetic mapping and implications of crossing-over during meiosis. Engage with key concepts and quizzes to test your understanding.
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Welcome What is a dihybrid cross?
Agenda • Quiz • Sex Linked Traits • Mutations
Sex Determination • Thomas Hunt Morgan – studied fruit flies in the early 1900’s
Sex Determination • Observed that one pair of chromosomes was different between males and females • Large one named “X” chromosome • Smaller one named “Y” chromosome • XX = female; XY = male
XY XX X Y X X X X XX Female XX Female X XY Male XY Male Y 50% Female; 50% Male
Sex Linkage • Sex Linkage: the presence of a gene on a sex chromosome (X or Y)
Sex Linkage • X-linked genes: genes found on the X chromosome • X chromosome carries more genes • Y-linked genes: genes found on the Y chromosome
Fruit Fly Eye Color • Fruit flies normally have red eyes • A few males have white eyes • Red is dominant; white is recessive
Morgan’s Fruit Fly Experiments • Red-eyed female (XRXR) x White-eyed male (XrY) XR XR XRXr XRXr RESULTS: F1 generation – all red-eyed Xr XRY XRY Y
Morgan’s Fruit Fly Experiments • Red-eyed female (XRXr) x Red-eyed male (XRY) XR Xr RESULTS: F2 generation – 3 red-eyed and 1 white-eyed ** all white-eyed where males…why? XRXR XRXr XR XRY XrY Y
Morgan’s Conclusions • Gene for eye color is carried on the X chromosome = eye color is an X-linked trait • Y chromosome does not carry a gene for eye color • Red-eyed = XRXR, XRXr , XRY • White-eyed = XrXr, XrY
In humans colorblindness (b) is an example of a sex-linked recessive trait. A male with colorblindness marries a female who is not colorblind but carries the (b) allele. Using a Punnett square, determine the genotypic and phenotypic probabilities for their potential offspring.
Welcome What is an X-Linked trait?
Agenda • Review sex linked traits • Mutations • Movie
In fruit flies red eye color (R) is dominant to white eyes (r). In a cross between two flies, 50% of the male and 50% of the female offspring had red eyes. The other half of the males and females had white eyes. What are the phenotype, and all possible genotypes, of the offspring?
Linkage Groups • 2 or more genes that are on the same chromosome are “linked” • Linked genes tend to be inherited together
More Fruit Fly Experiment • Gray, long-winged (GGLL) x black, short-winged (ggll) • F1 generation = all heterozygous gray, long-winged (GgLl)
Morgan’s Fruit Fly Experiment • Cross F1 flies: GgLl x GgLl • F2 generation • If alleles on different chromosomes, they assort independently and get a 9:3:3:1 ratio • If alleles on same chromosome, get 3 gray, long-winged: 1 black, short-winged ratio • Morgan saw roughly the 3:1 ratio
Morgan’s Fruit Fly Experiment • Unexpected results • Some gray, short-winged (Ggll) • Some black, long-winged (ggLl)
Morgan’s Fruit Fly Experiment • How were these alleles separated? • Alleles were rearranged through crossing-over during meiosis • Genes that are farther apart are more likely to be separated by crossovers
Chromosome Mapping • Chromosome Map: diagram that shows the possible genes on a chromosome • Made using crossing-over data
Chromosome Mapping • The percentage of crossing-over between the genes for 2 traits is equal to the distance between them on a chromosome • This distance is measured in map units
