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 X Y Female XX Female XY Male XY Male 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 (X R X R) x White-eyed male (X r Y) XRXR XRXR XrXr Y XRXrXRXr XRXrXRXr XRYXRYXRYXRY RESULTS: F 1 generation – all red-eyed

Morgan’s Fruit Fly Experiments Red-eyed female (X R X r) x Red-eyed male (X R Y) XRXR XrXr XRXR Y XRXRXRXR XRXrXRXr XRYXRYXrYXrY RESULTS: F 2 generation – 3 red-eyed and 1 white-eyed ** all white-eyed where males…why?

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 = X R X R, X R X r, X R Y White-eyed = X r X r, X r Y

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.

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?

Worksheet

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) F 1 generation = all heterozygous gray, long-winged (GgLl)

Morgan’s Fruit Fly Experiment Cross F 1 flies: GgLl x GgLl F 2 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

MUTATIONS

Mutations Mutation: change in DNA VIDEO

Mutations Germ-cell mutation: occurs in sex cells –Affect the offspring –Example – Down Syndrome Somatic mutation: occurs in body cells –Affect the individual –Example - Cancer

Possible Effects of Mutations Lethal mutations: cause death, often before birth Beneficial mutations: provide variation needed for evolution No effect

Chromosome Mutations Chromosome Mutations: a change in the chromosome structure or loss/addition of entire chromosome

Chromosome Mutations Deletion: loss of piece of chromosome due to breakage

Chromosome Mutations Deletion: loss of piece of chromosome due to breakage –Example – Cri du chat; deletion of part of chromosome 5

Chromosome Mutations Duplication: part of chromosome is duplicated/copied

Chromosome Mutations Duplication: part of chromosome is duplicated/copied –Example – Charcot-Marie Tooth Disease

Chromosome Mutations Inversion: piece breaks off and reattaches to SAME chromosome in wrong order

Chromosome Mutations Inversion: piece breaks off and reattaches to SAME chromosome in wrong order –Example – Hemophilia (this disorder is also X linked)

Chromosome Mutations Translocation: piece breaks off and reattaches to DIFFERENT chromosome

Chromosome Mutations Translocation: piece breaks off and reattaches to DIFFERENT chromosome –Example – Cancer

Chromosome Mutations Nondisjunction: chromosomes do not separate during meiosis

Chromosome Mutations Nondisjunction: chromosomes do not separate during meiosis –Example – trisomy, monosomy

Gene Mutations Gene Mutations: involve large segments of DNA (genes) or a single nucleotide –Causes possible changes in the amino acid sequence

Gene Mutations Point Mutation: substitution, addition, or deletion of a single nucleotide

Gene Mutations Point Mutation: substitution, addition, or deletion of a single nucleotide –Example – sickle cell anemia

Gene Mutations Frame Shift Mutation: occurs when the number of nucleotides inserted or deleted is not a multiple of 3