Gene Mapping Linked Unlinked Genes Packet 40 Chapter

Gene Mapping, Linked & Unlinked Genes Packet #40 Chapter #15 Friday, November 27, 2020 1 S

Linked Genes vs. Unlinked Genes Friday, November 27, 2020 2 S

Linkage S Each chromosome behaves genetically as if it consisted of genes arranged in a linear order S Linkage is the tendency for a group of genes, on the same chromosome, to be inherited together via crossing over Friday, November 27, 2020 3

Linkage II S However, for linkage to occur, the genes, located on the same chromosome, must be close enough to each other. S If the genes are close enough to each other, and have a high probability of crossing over together, then they are considered to be linked genes. Friday, November 27, 2020 4

Linkage III S If genes are linked together, then Mendel’s law of independent assortment does not apply. S Genes, in this case, are not passed on independent of another gene. S However, linked genes may recombine during crossing over. Friday, November 27, 2020 5

Determining Whether Genes are Linked or Unlinked Friday, November 27, 2020 6 S

Recombination of Genes S Recombination of unlinked genes is achieved via independent assortment of chromosomes S Mendel’s Law S Recombination of linked genes is only possible during crossing over. Friday, November 27, 2020 7

Two-point Cross I S The two-point test cross allows scientists to determine whether genes are linked or unlinked. S In order to perform a two-point test cross one parent must be heterozygous while the other must be homozygous recessive. Friday, November 27, 2020 8

Two-point Cross II S If the majority of the offspring have a genotype similar to one of the parents, then the genes are linked. S If the majority of the offspring have a recombinant genotype, then the genes are unlinked. Friday, November 27, 2020 9

Two Point Cross Example S Parent #1 S Bb. Vv S Grey with normal wings S Parent #2 S bbvv S Black with vestigial wings Friday, November 27, 2020 10

Two-Point Cross BV bv Bv b. V S bv Bb. Vv bbvv Bbvv bb. Vv Expec 575 ted Resul ts 575 575 Actua 965 l Resul ts 944 206 185 Friday, November 27, 2020 11 Calculations S Parental Genotypes S 965 (42%) +944 (41%) = 1909 S 1909/2300 = 83% S Recombinant Genotypes S 206 (9%)+185 (8%) = 391 S 391/2300 = 17% S If independent assortment was to occur, the percentages would be 25% a piece. S Based on the data, the recombinants arose because of crossing over

Gene Mapping Friday, November 27, 2020 12 S

Gene Mapping S By measuring the frequency of recombination between linked genes, it is possible to construct a linkage map of a chromosome S Scientists were able to develop a detailed genetic map of Neurospora (fungus), fruit fly, the mouse, yeast and many plants that are particularly important as crops via gene mapping. Friday, November 27, 2020 13

Gene Mapping II S The recombinant frequencies (percentages) are used as units that separate two alleles. S Recombinant = 58% S The genes are 58 units apart. Friday, November 27, 2020 14

Gene Mapping III Example S Create a map using the information provided below. S A & D = 2 units S B & D = 10 units S C & B = 3 units S C & A = 5 units Friday, November 27, 2020 15

Sex-Linked Genes Friday, November 27, 2020 16 S

Introduction I S Sex is determined by sex chromosomes S X and Y S XX = female S XY = male Friday, November 27, 2020 17

The X Chromosome S The X chromosome contains many important genes that are unrelated to sex determination S These genes are required for both males and females S A male receives ALL of his X-linked genes from his mother while a female receives her X-linked genes from both parents. Friday, November 27, 2020 18

Dosage Compensation S Female mammals display dosage compensation. S Only one of the two chromosomes is expressed in each cell S The other allele is inactive S Seen as a dark-staining Barr body at the edge of the interphase nucleus. S Equalizes the expression of xlinked genes for both genders. Friday, November 27, 2020 19

Dosage Compensation II S A random event that occurs in each somatic cells S A female that is heterozygous expresses one of the alleles in about half her cells and the other allele in the other half Friday, November 27, 2020 20

Practice Homework Assignments Friday, November 27, 2020 21 S

Practice Assignment #1 S Determine if the genes are bv Bv b. V Bb. Vv bbvv Bbvv bb. Vv Expect 300 ed Result s 300 300 Actual 310 Result s 315 287 288 bv linked together using the information provided. Friday, November 27, 2020 BV 22

Practice Assignment #2 S Determine if the genes are bv Bv b. V Bb. Vv bbvv Bbvv bb. Vv Expect 295 ed Result s 295 295 Actual 360 Result s 380 230 bv linked together using the information provided. Friday, November 27, 2020 BV 23