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GENERAL GENETICS (PRACTICAL SESSION) BIO221 LECTURER ALSHEHRI, DOKHNAH SAEED [email protected] Alshehri, Dikhnah Saeed LINKAGE AND CROSSING OVER TH 8 Alshehri, Dikhnah Saeed LAB OUTLINES: • What is Linkage?. • Genes Linkage and Crossing Over. • Gene Mapping. • Some practical exercise And class activates. Alshehri, Dikhnah Saeed WHAT IS LINKAGE? • Linkage is defined genetically: the failure of two genes to assort independently. • Linkage occurs when two genes are close to each other on the same chromosome. • Linked genes are syntenic, but syntenic genes are not always linked. Genes far apart on the same chromosome assort independently: they are not linked. • Linkage is based on the frequency of crossing over between the two genes. Crossing over occurs in prophase of meiosis 1, where homologous chromosomes break at identical locations and rejoin with each other. Alshehri, Dikhnah Saeed Gene Linkage & Crossing Over • Genes that are found on the same chromosome are said to be LINKED genes • Linked genes are inherited TOGETHER. • Genes will not separate during meiosis… • Unless...Crossing over occurs Alshehri, Dikhnah Saeed RECOMBINATION PROCESS Alshehri, Dikhnah Saeed Alshehri, Dikhnah Saeed GENE MAPPING • • A CHROMOSOME MAP A LINEAR MAP OF GENES ON THE CHROMOSOME. IT CONTAINS: 1. THE ORDER IN WHICH SPECIFIC GENES OCCUR ON A CHROMOSOME 2. THE DISTANCES BETWEEN THE GENES A MAP IS DERIVED FROM CROSSOVER FREQUENCIES. Alshehri, Dikhnah Saeed MAPPING CHROMOSOMES • THE CLOSER GENES ARE TOGETHER ON A CHROMOSOME THE FEWER THE CROSSOVERS THAT OCCUR. • GENES FARTHER APART (MORE MAP UNITS APART) AFFECTED BY MORE CROSSOVERS (HIGHER CROSSOVER FREQUENCY) • FOR EG. CROSSOVER VALUE OF • 1% = 1 MAP UNIT APART - GENES ARE CLOSE TOGETHER; • 12% = 12 MAP UNITS APART - GENES ARE FURTHER APART Alshehri, Dikhnah Saeed Alshehri, Dikhnah Saeed EXERCISE 1 • . RESULTED FROM TESTCROSS AB/AB × AB/AB: 40% AB/AB 40% AB/AB 10% AB/AB 10% AB/AB • A-WHAT IS THE PERCENT OF THE CROSSING OVER? • B-WHAT IS THE PERCENT OF THE CHIASMA? • C-IS THE GENOTYPE AB/AB IN THE CASE OF ATTRACTION OR REPULSION? Alshehri, Dikhnah Saeed EXERCISE 2 • 5.FEMALE FROM F1HETEROZYGOUS WITH GRAY BODY AND LONG WING (DOMINANT) WERE MATED TO MALE WITH BLACK BODY AND REDUCTASE WING (RECESSIVE) • 23 GRAY BODY AND LONG WING • 127 GRAY BODY AND REDUCE WING • 124 BLACK BODY AND LONG WING • 26 BLACK BODY AND REDUCE WING • A-ARE THE GENES LINKED? • B-IF THE GENES ARE LINKED, WHAT IS THE PERCENT OF THE CROSSING OVER ? • C-DRAW THE MATING IN CHROMOSOMES? • Alshehri, D-IS Dikhnah THE Saeed MATING IN THE CASE OF ATTRACTION OR REPULSION? EXERCISE 3 • 1-THE RECESSIVE GENE (G,L) IN DROSOPHILA RESPONSIBLE FOR THE BLACK BODY COLOR AND REDUCE WING RESPECTIVELY. FEMALE WITH GRAY BODY AND LONG WING (G,L) WERE MATED TO MALES WITH BLACK BODY AND REDUCE WING. THE F1 WERE ALL GRAY BODY AND LONG WING. WHEN THE F1 FEMALES WERE TEST CROSSED WITH MALES HOMOZYGOUS THE FOLLOWING RESULT WERE OBTAINED • 664 GRAY BODY AND LONG WING • 888 BLACK BODY AND REDUCE WING • 134 BLACK BODY AND LONG WING • 122 GRAY BODY AND REDUCE WING • FIND THE FOLLOWING: • ARE THE GENES (GL,GL) LINKED? Dikhnah Saeed • IF Alshehri, THE GENE IS LINKED, CALCULATE THE DISTANCE BETWEEN THEM? EXERCISE 4 • . DRAW THE CHROMOSOME MAP, IF YOU KNOW THAT THE DISTANCE BETWEEN PAIRS OF GENES: • 16 S-C • 10 R-C • 6 S-R • 13 C-P • 3 P-R Alshehri, Dikhnah Saeed Alshehri, Dikhnah Saeed