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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