LINKAGE AND CROSS OVER

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Polytene Chromosome

2

LINKAGE

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F

eE

cC

bBaA

f

4

F

eE

cC

f

B

A

5

Thomas Hunt Morgan

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7

Red - dominant (R)Purple - recessive (r)

Normal wings - dominant(N)Vestigial wing - recessive (n)

Eye colour

Type of wing

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9

Normal Vestigial

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RrNn x rrnn

RRNN X rrnn

RrNn

RN rn

Parent

Gamete

F1

Test cross

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Expected results were:25% Red Normal25% Red vestigial25% purple Normal25% purple vestigial

The actual results were: 47% Red Normal43% Purple vestigial5% Red vestigial5% purple normal

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Types of linkage

1.Complete linkage

Closely associated

Pass to 2/ more generations

2.Incomplete linkage

Widely located

High chance of separation by cross over

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CROSS OVER

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15

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Crossover is the process by which two chromosomes paired up duringprophase I of meiosis exchange a distal portion of their DNA

Crossover occurs when two chromosomes, normally two homologousinstances of the same chromosome, break and connect to each other'sends

If they break at the same locus, this merely results in an exchange ofgenes

This is the normal way in which crossover occurs

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If they break at different loci, the result is a duplication of genes on onechromosome and a deletion on the other

If they break on opposite sides of the centromere, this results in onechromosome being lost during cell division

Any pair of homologous chromosomes may be expected to cross over three orfour times during meiosis

This aids evolution by increasing independent assortment, and reducing thegenetic linkage between genes on the same chromosome

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Dihybrid cross – one point

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Trihybrid cross – 2 point

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Types of cross over product

1. Parental/ Non cross over type

Coupling phase

2. Recombinant/ Cross over type

Repulsion phase

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Genetic mapAny method used for determining the location of and relativedistances between genes on a chromosome

Map distance =Number of recombinant offspring

Total number of offspring× 100

Map unit – centiMorgan (cM)

1 Map unit = 1% recombination frequency

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Parental phenotypes – most frequently

Double cross over – least frequently

Single cross over – Intermediate frequently

Variation in the number of chromosomes

Variation in the arrangement of chromosomes

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Why it is not possible to use a homozygous dominant organism

(such as TT) in a test cross experiment to determine the

genotype of an organism showing the dominant phenotype.

Illustrate your answer with fully using appropriate genetic

symbol?

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In maize the following cross between pure breeding plants were made,coloured shrunken starchy × colourless full waxy. The F1 plants were allfound to be coloured full starchy. The F1 plants were then test crossedand the following progenies were obtained. Explain the results andobtain the necessary information regarding the inheritance of thesecharacters.

Coloured shrunken starchy 2538

Colourless full waxy 2708

Coloured full waxy 116

Colurless shrunken starchy 113

Colured shrunken waxy 601

Colourless full starchy 626

Colured full starchy 4

Colourless shrunken waxy 2