lecture 001, 204

8/13/2019 Lecture 001, 204

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Chromosomal Basisof Inheritence

The chromosome theory

of heredity

8/13/2019 Lecture 001, 204


Although it was suspected that genes were situated

in chromosomes, no definite proof was there yet.

Thomas H. Morgan, discovered a particular eye colourmutation in the fruit fly Drosophila melanogaster .

Advantages of working with this fly were:

Quick reproduction,

inexpensive to rear in laboratory and

it had only 4 pairs of chromosomes.

One pair being the sex chromosomes, were

morphologically distinguishable from each other and

the autosomes.

Morgan was able to show that eye colour mutation of

fruit fly was inherited along with the X chromosome.

This suggested that a gene for eye colour was

physically situated on that chromosome.

This theory was later definitely proved by one of hisstudents named Calvin B. Bridges.

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Experimental Evidence Linking the Inheritence of

genes to chromosomes

Mutant Male flywith white eyesX

Wild type femalewith Red eyes

All progeny had Red eye colour

Progeny (F1) Malefly with Red eyes

Progeny (F1) femalefly with Red eyes

X

All (100%) the daughters had Red eyes

Only half (50%) the sons had Red eyes

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XXX Y

W+W+ W

X

Red Eyed Female White Eyed male

P

Red Eyed FemaleRed Eyed male

W+W+ W

XF1

W+W+ W+W+WW

Red Eyed FemaleRed Eyed Female Red Eyed male White Eyed male

F2

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X

Red Eyed Female White Eyed male

W+W+ W

F1

W+W+ W+W+WW

White Eyed FemaleRed Eyed Female Red Eyed male White Eyed male

F2

Cross between a heterozygous female and ahemizygous mutant male

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XXX Y

W+W+ W

X

White Eyed Female Red Eyed male

P

Red Eyed FemaleWhite Eyed male

W+W+ W

XF1

W+W+ W+W+WW

Red Eyed FemaleWhite Eyed Female White Eyed male Red Eyed male

F2

Cross between a homozygous mutant female and ahemizygous wild-type male

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Morgan and his students soon identified other X-linked

genes in Drosophila.

Simple breeding experiments demonstrated that recessivemutations of these genes were transmitted along with the X

chromosome.

They found out that many genes were located on the

X-chromosome.

They also found genes not on the X-chromosome. These

genes followed the Mendelian Principle and did not segregate

with sex (unlike gene of the eye colour).

Morgan correctly concluded : Such genes were located on one

of the three autosomes in the Drosophila genome. Thus each

Drosophila chromosome appeared to contain a different set of

genes.

Morgan’s team experimented with relationship among genes

on a particular chromosome. Cytological study already

indicated that chromosomes were long threadlike structures.

They showed that genes were situated at different loci (sites)

on the linear structure. They pioneered the first genetic

mapmaking and studying the physical structure of

chromosomes. All their observations lead to the Chromosome

Theory of Heredity.

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Chromosome Theory of Heredity

All genes are located on the chromosomes.

Mendel’s principles could be explained by the transmission

properties of chromosomes during reproduction.

This theory stands as one of the most important achievements

in Biology.

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Proof of the Chromosome Theory by Nondisjunction.

Morgan showed that the gene for eye colour was on the X-

chromosome in Drosophila by correlating the inheritence of

the gene with the transmission of X-chromosome during

reproduction.

Later, his student C. B. Bridges secured proof of thechromosome theory by showing that exceptions to the rules

of inheritence could also be explained by chromosome

behaviour.

Bridges performed Morgan’s experiments on a larger scale.

Bridges crossed white-eyed female Drosophila with

red-eyed males and examined the F1 progeny.

As expected, nearly ALL the F1 flies were either red-

eyed females or white-eyed males.

However, he found a few exceptions: A few werewhite-eyed females and red-eyed males.

He crossed between these exceptional males and

females and got the following results:

1. The exceptional males were ALL proved to be sterile.

2. However, the exceptional females were fertile.

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3. When these fertile white eyed females were crossed with

normal red-eyed males, they produced many progeny,

including large numbers of white-eyed daughters and red-

eyed sons.

4. Thus, the exceptional F1 females, though rare in their

own right, were prone to produce many exceptional progeny.

Bridge’s conclusion according to these observations were as

follows:

1. The exceptional F1 flies were the result of abnormal X

chromosome behaviour during meiosis in the females

of P generation.

The normal disjoining (separation of the XX chromosomes)

did not occur during meiosis.

Fertilization of such abnormal eggs would produce zygotes

with abnormal number of sex chromosomes.

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W

X

White-eyed

female

W

X

XW+

X Y

Red-eyed

male

W

Normal eggs

W W

Nondisjunctional eggs

nullo-X

W

W

W+

XX Red-eyedfemale

XY white-eyed

male

WW

WW W+W+

XXX metafemale

(usually dies)

XXY exceptionalwhite-eyed

female

XO exceptionalRed-eyed male

YO

(dies)

sperm

W+

lecture 001, 204

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