LINKAGE INTRODUCTION The hereditary units or genes which determines the character of an - PowerPoint Presentation

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LINKAGE

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INTRODUCTION

The hereditary units or genes which determines the character of an individual are present on the chromosome and an individual usually have many genes for the determination of various different characters.

As there are more genes than chromosomes, it can be expected that each chromosome contains more than one gene.

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The genes for different characters may be either situated on the same chromosome or on different chromosome.

When the genes are situated on different chromosome, the characters they control may appear in the next generation either together or apart depending on the chance alone.

They assort independently according to…

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But if the genes are present on the same chromosome and fairly close to each other, they tend to be inherited together.

This type of coexistence of two or more genes on the same chromosome is known as

LINKAGE

.

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DEFINITION

LINKAGE

is the tendency for two alleles of two or more genes to pass from one generation to another generation in the same combination.

This usually means that the closer together any two genes lie on the same chromosome, the more likely they show linkage…

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Linked Genes Do Not Assort Independently…

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LINKAGE PHASE

Crosses involving linked genes are usually diagrammed to show the

linkage phase…

It is the way in which the alleles are arranged in heterozygous individuals.

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In Bateson and

Punnett’s

sweet pea experiment, the heterozygous F1 plants received two dominant alleles, R & L, from one parent and two recessive alleles, r & l, from the other.

Thus we write the genotype of these plants RL/r l, where slash (/) separates alleles inherited from different parents.

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Another way of interpreting this symbolism is to say that the alleles on the left and right of the slash entered the genotype on different homologous chromosomes, one from each parent.

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TYPES OF LINKAGE PHASE

COUPLING L.P.

REPULSION L.P.

Whenever the dominant alleles are all on one side of the slash, the genotype has the

coupling

linkage phase.

RL/r l

When the dominant and recessive alleles are split on both sides of the slash, the genotype has the

repulsion

linkage phase.

R l/r L

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HISTORY OF LINKAGE

The effects of linkage were first evident in the results of a dihybrid cross in sweet peas that were reported by Bateson and Punnett in

1906

.

However, Bateson & Punnett did not interpret their results in terms of the behavior of genes located on the same chromosome.

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T. H. Morgan was the first to relate linkage to the segregation of homologous chromosomes and the occurrence of crossing over between homologous chromosomes during meiosis.

Morgan’s interpretation of linkage was published in 1911 in a paper in which he reported the results of crosses involving linked genes in the fruit fly Drosophila melanogaster.

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Chromosome Theory of Linkage

Morgan, along with Castle formulated the chromosome theory of linkage. It has the following postulates;

1. Genes are found arranged in a linear manner in the chromosomes.

2. Genes which exhibit linkage are located on the same chromosome.

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3. Genes generally tend to stay in parental combination, except in cases of crossing over.

4. The distance between linked genes in a chromosome determines the strength of linkage. Genes located close to each other show stronger linkage than that are located far from each other, since the former are less likely to enter into crossing over.

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Is “independent assortment” always the case?

No it depends on whether the genes are linked or not.

(a) Genes located on different chromosomes are not linked.

This allows independent assortment – in a

di

-hybrid cross

the traits show the classic

9:3:3:1 inheritance pattern.

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(b) Genes that are located very close together on the same chromosome may show complete linkage.

They may be so close to each other that they cannot be separated by

recombination during meiosis.

(c) Genes located far apart on the same chromosome

typically show

incomplete (partial) linkage

because they are easily separated by

recombination.

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TYPES OF LINKAGE

On the basis of the above discussion and research, linkage can be classified into the following two types:

COMPLETE LINKAGE

INCOMPLETE LINKAGE

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COMPLETE LINKAGE

The genes closely located in the chromosome show complete linkage as they have

no chance of separating

by crossing over and are always transmitted together to the same gamete and the same offspring.

Thus, the

parental combination

of traits is inherited as such by the young one.

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INCOMPLETE LINKAGE

The genes distantly located in the chromosome show incomplete linkage because they have a chance of separation by crossing over and of going into different gametes and offspring.

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The linked genes don’t always stay together because homologous non-sister chromatids may exchange their segments of varying length with one another during the meiotic prophase I.

This process is known as

crossing over

and the phenomenon of inheritance caused by it is known as

incomplete linkage.

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LINKAGE GROUPS

All the genes located on a particular chromosome, form a linkage group.

Since the genes present on a particular chromosome have their alleles located on its homologous chromosome, genes on a pair of homologous chromosomes.

Hence, the number of linkage groups corresponds to the number of haploid chromosomes found in a species.

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Drosophila melanogaster has four linkage groups which can be distinguished into three large and one small linkage groups corresponding to the four pairs of chromosomes.

Twenty-three linkage groups are present in humans corresponding to 23 pairs of chromosomes.

Pea plant has seven linkage groups, corresponding to the seven pairs of chromosomes.