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Powerpoint Presentation:Linkage

 

Genetics Index

Chromosomes
Meiosis
Meiosis: Crossing over
Mitosis and Meiosis
Introduction to Mendelian Genetics
Test Cross
Codominance
Multiple Alleles
Pedigree Charts
Twin Studies
The dihybrid cross
Dihybrid Cross : Test Cross
The Genetic Diagram for Linked Genes
Calculating the cross over value using a test cross
Sex determination and sex linkage
Sex linkage
Genetic diagram for sex linked genes
Blood Clotting and Haemophilia
The Retina and Daltonism
Genetic Modification
Cloning Animals
Cloning Plants

Topic Chapters Index

 

Reminder

In monohybrid inheritance the F2 generation gives a 3:1 ratio because of the segregation of the alleles during meiosis and their random fusion during fertilisation. (Mendel's Law of Segregation).

A back cross with a heterozygote (Aa) gives a 1:1 ratio.

 

In dihybrid inheritance the F2 generation gives a 9:3:3:1 ratio because of the independent assortment of the pairs of alleles on their homologous chromosomes during meiosis I and their random fusion during fertilisation. (Mendel's Law of Independent Assortment).

A back cross with a heterozygote for both pairs of alleles (AaBb) gives a ratio of 1:1:1:1

 

 

GENETICS

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Autosomal Linkage

Bateson and Punnett discovered genes which did not obey Mendel's Second Law

Bateson and Punnett

 

Batson and Punnet II

The parental combinations of alleles (purple long and red round) seem to be inherited as almost a 3:1 ratio (i.e. as though they were behaving as a single character). These genes are called linked genes.

That is the LOCI for these genes are linked on the same chromosome.

But if these genes were perfectly linked together they would stay in their parental combinations. There would be no Purple Round or Red Long. These combinations have come about because of crossing over between the linked alleles on their chromosomes during Meiosis I. These are called recombinants.

 

Are there any other recombinants?

Yes, hidden amongst the Purple Long plants:

The genotype FfLl does not tell us enough about linked genes.

Genotypes for linked genes can be shown as Purple Long plants. This is an example of a parental combination. This genotype would give the same phenotype as Purple Long plants, which is an example of a recombinant.

 

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