linked genes Watch

biodude
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#1
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can anyone explain to me what linked genes are and how they go against Mendel's laws?

thanks.
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shorty.loves.angels
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(Original post by biodude)
can anyone explain to me what linked genes are and how they go against Mendel's laws?

thanks.
Some genes have a tendancy to 'stay together' during segregation, which according to Mendel's law is supposed to be independant as I'm sure you know.

http://en.wikipedia.org/wiki/Genetic_linkage

Try that site, it was a simple search for genes and linkage, there are very good websites out there. Come back if you need a bit more help and can't find it.
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emitothelee
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Here are my notes about sex linkages:
Sex Linked Characteristics

Chromosomes which are not sex chromosomes are known as autosomes. The x chromosome is larger than the Y chromosome. The sex chromosomes are therefore not always homologous.

Phenotypes: Male Female
Genotypes: XY X X

Sex Linkage – Haemophilia

The X chromosome carries a gene that codes for the production of a protein needed for blood clotting called factor VIII. The factor VIII gene is said to be sex-linked. A sex-linked gene is one which is found on a part of the X chromosome not matched by the Y, it is not found on the Y chromosome.

Males can only obtain their male Y chromosome from their father, so the X chromosome comes from their mother. As mothers do not suffer from the disease they are heterozygous for the character XX. Such females are called carriers because they carry the allele without showing any signs of the character in their phenotype. As males pass the Y chromosome on to their sons, they cannot pass haemophilia to them. However, they can pass the allele to their daughters via the X chromosome, who then become carriers of the disease.
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emitothelee
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What is Linkage?

The loci are said to be linked when two or more gene loci are on the same chromosome so they do not assort independently in meiosis.

An example of “linkage” can be seen in Drosophila, a fly. The gene for body colour and the gene for antennal shape are close together on the same chromosome and so are linked. A black body, with no stripes results from a recessive allele called ebony. A recessive allele for antennal shape, aristopedia, gives an antenna which looks like a leg.

When a homozygous fly with a striped body and normal antennae was crossed with a homozygous ebony bodied fly with aristoepdaia antenna.

Genes:
E= allele for striped body, e=allele for ebony body, A=allele for normal antennae, a=allele for aristopeida antennae.

(i couldnt paste the test cross table in here - sorry!)

The test cross gives a 1:1 ratio of the two original parental types and not the 1:1:1:1 ratio expected. If the genes were NOT linked it would have resulted in 1EeAa:1Eeaa:1eeAa:1eeaa. This is because the parental genotypes would have been EeAa x eeaa.
Due to linkage the dihybrid cross had behaved as a monohybrid cross. Total linkage is very rare as linkage groups are broken by crossing over during meiosis. There is no crossing over in chromosomes of male Drosophila.
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emitothelee
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GRR COPY AND PASTE ISNT WORKING
the 11:1 is meant to be 1 : 1 : 1 : 1 and the 1EeAa1eeAa:1eeaa should be 1EeAa :1 Eeaa : 1eeAa : 1eeaa
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biodude
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#6
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(Original post by emitothelee)
What is Linkage?

The loci are said to be linked when two or more gene loci are on the same chromosome so they do not assort independently in meiosis.

An example of “linkage” can be seen in Drosophila, a fly. The gene for body colour and the gene for antennal shape are close together on the same chromosome and so are linked. A black body, with no stripes results from a recessive allele called ebony. A recessive allele for antennal shape, aristopedia, gives an antenna which looks like a leg.

When a homozygous fly with a striped body and normal antennae was crossed with a homozygous ebony bodied fly with aristoepdaia antenna.

Genes:
E= allele for striped body, e=allele for ebony body, A=allele for normal antennae, a=allele for aristopeida antennae.

(i couldnt paste the test cross table in here - sorry!)

The test cross gives a 1:1 ratio of the two original parental types and not the 1:1:1:1 ratio expected. If the genes were NOT linked it would have resulted in 1EeAa:1Eeaa:1eeAa:1eeaa. This is because the parental genotypes would have been EeAa x eeaa.
Due to linkage the dihybrid cross had behaved as a monohybrid cross. Total linkage is very rare as linkage groups are broken by crossing over during meiosis. There is no crossing over in chromosomes of male Drosophila.

thanks this really helped.
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leylabez
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check this animation

http://www.biostudio.com/d_%20Meioti...ed%20Genes.htm
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