10.2 Inheritance

An example of a cross between linked genes

An example of linkage that actually led to the discovery of this idea was the classic experiment by Bateson,W., Saunders, E.R. and Punnett, R.C. In 1904.

They experimented on genes for flower colour in this case P for purple and p for Red and the gene influencing the pollen grain shape L for long and l for round.

They crossed PPLL and ppll and then self crossed the resulting PpLl

Phenotype and genotype

Observed

Expected from 9:3:3:1 ratio

Purple, long (PpLl)

284

216

Purple, round (Ppll)

21

72

Red, long (ppLl)

21

72

Red, round (ppll)

55

24

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

The discrepancy between the results and prediction was clear to the Authors and they suggested some sort of explanation. Bateson, W., et al. p89

“The peculiarities of this distribution are without doubt significant ………………….. There is, therefore some coupling of pollen-shape and colours.”

Their experimental results would be explained now as revealing linkage between the P and L alleles and the p and l alleles.

Parental Phenotypes

Purple Long pollenRed short pollen

Parental Genotypes

Gamete image

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F1 Genotype

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F1 test crossed with a homzygous recessive Red short pollen

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Gametes without crossing over

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Possible offspring

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Gametes with crossing over (notice that crossing over cannot change the alleles for the homozygous recessive)

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Possible offspring with crossing over

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HL Topic 10 Genetics Copyright © by Alex Black. All Rights Reserved.

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