Clinical genetics / edited by Arnold Sorsby.

Date:
1953
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Clinical genetics / edited by Arnold Sorsby. Source: Wellcome Collection.

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    CHAPTER 7 EVALUATION OF LINKAGE Alastair G. Macgregor The term linkage is applied to any two pairs of genes which are situated upon the same pair of chromosomes. The word is applicable whether or not the genes are on the same, or opposite, chromosomes of the pair. If the two genes for the characters in question are upon the same chromosome of the pair, with their normal allelomorphs at the corresponding sites on the other chromosome of the pair, then the genes are said to be linked in coupHng. If, on the other hand, they are on opposite chromosomes, and each chromosome of the pair carries one abnormal gene, and one normal allelomorph, then the genes are said to be linked in repulsion. Fig. 11 shows that when genes are hnked in coupling they tend when transmitted to remain together, whereas, if they are linked in repulsion they invariably separate. PARENTS. GAMETES. LINKAGE IN COUPLING Affected Normal 5) (51 (o LINKAGE IN REPULSION Affzclid Normal • О CROSSING OVER Affected Normal • ÍO Affected Normal О GO OFFSPRING. Fig. 11.—Diagram illustrating mode of transmission of linked genes in the absence, or presence, of crossing over. The effects of crossing over Crossing over may disturb this simple arrangement (Fig. 11). It is obvious that the further apart are a pair of genes the more hkely it is that the chromosome 65 Clin. Gen.—5