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Higher Biology

Higher Biology. Linkage & Recombination. Linkage & Recombination. By the end of this lesson you should be able to:. Know what is meant by linked genes. Know the consequences of genes being linked. Understand the effect crossing over has on linked genes.

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Higher Biology

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  1. Higher Biology Linkage & Recombination

  2. Linkage & Recombination By the end of this lesson you should be able to: • Know what is meant by linked genes. • Know the consequences of genes being linked. • Understand the effect crossing over has on linked genes. • Know the effect that crossing over has on distantly located genes. • Know what cross-over values (COV) are.

  3. Introduction • In the early 1900s it was noticed that the 9:3:3:1 ratio from a dihybrid cross did not always occur. • In some F2 generations the parental phenotypes appeared more frequently than expected. • Sometimes recombinant phenotypes did not appear at all.

  4. Linked Genes • Two or more genes which are carried on the same chromosome are said to be linked. • Insert linked picture Fig 3.1 p18 Scholar

  5. Linked Genes • If there is gene linkage there is a lower probability that the alleles will be separated during meiosis. • This means that more of the parental genotypes appear in the F2 generation. • Different linked genes are inherited as a group rather than as separate units. • The closer 2 genes are on a chromosome, the more likely it is that they will be inherited together.

  6. Linked Genes • The opposite is also true: • The further apart 2 genes are on a chromosome, the less likely it is that they will be inherited together as crossing over is more likely to separate them. • REMEMBER THAT: When the homologous chromosomes pair up during the first meiotic division, non-sister chromatids are able to exchange pieces of DNA.

  7. Linked Genes • Consider two genes A and B that are linked on the same chromosome. • A cross is carried out X • The F1 would all be AaBb • At this stage there is no indication that genes A and B are linked (on the same chromosome) AABB aabb

  8. Linked Genes • If two F1 individuals are then self-crossed: X • If A and B are very tightly linked, and there has been no crossing over, then the gametes will be: • Draw a Punnett Square: AaBb AaBb AB ab AB ab

  9. Linked Genes We sometimes use brackets around the gametes to show they are linked together. (AB)(AB) (AB)(ab) (AB)(ab) (ab)(ab) The F2 will be: AABB AaBb aabb Ratio 1 : 2 : 1

  10. Linked Genes • Complete the “Dihybrid Cross- Linked Genes” sheet from your diagram pack.

  11. Practice Questions • Torrance • TYK page 93 Q2 • AYK page 89 Q3 • AYK pages 101 Q2.

  12. Linkage & Recombination Can you do it? • Know what is meant by linked genes. • Know the consequences of genes being linked. • Understand the effect crossing over has on linked genes. • Know the effect that crossing over has on distantly located genes. • Know what cross-over values (COV) are.

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