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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

Higher Biology

Linkage & Recombination

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.

  • Know the effect that crossing over has on distantly

located genes.

  • Know what cross-over values (COV) are.
introduction
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.
linked genes
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
linked genes1
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.
linked genes2
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.

linked genes3
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

linked genes4
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

linked genes5
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

linked genes6
Linked Genes
  • Complete the “Dihybrid Cross- Linked Genes” sheet from your diagram pack.
practice questions
Practice Questions
  • Torrance
  • TYK page 93 Q2
  • AYK page 89 Q3
  • AYK pages 101 Q2.
linkage recombination1
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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