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Linkage and Crossing Over

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Linkage and Crossing Over Linkage Mr G Davidson Linkage Because a chromosome can have hundreds of genes, genes found in the same chromosome are said to be linked. – PowerPoint PPT presentation

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Title: Linkage and Crossing Over


1
Linkage and Crossing Over
  • Linkage

Mr G Davidson
2
Linkage
  • Because a chromosome can have hundreds of genes,
    genes found in the same chromosome are said to be
    linked.
  • During a cross, therefore, the linked genes will
    be transmitted together.
  • e.g. peas TTall tdwarf RRed rwhite
  • These genes are linked - i.e. on the same
    chromosome.

3
Diagram of Linked Genes
4
Linkage
  • The position of the gene on the chromosome is
    called the locus.
  • e.g. Parents TtRr x TtRr
  • Gametes TR or tr TR or tr
  • Because T and R are linked, and t and r are
    linked, there are no recombinants.

TR tr
TR TTRR TtRr
tr TtRr ttrr
5
Cross Overs
  • In dihybrid crossed, the two genes were inherited
    independently of each other.
  • Where the two genes are linked they are inherited
    together, therefore a cross involving a linkage
    group should behave like a monohybrid cross,
    however, life is never easy.

6
Cross Overs
  • Usually crosses involving linked genes produce a
    small proportion of offspring with new allele
    combinations (recombinants) in addition to the
    parental combinations.
  • This is because there is an exchange of material
    between homologous pairs during meiosis.
  • The chromosomes cross over at chiasmata and the
    exchange takes place.

7
Cross Overs
8
Cross Overs
  • These breaks and swapping of chromatid material
    are called crossovers.
  • If two genes are far apart on a chromosome, then
    the chances of them being separated are greater
    than two genes which are close together.
  • Therefore, if in a breeding experiment, a high
    percentage of offspring show that two genes have
    separated we can conclude that they are far apart
    or vice versa, if there is a low percentage.

9
Cross Overs
  • The percentage of F2 recombinants is called the
    cross over value (COV) and is calculated as
  • COV No. of F2 recombinants x 100
  • Total F2 Offspring

10
Chromosome Mapping
  • Cross overs allow geneticists to work out the
    relative positions of genes on chromosomes.
  • The crossover values are used to create
    chromosome maps.
  • e.g.

11
Sex Linkage
  • Homologous chromosomes are normally identical in
    appearance and are called autosomes.
  • Exceptions to this are the sex chromosomes called
    heterosomes.
  • These determine the sex of the organism
    concerned.

12
Sex Linkage
  • In mammals, the possession of 2 different sex
    chromosomes, i.e. X and Y results in male
    characteristics.
  • 2 identical chromosomes, both XX, results in
    female characteristics.
  • However, in birds, the reverse is true, with
    females being the heterogametic sex (XY).

13
Sex Linkage
  • Experiments and observations have shown that
    other genes in addition to sex determination are
    carried in the sex chromosomes.
  • As the Y chromosome is smaller than the X, it
    carries less information.

14
Sex Linkage
  • This means in females, the genes on the X
    chromosomes have two alleles in the normal way.
  • However, in males, some of the genes on the X
    chromosome have only one allele, because these
    genes are not present on the Y chromosome.

15
Sex Linkage
  • Where that allele is recessive, the recessive
    trait will be expressed.
  • In males, there are many examples where a
    recessive trait shows in the phenotype.

16
Sex Linkage
  • In a female carrying the same recessive allele,
    its effect might not show in the phenotype
    because it could be masked by a dominant allele
    on the other X chromosome.
  • As the genes found in the sex chromosomes are
    linked to those determining sex, they are said to
    be sex-linked.

17
Sex Linkage
  • In humans, colour blindness is a sex linked
    trait.
  • The gene for colour blindness is carried on the
    non-homologous portion of the X chromosome.
  • The convention for describing a cross involving
    sex linkage is as follows
  • The chromosome is shown as X or Y and the allele
    as superscript, e.g. Xn or XN or Y.

18
Sex Linkage
19
Example
  • Colour blindness (n) is recessive to the normal
    sight gene (N).
  • If a man with normal sight marries a woman with
    normal sight, but a carrier for colour
    blindness, what are the likely phenotypes of any
    children they have?

20
Example
  • Female Male
  • Parents XNXn x XNY
  • Gametes XN or Xn XN or Y

21
Example
XN Y
XN XNXN XNY
Xn XNXn XnY
Therefore XNXN girl with normal sight XNXn
girl with normal sight but a carrier XNY boy
with normal sight XnY colour blind boy
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