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Chapter 6 genetic linkage and mapping in eukaryotesTwo genes that are Each species of organism must contain hundreds to thousands of genes. Yet most species have at
located on the same most a few dozen chromosomes. Therefore, each chromosome carries many hundred or even
chromosome are said thousands of different genes. The transmission of genes close to one another on the same
to ben physically chromosome will violate Mendel’s law of independent assortment
linked.
In eukaryotic species, each linear chromosome contains a long piece of DNA. A typical
Locus is the physical chromosome contains many hundred or even a few thousand different genes. The term synteny
place of a gene on a means two or more genes are located on the same chromosome and are physically linked.
chromosome. Genetic Linkage is the phenomenon that genes close together on a chromosome tend to be
transmitted as a unit, which influences inheritance patterns
Another name for a Chromosomes are called ‘linkage groups’. They contain a group of genes that are linked
chromosome is a together. The number of linkage groups is the number of types of chromosomes of the species
linkage group, since it – For example, in humans
contains genes that • 22 autosomal linkage groups
are often inherited • An X chromosome linkage group
together. • A Y chromosome linkage group
Genes that are far apart on the same chromosome may independently assort from each other.
This is due to crossing overs
• A dihybrid cross studies linkage between two genes
• A trihybrid cross studies linkage between three genes
Crossing over: change Crossing over may produce recombinant phenotypes. In diploid eukaryotic species, linkage can
arrangement of alleles be altered during meiosis as a result of crossing over. Crossing over
– Occurs during prophase I of meiosis, replicated sister chromatid homologues
associate as bivalents.
– Non-sister chromatids of homologous chromosomes exchange DNA segments
Crossing over in Without crossing over, linked alleles segregate
meiosis together. The haploid cells contain the same
combination of alleles as the original
If a crossover occurred chromosomes. The arrangement of linked
in the short region alleles has not been altered. <-
between A and the tip
of the chromosome, Crossing over can reassort linked alleles. These
this would not affect haploid cells contain a combination of alleles
the arrangement of NOT found in the original chromosomes. This
the A and B alleles. new combination is a result of genetic
recombination. This are termed nonparental or
recombinant cells. ->
, Recall that pleiotropy refers In 1905, William Bateson and Reginald Punnett
to the phenomenon in which
a single gene contributes to conducted a cross in sweet pea involving two
multiple phenotypic traits. If different traits: Flower color and pollen shape.
a single, pleiotropic gene
controls both seed coat color This is a dihybrid cross that is expected to yield a
and flower coat color, then a 9:3:3:1 phenotypic ratio in the F2 generation.
testcross of a heterozygous
plant should yield a 1:1 ratio However, Bateson and Punnett obtained
of the phenotypic classes. surprising results.
However, if these two traits
are controlled by two closely
linked genes, you would
expect a small proportion of
recombinant progeny.
The first direct evidence of linkage came from
studies conducted by Thomas Hunt Morgan.
Morgan investigated several traits that followed
an X-linked pattern of inheritance in Drosophila
• Figure 6.3 illustrates an experiment
involving three traits
– Body color
– Eye color
– Wing length
Both X chromosomes in Dros. Are active.
Morgan observed a much higher
proportion of the combinations of traits
found in the parental generation.
Morgan’s explanation:
All three genes are located on the X chromosome. Therefore, they tend to be transmitted
together as a unit .
Morgan made three important hypotheses to explain his results
1. The genes for body color, eye color and wing length are all located on the X-
chromosome. They tend to be inherited together.
1. Due to crossing over, the homologous X chromosomes (in the female) can exchange
pieces of the chromosomes. This created new combinations of alleles.
2. The likelihood of crossing over depends on the distance between the two genes. Crossing
over is more likely to occur between two genes that are far apart from each other.
Genetic 1. Two/+ genes are linked on same chromsome -> crossing over.
recombination, two 2. Two/+ genes are on different chromsomes -> inderpendent assortment.
ways. Recombinent offspring: produced by crossing over between two homologous chromosomes
leading to different combination of alleles compared to the parent.
Genetic mapping Also known as gene mapping or chromosome mapping. Its purpose is to determine the linear
order of linked genes along the same chromosome. Each gene has it own unique locus at a
particular site within a chromosome.
Genetic maps useful: 1. They allow us to understand the overall complexity and genetic organization of a
particular species
2. They can help molecular geneticists to clone genes
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