Gene linkage and Gene maps
Objectives:
identify the structures that actually
assort independently
explain how gene maps are produced
the # of genes in a cell is FAR greater
than the # of chromosomes
each chromosome has hundreds –
thousands of genes
genes located on the same chromosome
that tend to be inherited together = linked
genes
when geneticists follow linked genes in
breeding experiments – the results
deviate from the expected from Mendel’s
law of independent assortment
Thomas Hunt Morgan – experimental
embryologist (1907) Columbia
University – a skeptic of Mendelian
Genetics – his work provided evidence
that chromosomes are the location of
Mendel’s heritable factors
Morgan studied Gene linkage – with
drosophila – fruit flies
Morgan studied body color and wing size
Wild type flies - gray bodies/normal
size wings
Mutants – black bodies/vestigial
wings
Mutant alleles recessive to wild type
Gray body – b+b+
Normal wings – vg+ vg+
Crossed with
Black body - bb
Vestigial wings - vg vg
Morgan observed a higher proportion of
parental pheotypes than would be
expected if the two genes assorted
independently – he concluded these
genes must be located on the SAME
chromosome
Offspring _ b+bvg+vg
bbvgvg
b+bvgvg
bbvg+vg
since Morgan’s results show that both of
the nonparental pheonotypes were also
produced, Morgan suggested that body
color and wing size genes are only
partially linked genetically
Genetic Recombination
When the offspring’s pheonotypes do not
match the parent’s phenotype – the
offspring are said to be Nonparental
phenotypes – due to NEW
COMBINATIONS of genes –
RECOMBINANT TYPES or
RECOMBINANTS
Morgans results revisited
Most of the offspring from the test
cross for body color and wing size had
Parental Phenotypes
Suggests that the 2 genes are on the
same chromosome
Frequency of parental types > 50%
suggests that the genes are linked
17% of offspring – nonparental types
= recombinants
Morgan deduced that some process
must occassionaly break the connection
between alleles on the same chromosome
Crossing over – accounts for the
recombination of linked genes – end
portions of two nonsister chromatids
trade places
Recombination frequency is
dependent on the distance between the
linked genes
Morgan’s student, Alfred Sturtevant,
developed a method for constructing a
gene map(an ordered list of genetic loci
along a particular chromosome)
Sturtevant(1911): based on his
experiments –the farther apart two genes
are, the higher the probability that a
crossover will occur between them and
therefore the higher the recombination
frequency
Linkage map = a genetic map based on
recombination frequencies
Linkage maps based on recombinant
frequencies = approximate picture of a
chromosome
Frequency of crossing over is not
uniform over the length of the
chromosome – map units DO NOT
correspond to the actual physical
distances .
Linkage maps DO portray the order of
genes along a chromosome
Mapping of the Human Genome
employed cytogenetic maps –
chromosome banding pattern and
location of specific genes by
fluorescence in situ hybridization(FISH)
Along with linkage maps and physical
mapping(utilizing yeast and bacterial
artificial chromosomes(YAC,BAC) and
finally DNA sequencing – determination
of the nucleotide sequence