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Chromosome variation
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A summary of the lectures on how chromosomes can vary in structure and number and the linkage of genes on chromosomes.
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Changes in chromosome number2) Organisms with multiples of the
basic chromosome set (genome)
are referred to as euploid.
Organisms with more or fewer
than normal are aberrant
euploids.
Polyploids – more than 2
chromosome sets e.g. tetraploid
An individual of a typically
diploid species but with only 1
set of chromosomes =
monoploid.
Polyploidy and monoploidy can 1) Extra chromosomes can cause drastic effects due to the disruption
cause problems in humans , but of gene balance – genes have evolved to function in a diploid
common in some insects. E.g genetic background, and expression of deleterious alleles on
males are haploid (from monosomic autosome don’t have any other chromosomes to hide
unfertilised eggs) (polyploidy is them.
very common in plants)
3) Individuals whose chromosome
number differs by a small
number of chromosomes = Changes in chromosome structure
aneuploid. Trisomic = 2n +1. 1. Chromosomes have extra pieces: duplications – these have played
Monosomic = 2n -1. Nullisomic = an important role in evolution. A section of DNA is duplicated in
2n -2. the chromosome.
4) These can be cause by non- 2. Chromosomes can have missing pieces: deletions (small or large).
disjunction during mitosis or Large deletions can be seen in a karyotype. Small deletions can be
meiosis. visualised via special methods – fluorescent in situ hybridisation
3. Chromosomes can have missing or extra pieces on one
Mammals have an XY sex- chromosome: copy number variations (CNVs) – around a few
determination system. Males are thousand bp long. E.g. caused by a large deletion. Can cause
heterogametic. Presence of Y various diseases e.g. autism.
determines maleness. 4. Chromosomes can have mixed-up pieces: inversions –
Y chromosome discovered by Nettie chromosome segment is cut out, flipped and reinserted into the
Stephens in 1905. Mostly of chromosome in the opposite orientation- and translocations – a
repeated sequences, very few rearrangement involving a part of a chromosome that is broken off
genes, SRY maleness determining and reattached to a different chromosome. E.g reciprocal or
gene. Robertsonian
X-chromosome. Many genes
Linkage: linkage of genes on chromosome was discovered because in
unrelated to sex. Males are
certain crosses, a greater number of offspring with ‘parental’
hemizygous for x-linked traits. In
genotypes were generated than expected. 1905 Bateson, Saunders
females one of the X chromosomes
and Punnet: looking at flower genes – colour (red & purple, and pollen
randomly becomes a ‘Barr’ body –
shape – round or long).
condensed state. X-chromosome
Recombination frequencies can be used to produce a linkage map of a
aneuploidy is also more tolerated
chromosome as the frequency of recombination between two genes
than autosome aneuploidy.
will depend on the distance between them.
Some organisms have a ZW system.
Note: Drosophila use a 2-letter abbreviation for alleles, and cis means
Males are the homogametic sex. E.g.
homozygous and trans means heterozygous. + mean wildtype.
butterflies.
Genetic maps are linear and additive
Gene order can be deduced by comparing the parental genotypes to
the genotypes of the double recombinant offspring.
For multiple genes, e.g 3 genes, use a 3-point cross. 3 classes of
offspring tell you that all 3 genes are linked. 2 classes = one gene on a
separate chromosome.
Unlined genes have an RF of 50 or more.
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