Transcript Sources of Genetic Variation - University of Evansville Faculty Web

Sources of Genetic
Variation
Two important criteria for
evolution to proceed include:
• Heritable genetic variation
• Continuous origin of new genetic
variation
Heritable Genetic Variation and an
Acceptable Theory of Inheritance
• Blending Theory
– Heredity “stuff” of parents blend together to
produce characteristics observed in the
offspring
• Particulate Theory
– Heredity factors from the parents (=genes) will
remain unchanged in the hybrid
Blending versus Mendelian Heredity
Blending
versus
Mendelian
Heredity
Sources of Genetic Variation
I. Point Mutations
A. Base pair substitutions
B. Insertions and deletions
II. Chromosomal Mutations
A. Changes in the structure of a chromosome
1. Deletions
2. Duplications
3. Inversions
4. Translocations
B. Changes in the number of chromosomes
1. Centric fusion
2. Centric fission
3. Aneuploidy
4. Polyploidy: . Autopolyploidy and Allopolyploidy
Point
Mutations
Chromosomal
Mutations: Change
in Structure
Aneuploidy
• One or more chromosomes of a
normal set are lacking or present in
excess
• Due to meiotic nondisjunction members of pairs of homologous
chromosomes do not separate during
meiosis I or sister chromatids fail to
separate during anaphase of meiosis II
POLYPLOIDY - the duplication of chromosome sets such that
individuals have more than 2 of each chromosome.
• DIPLOIDY is the normal state (2 of each chromosome, 2N), but
some animals are TRIPLOID (3N) and TETRAPLOID (4N).
• There are 2 types of polyploidy: autopolyploidy and
allopolyploidy
Autopolyploidy
• The multiplication of chromosomes sets within a species
• For example, a failure of meiosis during gamete production can
double chromosome number from the diploid count (2N) to a
tetraploid number
• The tetraploids formed can mate with themselves (self-pollinate) or
with other tetraploids
Allopolyploidy
• It refers to the contribution of two different species to the
polyploid hybrid
• It begins with 2 different species interbreeding and
combining their chromosomes
• Interspecific hybrids are usually sterile because the
haploid set of chromosomes from one species cannot pair
during meiosis with the haploid set from the other species
Formation of an Allopolyploid
• During the history of the clone, a mitotic error affecting the
reproductive tissue occurs, doubling chromosome number
• The hybrid will be able to make gametes because each chromosome
has a homologue with which to synapse during meiosis
• The union of
gametes from this
hybrid may give rise
to a new species of
interbreeding plants,
reproductively
isolated from both
parent species
Rates of Mutation
• Most mutations are either neutral or deleterious; few are advantageous
• There are only approximately 1 X 10-5 mutations per
locus/individual/generation (i.e. 1/100,000); However, when summed over all
loci, over all gametes, over all individuals, the rates of mutation are rather high
Example:
– Assume that there are at least 100,000 pairs of genes in humans, and that the
average mutation rate/gene/generation is 10-5
– The average number of mutations arising per generation would then be estimated
as at least: 2 X 105 X 10-5 mutations/gene = 2 mutations for a human zygote
– There are about 6 X 109 humans in the world; with 2 mutations/individual, the
total number of mutations newly arisen in the human population is 12 X 109
• Clearly, the number of mutations in a population or species in any given
generation can be quite large
• Even if only a very small fraction of these were advantageous, there are
clearly enough mutations to provide the raw material for evolutionary change.
Random versus Non-random Mutations
• The probability of a mutation is not random with
respect to sites within the genome: some locations
and base pair sequences have very high mutation
rates while others have very low mutation rates
• Also, the mutation rates for organisms can be
increased by exposing organisms to certain
chemicals called mutagens
• Mutations are, however, random with respect to
the direction of adaptation
• There appears to be no evidence for the notion of
directed mutations
Maintaining Genetic Variation
• Meiosis
– Independent
assortment
– Chromosomal
crossing over
• Syngamy
Crossing Over the Chromosomal
Material
• If the 2 genes were not
linked, the hybrid AaBb
would yield 4 different
gametes
• A hybrid with 2 pairs of
linked genes can also yield
4 different gametes due to
crossing over
• However, new
recombinants (Ab and aB)
will arise less frequently,
than original combination
types (AB and ab)