(Lofts and Evergreen, 2015)
8.describe
how meiosis contributes to genetic variation
Most variation occurring in each generation relates to how chromosomes behave during meiosis and fertilization.
3 mechanisms contribute to genetic variation:
Most variation occurring in each generation relates to how chromosomes behave during meiosis and fertilization.
3 mechanisms contribute to genetic variation:
- Indpendent assortment of
chromosomes
- Crossing over of homologous
chromosomes
- Random fertilization of an
ovum by a sperm
- INDEPENDENT ASSORTMENT OF
CHROMOSOMES
- Contributes to genetic
variation because of the way homologous
chromosomes (tetrads)independently and randomly position themselves at
metaphase I.
- There is one pair of
maternal chromosomes and one pair of paternal chromosomes.
- The first meiotic division
results in independent assortment of maternal and paternal chromosomes into
daughter cells. See picture below.
- This independent assortment of chromosomes produces new genetic combinations
- The number of combinations
possible when the chromosomes undergo independent assortment into gametes is
2n.(n = the haploid number of the organism.)
- FOR EXAMPLE:-
- If n=3, there are 8 possible combinations
-
or
approximately 8 million possible combinations of chromosomes.
this will produce new genetic combinations that will contribute to genetic variation.
- CROSSING OVER
- Begins very early in Prophase I.
- Homologous chromosomes pair up gene by gene.
- Homolgous portions of 2 nonsiste chromatids trade
places.(in humans, this occurs 2-3 times per chromosome pair).
- The chromatids
of each pair will twist around each other
- During this process, tension is created on the
chromosomes and portions of the chromatids break off.
- One sister chromatid can undergo different patterns
ofcrossing over than the other.
- The broken pieces then will rejoin with the chromatid of its homologous pair to
produce new genetic combinations.
- This process of the broken pieces of chromatids
recombining with another chromatid is called RECOMBINATION.
**Crossing over increases genetic variety because 4 cells are produced that have different genetic composition
3. RANDOM NATURE OF FERTILIZATION
In conclusion, all three mechanisms described above, produce genetic variability during meiosis by
reshuffling the different genes carried by an individual in a population.
9. gene flow and role in evolution (population gene frequency)
-
In
the book called Introduction to Quantitative Genetics by D.S. Falconer
(1960) population is defined in the genetic sense.
The main points are:-
A Population :-
- A group of breeding individuals
- Who transmit genes from one
generation to the next
- The parental gene types are
broken down to make a new set of
genotypes
- Transmitted to the offspring by
the genes in the gametes
- Genes carried by population will continue from generation to
generation
- The genetic constitution of the
population – ie the genes it carries is called GENE FREQUENCIES.
- Gene frequencies refers to
- Presence of specific alleles at
every locus
- The numbers/proportions of
different alleles at each locus.
(McClean, P. 1998).
Gene flow :-
- The transfer of alleles or
genes from one population to another is an important factor to evolution.
- The flow of alleles can be due
to migration of individuals or gametes.
- This flow of people in and out
of a group causes :-
- Change
in gene structure of population
- Introduction
of new genetic variation in different
geographic locations and habitats.
- New
mutations may appear and bring about new genotypes and phenotypes variations.
- Harmful
mutations are eliminated from the population
by natural selection
- Favourable/beneficial
mutations will remain and spread through the population.
- A group of breeding individuals
Genetic population – total gene
or allelic frequencies for all the genes in that population.
EVOLUTION occurs when the gene frequencies in a population change
Gene frequencies change when:
EVOLUTION occurs when the gene frequencies in a population change
Gene frequencies change when:
- There
are traits that enable organism to survive long enough to reproduce
- Mutation
of alleles present
- Migration
of individuals with new alleles occurs
ß
Creates variation in the population.→SELECTION CHOOSES THE INDIVIDUALS WHO ADAPT BETTER INTO POPULATION
ßPOPULATION EVOLVES.
For population changes to occur, a change in gene frequencies is needed:-
3 methods of change/ FORCES THAT LEAD TO CHANGES IN GENE FREQUENCIES IN POPULATION
- MUTATION - beneficial – fitness is improved so
increase frequency of alleles in population
and can become the dominant alleles in the population.
- Harmful- will be lost
- Neutral
- MIGRATION - Immigration – brings in more alleles
into population so gene frequencies change. Introduction of new alleles in
population = gene flow.
- Emigration
- loss of alleles
- SELECTION
- new mutations can occur in the population that could increase
fitness of the individual. If this fitness leads to reproductive advantage then
these alleles will become more prevalent in the population. These new alleles
are then selected.
New traits introduced into population contributes to the fitness of the population in the current or future environment.