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L2 BIO: GENETIC VARIATION AND CHANGE…… DEFINITIONS
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Homozygous
(1) an individual (or a condition in a cell or an organism) containing two copies of the
same allele for a particular trait located at similar positions (loci) on
paired chromosomes (see homologous chromosomes).
(2) Having two identical alleles that code for the same trait.
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A ‘homozygous’ organism for a particular trait is described to possess either a pair
of dominant alleles (e.g. AA) or a pair of recessive alleles (e.g. aa).
Heterozygous
(1) an individual (or a condition in a cell or an organism) containing two different alleles for a
particular trait.
(2) Having dissimilar alleles that code for the same gene or trait.
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Example: a zygote having one dominant allele and one recessive allele, i.e. Aa, for a
particular trait.
Linked genes
Genes that are inherited together with the other gene(s) as they are located on the
same chromosome.
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When a pair or set of genes are on the same chromosome, they are usually inherited together
or as a single unit. For example, in fruit flies the genes for eye color and
the genes for wing length are on the same chromosome, thus are inherited together.
Codominance
A condition in which the alleles of a gene pair in a heterozygote are fully expressed thereby
resulting in offspring with a phenotype that is neitherdominant nor recessive.
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A typical example showing codominance is the ABO blood group system. For instance, a
person having A allele and B allele will have a blood type AB because both the A and
B alleles are codominant with each other.
Incomplete dominance
A kind of dominance occurring in heterozygotes in which the dominant allele is only partially
expressed, and usually resulting in an offspring with an intermediate phenotype.
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In incomplete dominance, a heterozygous organism carrying two alleles wherein one
is dominant and the other one is recessive, (e.g. Aa), the dominant allele will only be partially
expressed. Hence, the heterozygote (Aa) will have an intermediate phenotype.
A typical example of incomplete dominance is the colour of the flower in which R symbolizes
the dominant allele for red pigment and r is the recessive allele for no pigment. In incomplete
dominance, the heterozygous plant carrying both alleles, Rr, will not be able to produce
enough red pigment (since the dominant allele is only partially expressed) and therefore will
appear pink.
Meiosis
A form of cell division happening in sexually reproducing organisms by which two consecutive
nuclear divisions (meiosis I and meiosis II) occur without the chromosomal replication in
between, leading to the production of four haploid gametes (sex cells), each containing one of
every pair of homologous chromosomes (that is, with
the maternal and paternal chromosomes being distributed randomly between the cells).
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Meiosis encompasses interphase, meiosis I and meiosis II (both of these consist of four
major subphases - prophase, metaphase, anaphase and telophase).
The process of meiosis is briefly described as follows. During the interphase,
the chromosomes in a cell are duplicated. This is followed by meiosis I wherein
the chromosomes condense along the center of the nucleus, and pair with
their homologues during crossing over.
Next, the pairs of chromosomes separate and move to opposite ends of the cell.
The cell divides for the first time producing two cells. The two cells will undergo meiosis
II wherein both of them divides further into two cells, each containing one of every decoupled
chromosome’s sister strands (chromatids), thus, producing four genetically
different, haploid cells.
Meiosis is a vital process because it reduces the original number of chromosomes to half,
and allows genetic variability by genetic recombination and independent assortment.
Meiosis produces four haploid cells that may develop into potential gametes so that when
fertilization occurs, a new individual with the full number of genes results, thereby maintaining
the integrity of chromosomal number across generations while promoting genetic diversity and
variability in forms in the population.
Independent assortment
The process of random segregation and assortment ofchromosomes during anaphase
I of meiosis resulting in the production of genetically unique gametes.
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The Law of Independent Assortment speaks of alleles of a gene separating independently
from alleles of another gene (unless they are linked). Hence, the inheritance pattern of
one trait will not affect the inheritance pattern of another.
For instance, the gene for the eye colour is inherited independently from the gene for hair
colour. That is, not all individuals with brown eyes will always have a black hair colour; others
may still have a different hair colour. It is because the gene coding for the eye colour
separates independently (and randomly) from the gene coding for the hair colour
during formation of gametes (meiosis).
Independent assortment of genes is important to produce new genetic combinations that
increase genetic variations within a population.
Law of Segregation
Paired alleles separate during gamete formation. Consequently, each gamete would contain
only one copy of every paired gene.
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During anaphase II of meiosis, the genes on the homologous chromosomes separate as
the homologous chromosomes move apart from each other toward the opposite sides of the
dividing cell.
Crossing over
A process occurring during meiosis wherein two chromosomes pair up and exchange
segments of their genetic material.
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This occurs at the stage when chromatids of homologous chromosomes pair up
during synapsis, forming X-structure (chiasma). The chromatids break into segments (of
matching regions), which are then exchanged with one another.
Crossing over is important because it results in new combinations of genes that are
different from either parent, contributing to genetic diversity.
Recombination
The process or act of exchanges of genes between chromosomes during meiosis
when chromosomes exchange genes. This process results in a different genetic combination
and ultimately to the formation of unique gametes with chromosomes that are different from
those in parents.
Thus, recombination is one of the important ways to promote and
increase genetic diversity between generations.
Mutation
A permanent, heritable change in the nucleotide (base) sequence in a gene or a chromosome.
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Mutation may be small scale (affecting the nucleotide (gene) sequence of a gene) or large
scale (involving a change in the chromosome). It may arise from exposure to
ultraviolet or ionizing radiation, chemical mutagens, viruses, etc. Such a change may result in
the creation of a new character or trait.
Mutations can be SILENT (if they do not change the amino acid sequence or the final protein
produced); HARMFUL (because they stop or alter the production of an essential protein such
as an enzyme); BENEFICIAL (because they help the survival of an organism).
Genetic variation
1. Variations of genomes (all the genetic material) between members of species, or between
groups of species thriving in different parts of the world as a result of genetic mutation.
2. Genetic diversity in a population or species as a result of new gene combinations
(e.g. crossing over of chromosomes), genetic mutations, genetic drift, etc.
3. Important in maintaining biodiversity among species.
Gene pool
The total number of genes of every individual in an interbreeding population.
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A large gene pool indicates high genetic diversity, increased chances of biological fitness,
and survival. A small gene pool indicates low genetic diversity, reduced chances of acquiring
biological fitness, and increased possibility of extinction.
Gene pool increases when mutation occurs and survives. Gene pool decreases when
the population size is significantly reduced (e.g.famine, genetic disease, etc.). Some of the
consequences when gene pool is small are low fertility, and increased probability of acquiring
genetic diseases and deformities.
Gene pool gives an idea of the number of genes, the variety of genes and the type
of genes existing in a population. It can be used to help determine gene frequencies or
the ratio between different types of genes in a population.
Evolution
(1) The change in genetic composition of a population over successive generations, which
may be caused by natural selection, inbreeding, hybridization, or mutation.
(2) The sequence of events depicting the evolutionary development of a species or of a group
of related organisms.
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In order for evolution to occur, there must be genetic variation. Genetic variation brings about
evolution. Without it there will be no evolution. There are two major mechanisms that drive
evolution. First is natural selection. Individuals with advantageous traits are more likely
to reproducesuccessfully, passing these traits to the next generation. This kind of evolution
driven by natural selection is called adaptive evolution. Another mechanism involves genetic
drift, which produces random changes in the frequency of traits in a population.
Genetic drift
The process of change in the genetic composition of a population due to chance or random
events rather than by natural selection, resulting in changes in allele frequencies over time.
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The effect of genetic drift in large populations is usually negligible whereas in
small populations, it predominates. In a small population, genetic drift results in
some alleles becoming more common while others become less common or lost over time.
Natural selection
A process in nature in which organisms possessing certain genotypic characteristics that make
them better adjusted to an environment tend to survive, reproduce, increase in number
or frequency, and therefore, are able to transmit and perpetuate their essential genotypic
qualities to succeeding generations.
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It is the process by which heritable traits that increase an organism’s chances
of survival and reproduction are more favoured than less beneficial traits. Originally proposed
by Charles Darwin, natural selection is the process that results in the evolution of a species.