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Genetic Variation (Discuss, in depth, how three processes of meiosis…
Genetic Variation
Discuss, in depth, how three processes of meiosis result in new combinations of alleles (variation)
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The process of independent assortment is where the homologous pairs line up in a random order along the cell equator. Therefore, only one chromosome from each homologous pair is placed in the gametes. Therefore, genetic variation is achieved when the chromosomes pairs are separated because each new cell has a different combination of chromosomes from each.
During segregation, only one chromosome from each homologous is placed into the new gametes made. During gamete formation alleles for each gene separate from each other so that each gamete carries one allele per gene. Therefore, genetic variation is achieved because each new cell has a different combination of alleles from each other and only half the chromosomes as the parent cell.
Crossing over is the exchange of segments of DNA between homologous chromosomes. Therefore, the resulting gametes have chromosomes with different combinations of alleles from each other.
Sources of Variation
Meiosis
Meiosis is a type of cell division that occurs in the ovaries and testes to make gametes for sexual reproduction. It produces 4 haploid cells (half the number of chromosomes as the parent cell). Meiosis results in variation due to crossing over, independent assortment and segregation
INDEPENDENT ASSORTMENT occurs when homologous pairs of chromosomes randomly line up at the equator and are assorted into gametes (meiosis I). This means it is completely random which combination of alleles end up in a particular gamete. This results in cells having different combinations of chromosomes.
CROSSING OVER is where homologous chromosomes line up at the cell equator and information is physically swapped. Segments of chromatids break and reattach to the other chromatid. Because of crossing over each gamete produced will have different combinations of alleles
SEGREGATION is when homologous pairs are pulled apart by a spindle to opposite poles of the cell. This is called segregation, and each chromosome pair segregates independently of each other pair resulting in each pole having a random mix of chromosomes
Sexual Reproduction
Sexual reproduction causes genetic variation because 2 unique gametes, which contain a random sample of half the genetic material from 2 unique individuals come together in fertilisation to produce a genetically unique individual, containing a combination of alleles from its mother and father.
Mutation
Mutation is the ultimate source of variation. Mutations are permanent and random changes to the base sequence of DNA. Mutations create new alleles.
Mutations can be heritable or inheritable. If the mutation occurs in a somatic cell, then it cannot be inherited. If however the mutation occurs in a germ cell or gametic cell, then it has the potential to be inherited by offspring.
This can only occur if the gamete that the mutation occurs in is randomly selected in fertilisation. Also, the mutation should not affect the survival or reproduction of the new individual.
Mutations create changes in the DNA base sequence which causes a change in amino acid sequence, resulting in a different protein. This could potentially change the phenotype.
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