A2 OCR Biology: Meiosis and Variation

A2 OCR Biology: Meiosis and Variation

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  • Created on: 25-05-11 00:08
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Meiosis and Variation
Variation is the sum of differences between species and within species. Intraspecific
variation is variation within a species. There are two form of variation in phenotypic
In continuous variation there is a range of variation between extremes (i.e. Mass,
height, etc.) The appearance of the phenotype is quantitative and they're polygenic.
In discontinuous variation there are distinct categories with no range of intermediates
in between (i.e. presence or absence of a feature). The appearance of the phenotype is
qualitative and they're monogenic.
Genotype and Environment Contribute to Phenotype
Milk yield is an example of continuous variation. it is influenced by the environment and by
genes. The volume of milk produced by a cow is determined by the genes inherited from her
parents and by environmental factors, such as quality and quantity of food. The
improvement in milk yield is the result of selective breeding and improved feedings.
Without variation there can be no selection; variation provides the `raw material' for
Role of Meiosis in Variation
New genetic material is generated by gene mutation. in most organism, meiosis occurs
before the fusion of gametes. Whenever meiosis occurs, it shuffles most genes the
organism has inherited; which gives new combinations of alleles. This happens by:
gene mutation is gamete-forming cells
chromosome mutation
crossing over between non-sister chromatids during meiosis I
Without meiosis the chromosome number would double with each generation. Meiosis
occurs during the formation of gametes:
Meiosis I
Prophase I ­ Chromosome condenses becoming shorter and thicker; Homologous
chromosomes pair to form bivalents; Chiasmata form to hold chromosomes
together; non-sister chromatids undergo crossing over; Nuclear membrane breaks up;
centrioles replicate and move to opposite poles and form spindles.
Metaphase I ­ Bivalents move to the equator across the cell; paternal and maternal
chromosomes in each bivalent position themselves independently; Microtubules
attach to the centromere of each chromosome.

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Anaphase I ­ Chromosomes are pulled towards the poles by the shortening of the
Telophase I ­ Chromosomes reach opposite poles; nuclear membrane reforms to make
two haploid daughter cells; Cytokinesis occurs.
Meiosis II
Prophase II ­ Centrioles replicate and move to opposite poles; Nuclear membrane breaks
Metaphase II ­ Individual chromosomes align on the equator with their chromatids
randomly arranged (crossing over occurred in Meiosis I); Microtubules attach to the
centre.…read more

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where E = expected value and O =
observed value…read more


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