Supporting documents for OCRA Biology Alevel Module 6(6.2) Patterns of inheritance Revision notes - Sexual reproduction Summary notes.
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Course
OCRA Biology Module 6 Patterns of inheritance
Institution
OCR
Sexual reproduction revision notes/ summary notes. Featured topic in OCRA Alevel Biology, Module 6. Module 6.2 Patterns of inheritance.
OCRA Biology ALevel and OCRA Biology AS Level. H020 and H420.
OCR, OCRA Specification. Breadth and depth of biology.
Featured in Paper 1,2,3.
VARIATION: SEXUAL REPRODUCTION
Crossing over – The process by which non-sister chromatids exchange alleles.
1. During meiosis I homologous chromosomes pair up and are in very close proximity to each other
2. The non-sister chromatids can cross over and get entangled
3. These crossing points are called chiasmata
4. The entanglement places stress on the DNA molecules
5. As a result of this a section of chromatid from one chromosome may break and rejoin with the
chromatid from the other chromosome
This swapping of alleles is significant as it can result in a new combination of alleles on the two chromosomes.
Mutations
Although not specific to sexual
reproduction, genetic variation can also be
caused by mutations, as mutations can
result in the generation of new alleles.
New allele may be advantageous,
disadvantageous or have no apparent
effect on phenotype (since the genetic
code is degenerate.
They can remain hidden (not expressed)
within a population for several generations
before they contribute to phenotypic
variation.
Independent assortment - the production of different combinations of
alleles in daughter cells due to the random alignment of homologous
pairs along the equator of the spindle during metaphase I.
Different combinations of chromosomes in daughter cells increases
genetic variation between gametes.
In prophase I homologous chromosomes pair up and in metaphase
I they are pulled towards the equator of the spindle.
Each pair can be arranged with either chromosome on top, this is
completely random. The orientation of one homologous pair is
independent / unaffected by the orientation of any other pair.
The homologous chromosomes are then separated and pulled
apart to different poles.
The combination of alleles that end up in each daughter cell
depends on how the pairs of homologous chromosomes were lined
up.
To work out the number of different possible chromosome
combinations the formula 2n can be used, where n corresponds to
the number of chromosomes in a haploid cell.
Random fusion of gametes at fertilization creates genetic variation between zygotes as each will have a unique
combination of alleles.
Meiosis creates genetic variation between the gametes produced by an individual through crossing over and
independent assortment.
This means each gamete carries substantially different alleles.
Around zero chance of individual organisms resulting from successive sexual reproduction being genetically
identical.
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