Meiosis and Genetics OCR A2 Biology

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Biology F215
Meiosis and Genetics
Key Words
Autosomes - All the chromosomes expect X and Y
Bivalent - The structure formed when homologous chromosomes pair up
Centromere - The structure that holds two sister chromatids together
Chiasma(ta) - The structural point where non0sister chromosomes in a bivalent join during crossing over
Chi-Squared test - A statistical test that can be used to determine if any difference between the observed and the expected is
significant or due to chance
Chromatid - The name given to each of the two strands of a replicated chromosome
Chromosomal mutation - A random and unpredictable change in the structure or number of chromosomes in a cell
Complementary genes - Genes that interact to govern the expression of a single characteristic
Crossing over - An event during meiosis I, where non-sister chromatids break and rejoin so that part of one of the chromatids
has swapped places with the same part of the other
Dihybrid cross- A genetic cross in which two different genes are considered
Diploid - An organism or cell which has two sets of chromosomes - denoted by 2n
Epistasis - The interaction of genes concerned with the expression of one characteristic, one gene may mask the expression of
another gene
Gamete - A specialised sex cell produced by meiosis
Genotype - Alleles for a particular characteristic that are present in an organism
Haploid - An organism or cell which has one set of chromosomes - denoted by n
Heterogametic - A sex that has non-homologous (XY) sex chromosomes
Homogametic - A sex that has homologous (XX) sex chromosomes
Homologous - Two chromosomes that carry the same genes in the same positions
Independent assortment - This results from the random alignment of homologous pairs on the equator during Meiosis I
Karyotype - All the chromosomes possessed by an organism arranged in order
Linkage - When two or more genes are located on the same chromosome
Locus - The position on a chromosome where a gene is located
Meiosis - A type of nuclear division that halves the chromosome number
Non-disjunction - This is the failure of members of a homologous pair of chromosomes or a pair of chromatids to separate
during nuclear division
Phenotype - The observable characteristics of an organism
Sex -Linked - A gene with its locus on one of the sex chromosomes, X or Y

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Zygote - A cell formed from the fertilization of an egg cell by a sperm cell
What are chromosomes?
In eukaryotes, chromosomes are linear DNA molecules, packaged with histone proteins to form chromatin
o Prior to nuclear division, the chromatin condenses to form discrete chromosomes
What are the male and female karyotypes?
The first image is of the male karyotype, the second therefore is the female karyotype
What are homologous chromosomes?
Homologous chromosomes, are chromosomes which carry the same genes at the same position or…read more

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What are diploid and haploid cells?
Diploid cells have two copies of each chromosome so are annotated as (2n)
Haploid cells have only one copy of each chromosome so are annotated as (n)
What happens during interphase?
During interphase new proteins are synthesised and
alongside the production of organelles and
o Also during this S-phase, DNA is replicated to
produce two sister chromatids which are
joined together at the centromere
What is asexual reproduction?
Asexual reproduction is the formation of genetically
identical offspring from…read more

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The chiasmata separate and lengths of the chromatid which had been crossed
over remain with their newly attached to chromosome
o Telophase 1
In animal cells the chromosomes uncoil
Two nuclear envelope forms in most animal cells
o Cytokinesis occurs as the plasma membrane pinches inwards
o Prophase 2
The chromosomes condense
The nuclear envelope breaks down if it had reformed
o The centrioles replicate and move towards the poles of the cell forming a
spindle at right angles to the original spindle
o Metaphase…read more

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Resulting in genetic diseases such as Trisomy 21 - Down's Syndrome
o Leads to polyploidy if whole sets of chromosomes fail to separate
What other sources of variation arise from sexual reproduction?
The random mating within the population results in variation as different individuals have a different
combination of alleles
The random fertilisation between two unique gametes results in variation as the gametes are different due to
cross over and independent segregation and assortment in meiosis
What are DNA mutations?
DNA mutations are random mutations…read more

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If there are more than one allele for a gene, the gene represented by an upper case letter and the alleles are in
superscript e.g.…read more

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Why does the allele for sickle cell globin remain at high levels (up to 20%) in some populations?
In populations in areas of malaria sickle cell anaemia is an example of heterozygous advantage
o This is because sickle cell anaemia reduces the severity of malaria
As the Plasmodium is unable to infect it like normal red blood cells
Taking this to a more in-depth level, in areas where malaria is endemic, there are two strong selection pressures:
o Against HS HS - have sickle cell…read more

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When writing this out always choose symbols that show the gene is present on the X chromosome e.g.…read more

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What is autosomal linkage?
If two genes are located on different chromosomes
o There are four different gametes
This can then lead to four possible phenotypes
o If two genes are located on the same chromosome (are linked)
They are usually inherited together so there are only two possible gametes
This means that fewer phenotypes are obtained
o Sometimes all the expected phenotypes are obtained but in an unusual ratio due to separation at cross
over or being far apart on the chromosome
What is…read more

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All the F1 generation have purple flowers if these are breed to produce the F2 generation they
have purple and white flowers in the ratio 9:7
This must be because at least one dominant allele for both gene loci C-R- has to be
present for the flowers to be purple
o All other genotype combinations such as ccR- and C-rr produce white flowers
this is because the homozygous recessive condition at either locus masks the
expression of the dominant allele at the other locus…read more


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