This document contains summarised notes taken for Topic 8 Origins of genetic variation, using the Pearson Edexcel B biology activate textbook. Notes taken with referencing to specification.
8.1 – GENETIC INFORMATION
1 – ORIGINS OF GENETIC VARIATION
DNA – genetic info, series of bases in order to code for proteins
MUTATIONS – changes to the genetic material of a cell
~ They can be small-scale (gene mutations) or large-scale (chromosome mutations)
GENE – length of DNA coding for a specific protein
GENE MUTATIONS – changes to the base sequence or quantity of DNA within a gene or section of
DNA.
~ These can occur randomly during DNA replication
CHROMOSOME MUTATIONS – changes to the structure or number of whole chromosomes
Usually occur in meiosis
MEIOSIS AS A SOURCE OF GENETIC VARIATION
MEIOSIS – a reduction division that occurs only in the sex organs.
~ It halves the chromosomes number in the cells from diploid to haploid.
~ Meiosis introduces genetic variation in two ways:
INDEPENDENT ASSORTMENT (RANDOM ASSORTMENT)
~ In meiosis the components of the chromosome pairs from the father and mother are
distributed into the gametes randomly.
RESULT: many new combinations of alleles and introduces genetic variation.
CROSSING OVER (RECOMBINATION)
~ Takes place during meiosis when large multienzyme complexes ‘cut and join’ bits of the
maternal and paternal chromatids together at the chiasmata.
~ This leads to genetic variation as many new combinations of alleles arise.
~ Crossing over is a potential source of mutation, which introduces new combinations into the
genetic make-up of a species.
INHERITENCE OF MUTATIONS
~ If a mutation occurs during the formation of gametes, then it may be inherited by offspring
~ Mutations are important for evolution as it increases genetic variation within a species. (eye
colour)
~ If a mutation occurs during mitotic division of somatic cells, then it will not be inherited by
offspring.
MITOTIC DIVISION – happening during mitosis where they aren’t forming gametes just forming
more of themselves.
SOMATIC CELLS = body cells and not gametes
Mutations that occur in somatic cells can often result in cancers
DIFFERENT TYPES OF MUTATIONS
POINT OR GENE MUTATIONS – caused by the miscopying of just one or a small number of
nucleotides.
, Include substitutions, where one base is substituted for another, deletions, where a base is
completely lost in the sequence, or insertions, when an extra base is added in that it is a repeat of
one of the bases already there or a different base entirely.
CHROMOSOMAL MUTATIONS – involve changes in the positions of genes within chromosomes.
CHROMOSOME MUTATIONS – occur when an entire chromosome is either lost during meiosis or
duplicated in one cell by error in the process. Usually have a major impact on the organism.
CAUSES OF MUTATION
~ The rate of mutation is rapid.
~ This rate is increased by mutagens
MUTAGEN – is a chemical, physical or biological agent that causes mutations
Many mutagens cause mutations in DNA that lead to the cell becoming cancerous – CARCINOGENS
RANDOM FERTILISATION AS A SOURCE OF GENETIC VARIATION
~ In sexual reproduction, the male and female gametes from two unrelated individuals fuse to
form a new genetic individual.
~ This introduces considerable genetic variation into the offspring.
~ A combination of all these sources of genetic variation – meiosis, mutation and random
fertilisation – ensures genetic variation in each new organism produced by sexual
reproduction.
2 – TRANSFER OF GENETIC INFORMATION
A MONOHYBRID CROSS WITH PURE BREEDING PEA PODS
GENOTYPE - partly the result of the genotype (the genetic information) passed from parents to
their offspring and the effects of the environment in which the organism lives.
PHENOTYPE – the physical and chemical characteristics that make up the appearance of an
organism.
HOMOZYGOUS – if both the alleles coding for a particular characteristic are identical, then the
individual is homozygous – it is a homozygote. (‘homo’ = same)
HETEROZYGOUS – If the alleles coding for a characteristic are different, the individual is
heterozygous for that characteristic and is called a heterozygote. (‘hetero’ = different)
DOMINANT – When a phenotype shows dominance it is expressed whether the individual is
homozygous for the characteristic or not.
RECESSIVE – When a phenotype is recessive, it is only expressed when both alleles code for the
recessive feature, the individual is homozygous recessive for that trait.
Dominant allele for colour = green allele (G) gg = recessive, no dominant allele to show itself -
Recessive allele = yellow allele (g) YELLOW
Dominant – shows itself GG/Gg = show = GREEN
Pure breeding pea pods are always HOMOZYGOUS – pure breeding green pods will have a GG
genotype and pure breeding yellow pods will have a gg phenotype
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