Edexcel A level Biology B Student Book 2 ActiveBook
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
The benefits of buying summaries with Stuvia:
Guaranteed quality through customer reviews
Stuvia customers have reviewed more than 700,000 summaries. This how you know that you are buying the best documents.
Quick and easy check-out
You can quickly pay through credit card or Stuvia-credit for the summaries. There is no membership needed.
Focus on what matters
Your fellow students write the study notes themselves, which is why the documents are always reliable and up-to-date. This ensures you quickly get to the core!
Frequently asked questions
What do I get when I buy this document?
You get a PDF, available immediately after your purchase. The purchased document is accessible anytime, anywhere and indefinitely through your profile.
Satisfaction guarantee: how does it work?
Our satisfaction guarantee ensures that you always find a study document that suits you well. You fill out a form, and our customer service team takes care of the rest.
Who am I buying these notes from?
Stuvia is a marketplace, so you are not buying this document from us, but from seller amina101. Stuvia facilitates payment to the seller.
Will I be stuck with a subscription?
No, you only buy these notes for $7.17. You're not tied to anything after your purchase.
