A summary of TOPIC 8 Biology B edexcel course, including bottleneck effect and ecosystem definitions and explanations. The notes are written from the spec point, with a full explanation under the heading. This document got me an A* so I recommend!
i Understand that mutations are the source of new variations and that the processes of random
assortment and crossing over during meiosis give rise to new combinations of alleles in gametes.
Mutations (change in nucleotide base sequence of an organisms DNA) occur randomly during DNA
replication. They can:
Change allele frequency of a gene pool
Be passed onto offspring (germ line mutations)
Asexual reproducing animals pass on mutations via
o Vertical transfer i.e. binary fission
o Horizontal transfer i.e. antibiotic resistance by conjugation
ii Understand how random fertilisation during sexual reproduction brings about genetic variation.
Meiosis produces genetically different cells as a result of:
Independent assortment of homologous chromosomes (occurs when bivalents line up at
equator in meiosis I in entirely random fashion)
Crossing over of segments of non-sister chromatids result in the exchange of alleles
Random fusion of gametes during fertilisation
Transfer of geneti c informati on
i Understand the terms ‘genotype and phenotype’,’ homozygote and heterozygote’, ‘dominance’,
‘recessive’, ‘codominance’ and ‘multiple alleles’.
Genotype - the combination of alleles of a particular gene, or genes, present in a haploid gamete or
diploid organism.
Phenotype is a measurable feature of an organism. It might be visible, for example, fur colour, or
detectable only by chemical or immunological analysis, for example, ABO blood groups. Although the
phenotype is affected by the genotype, it is also affected by environmental factors.
Homozygous cells or organisms are diploid and have the same allele of a gene under consideration
on both copies of a pair of homologous chromosomes. The cell or organism is a homozygote for this
gene.
Heterozygous cells or organisms are diploid and have different alleles of a gene under consideration.
The cell or organism is a heterozygote for this gene.
Dominant alleles, when present in the genotype, always show their effect in the phenotype.
Recessive alleles, when present in the genotype, do not show their effect in the phenotype if a
dominant allele of the same gene is also present. Recessive alleles only show their effect in the
homozygous condition.
, Codominant alleles, when present in the genotype, show their effect in the phenotype regardless of
the other allele of the gene.
Multiple alleles occur when there are more than two alleles of a single gene, for example there
three alleles of the human ABO blood group (IA, IB and Io)
Polygenic inheritance – more than one gene for a specific characteristic on more than one locus
ii Be able to construct genetic crosses and pedigree diagrams.
Genetic cross
Pedigree Diagram
iii Understand the inheritance of two non-interacting unlinked genes.
Genes that are unlinked occur on different chromosomes
Non-interacting genes control two different phenotypic features
The non-interacting unlinked genes is a dihybrid cross (describes a mating experiment between two
organisms that are identically hybrid for two traits. A hybrid organism is one that is heterozygous, so
carries two different alleles at a particular locus)
If you get two organisms with homozygous alleles for two different characteristics - one organism
with all dominant alleles, the other with all recessive alleles - and breed them you will get a 9:3:3:1
ratio of phenotypes in the second generation of offspring.
Example: Two unrelated characteristics (i.e. the alleles for which are NOT linked) could be their
height (tall or dwarf) and the shape of their seeds (round or wrinkled).
The alleles are as follows:
T - tall (dominant) t - dwarf (recessive)
R - round seeds (dominant) r - wrinkled seeds (recessive)
We take one tall pea plant with round leaves (TTRR - remember they are homozygous) and one
dwarf pea plant with wrinkled leaves (ttrr) and breed them.
Parents: TTRR x ttrr
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