Biology Unit 4 Inheritance
For AQA
- Created by: Hannah
- Created on: 08-06-11 14:10
Definitions
Gene- A sequence of bases that codes for a protein (polypeptide) which results in a characteristic
Allele- A different form of a gene
Genotype- The genetic compositionof an organism, the alleles an organism has e.g. BB, Bb, bb
Phenotype- The expression of the genetic composition of an organism, and organisms characteristics e.g. brown eyes
Dominant- An allele whose characteristic appears in the phenotype even when only one copy is present- shown by a capital letter
Recessive- An allele whose characteristic only appears in the phenotype if two copies are present- shown by a lower case letter
Codominant- Alleles that are both expressed in the phenotype, neither is recessive e.g. alleles for haemoglobin
Punnet Square/ Genetic Cross
Homozygote- An organism that carries two copies of the same allele e.g. BB, bb
Heterozygote- An organism that carries two different alleles e.g. Bb
Monohybrid Crosses:
Codominance
Example-
People who are homozygous for normal haemoglobin don't have the disease
People who are homozygous for sickle haemoglobin have sickle-cell anaemia
People who are heterozygous have sickle-cell trait:
Sex link
Females have ** and Males have XY
Example:
Blood group example
Cystic fibrosis example
Inherited disorder caused by faulty recessive allele (f)- faulty chloride ion channel
Populations share a gene pool:
Species- similar organisms that reproduce to give fertile offspring
Population- group of the same species living in a particular area
Species can exist as one or more different populations e.g. black bear in Canada and USA
Gene pool- complete range of alleles present in a population
New alleles are usually generated by mutations in genes
Allele frequency- how often an allele occurs (usually given as a percentage)
Natural Selection
Evolution- When the frequency of an allele within a population changes. This can happen when the allele codes for a characteristic that affects the chances of an organism surviving
Variation means not all individuals are at the same likeliness to reproduce.Means there is different reproductive success in a population, as those with the beneficial allele are more likely to survive.
They reproduce and a greater proportion of the next generation inherit the beneficial allele. They reproduce etc. and frequency of beneficial allele increases
This is called natural selection
Stabilising selection- individuals in middle of bell graph most likely to survive. Happens when environment stays the same, reduces range of phenotypes
Directional selection- individuals with alleles for extreme characteristics (end of a graph) more likely to survive and reproduce. Response to environmental change, causes genetic change in population
Hardy- Weinberg Principle
Predicts that the frequency of alleles in a population won't change from one generation to the next
Only true under certain conditions:
Large population, no immigration, no emigration, no mutations, no natural selection.
Needs to be random mating- all possible genotypes can breed with all others
Hardy-Weinberg equations can be used to estimate the frequency of particular alleles and genotypes within populations.
Laso to test whether the principle applies to particular alleles in particular populations.
I.E to test whether selection or any other factors are influencing allelic frequencies- if frequencies change between generations then theres a pressure
Equations:
To predict Allelic Frequency:
p + q = 1 p= frequency of dominant allele
q= frequency of recessive allele
Genotype Frequency:
p2 +2pq + q2 = 1 p2= frequency of homozygous dominant genotype
2pq= frequency of heterozygous genotype
q2= frequency of homozygous recessive genotype
Percentage population that has a certain genotype:
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