Biology Unit 4 Inheritance


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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

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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:

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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:

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Sex link

Females have ** and Males have XY


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Blood group example

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Cystic fibrosis example

Inherited disorder caused by faulty recessive allele (f)- faulty chloride ion channel 

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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)

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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 

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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

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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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