Biology Unit 4- Inheritance and selection

Unit 4, Chapter 8

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  • Created on: 05-04-14 15:04

8.1- studying inheritance

Genotype- the genetic make-up of an organism, describing all the alleles that an organism contains
Phenotype- the observable characteristics of an organism as a result of the interaction between the expression of the genotype and the environment 
Gene- a section of DNA (a sequence of nucleotide bases) that determines a single characteristic of an organism
Allele- one of the different forms of a gene 
Homozygous- when an individual has the same alles on the same gene
Heterozygous- when an individual has different alles on the same gene
Dominant- the allele which presents itself in the phenotype
Co-Dominance- when both alleles contribute to the phenotype 
Recessive- this allele only presents itself in the phenotype when it is not in the presence of a dominant allele

All individuals of the same genes- but not necessarily the same alleles. Genetic diversity and allele frequency in the gene pool of a population vary between different species.

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8.2- monohybrid inheritance

Genetic cross diagram:

parents + their alleles
possible gametes
male and female gametes
state the phenotypes and genotypes of each offspring, and whether they are heterozygous or homozygous dominant/recessive


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8.3- sex inheritance and sex linkage

All humans have 22 chromosome pairs in common
Final pair is the sex pair- females have two X chromosomes and males an X and a Y
X chromosome is longer than Y chromosome- there is a portion to which the Y has no homologous section.
Any allele on this part of the X chromosome will, therefore, manifest itself in the male phenotype, even if recessive.
Therefore males only have to inherit one allele on the X chromosome from their mother in order for a disease or genetic problem to be present in their phenotype. Therefore, sex-linked diseases etc. are passed on from mother to son on the X chromosome, but daughters may inherit this gene on one X chromosome from their fathers and therefore be carriers and then pass it onto their sons- so it could skip a generation.
Females have to inherit either a dominant allele, or two recessive alleles on this section. 

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8.4- co-dominance and multiple alleles

Co-dominance occurs when two different alleles are both presented partially in the phenotype- e.g. pink flowers with red and white alleles.

Multiple alleles are when a gene has more than two alleles, although only two can be presented in the genotype at one time as there are two homologous chromosomes. This leads to a dominance hierachy, whereby certain alleles are dominant over others.

e.g. human blood types- A and B are co-dominant and result in a blood type AB, but O is recessive.  

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8.5- allelic frequency and population genetics

All the alles of all the individuals in a population at one time is known as the gene pool

The number of times an allele occurs within the gene pool is referred to as the allelic frequency

The Hardy-Weinberg Principle:

This predicts that the proportion of dominant and recessive alleles of any gene in a population remains the same from one generation to the next provided that:

  • no mutations arise
  • the population is isolated
  • there is no selection of which allees are passed onto the next generation
  • the population is large
  • mating is random

p = frequency of allele A
q= frequency of allele a

p + q =1      and therefore p² + 2pq = q² = 1 (expressed as frequency) 


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8.6- selection

Competition between individuals in a species to survive means that those with the most advantageous alleles and therefore phenotypes will be more likely survive to breed and pass on these alleles to their offspring. Those with less advantageous alleles are less likely to survive and breed to pass on their alleles to offspring- therefore the frequency of advantageous alleles increases. 
Which alleles are advantageous to a population depends on the environement and habitiat of the population. 
Sometimes mutations arise which are useful to a population- e.g. a colour change which results in better camoflague from predators.

Types of selection:
- Directional: selection favours individuals that vary in one direction from the mean
- Stabilising: where average individuals are favoured

(look in AQA book pg 129-130 for diagrams) 

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8.7- speciation

The evolution of new species from existing species is known as speciation.

This occurs when a species becomes separated due to geographical isolation, whereby a physical barrier prevents two populations from breeding with one another.
The individuals in each population interbreed and form a single gene pool.
Climate changes over centuries or other changes in environment lead to certain alleles becoming more frequent due to natural selection. This leads to the two species becoming so different over time from how they originally were, that they can no longer breed to produce fertile offspring and are therefore different species.

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