Inheritance and Selection

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Genotype is the genetic composition / make up of an organism

Phenotype is the characteristics of an organism, resulting from its genoype and the effects of the environment

Gene is a section of DNA that determines a signle characteristic of an organism

Allele is one form or variation of a gene

Homologous Chromosomes are a pair of chromosomes that have the same gene loci and so determine the same features. They are not necessarily identical

Homozygous is when the alleles of a gene are identical

Heterozygous is when the alleles of a gene are different

Monohybrid Inheritance is the inheritance of a single gene

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Sex Linkage

Any gene that is carried on the X or the Y chromosome is sex linked. As the X chromosome is longer than the Y chromosome, there is no equivalent homologous portion of the Y chromosome. Therefore, those characteristics that are controlled by recessive alleles on this non-homologous portion of the X chromosome will appear more frequently in the male. For example, haemophilia.(         (

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Co-Dominance and Multiple Alleles

Co-dominance: both alleles are equally dominant so both alleles are expressed in the genotype

Multiple alleles: there are more than two alleles, yet only two may be present at the loci

Sometimes a gene has more than two alleles, e.g ABO blood groups. Although there are multiple alleles, only two can be present in an individual. There are multiple possible genotypes: AA/AO, BB/BO, AB, OO. A and I are co-dominant and O is recessive.


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Hardy-Weinberg Principle

Allelic Frequency is the number of times an allele occurs within the gene pool

Conditions of Hardy-Weinberg Principle:

  • no mutations arise
  • population is isolated: no flow of alleles into or out of it
  • no selection: all alleles are equally likely to be passed on
  • population is large
  • mating is random

A gene has to alleles a dominant allele (A) and a recessive allele (a):

  • is the frequency of allele A
  • is the frequency of allele a

p + q = 1

p2 + 2pq + q2 = 1

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Reproductive success affects allele frequency in populations:

  • all organisms produce more offspring than can be supported by the supply of resources
  • despite this, most populations remain constant in size
  • so there is competition between members of species
  • there is a gene pool containing a wide variety of alleles
  • some individuals will posses combinations of alleles that make them better able to survive in their competition with others
  • these individuals are more likely to obtain the available resources and so grow more rapidly and live longer. They will have a better chance of successfully breeding and producing offspring
  • only individuals that successfully reproduce will pass on their alleles to the next generation
  • so it is the alleles that gave the parents an advantage in the competitions for survival that are most likely to be passed on
  • as these new alleles have 'advantageous' alleles, they in turn are more likely to survive and reproduce successfully
  • over many generations, the number of individuals with the 'advantageous' alleles will increase, and the 'disadvantageous' alleles will decrease
  • over time, the frequency of the 'advantageous' alleles will increase and vice versa
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Types of Selection


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Speciation is the evolution of new species from existing species

If two populations become separated, the flow of alleles between them may cease. The environmental factors of each group may differ and selection will affect the two populations in different ways. So the type and frequency of alleles will change. Gene pools of the two populations will become so different that they would be incapable of successfully breeding. So they have become seperate species, each with its own gene pool.

Geographical Isolation can cause this:

  • The individuals of species X form a single gene pool and freely interbreed
  • Climate changes over the centuries lead to drier conditions that separate it in two
  • Further climate changes causes A's region to be much colder and wetter, and B's region to become warmer and drier
  • In region A, phenotypes are selected that are adapted to wetter and colder conditions
  • In region B, phenotypes are selected that are adapted to drier and warmer conditions
  • The type and frequency of the alleles in the gene pools of each group become so different that they are now different species and cannot interbreed.
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