Biology unit 4 module 4.6 revision

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Unit 4.6
Genetic variation within a species and geographical isolation leads to accumulated different genetic
information in a population and the potential to form new species.
Know the terms genotype, phenotype, gene, allele and locus.
Understand alleles can be dominant, recessive or co-dominant
Know that there may be multiple alleles of a gene e.g. blood groups
Predict the result of monohybrid crosses involving dominant/recessive/co-dominant alleles and
compare with observed results using X2.
Understand the behaviour of chromosomes during meiosis (reduction division) explain the ratios
and combinations of alleles seen in monohybrid crosses.
Predict the results of monohybrid crosses involving multiple alleles and sex linked characteristics.
Understand the concept of gene pool and allele frequency in a population.
Calculate allele/genotype/phenotype frequencies using Hardy-Weinburg equations
(p2 +2pq + q2 = 1.0 and p + q = 1.0)
Appreciate the conditions needed before Hardy-Weinburg equations can be used.
Understand this is a mathematical model which predicts allele frequencies will not change from
one generation to the next.
Appreciate selection results from differential reproductive success and that this does alter allele
frequencies within a gene pool.
Interpret data which shows selection occurring.
Know selection can be directional and stabilising.
Understand that when populations are separated geographically selection for different
characteristics results in differences in their gene pool.
Know if individuals from these populations try to breed together they may not now be physically
capable or their offspring are infertile and so they are now separate species.
Genotype: the genetic composition of an organism
Phenotype: the characteristics of an organism ­ results from interaction of genes and the
Gene: length of DNA coding for a polypeptide
Allele: a version or form of a gene.
Dominant: allele which is always able to express itself in the phenotype.
Recessive: An individual must have 2 copies of this allele for it to be expressed in the phenotype.
Locus: the position of a gene on a chromosome.
Homologous: a pair of chromosomes which have genes controlling the same characteristics at the
same loci. They pair up then separate during meiosis.
Homozygous: both alleles of a pair of genes are identical.

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Heterozygous: both alleles of a pair of genes are different.
Co-Dominance and multiple alleles:
Co-dominance ­ in which both alleles are equally dominant
o Both alleles are expressed in the phenotype.
Multiple alleles ­ where there are more than two alleles, of which only two may be present
at the loci of an individual's homologous chromosomes.
o E.g. blood type.…read more

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Directional selection therefore results in phenotypes at one extreme of the
population being selected for and those at the other extreme being
selected against.
o Stabilising selection: selection may favour average individuals. This preserves
characteristics of a population.
If environmental conditions stay stable, the individuals with phenotypes
closest to the mean are favoured. This tends to eliminate phenotypes at the
o Speciation is the evolution of a new species from existing species.…read more

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Changes in ecosystems can lead to speciation; in southern California 10000 years ago a number of
interconnecting lakes contained a single species of pupfish. Increasing temperatures caused
evaporation and the formation of separate, smaller lakes and streams. This led to the formation of a
number of different species of pupfish. Explain how these different species evolved.
Geographical isolation
Separate gene pools/no interbreeding between the two populations.
Different environmental/abiotic/biotic conditions/ which therefore mean that there are
different selection pressures.
Selection for different or advantageous features/characteristics/specific alleles.…read more


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