iGCSE Biology Section E


Chromosomes, genes and DNA

The nucleus of the cell contains the chromosomes, where the genes are located.
Gene - a section of DNA that controls certain characteristics.
Allele - a type of gene which give rise to inherited characteristics. Some alleles are dominant - they show up over another allele. Recessive alleles only show up if the allele is present on both chromosomes.
Normal body cells are diploid. They have 46 chromosomes each arranged as 23 pairs of matching (homologous) chromosomes.
Sex cells are haploid. They only have 23 chromosomes each.

Mutations happen when the DNA is replicating as a cell divides.

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Mitosis and meosis

Mitosis is the division of a diploid cell creating two cells with identical sets of chromosomes.
Meosis creates 4 haploid cells that are not genetically identical.(http://ghr.nlm.nih.gov/handbook/illustrations/mitosismeiosis.jpg)look in book

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Sexual reproduction
Two sex cells combine to form a new diploid cell (zygote) at fertilisation.
There is genetic variety because of the variety in the sex cells as a result of meiosis.

Asexual reproduction
A part of the organism grows and forms a genetically identical organism by mitosis, known as a clone.

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Genes and inheritance

Dominant allele - the characteristic will show up in the offspring even if another allele is present
Recessive allele - the characteristic will only show up if both alleles are inherited.
Homozygous - both alleles are the same
Heterozygous - it has two different alleles
Phenotype - describes physical characteristics of an organism
Genotype - describes the alleles that a cell or organism has
Codominance - when neither allele is dominant

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Genetic cross

Phenotype of parents:       tall           dwarf
Genotype of parents:         TT               tt
Gametes (sex cells)            T                 t
Genotype                                      Tt
Phenotype                                 all tall
Gametes                   male gametes  female gametes
                                        T or t            T or t
                                            Punnet square
                                            female gametes
                                           T                   t
  male gametes            T   TT                 Tt
                                     t    Tt                   tt
3 tall : 1 dwarf

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Family pedigree

Also known as a family tree

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Codominance and sex determination

If a plant had a red allele and a white allele, if the alleles are codominant then the plant would be pink.

XY - male
** - female

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Natural selection and selective breeding

Evolution - organisms on earth changing
Natural selection - survival of the fittest - e.g. peppered moth
People selectively breed animals and plants for desired characteristics.
Micropropagation - small pieces of plants that are grown to produce commercial quantities of clones with desirable characteristics.

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