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.
Mitosis and meosis
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.
A part of the organism grows and forms a genetically identical organism by mitosis, known as a clone.
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
Phenotype of parents: tall dwarf
Genotype of parents: TT tt
Gametes (sex cells) T t
Phenotype all tall
Gametes male gametes female gametes
T or t T or t
male gametes T TT Tt
t Tt tt
3 tall : 1 dwarf
Also known as a family tree
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
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.