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Genetics is the study of DNA and hereditary changes in living organisms. The idea that species of organisms can change over time was first thought about and later published by the famous English naturalist, Charles Darwin in 1859. He wrote a book called “On the Origin of Species”, which continued to highlight views such as natural selection and survival of the fittest. These terms are now accepted by the wider community and are common phrases. At the time of writing the origin of species, DNA and other genetic science had not yet been discovered and it wasn’t until 1953, nearly 100 years later, that Francis Crick and James Watson discovered the DNA molecule and nucleic acids.


  • Allele – Alternative forms of a gene occupying a position on a chromosome.
  • Heterozygous – has two different alleles (example Rr).
  • Homozygous – has two alleles the same (example RR or rr).
  • Genotype – The genetic constitution of an organism.
  • Phenotype – the appearance determined by the genotype (example red)
  • Dominant – out of the pair of alleles this is the one which takes effect or appearence and is usually represented by a capital letter (example R)
  • Recessive – An allele that affects the phenotype of the organism only if a dominant allele is not present.

Mendels Laws

First Law – The Law of Segregation

Characteristics of an organism are controlled by pairs of alleles that separate in equal numbers into different gametes as a result of meiosis.

Second Law – The Law of Independent Assortment

Two or more pairs of alleles segregate independently of each other as a result of meiosis, provided the genes concerned are not linked by being on the same chromosome.

Monohybrid Inheritance

Crossing two pure bred

Phenotype – Tall x Short

Genotype – TT x tt

Gametes T T x t t


F1 – All tall heterozygous offspring


F2 – Three tall, One Short Offspring. 3:1 ratio

Dihybrid Inheritance

Below is a video I found truly unique in the ways it looks at dihybrid inheritance.