People inherit different traits from parents, they are determined by

• Set of genes inherited from parents (nature)
• The environment where they developed (nurture)

Characteristics

Some characteristics (for example, height, eye colour, ear shapes etc.) are dependent on genes

They can be acquired through environmental factors (e.g. hair length, scars) or determined by your genes in your DNA

These observable characteristics are called phenotypes

A full set of genes is called a genotype (a combination of alleles that you have)

These are represented through capital and lower case letters. So the possibilities are AA (inherited an A from both parents), aa (inherited an a from both parents), and Aa (inherited A from one parent and an a from the other)

AA - homozygous dominant, aa - recessive dominant, Aa - heterozygous

There are two types of characteristics: continuous and discontinuous

Continuous are measured in a range of values (e.g. height); discontinuous means that there are one of a few options (e.g. eye colour)

Variation

Sexual reproduction drives this

Independent assortment, random fertilisation (the sperm that reaches the egg is random) and crossing over cause genetic variation

Mutations also result in genetic variation

• Changes in type/amount of DNA
• Can be caused by environmental factors (e.g. radiation, chemicals)

  things that increase the chance of mutations are called mutagens

Alleles

Homologous pair contains two genes (code same characteristics)

• May have different versions; different versions of the same gene is called an allele -> e.g. there are brown eye alleles and blue eye alleles etc.

Homozygous means there is the same allele for a characteristic (2 blue eyes alleles for example); this results in a 100% chance to express the trait (so 2 blue eyes alleles = 100% chance to have blue eyes)

Heterozygous means that are two different alleles (one brown eye allele and one blue eye allele); this now means that whatever trait is expressed is due to dominance

Dominant and Recessive traits

• Dominant traits are represented with capital letters (e.g. A)
• Recessive traits are lower case (e.g. a)
• Dominant will always be expressed over recessive traits (so if brown eyes were dominant, then if there was a brown eye allele then the child would have brown eyes. The blue eyes can only be expressed if there are 2 blue eyes and no brown eyes alleles. The blue eyes would be recessive in this case)

Co-dominance and Incomplete

Co-dominance

• Make extra phenotype; 3 phenotypes
• For example, there are 3 types of blood cells, A, B and O
• A and B are dominant while O is recessive
• AB is co-dominant; it is when both A and B are expressed (because they are both dominant and are expressed)

Incomplete

• Neither are fully dominant over the other
• A and B are neither dominant nor recessive
• Blend together into a mix; combine for a new outcome

Comparing the two:

Here is an example… with my favourite animal, a kitten!

Co-dominant: White cat and black cat produce a cat with black and white patches

Incomplete: white cat and black cat blend together and form a gray cat