Patterns of inheritance

Gene

A unit of heredity made up of a unique sequence of DNA that determines a characteristic of a trait

Allele

The alternative form of a gene which can make it appear in a variety of forms

Genotype

The set of alleles present in the DNA of an individual organism.

It is the result of inheritance and is represented with letters.

Dominant Alleles

Only require at least one in a genotype to show the trait (represented in upper case)

Recessive Alleles

Require both in the genotype to show its trait (represented in lower case)

Homozygous gene

When an individual inherits two identical alleles from both their mother and their father

Heterozygous gene

If an individual inherits two different alleles from both their mother and their father

Phenotype

All of an organisms observable characteristics.

It is the result of inheritance and the effects of an organisms environment

Codominance

When an organism contains and expresses the phenotypes of both different allele as neither allele can mask the expression of the other

Incomplete Dominance

Organism expresses a blending of the two phenotypes as neither genotype is fully expressed.

Also known as partial dominance

Polygenetic inheritance

Refers to phenotypes that require input from multiple alleles for a particular characteristic

Monogenetic inheritance

Phenotypes which use only one set of alleles to determine a characteristic

Epigenetics

Where genetics are influenced by internal as well as external sources

Internal factors

Can influence gene expression such as the over saturation of enzymes, or temperature, or through signalling molecules, we can see some genes get switched on or off

External factors

Can influence gene expression by adding methyl groups to DNA, typically through food

Monohybrid crosses

A cross between two individuals with different alleles at a single locus

Autosomal Dominant Inheritance

Seen through 3 stages: the cross of homozygous parents, the first filial generation, the second filial generation

Ratios: 1GG:2Gg:1GG (genotypic)

3 White Petal:1 Red Petal (phenotypic)

Test cross

Crossing the dominant parent (with an unknown genotype) with a homozygous recessive parent

Codominant inheritance

When neither allele is dominant and both alleles are expressed at the same time

E.g. CʳCʷ, CʳCʳ, CʷCʷ

Incomplete dominance inheritance

When neither allele is dominant and the phenotype of the heterozygous individual is a blend of the 2 pure breeding phenotypes

Sex linked inheritance

The inheritance of any trait from genes located on the sex chromosomes, however many of these traits are X linked

X linked conditions

• Males can only be affected or not affected, and cannot ever be a carrier

• In men, the allele on their one X chromosome will ALWAYS be expressed