Mendelian Inheritance & Monohybrid Crosses

Genetics

The study of heredity – the passing of traits from parents to offspring

Trait

an inherited characteristic such as hair colour or eye colour

Genes

Genes are the basic units of heredity, which are sections of chromosomes.

Alleles occur in pairs

One allele comes from each parent

Genes code for traits – the characteristics that make us unique!

Alleles

Each gene may have different versions. The different versions are called alleles.

Different alleles account for most of the variation in the traits of organisms within a species.

Dominant & Recessive Alleles

The traits of the dominant alleles are always expressed if present – even if a recessive allele is also present.

The traits of recessive genes appear only when the dominant genes are not present.

Genotype

Genotype is what the genetic code of an individual is for a specific trait.

For simple inheritance, a genotype is made up of two alleles – one from one parent, and one from the other.

Heterozygous:

Two different alleles (Bb) - said to be a carrier of the recessive gene

Homozygous

Two of the same alleles (bb or BB)

Homozygous dominant: BB

Homozygous recessive: bb

Phenotype

The phenotype is what is expressed on the outside.

For eye colour, the phenotype would be brown or blue eyes.

The phenotype depends on the genotype - a genotype of Bb tells us a person has a brown-eyed phenotype. A genotype of bb tells us a person has a blue-eyed phenotype.

Gregor Mendel

Tried to explain the scheme of inheritance in the middle of the nineteenth century.

Known as the ‘father of genetics’

Pea Plant Experiment

The Law of Segregation

The alleles (formerly known as FACTORS by Mendel) for each trait are inherited separate from one another

This means they split during gamete formation (meiosis)

Recall: Gametes = sperm or egg cells

Punnett Squares

Reginald C. Punnett discovered this approach in 1905

Based on Mendel’s pea plants!

Used to determine the probability of offspring from two parents having a certain genotype

Monohybrid Cross: a cross that involves only one trait

Generations

Parent generation: P1

We cross P1 x P1 to get Generation 1: F1

We cross F1 x F1 (two offspring of the original parents) to get Generation 2: F2

ETC!