AQA GCSE Genetics & inheritance(Biology)

Genetics & inheritance

There are some key definitions that you need to learn

Key term	Definition
Gamete	A gamete is a sex cell. Examples: Sperm and ovum(egg) in animals Pollen and ovum(egg) in plants
Chromosome	A chromosome is a long, coiled molecule of DNA that contains many genes.
Gene	A short length of DNA that codes for a protein
Allele	An alternative form of a gene
Dominant	A dominant allele is an allele that is always expressed in the phenotype
Recessive	A recessive allele is an allele that is only expressed in the phenotype when two copies are present.
Homozygous	Homozygous means having two identical alleles for a particular gene.
Heterozygous	Heterozygous means having two different alleles for a particular gene.
Genotype	Genotype is the combination of alleles an organism has for a particular gene.
Phenotype	Phenotype refers to the observable features of an organism, which are influenced by its genotype and the environment.

Controlling Characteristics

Some characteristics are controlled by one gene:

e.g. fur colour in mice or red-green colour blindness in humans.

Most characteristics are controlled by many genes (polygenic):

e.g. height, skin colour

You may have to classify if a characteristic is controlled by a single gene or is polygenic.

Single gene	Polygenic
Characteristic controlled by a single gene	Characteristic controlled by multiple genes
Environment does not affect the outcome	Environment can influence the phenotype (nutrition, sun, exercise, etc.)
There is a limited number of outcomes e.g. Yes or No, blood group A,B,AB or O,	There is a whole range out outcomes. Normally a minimum and maximum value with lots of intermediate values. e.g. height or weight
Results in discrete data that can be represented by a bar chart	Results in continuous data that can be represented in a graph that shows a normal distribution curve

Alleles

An allele is an alternative form of a gene.

Lets take eye colour as an example being blue or brown.

Allele	Eye colour
B	Brown
b	Blue

The diagram shows three pairs of homologous chromosomes, each representing a different genotype for a single trait: BB, Bb, and bb.

In the first pair (BB), both chromosomes carry the same dominant allele, so the organism is homozygous dominant and will express the dominant trait in the phenotype(brown eyes).

In the second pair (Bb), one chromosome has the dominant allele and the other has the recessive allele, so the organism is heterozygous. The dominant trait will still be expressed in the phenotype(brown eyes).

In the third pair (bb), both chromosomes carry the recessive allele, so the organism is homozygous recessive and the recessive trait will be expressed in the phenotype (blue eyes).

Genotype vs Phenotype

Genotype is the combination of alleles an organism has for a particular gene.

For eye colour this could be BB, Bb or bb

Phenotype refers to the observable features of an organism, which are influenced by its genotype and the environment.

For eye colour this could be Brown eyes or blue eyes

Dominant and Recessive alleles

Dominant allele:Expressed even if only one copy present (B)

Recessive allele:Only expressed if two copies present (bb)

This means that BB or Bb will give brown eyes

Only bb will give blue eyes.

A recessive trait can skip generations because a person only shows it if they have two recessive alleles. If they have one dominant and one recessive allele, the dominant trait is shown and the recessive one is hidden, but it can still be passed on to their children.

Homozygous vs Heterozygous

Homozygous = two same alleles e.g. BB or bb

Heterozygous = two different alleles e.g. Bb

From Genes to Traits

Every observable characteristic, called a phenotype, comes from instructions in our genes. The process can be understood as a chain:

Step 1: Gene

A gene is a section of DNA that controls a trait. For example, a gene might control eye colour or fur colour in mice.

Step 2: Allele

A gene can have different versions, called alleles. For example, the eye colour gene might have a brown allele (B) or a blue allele (b).

Step 3: Genotype

The genotype is the combination of alleles an organism has.

BB = two dominant alleles
Bb = one dominant, one recessive allele
bb = two recessive alleles

Step 4: Protein

Genes contain instructions to make proteins. The alleles determine which version of the protein is made. Proteins affect traits like pigment for eye colour.

Step 5: Phenotype

The phenotype is the observable trait. For example:

BB or Bb = brown eyes (dominant allele expressed)
bb = blue eyes (recessive trait only shows if there are two recessive alleles)

Practice Questions

1.Define an allele.

2.Explain why a recessive trait can skip generations.

3.What is meant by phenotype?