Learning outcomes:
understand that the nucleus of a cell contains chromosomes on which genes are located
understand that a gene is a section of a molecule of DNA and that a gene codes for a specific protein
describe a DNA molecule as two strands coiled to form a double helix, the strands being linked by a series of paired bases: adenine (A) with thymine (T), and cytosine (C) with guanine (G)
understand that genes exist in alternative forms called alleles which give rise to differences in inherited characteristics
understand the meaning of the terms: dominant, recessive, homozygous, heterozygous, phenotype, genotype and codominance - b - c
describe patterns of monohybrid inheritance using a genetic diagram - b
understand how to interpret family pedigrees - b
predict probabilities of outcomes from monohybrid crosses - b
understand that the sex of a person is controlled by one pair of chromosomes, XX in a female and XY in a male
describe the determination of the sex of offspring at fertilisation, using a genetic diagram
understand that division of a diploid cell by mitosis produces two cells which contain identical sets of chromosomes - b - c
understand that mitosis occurs during growth, repair, cloning and asexual reproduction
understand that division of a cell by meiosis produces four cells, each with half the number of chromosomes, and that this results in the formation of genetically different haploid gametes
understand that random fertilisation produces genetic variation of offspring
know that in human cells the diploid number of chromosomes is 46 and the haploid number is 23
understand that variation within a species can be genetic, environmental, or a combination of both
understand that mutation is a rare, random change in genetic material that can be inherited
describe the process of evolution by means of natural selection
understand that many mutations are harmful but some are neutral and a few are beneficial
understand that resistance to antibiotics can increase in bacterial populations, and appreciate how such an increase can lead to infections being difficult to control
understand that the incidence of mutations can be increased by exposure to ionising radiation (for example gamma rays, X-rays and ultraviolet rays) and some chemical mutagens (for example chemicals in tobacco).
The nucleus of every cell contains DNA. DNA is a genetic code. Each instruction in the code is called a gene. Each gene tells the cell how to make a specific protein. The proteins are what control the cell (e.g. enzymes are proteins, so are structural proteins like collagen). Sometimes more than one version of a gene occur. The different versions are called alleles (i.e. we all have the gene for iris pigment, but there are different colours of iris pigment, same gene but different alleles).

DNA is a very long molecule. To stop it from breaking it is coiled up inside the nucleus. The coiled up DNA forms a chromosome. Humans have 23 different chromosomes inside their cells. We have two copies of each chromosome, therefore, each cells contains 46 chromosomes. The haploid number is the number of different chromosomes (i.e. 23) and the diploid number is the total number of chromosomes in the cell (i.e. 46)

Key Word Summary:

This topic, more than any other, confuses people. Learn these thoroughly!

DNA: A genetic code
Gene: One instruction in the code telling a cell how to make a specific protein
Allele: A different version of a gene
Chromosome: Coiled up DNA
Haploid number: the number of different chromosomes in a cell (23)
Diploid number: the total number of chromosomes in a cell (46)

Cell Division:
There are two types of cell division;

- Mitosis – used for growth, repair & asexual reproduction
- Meiosis – used to produce gametes for sexual reproduction


1. Produces 4 daughter cells
2. Daughter cells are diploid (i.e. only have 23 chromosomes)
3. Daughter cells are genetically identical to each other
4. Daughter cells are genetically identical to parent cell
5. Occurs in one stage
6. Happens everywhere in the body


1. Produces 2 gametes
2. Daughter cells are haploid (i.e. have 23 pairs of chromosomes)
3. Gametes are genetically different to each other
4. Gametes are genetically different to parent cell
5. Occurs in two stages
6. Happens in reproductive organs only

Therefore, fertilization produces a diploid cell (which will grow by mitosis) from two haploid gametes.

Each parent gives only one of each of the pairs of chromosomes to their gametes. A pair of chromosomes will have exactly the same genes on them, but not necessarily the same alleles! This is the source of genetic variation in gametes.

Alleles for the same gene can be;

- Dominant – always affect the phenotype (allele represented with capital letter)
- Recessive – never affect the phenotype in the presence of a dominant allele (allele represented with lower case letter)
- Co-dominant – affect the phenotype equally in the presence of another co-dominant allele (both alleles have capital letters)


Inheritance patterns are always given using a genetic diagram. If this comes up you get loads of marks for it, but only if you use the genetic diagram!

More Key Words:

Phenotype: physical appearance
Genotype: the combination of alleles an individual possesses
Heterozygous: two different alleles in genotype (i.e. B b)
Homozygous: both alleles the same in genotype (i.e. B B or b b)

Inheritance of gender is governed by the 23rd chromosome. Boys have an X and a Y, girls have two X chromosomes

Variation within a species is produced by two factors

1. The environment
2. The genotype.

New alleles arise in the population through mutation

Mutation - a rare, random change in the genetic code of a gene. The mutated gene will therefore produce a slightly different protein to the original non-mutant gene. The new protein might;

1. Work just as well as it did before (neutral mutation)
2. Work better than before (beneficial mutation)
3. Work worse / not at all (harmful mutation)

Beneficial mutations give a selective advantage to the individual. Individuals with this kind of mutated allele are more likely to survive, reproduce and pass their alleles on. This is the basis of Natural Selection

Natural Selection:

Darwin came up with this theory.

Darwin’s 1st Observation: Not all individuals survive
Darwin’s 2nd Observation: There is variation in a species
Darwin’s Conclusion: The better adapted individuals survive (the “fittest”) and reproduce, passing their alleles onto the next generation. Over time this process leads to evolution.

Evolution: the formation of a new species from an original species.

Mutations can be inherited or happen on their own. The frequency that mutation occurs naturally can be increased by exposure to radiation (e.g. gamma rays, X-rays and ultraviolet rays) and some chemical mutagens (e.g. chemicals in tobacco).