The genome, DNA & the double helix

The genome is the entire genetic material of an organism. It is stored as chromosomes — linear arrangements of genes, found in pairs in body cells, made from strands of DNA.

• DNA is a polymer: two strands that twist into a double helix.
• DNA is built from repeating units called nucleotides. Each nucleotide = a common sugar + a phosphate group + one of four bases.
• The four bases are Adenine, Thymine, Cytosine and Guanine. The order of the bases forms a code.

Non-coding DNA: much of our DNA is not inside genes. Some of this non-coding DNA helps to control how and when genes are switched on.

Complementary base pairing & the code

The two DNA strands are held together by base pairs. The pairing rule is fixed:

The order of bases determines the order in which amino acids are linked together to make a protein. The bases are read in groups of three — this is the triplet code. Different forms of a gene (alleles / variants) have different base sequences.

Which base pairs with cytosine?

Genes, alleles, genotype & phenotype

Genes are sections of DNA that determine inherited characteristics. In body cells they occur in pairs — one from each parent. The different forms of a gene are called alleles (variants).

• Dominant allele — shown even if only one copy is present (written as a capital, e.g. B).
• Recessive allele — only shown when both alleles are recessive (lower case, e.g. b).
• Homozygous — the two alleles are the same (BB or bb).
• Heterozygous — the two alleles are different (Bb).
• Genotype — the alleles an organism has (e.g. Bb).
• Phenotype — the characteristics actually shown (e.g. brown eyes).

Watch out: most features (like height) are controlled by many genes, not a single gene. Single-gene examples are used because their inheritance is easy to predict.

Homozygous or heterozygous?

Genotype or phenotype?

Tap a term, then tap where it belongs. (Some terms fit both ideas.)

Punnett squares & the 3:1 ratio

A Punnett square predicts the offspring of a single-gene (monohybrid) cross. Put one parent's alleles across the top, the other's down the side, then combine them.

Two more crosses to know: Bb × bb gives 1 Bb : 1 bb (a 1:1 ratio, 50% each). BB × bb gives all Bb — every offspring is heterozygous and shows the dominant phenotype.

What phenotype ratio?

Your turn — percentage recessive

Your turn — percentage heterozygous

Determining sex: XX and XY

Sex is decided by one pair of chromosomes. Females are XX; males are XY. Egg cells always carry an X. Sperm carry either an X or a Y, and combine randomly at fertilisation.

Mendel: Gregor Mendel worked out the basic rules of inheritance from pea plants long before DNA was known. The significance of his work was not recognised for many years.

Boy or girl?

Variation & mutation

Differences between individuals are called variation. A phenotype is influenced by both the genome and the environment, so variation may be:

• Genetic — caused by different alleles (e.g. blood group, eye colour).
• Environmental — caused by surroundings (e.g. a scar, or a plant grown in the shade).
• Both — e.g. height and body mass are affected by genes and diet.

There is extensive genetic variation within a population. New variation arises from mutations — changes in existing genes that occur at random.

Effect of mutations: most have no effect on the phenotype; some influence it; very few determine a characteristic. The mutation rate is increased by ionising radiation.

What is a mutation?

Evolution by natural selection

Evolution is a change in the inherited characteristics of a population over time, through the process of natural selection, which may result in the formation of a new species.

Evidence & today: fossils record change over time. Evolution is still happening now — e.g. antibiotic resistance in bacteria and Warfarin resistance in rats. If selection is too slow to match a changing environment, a species may become extinct. Darwin and Wallace proposed the theory of evolution by natural selection.

Antibiotic resistance

The three-domain system

As we learn more about organisms (including differences in RNA), classification systems are updated. The modern three-domain system groups all life into:

• Two domains of prokaryotes (organisms with no true nucleus).
• One domain of eukaryotes, which contains four kingdoms: Protists, Fungi, Plants and Animals.

Why it changes: classification is based on evidence. As tools like RNA analysis improve, scientists find better-supported groupings — so schemes are revised over time.

Selective breeding & genetic engineering

Humans can deliberately change the characteristics of organisms in two ways:

• Selective breeding — over many generations, choose the parents with the desired characteristics (e.g. high-yield crops, docile animals) and breed them together.
• Genetic engineering — modifying an organism's genome to introduce a desirable characteristic, by transferring a gene from one organism into another.

Benefits & risks: genetic engineering can raise yields or make medicines, but there are practical and ethical concerns to evaluate (e.g. effects on wild populations and long-term safety). Selective breeding can reduce genetic variation, making a population more vulnerable to disease.

Match each term to its definition

Tap a term on the left, then its matching definition on the right.

The big ideas to know

You've covered all four parts of Eduqas Topic 7 — the genome & DNA, inheritance, variation & evolution, and gene technology. Press Finish to see your score.