Inheritance — Edexcel Linear - OL Revision Notes & Topical Questions

The entirety of an organism's DNA is its genome. A gene is a section of a DNA molecule that codes for a particular sequence of amino acids, and these sequences form proteins. Genes control our characteristics because proteins do important jobs in cells: structural proteins such as collagen in skin, enzymes and hormones.

In the nucleus the DNA double helix supercoils to form chromosomes, which are only visible during cell division.

DNA (deoxyribonucleic acid) is the genetic material in the nucleus.

RNA (ribonucleic acid) is also a polynucleotide, a long chain of nucleotides.

Turning a gene into a protein has two stages.

Transcription (in the nucleus):

Translation (in the cytoplasm, on ribosomes):

Alleles are alternative forms of the same gene; they differ slightly in base sequence, which is why they produce different versions of a characteristic.

Alleles are different forms of the same gene, with small differences in base sequence.

The phenotype is the observable characteristic.

You do not have to memorise PTC. In exams you will be given an unfamiliar characteristic: use capital and lower-case letters for the alleles and always state the phenotype that matches the genotype.

In codominance both alleles are expressed in the phenotype. Human ABO blood group is an example, with three alleles of one gene: I^A, I^B and I^O. I^A makes antigen A and I^B makes antigen B; they are codominant with each other but both are dominant to I^O, which makes no antigen.

Worked idea

• a group A parent and a group B parent can have a group O child only if they are I^AI^O and I^BI^O, since the child must inherit I^O from each.
• Their Punnett square gives a 25% chance of group O (and 25% each of A, B and AB).

Most characteristics result from several genes interacting, not one gene.

Monohybrid inheritance is the inheritance of a characteristic controlled by a single gene. A Punnett square shows the possible allele combinations in offspring, from which you can work out ratios and percentage chances.

Example

• in pea plants the tall allele T is dominant to the short allele t.
• Crossing a pure-breeding tall plant (TT) with a short plant (tt) gives offspring that are all Tt and all tall (the F1 generation).
• Letters for alleles should make it clear which is dominant.

In pea plants the tall allele T is dominant and short t is recessive.

A cross between a heterozygous tall plant (Tt) and a short plant (tt) gives offspring Tt, Tt, tt, tt: a 1:1 ratio of tall to short. This is a useful test cross.

A Punnett square shows possible allele combinations and their ratio, and the ratio can be converted to probabilities.

A pedigree diagram traces a characteristic, usually a genetic disorder, through the generations of a family and helps to work out the probability that someone inherits it.

Sex is determined by one whole chromosome pair, not by one or a few genes.

Mitosis is nuclear division giving rise to genetically identical cells. Most body cells are diploid, with two copies of each chromosome. Before dividing the chromosomes duplicate (forming X-shaped chromosomes), so after the cell divides each new cell still has two copies of every chromosome.

All body cells except gametes come from mitosis of the zygote. It is used for growth (new cells, for example when a zygote becomes an embryo), repair of damaged tissue, replacement of cells such as skin and red blood cells, and asexual reproduction, where the offspring are genetically identical to the parent.

Meiosis is a type of nuclear division that gives cells that are genetically different.

Exam tip

• learn two or three differences and be specific;
  • know the reason for each type of division and the cells where it happens.

A diploid cell contains two complete sets of chromosomes (2n).

Variation means differences between individuals of the same species.

Mutations are rare, random changes in the sequence of bases in a gene or chromosome.

Most mutations do not alter the protein, or alter it so slightly that its function is not changed, so most have no effect on the phenotype.

Occasionally a mutation creates a new allele and a new phenotype.

Worked reasoningsuppose a mutation replaces one base in the gene for an enzyme.

Mutations occur spontaneously and continuously, but the rate increases with exposure to ionising radiation (gamma rays, X-rays and ultraviolet rays), which damages bonds and changes base sequences, and to chemical mutagens such as tar in tobacco smoke. A higher mutation rate can make cells cancerous, which is why these agents are linked with cancers.

Charles Darwin proposed that evolution has occurred and that natural selection is the process that drives it. The stages are:

Examples are snail shell colour and peppered moth colour. Exam questions often give an unfamiliar example, so learn these four steps and apply them to whatever organism is described.

Key idea

• natural selection acts on variation that already exists.
• Individuals do not change their genes because they need to;
  • mutation creates new alleles at random and the environment then selects.
• Natural selection can be contrasted with selective breeding, where humans choose which individuals reproduce.

Natural selection can produce bacteria that are resistant to antibiotics.

A resistant population can only be treated with a different antibiotic, sometimes several, so resistance makes bacterial infections harder to control. In an exam answer include the natural selection steps: variation (mutation), increased survival, increased reproduction and increased allele frequency.