Summary — What This Chapter Covers

1. Mutations

What is a mutation, really?

A gene mutation is simply a change in the sequence of bases in a DNA molecule. Think of a gene as a recipe written in a 3-letter code (each triplet of bases codes for one amino acid). A mutation is like a typo in that recipe. Sometimes the typo doesn't change the meaning at all. Sometimes it changes one word. And sometimes it shifts every single word after the typo, turning the whole recipe into nonsense.

Mutations happen spontaneously during DNA replication — they're not something that only happens when something goes wrong from outside; copying over 3 billion bases perfectly, every single time, is just statistically hard. Mutagens (ionising radiation, X-rays, certain chemicals) increase how often mutations occur, but they aren't required for a mutation to happen.

The three point mutations

Substitution, insertion, and deletion are all called point mutations because each one involves a change at a single location in the DNA sequence — but their effects are very different.

Substitution mutations get split into three sub-types based on what happens to the resulting amino acid:

Effects of mutations — mostly nothing, sometimes everything

Here's the pattern to remember: most mutations do nothing noticeable. Either they're silent, or the small change in the protein doesn't affect its shape or function enough to matter. Only a small number of mutations produce a significantly altered polypeptide — and that follows a chain reaction:

1. DNA base sequence changes
2. Amino acid(s) in the primary structure change
3. Different bonds form in the secondary/tertiary structure
4. The final 3D shape of the protein is altered
5. Function changes — for better, or (much more often) for worse

Very rarely, an altered protein gives an organism an advantage (e.g. antibiotic resistance) — this is the raw material that drives evolution via natural selection. Far more often, altered proteins are harmful, as in cystic fibrosis (non-functional chloride channel) or sickle-cell disease (haemoglobin proteins clump together, distorting red blood cells).

One more crucial distinction: mutations that occur in gametes (sperm/egg cells) can be passed to offspring — every cell of the child will carry it. Mutations in ordinary body cells (somatic mutations) die with that individual and are never inherited, though they can still cause problems like cancer if they affect genes controlling cell division.

2. Patterns of Inheritance & Key Genetics Vocabulary

Genes, alleles, and loci

A chromosome is a long DNA molecule carrying many genes. A gene is a length of DNA that codes for a single polypeptide, and its physical position on the chromosome is called its locus (plural: loci). Every gene can exist in two or more different versions — these are alleles. Different alleles have slightly different base sequences (they arose by mutation) but sit at exactly the same locus.

You'll always see genes written with a single letter, and alleles shown with upper/lower case of that letter — e.g. gene "A", alleles A (dominant) and a (recessive).

Homozygous vs. heterozygous

Every individual has two copies of every gene — one on each chromosome of a homologous pair (one chromosome from each parent, matching in size, shape, and gene content, though not necessarily the same alleles).

Genotype vs. phenotype

Dominant, recessive, and codominant alleles

Codominance uses different notation: the gene gets a capital letter, and alleles are shown as superscripts. Classic example — cattle coat colour, gene C, alleles R (red) and W (white):

• C^RC^R = red coat (homozygous)
• C^WC^W = white coat (homozygous)
• C^RC^W = roan coat — a genuine mixture of red and white hairs (heterozygous, both alleles visible at once)

Reading pedigree diagrams

A pedigree diagram traces a trait through generations of a family. Squares = males, circles = females. Shaded/coloured shapes = affected individuals; unshaded = unaffected. Horizontal lines connect partners who had children together; vertical lines connect parents to their children.

This single rule unlocks almost every pedigree question. If you ever see an affected child appear from two seemingly-normal parents, recessive inheritance is confirmed immediately, and you instantly know both parents' genotypes are heterozygous.

3. Sex Linkage

Why sex chromosomes are special

Sex-linked genes sit on the sex chromosomes rather than on the other 22 pairs (autosomes). Since males are XY and females are XX, and the X chromosome carries far more genes than the tiny Y, sex-linked genetics almost always refers to genes on the X chromosome. This creates a fundamentally different inheritance pattern:

• Females (XX) have two copies of every X-linked gene — just like an ordinary autosomal gene. They can be unaffected, a carrier, or affected.
• Males (XY) have only ONE copy of every X-linked gene (their Y chromosome doesn't carry an equivalent allele). Whatever single allele they have on their one X, that's what shows in their phenotype — there's no second copy to mask it.

Worked example: red-green colour blindness

Colour blindness is X-linked recessive: dominant allele B = normal vision, recessive allele b = colour blind. Question: how can two parents with normal vision have a colour-blind child?

Step 1 — genotypes of parents: Father has normal vision with only one X, so he must be X^BY. Since the child must be able to inherit colour blindness, the mother must carry the b allele — she's a carrier, X^BX^b.

Step 2 — gametes: Father produces X^B or Y gametes. Mother produces X^B or X^b gametes.

The X^bY offspring is a colour-blind son — he only has one X, and it carries the recessive allele, so nothing masks it.

4. Cystic Fibrosis — A Real Genetic Disorder

The molecular cause

Cystic fibrosis (CF) is a genetic disorder of cell membranes, caused by a recessive allele of the CFTR gene (Cystic Fibrosis Transmembrane Conductance Regulator) on chromosome 7. Normally, this gene codes for chloride ion channel proteins that sit in cell membranes and help control secretion of sweat, mucus, and digestive juices.

Here's the chain of cause-and-effect you need to be able to explain in an exam:

Inheritance pattern

Because CF is caused by a recessive allele:

• Heterozygous individuals (Ff) are unaffected carriers
• You need to be homozygous recessive (ff) to actually have the disorder
• If both parents are carriers: 1 in 4 (25%) chance of a child with CF
• If only one parent is a carrier (other is FF): 0% chance — the dominant F allele always masks the recessive f

How it affects three body systems

5. Genetic Screening

The three uses of genetic screening

Genetic screening lets people find out whether a particular allele is present in a genome. There are three main contexts where it's used:

Preimplantation Genetic Diagnosis (PGD)

During IVF (in vitro fertilisation), fertilisation happens in a lab rather than the body. Couples at increased risk of passing on a genetic disorder can have the resulting embryos' DNA analysed before any embryo is implanted into the uterus.

• ✅ Evidence suggests the sampling process doesn't harm the embryo
• ✅ Reduces the chance of having a baby with the genetic disorder
• ✅ Avoids abortion entirely, since screening happens before implantation
• ⚠️ Not ethically simple for everyone — some believe an embryo already has the same moral status as a foetus or baby, making the discarding of "affected" embryos after PGD deeply controversial

Prenatal testing: two methods, two very different risk profiles

This is one of the most exam-tested comparisons in the whole chapter — know these numbers.

6. Ethical & Social Issues of Genetic Screening

Why this always needs balance

Genetic screening raises questions that don't have a single "correct" answer — views depend heavily on someone's religious, moral, and cultural position. In an exam, you must present multiple perspectives, not your personal opinion.

What to Memorise

Concepts Checklist

Exam Tips — Common Mistakes & Mark-Scheme Traps