1. Chromosomes, Genes & Epigenetics

In brief:Genes on chromosomes control inherited traits. Epigenetics studies how environment and behaviour affect gene expression without changing DNA.

Heredity

Heredity is the passing on of characteristics from parents to offspring via genes. A species is a group of organisms that can interbreed to produce fertile offspring.

Key Terms

Locus: the location of a gene on a chromosome
Allele: alternative forms of a gene (e.g. B = brown eyes, b = blue eyes)
Homozygous: both alleles the same (BB or bb)
Heterozygous: alleles are different (Bb)
Genotype: the genetic make-up (e.g. Bb)
Phenotype: the physical appearance (e.g. brown eyes)
Dominant: allele expressed when present (capital letter)
Recessive: only expressed in homozygous state (lowercase)

Nuclear vs Non-nuclear Inheritance

Nuclear DNA: inherited from both parents via meiosis (46 chromosomes).

Mitochondrial DNA: circular, ~37 genes, inherited only from the mother. Mutations cause mitochondrial diseases.

Chloroplast DNA: ~120 genes controlling photosynthesis, also maternally inherited.

Epigenetics

Epigenetics studies how behaviour and environment affect gene expression without changing the DNA sequence. Mechanisms include:

DNA methylation: chemical group added to DNA, blocking gene expression
Histone modification: changes how tightly DNA is packed, affecting gene access

Epigenetic changes are reversible and can be influenced by diet, stress, smoking, and pollution.

DNA structure — Chemical structure of DNA
Wikimedia Commons (CC)

Punnett square showing monohybrid inheritance
Wikimedia Commons (CC)

Key Points

1Heredity: passing traits from parents to offspring via genes
2Alleles are alternative forms of a gene; dominant masks recessive
3Homozygous = same alleles; heterozygous = different alleles
4Mitochondrial DNA is inherited only from the mother
5Epigenetics: environment/behaviour affects gene expression without changing DNA
6DNA methylation and histone modification are key epigenetic mechanisms

Learning Outcomes

Describe the structure of a chromosome and the role of a gene
Explain what is meant by nuclear inheritance; compare to non-nuclear inheritance
Compare genetic and epigenetic mechanisms; research one example of epigenetic inheritance in nature

2. Genetic Crosses & Sex Linkage

In brief:Punnet squares predict offspring genotypes and phenotypes. Sex-linked traits are carried on the X chromosome.

Monohybrid Crosses

A monohybrid cross involves parents differing in one trait. Use a Punnet square to predict offspring.

Example: Tall (T) is dominant over short (t).

Tt × tt → 50% Tt (tall), 50% tt (short) → 1:1 ratio

Tt × Tt → 25% TT, 50% Tt, 25% tt → 3 tall : 1 short

Incomplete Dominance

Neither allele is fully dominant. The heterozygous phenotype is a blend. Example: Red (RR) × White (WW) → Pink (RW).

Mendel's Laws (HL)

Law of Segregation: Each trait is controlled by a pair of alleles that separate during gamete formation
Law of Independent Assortment: Genes for different traits are inherited independently (unless linked)

Sex Determination

Females = XX, Males = XY. The Y chromosome determines maleness.

Sex-linked Traits

Genes on the X chromosome (e.g. colour blindness, haemophilia). Males are more affected because they have only one X - a single recessive allele will be expressed. Females need two recessive alleles to show the trait; one recessive makes them a carrier.

Key Points

1Punnet squares predict offspring genotype and phenotype ratios
2Tt × Tt gives 3:1 phenotype ratio (dominant:recessive)
3Incomplete dominance: heterozygous shows a blended phenotype
4Law of Segregation: allele pairs separate into different gametes
5Sex determined by X and Y chromosomes: XX = female, XY = male
6Sex-linked traits (on X) affect males more - they only have one X

Learning Outcomes

Model inheritance to the first generation of a single unlinked trait in crosses involving homozygous and heterozygous parents
Model a cross involving incomplete dominance
Illustrate and state Mendel's Laws of Segregation and Independent Assortment
Model inheritance to the second generation of two unlinked traits in crosses involving homozygous and heterozygous parents
Explain how linkage affects Mendel's Law of Independent Assortment (knowledge of crossing over not required)
Model sex determination by X and Y chromosomes in humans
Model the inheritance of sex-linked traits from known examples
Identify benefits and limitations of Mendelian genetics to our understanding of heredity in the modern world

3. Epigenetics: Gene Expression Without Changing the DNA

In brief:Epigenetics explains how the environment and behaviour can switch genes on or off while leaving the base sequence untouched.

Every cell in your body carries the same genes, yet a nerve cell and a liver cell behave completely differently. The difference is gene expression: which genes are switched on. Epigenetics is the study of changes in gene expression that do not alter the DNA base sequence.

Two main mechanisms

DNA methylation: methyl groups attach to the DNA, usually silencing that gene so it cannot be transcribed.

Histone modification: DNA is wound around histone proteins. Chemical tags on the histones tighten the coil, hiding the gene, or loosen it so the gene can be read.

What triggers epigenetic change

Diet, stress, exercise, smoking, pollutants and even temperature can add or remove these marks. Studies of people conceived during the Dutch Hunger Winter found methylation differences decades later, and identical twins accumulate different epigenetic marks as their lifestyles diverge, which is why one twin can develop a condition the other does not.

Genetic versus epigenetic

	Genetic change	Epigenetic change
DNA sequence	Altered (mutation)	Unchanged
Cause	Copying error, radiation, chemicals	Environment, diet, lifestyle
Reversible?	Generally not	Often yes
Inherited?	Yes, through gametes	Sometimes, some marks persist

Epigenetics matters in medicine because some cancers involve silenced tumour suppressor genes, and drugs that reverse methylation are already in use.

svg

DNA methylation and histone modification

Key Points

1Epigenetics: changes in gene expression without any change in the DNA base sequence
2DNA methylation usually switches a gene off
3Histone modification tightens or loosens the coiling of DNA, hiding or exposing genes
4Diet, stress, smoking and exercise can all add or remove epigenetic marks
5Epigenetic changes are often reversible and some can be passed to offspring

Learning Outcomes

Compare genetic and epigenetic mechanisms and explain how the environment influences gene expression

Interactive: Punnett Square

Build a cross to predict genotype and phenotype ratios.

Interactive Punnett Square

Choose alleles for each parent. B = Brown eyes (dominant), b = Blue eyes (recessive). Ratios update live.

Parent 1

= Bb

Parent 2

= bb

	b	b
B	Bb	Bb
b	bb	bb

Genotype ratio

Bb2/4 (50%)
bb2/4 (50%)

Phenotype ratio

Brown eyes (dominant)50%
Blue eyes (recessive)50%

Presets:

Interactive: Sex-Linked Cross Generator

Model X-linked inheritance for haemophilia, colour blindness and DMD - see male vs female offspring outcomes.

TraitMother (XX)Father (XY)

	XH	Y
XH	XHH Unaffected female	XHY Unaffected male
Xh	XhH Carrier female	XhY Affected male (Haemophilia)

Predicted offspring ratios

Unaffected females: 1/4 (25%)
Carrier females: 1/4 (25%)
Affected females: 0/4 (0%)
Unaffected males: 1/4 (25%)
Affected males: 1/4 (25%)

Capital letter = dominant allele on the X chromosome. Lower case = recessive. The Y chromosome carries no allele for this gene, so a single recessive allele expresses the trait in males.