Inheritance

Welcome to your study notes on Inheritance. In this topic, we will explore how genetic information is passed down from parents to offspring, how we can predict the characteristics of future generations, and how modern genetics has developed.

What you'll learn

How genetic material is structured inside your cells, from chromosomes to DNA.
How to define and use key genetic terms like alleles, genotype, and phenotype.
How to construct and interpret Punnett squares to predict genetic crosses and sex determination.
The differences between sexual and asexual reproduction, and how meiosis creates genetic variation.

The Basics of Genetic Material

To understand how inheritance works, we must first understand the physical layout of our genetic material. It is easy to confuse terms like "nucleus", "chromosome", "DNA", and "gene", but they fit together in a very specific hierarchy.

Definition

Genome

The genome is the entire genetic material of an organism.

Inside almost every cell in your body, there is a nucleus which acts as the control centre. Inside the nucleus, your genome is packaged into long, thread-like structures called chromosomes.

Chromosomes are made of a chemical called DNA (deoxyribonucleic acid). DNA is a double-stranded polymer wound into a double helix shape.

A gene is a small section of DNA on a chromosome. Each gene contains the code for a specific sequence of amino acids, which fold to make a specific protein. It is these proteins that determine your biological characteristics.

Genetic Hierarchy

Common Mistake

Confusing the genetic scale

Students often write that "genes are made of chromosomes" or that "DNA is inside a gene". Always remember the correct size order from largest to smallest: Cell $→\rightarrow$ Nucleus $→\rightarrow$ Chromosome $→\rightarrow$ DNA $→\rightarrow$ Gene.

Genotypes, Phenotypes, and Alleles

Your characteristics are not just determined by having a gene; they depend on the version of the gene you inherit.

Definition

Allele

An allele (or variant) is a different version of the same gene.

For example, there is a gene for eye colour, but you might inherit an allele for brown eyes or an allele for blue eyes.

Because you inherit one set of chromosomes from your mother and one set from your father, you have two alleles for every gene.

Key Genetic Terminology

Genotype: The combination of alleles you have for a particular gene (e.g., $B b$ ).
Phenotype: The visible, physical characteristic or trait that develops as a result of your genotype and its interaction with the environment (e.g., brown eyes).
Homozygous: Having two of the same alleles for a gene (e.g., $B B$ or $bb$ ).
Heterozygous: Having two different alleles for a gene (e.g., $B b$ ).
Dominant: An allele that is always expressed in the phenotype, even if only one copy is present. We represent dominant alleles with capital letters (e.g., $B$ ).
Recessive: An allele that is only expressed in the phenotype if two copies are present (i.e., if no dominant allele is present). We represent recessive alleles with lowercase letters (e.g., $b$ ).

Key Idea

How alleles behave

A dominant allele does not physically "destroy" or "block" a recessive allele. Instead, the dominant allele usually codes for a fully functional protein, whereas the recessive allele codes for a modified or non-functional version of that protein.

Genome-Environment Interactions

Your phenotype is not purely dictated by your genes. Your development is influenced by your genome and its interaction with the environment. For example, you may have the genetic potential to grow very tall (genotype), but if you suffer from poor nutrition during childhood (environmental factor), you may not reach that height (phenotype).

How Mutations Create Variants

Every single genetic variant (allele) originally came from a mutation. A mutation is a spontaneous change in the DNA sequence of an organism.

The impact of mutations on the phenotype varies greatly:

Most mutations have no effect on the phenotype: This is because they occur in non-coding regions of the DNA, or they change a DNA base but still code for the exact same amino acid.
Some mutations influence the phenotype: They might slightly alter the shape of a protein, giving you a slightly different characteristic (like a different shade of hair colour).
A very few mutations determine the phenotype: These mutations significantly change a protein's structure, causing genetic conditions like cystic fibrosis.

How Variants Alter Phenotypes (Higher Tier Only)

If you are taking the Higher Tier exam papers, you need to understand the mechanism of how these mutations alter the phenotype depending on where they occur:

In coding DNA: A mutation in a coding region changes the sequence of amino acids in a protein. This changes how the protein folds into its 3D shape. If the protein is an enzyme, this can alter the shape of its active site so that the substrate can no longer bind, making the enzyme inactive.
In non-coding DNA: A mutation in non-coding DNA can affect how genes are expressed. It can alter the binding of the cellular machinery responsible for starting transcription (such as RNA polymerase). This can stop transcription of mRNA, effectively "switching off" the gene so that the protein is not made at all.

Sexual vs. Asexual Reproduction

If you are studying the separate Biology course (J247), you must be able to compare the advantages and disadvantages of sexual and asexual reproduction.

Sexual Reproduction

Sexual reproduction involves the fusion of male and female gametes (sex cells) during fertilisation. Because the offspring inherit half of their DNA from each parent, this process introduces genetic variation into the population.

Advantages: Introduces genetic variation, which helps populations adapt to changing environments or survive new diseases.
Disadvantages: It is slow, requires finding a mate, and produces fewer offspring per unit of time.

Asexual Reproduction

Asexual reproduction involves only one parent and does not involve the fusion of gametes. The offspring are genetically identical clones of the parent, produced via cell division (mitosis).

Advantages: Extremely fast and energy-efficient (no need to find a mate), allowing rapid colonisation of an area.
Disadvantages: Zero genetic variation. If the environment changes or a disease enters the population, every single clone is equally vulnerable and the entire population could be wiped out.

Meiosis and the Production of Gametes

To prepare for sexual reproduction, organisms must produce specialised sex cells called gametes (sperm and egg cells in animals; pollen and egg cells in plants).

To understand this process, we must define two terms:

Diploid: A cell that contains two complete sets of chromosomes, one set from each parent. In human body cells, the diploid number is $2 n = 46$ chromosomes.
Haploid: A cell that contains only a single set of chromosomes. In human gametes, the haploid number is $n = 23$ chromosomes.

Definition

Meiosis

Meiosis is a type of cell division that halves the chromosome number from diploid ( $2 n$ ) to haploid ( $n$ ) to produce genetically varied gametes.

The Role of Meiosis

Halving the chromosome number: If gametes were diploid ( $46$ chromosomes), then fertilisation would result in an offspring with $92$ chromosomes! Meiosis halves the chromosome number to $23$ so that when fertilisation occurs, the diploid number of $46$ is restored in the zygote.
Creating genetic variation: During meiosis, the maternal and paternal chromosomes swap segments of DNA and assort randomly into the gametes, ensuring that every gamete produced is genetically unique.

The Sexual Lifecycle

Single Gene Inheritance and Punnett Squares

Most of your physical traits are polygenic, meaning they are controlled by the interaction of multiple genes working together (for example, your height or skin colour). However, some simple traits are controlled by a single gene. We can predict the outcome of single-gene crosses using a Punnett square.

Look at the diagram below showing a cross between two individuals who are both heterozygous for eye colour ( $B b$ ).

Punnett Square

Because brown eyes ( $B$ ) are dominant to blue eyes ( $b$ ), any offspring with the genotypes $B B$ or $B b$ will have brown eyes. Only offspring with the homozygous recessive genotype $bb$ will have blue eyes. This gives us a classic $3 : 1$ phenotypic ratio (or a $75%75\%$ to $25%25\%$ chance).

Example

Predicting cystic fibrosis inheritance

Cystic fibrosis is an inherited disorder of cell membranes caused by a recessive allele ( $f$ ). A person must be homozygous recessive ( $f f$ ) to have the condition. Heterozygous individuals ( $F f$ ) are healthy but are "carriers".

If two carriers ( $Ff \times Ff$ ) decide to have a child, let us calculate the probability that their child will inherit cystic fibrosis.

Identify the parental genotypes: Both parents are carriers, so both have the heterozygous genotype $F f$ .
Determine the gametes each parent can produce: Each parent can pass on either the dominant allele ( $F$ ) or the recessive allele ( $f$ ).
Draw and fill in a $\times 2$ Punnett square: Place one parent's gametes on the top and the other parent's gametes on the left side, then cross them:

	F	f
F	$F F$	$F f$
f	$F f$	$f f$

Identify the phenotypes of the offspring genotypes:
- $F F$ (homozygous dominant) $→\rightarrow$ Healthy (non-carrier)
- $F f$ (heterozygous) $→\rightarrow$ Healthy carrier
- $f f$ (homozygous recessive) $→\rightarrow$ Has cystic fibrosis
Calculate the ratio and probability: Out of the $4$ possible genetic outcomes, only $1$ result ( $f f$ ) leads to the child having cystic fibrosis. The probability is calculated as:

\text{Probability} = \frac{1}{4} = 0.25 \text{ (or } 25\% \text{)}

Tip

Probability has no memory

Each pregnancy is an independent event! If a carrier couple has a child with cystic fibrosis, the probability of their next child having the condition is still exactly $\div 4 = 25\%$ . It does not decrease.

Sex Determination in Humans

Out of the $23$ pairs of chromosomes in human cells, one pair consists of the sex chromosomes which determine your biological sex:

Females have two identical, large chromosomes: $X X$ .
Males have one large and one small chromosome: $X Y$ .

When a father produces sperm, meiosis splits his sex chromosomes so that half his sperm contain an $X$ chromosome, and half contain a $Y$ chromosome. Because mothers ( $X X$ ) can only pass on an $X$ chromosome in their eggs, it is the father's sperm that determines the sex of the child.

By crossing maternal gametes ( $X$ and $X$ ) with paternal gametes ( $X$ and $Y$ ) in a Punnett square, we can easily see the $1 : 1$ ratio of sex determination:

	X	Y
X	$X X$ (Female)	$X Y$ (Male)
X	$X X$ (Female)	$X Y$ (Male)

There is always a $1 : 1$ ratio, or a $50%50\%$ probability, of having a male or female child.

Gregor Mendel and the History of Genetics (Separate Biology ☑)

If you are studying separate biology, you must know about Gregor Mendel, an Austrian monk who lived in the mid-19th century.

Mendel carried out thousands of meticulous breeding experiments on pea plants. He noticed that characteristics (like pea shape or flower colour) were passed down in clear, mathematical ratios (such as the $3 : 1$ ratio of dominant to recessive traits).

Mendel concluded that traits were determined by inherited "hereditary units" (what we now call genes) which were passed on unchanged from parents.

Why was Mendel's work ignored at first?

Scientists at the time believed in the "blending" theory of inheritance (e.g., a tall parent and a short parent make a medium-height child).
Mendel was a monk and not part of the mainstream scientific community.
He published his work in an obscure journal, and his use of advanced mathematics was unusual for biologists at the time.
Crucially, no one had discovered chromosomes, DNA, or meiosis yet, so there was no physical mechanism to explain how his "hereditary units" were transmitted. It was only after these cellular structures were observed under microscopes decades later that Mendel's work was finally recognised.

Exam technique

In the exam

Keep your letters distinct: When choosing letters for a Punnett square, avoid letters where the capital and lowercase versions look similar (like $C$ / $c$ , $S$ / $s$ , $O$ / $o$ ). Instead, use letters like $B$ / $b$ , $F$ / $f$ , or $A$ / $a$ to prevent self-marking errors.
Read the question carefully: Examiners often ask for the probability of a specific phenotype or the ratio of phenotypes. Make sure you don't accidentally write down the genotypic ratio instead!
Show your working: Always draw the full Punnett square even if you can do the math in your head. Marks are awarded for setting up the gametes correctly.

Self review

Check yourself

Why must gametes be haploid rather than diploid?
If two heterozygous brown-eyed dogs ( $B b$ ) mate, what is the probability that they will produce a puppy with blue eyes ( $bb$ )?
How does a genetic mutation in non-coding DNA prevent a protein from being made?

Inheritance

What you'll learn

How genetic material is structured inside your cells, from chromosomes to DNA.
How to define and use key genetic terms like alleles, genotype, and phenotype.
How to construct and interpret Punnett squares to predict genetic crosses and sex determination.
The differences between sexual and asexual reproduction, and how meiosis creates genetic variation.

The Basics of Genetic Material

Definition

Genome

The genome is the entire genetic material of an organism.

Inside almost every cell in your body, there is a nucleus which acts as the control centre. Inside the nucleus, your genome is packaged into long, thread-like structures called chromosomes.

Chromosomes are made of a chemical called DNA (deoxyribonucleic acid). DNA is a double-stranded polymer wound into a double helix shape.

Genetic Hierarchy

Common Mistake

Confusing the genetic scale