Genetics and Inheritance

Why you look the way you do

Why do you have your father's nose, your mother's eye colour, or your grandmother's curly hair? Why do puppies from the same litter look similar but not identical? Why does a sunflower seed always grow into a sunflower and never into a rose? The answers all come from one of the most powerful ideas in all of biology: inheritance, the passing of characteristics from parents to offspring. The science that studies how this works is called genetics, and it explains the unbroken thread connecting every generation of living things.

Before we begin, it helps to recall what you may have learned in DNA and genetics basics. DNA is the molecule inside every cell that carries the instructions for building and running a living thing. Those instructions are written in a chemical code, and a gene is a section of DNA that contains the instructions for one particular feature — like eye colour or the shape of a pea seed. In this lesson, we explore how those genes are passed on and how they decide the traits we see.

Genes, alleles and two copies of everything

Here is a key fact that unlocks much of genetics: in most plants and animals, including humans, you inherit two copies of every gene — one from your mother and one from your father. These copies are carried on structures called chromosomes, which come in matching pairs.

But the two copies of a gene are not always identical. A gene can come in different versions, and each version is called an allele. For example, there is a gene that influences eye colour, and it exists as different alleles — one allele might lead to brown eyes, another to blue. You inherit one allele from each parent, so you might end up with two of the same allele, or one of each.

This raises an obvious question. If you inherit a "brown" allele from one parent and a "blue" allele from the other, what colour are your eyes? They cannot be both. The answer lies in one of the most important rules in genetics.

Dominant and recessive: who wins?

When an organism carries two different alleles for a trait, often one of them is dominant and the other is recessive. A dominant allele always shows its effect, even if only one copy is present. A recessive allele only shows its effect when two copies are present — that is, when there is no dominant allele to mask it.

Scientists use letters to represent alleles: a capital letter for the dominant allele and the same letter in lowercase for the recessive one. Let's use eye colour as a simplified example, with B for the dominant brown allele and b for the recessive blue allele. There are three possible combinations:

• BB — two brown alleles. Eyes are brown.
• Bb — one of each. Because B is dominant, the eyes are still brown. The blue allele is present but hidden.
• bb — two blue alleles. Now there is no dominant allele to mask it, so the eyes are blue.

Notice something fascinating: a person with brown eyes could be either BB or Bb. From the outside they look the same, but their genetics differ. This is why two brown-eyed parents can sometimes have a blue-eyed child — if both parents are Bb, each can pass on their hidden b allele.

Genotype and phenotype

This brings us to two essential terms that scientists use to keep things clear.

• Genotype is the actual genetic makeup — the combination of alleles an organism carries, such as BB, Bb or bb.
• Phenotype is the observable characteristic — the trait you can actually see or measure, such as brown or blue eyes.

The relationship between them is the heart of genetics. The genotype Bb and the genotype BB produce the same phenotype (brown eyes), because the dominant allele determines what shows. Understanding this difference is what lets scientists trace traits through families and predict what offspring might inherit.

Mendel: the monk who founded genetics

None of this was understood until the 1860s, when an Austrian monk named Gregor Mendel carried out careful experiments in his monastery garden. Working with pea plants long before anyone knew DNA existed, Mendel is now called the "father of genetics."

Mendel chose pea plants for good reasons: they grow quickly, have clearly different traits (tall or short plants, round or wrinkled seeds, purple or white flowers), and he could control exactly which plants bred together. By cross-breeding thousands of plants and counting the offspring carefully, he noticed something remarkable. When he crossed a tall plant with a short one, all the offspring were tall — the short trait seemed to vanish. But when he bred those offspring together, the short trait reappeared in about one out of every four plants in the next generation.

From these precise patterns, Mendel worked out — without any knowledge of genes or DNA — that traits are passed on as separate "factors" (what we now call alleles), that each organism carries two of them, and that some are dominant over others. His mathematical thinking was decades ahead of its time, and it forms the foundation of everything in this lesson.

Predicting inheritance with Punnett squares

So how can we predict what offspring will inherit? Scientists use a simple grid called a Punnett square. It lays out all the possible combinations of alleles that two parents can pass on, and it shows the probability of each outcome.

Let's cross two brown-eyed parents who are both Bb. We put one parent's alleles along the top and the other's down the side, then fill in each box:

	B	b
B	BB	Bb
b	Bb	bb

Now we read the results. Out of four equally likely combinations:

• 1 is BB — brown eyes
• 2 are Bb — brown eyes
• 1 is bb — blue eyes

So this cross predicts a 3 to 1 ratio: a 75% chance of brown eyes and a 25% chance of blue eyes — exactly the pattern Mendel observed in his peas. It is crucial to understand that these are probabilities, not guarantees. Just as flipping a coin four times will not always give exactly two heads and two tails, real families do not always match the predicted ratio. But across many offspring, the patterns hold true.

Beyond simple inheritance

Mendel's rules are the perfect starting point, but nature is often more complex. Some traits show incomplete dominance, where neither allele fully dominates and the result is a blend — for example, a red flower crossed with a white flower producing pink offspring. In codominance, both alleles show at once, as in certain coat patterns where both colours appear.

Most importantly, many traits are polygenic, meaning they are controlled by many genes working together. Human height, skin colour and many other features are influenced by dozens or hundreds of genes, plus the environment — things like nutrition and lifestyle. This is why such traits vary smoothly across a whole range rather than falling into a few neat categories. Understanding inheritance also connects to the bigger story of evolution and natural selection, because inherited variation is exactly what natural selection acts upon over many generations.

Try this activity — Run your own genetics survey. Some human traits follow simple, near-Mendelian patterns you can observe yourself. Check whether you can roll your tongue into a tube (often linked to a dominant allele), whether your earlobes are attached or hang free, and whether you have a widow's peak hairline. Survey your classmates or family and tally how many people show each version of each trait. Then, for one trait, draw a Punnett square to predict what proportion of offspring would show the dominant version if two carriers had children, and compare it with the proportions in your survey. You will see genetics playing out in the real people around you — and you will also notice that real traits are messier than the textbook, a perfect reminder of how nature is more complicated than any single rule.

The thread that connects all life

Genetics and inheritance explain one of the deepest patterns in nature: how life passes its instructions from one generation to the next, keeping species recognisable while still allowing every individual to be unique. From Mendel's pea plants to the DNA in your own cells, the same principles apply across all living things. By understanding genes, alleles, dominance and probability, you hold the key to explaining not just why you look the way you do, but how life itself continues and changes over time.