Science
What is inheritance in biology, and how are traits passed on?
Quick answer
Inheritance is how characteristics pass from parents to children through genes. Each parent gives one copy of every chromosome, so you get one version of each gene from each parent.
In one sentence
Inheritance (or heredity) is the passing of characteristics from parents to their offspring through genes, and genetics is the science that studies it.
Explained
Why do you have your mum's smile or your grandad's curly hair? Because inside almost every one of your cells there is a set of instructions, and you got half of it from each parent.
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Those instructions are written in DNA, a long, twisted molecule shaped like a double helix. The DNA is packed into chromosomes in the nucleus of the cell. Most human body cells have 46 chromosomes in 23 pairs. A gene is a short section of DNA that carries the instructions for one characteristic, usually by telling the cell how to make a particular protein.
Genes come in different versions, called alleles. The gene for pea plant height has a tall allele and a short allele; the gene for blood group has A, B and O alleles. Because chromosomes come in pairs, you have two alleles for each gene: one from your mother and one from your father.
How does that happen? Sperm and egg cells (gametes) are made by a special cell division, so each one carries just 23 chromosomes, one from each pair. At fertilisation, a sperm and an egg join, and the baby has 46 again. Which chromosome of each pair ends up in a gamete is a matter of chance, which is why brothers and sisters (except identical twins) all have different mixtures of their parents' genes. This mixing is a big source of variation, and variation is what evolution works on.
Dominant and recessive. Some alleles hide others. A dominant allele (capital letter, T) shows its effect even if there is only one copy. A recessive allele (small letter, t) only shows when there are two copies. So a pea plant that is TT or Tt is tall; only tt is short.
Some key words:
- Genotype: the alleles an organism has (TT, Tt or tt).
- Phenotype: the characteristic you can see or measure (tall or short).
- Homozygous: two identical alleles (TT or tt). Heterozygous: two different alleles (Tt).
These rules were discovered by Gregor Mendel, a monk who bred thousands of pea plants and published his results in 1866, long before anyone knew about DNA. In 1953 James Watson and Francis Crick worked out the double-helix structure of DNA, using X-ray images made by Rosalind Franklin and data from Maurice Wilkins.
Sex chromosomes. One of the 23 pairs decides biological sex: XX in females and XY in males. Every egg carries an X; each sperm carries an X or a Y, so there is a 1 in 2 chance of each. The other 22 pairs are called autosomes.
Not everything is that simple. Many characteristics, such as height, skin colour and eye colour, are controlled by several genes (polygenic inheritance) and are also shaped by the environment: food, exercise, sunlight, illness. Identical twins have the same DNA, yet they can grow to slightly different heights. And characteristics you gain during your life, such as a scar, big muscles or the ability to play the piano, are not inherited, because they do not change your DNA.
Examples
A Punnett square shows every possible combination of alleles from two parents. Here two heterozygous tall pea plants (Tt × Tt) are crossed:
| T (from parent 2) | t (from parent 2) | |
|---|---|---|
| T (from parent 1) | TT (tall) | Tt (tall) |
| t (from parent 1) | Tt (tall) | tt (short) |
Result: 1 TT : 2 Tt : 1 tt, so 3 tall : 1 short. Each offspring has a 3 in 4 (75%) chance of being tall and a 1 in 4 (25%) chance of being short. Mendel's real experiments gave results very close to this ratio.
More inherited characteristics:
| Characteristic | How it is inherited |
|---|---|
| Pea plant height (Mendel) | one gene: tall (T) dominant over short (t) |
| ABO blood group | one gene, three alleles: A and B are both dominant over O, and A and B together give AB |
| Biological sex | XX or XY chromosomes |
| Cystic fibrosis | a recessive allele: a person must inherit two copies to have the condition |
| Height, eye colour, skin colour | several genes plus the environment |
Common mistakes
- "Dominant means most common." No: dominant only means it shows with one copy. A dominant allele can be rare in a population.
- Mixing up genotype and phenotype. Genotype is the letters (Tt); phenotype is what you see (tall).
- Treating a ratio as a promise. 3 : 1 is a probability for each offspring. A family with four children will not always have exactly three of one kind and one of the other.
- "Two blue-eyed parents can never have a brown-eyed child." The simple one-gene model says so, but real eye colour depends on several genes, so it can happen, though rarely.
Step by step
Step by step
- Choose the lettersUse a capital for the dominant allele and the same small letter for the recessive one (T and t).
- Write the parents' genotypesFor example Tt × Tt.
- Split each parent into gametesEach gamete gets one allele: T or t.
- Draw the Punnett squareOne parent's gametes along the top, the other's down the side. Fill in each box with one allele from each.
- Read the resultsCount genotypes, then turn them into phenotypes, a ratio (3 : 1), a fraction (3/4) or a percentage (75%).
Practice questions
Inheritance and Punnett squares
Try each one, then check your answer.
- What is a gene?
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A section of DNA that carries the instructions for a characteristic, usually by coding for a protein. - How many chromosomes are in a human egg cell?
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23 (one from each pair) - Brown eyes (B) are dominant over blue (b) in the simple model. What colour are the eyes of a person with genotype Bb?
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Brown - In the same model, what is the phenotype of bb?
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Blue eyes - Is Tt homozygous or heterozygous?
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Heterozygous (two different alleles) - Cross TT × tt. What are the offspring?
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All Tt, so all tall (100%). - Cross Tt × tt. What fraction of the offspring are short?
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Tt, Tt, tt, tt: 1/2 (50%) - Cross Tt × Tt. What is the ratio of tall to short?
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TT, Tt, Tt, tt: 3 tall : 1 short - What is the chance that a baby has XY sex chromosomes?
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The egg gives X; the sperm gives X or Y equally: 1/2 (50%) - One parent has blood group genotype AO and the other BO. Which blood groups can their children have?
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AB, AO, BO, OO: AB, A, B or O, each with a 25% chance - Can a skill you learn, like swimming, be passed on to your children through your genes?
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No: acquired characteristics do not change your DNA. - Why do brothers and sisters look different, unless they are identical twins?
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Each gamete gets a random mix of chromosomes, so each child gets a different combination of alleles. - Identical twins have the same DNA but different weights. Why?
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The environment (diet, exercise, illness) also affects characteristics. - What is a mutation?
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A change in the DNA. It can create a new allele.
Word problems
Genetics in real life
Try each one, then check your answer.
- A gardener crosses two Tt pea plants and plants 400 of the seeds. About how many will grow into short plants?
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1/4 × 400 = about 100 - In one experiment Mendel counted 787 tall and 277 short plants. What is the ratio of tall to short, to one decimal place?
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787 ÷ 277 ≈ 2.8 : 1, close to the expected 3 : 1. - Both parents carry one cystic fibrosis allele (Ff). What is the chance that a child inherits the condition (ff)?
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FF, Ff, Ff, ff: 1/4 (25%) for each child - A family has three girls. What is the chance that their fourth child is a boy?
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Each birth is independent: 1/2 (50%) - Black fur (B) is dominant over white (b) in guinea pigs. A black guinea pig is crossed with a white one, and some babies are white. What is the black parent's genotype?
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Bb: a BB parent would give only Bb (black) babies. - A mother has blood group O (OO) and the father has group AB. Which groups can their children have?
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AO or BO: A or B, each 50%; never O or AB.
Questions people ask
What is inheritance in biology?
It is the passing of characteristics from parents to offspring through genes. Each parent gives half of the chromosomes, so offspring get one allele of each gene from each parent.
What is the difference between a gene, DNA and a chromosome?
DNA is the molecule that carries genetic information. A chromosome is a long, tightly packed strand of DNA. A gene is a short section of a chromosome that controls one characteristic.
What did Mendel discover?
By crossing pea plants, Mendel showed that characteristics are passed on as separate "factors" (we now call them genes), that each parent gives one, and that dominant ones can hide recessive ones, giving ratios such as 3 : 1.
Is eye colour inherited from the mother or the father?
From both. You get alleles from each parent, and eye colour depends on several genes, so it is hard to predict exactly.
How is blood group inherited?
By one gene with three alleles: A, B and O. A and B are both dominant over O, so AO is group A, BO is group B, AB is group AB and only OO is group O.
What is polygenic inheritance?
It is when several genes together control one characteristic, such as height. It gives a smooth range of values rather than a few clear types.
Quick quiz
Inheritance and genetics
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How many chromosomes are in most human body cells?
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