Key Stage 3 science · Biology – Genetics and evolution

KS3 DNA, genes and variation: a guide for parents

Heredity is how characteristics pass from parents to their children. In Key Stage 3 science children learn a simple model in which chromosomes in the nucleus carry genes, which are sections of DNA. They hear how Watson, Crick, Wilkins and Franklin worked out the structure of DNA, compare species, and study variation within a species, learning to measure it and to show it on graphs.

DNA, genes and variation, explained

Heredity is the process by which genetic information is passed from one generation to the next. Almost every cell in your body has a nucleus, and inside the nucleus are chromosomes, which are long threads of DNA. A gene is a short section of DNA that carries the instructions for building a particular protein, and together proteins help to determine many of our characteristics. The simple model is: nucleus, then chromosomes, then genes, then DNA. Children often picture a library: the chromosome is a book, the gene is a page of instructions and the DNA is the writing.

Offspring get their genes from both parents. A sperm or egg cell carries half the usual set of chromosomes, and when they join at fertilisation the new cell has a full set again, half from each parent. This is why children share features with both parents without being identical to either, and why brothers and sisters from the same parents differ from each other. In humans, body cells have 46 chromosomes in 23 pairs, and sex cells have 23.

The National Curriculum asks children to learn about the scientists who built the model of DNA. In the early 1950s Rosalind Franklin and Maurice Wilkins, working at King’s College London, used X-rays to produce images of DNA. Using that evidence, James Watson and Francis Crick in Cambridge built a model of DNA as a double helix, which they published in 1953. The story shows how science is a team effort that builds on others’ evidence, and it is also a chance to discuss whose contributions were recognised at the time. Watson, Crick and Wilkins shared the 1962 Nobel Prize. Franklin had died in 1958, and Nobel Prizes are not normally given to people who have died.

Variation means the differences between living things. Differences between species are the first level: species are groups of organisms that can breed together to produce fertile offspring, and different species have different characteristics. Differences between individuals within a species are the second level. Some variation is caused by genes, some by the environment (a plant in the shade grows differently from one in the sun) and many by both. Pupils learn to sort variation into two types. Continuous variation has any value in a range, such as height, mass or foot length. The data are grouped into equal classes and shown on a bar chart with touching bars, which many schools call a histogram. Discontinuous variation falls into clear categories, such as blood group or the number of petals on a flower, and is shown on a bar chart with gaps between the bars.

Measuring variation is part of the topic. Children take measurements from a sample, such as hand spans or leaf lengths, group them into classes, and draw conclusions about the spread of results. They should be able to state that a bigger sample gives a more reliable picture.

Words your child will hear

Heredity: The passing on of genetic information from parents to offspring.
Chromosome: A long thread of DNA in the nucleus, carrying many genes.
Gene: A section of DNA that contains the instructions for making a particular protein.
DNA: The molecule that carries genetic information, shaped like a twisted ladder called a double helix.
Variation: Differences between individuals of the same species or between species.
Continuous variation: Variation where any value in a range is possible, such as height.
Discontinuous variation: Variation that falls into distinct groups, such as blood group.

What the National Curriculum says

The programme of study for Key Stage 3 says:

Pupils should be taught about heredity as the process by which genetic information is transmitted from one generation to the next
Pupils should be taught about a simple model of chromosomes, genes and DNA in heredity, including the part played by Watson, Crick, Wilkins and Franklin in the development of the DNA model
Pupils should be taught about differences between species
Pupils should be taught about the variation between individuals within a species being continuous or discontinuous, to include measurement and graphical representation of variation

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

Try it: choosing how to show variation

A class of 30 pupils records two characteristics: hand span (in cm) and eye colour. Decide which is continuous and which is discontinuous, and say how each should be graphed.

Ask whether the characteristic can take any value in a range or only fall into groups. Hand span can be 19.2 cm, 19.3 cm or any value in between, so it is continuous. Eye colour is recorded in a few named groups, so schools treat it as discontinuous.
Group the continuous data into equal classes so the pattern is visible. For example: 16 to less than 18 cm: 3 pupils; 18 to less than 20 cm: 7; 20 to less than 22 cm: 11; 22 to less than 24 cm: 7; 24 to less than 26 cm: 2. The total is 30.
Graph the hand spans as a bar chart with the bars touching, because the classes run on from one another with no gaps. The graph rises to a peak in the middle class and falls away at both ends.
Tally the eye colours and draw a bar chart with gaps between the bars, because the categories are separate. For example: brown 14, blue 8, green 3, hazel 5. The total is 30.
Say what the graphs show. Most hand spans are close to the middle, with few very small or very large values. Eye colour has no middle value, and brown is the most common group in this class.

Hand span is continuous and is shown on a bar chart with touching bars. Eye colour is discontinuous and is shown on a bar chart with gaps. A graph of a continuous characteristic often peaks in the middle and tails off.

Common mistakes and how to help

Thinking you inherit characteristics from only one parent, usually the one of the same sex

Family sayings such as “he’s his father’s son” make the pattern look simple, and we notice the features we expect to see.

What helps: Half of a child’s chromosomes come from each parent, so features can come from either side. Look at old photographs and see how many family members share a feature.

Thinking each characteristic is controlled by a single gene, and that genes decide everything

Simple examples and the phrase “the gene for…” are used in the news, so the idea stays.

What helps: Many characteristics, such as height, depend on lots of genes plus the environment, such as food and health. Identical twins who live in different places can still differ.

Believing that DNA was discovered by Watson and Crick

Their names are the best known, and it is often shortened to “they discovered DNA”.

What helps: DNA had been identified by other scientists earlier. In 1953 Watson and Crick built a model of its structure, using X-ray evidence from Franklin and Wilkins, which explained how DNA could carry and copy information.

Assuming anything described with numbers is continuous variation

Children see numbers and a graph and assume the data run on without a break, even when the values come in steps.

What helps: Ask whether a value between two neighbours makes sense. A flower can have 4, 5 or 6 petals but not 4.7, so petal number is discontinuous. Foot length can be 23.4 cm, so it is continuous.

How to help at home

Make a family traits chart. Pick a few features, such as hair colour, whether hair is curly, straight or wavy, freckles or height, and see who in the family has each one. Which features seem to run through the family, and which appear in only one person?
Measure and graph something continuous. Measure the length of 20 or more leaves from one bush or tree of the same kind with a ruler, or the hand spans of willing family and friends. Group the results and draw a bar chart. Talk about why not every leaf is the same.
Compare species and individuals. Look at pictures of two different kinds of bird, then at two of the same kind. Which differences are between species and which are between individuals? Which could be caused by food or the environment?
Build a DNA model. Twist two long pipe cleaners or strips of paper into a double helix and join them with short “rungs” of four different colours. Ask your child to explain why a model helps scientists explain something they cannot see.

Builds on

Year 6: Fossils, inheritance and adaptation

Leads on to

Key Stage 4: The genome and inheritance
Key Stage 4: Variation, natural selection and classification
Key Stage 4: Selective breeding and biotechnology