Reproduction and genetics, Year 9: Genes and variation

nucleus; chromosome

提示Start with the cell's control centre, then name the threads inside it.

为什么The nesting order from largest to smallest is cell, nucleus, chromosome, gene. A chromosome is a single enormous DNA molecule, and a gene is one short stretch of it. Chromosomes can be seen with a light microscope only when a cell is dividing.

46

提示They come in pairs, one of each pair from each parent.

为什么Human body cells have 23 pairs of chromosomes. One chromosome of each pair came from the mother's egg and the other from the father's sperm, which is why eggs and sperm carry just 23 each.

It carries the instructions for one characteristic

提示Think of a single recipe in a very large cookbook.

为什么Each gene is a coded instruction that a cell reads to build part of the body or to control how it works. Together, the genes on all the chromosomes make up the full set of instructions for the organism.

Franklin

提示Her first name was Rosalind.

为什么Rosalind Franklin's X-ray images showed that DNA is a helix. Maurice Wilkins, who worked in the same laboratory, showed one to James Watson, and in 1953 Watson and Francis Crick used it to build the first correct model. Franklin died in 1958, before the Nobel Prize for the discovery was awarded.

A double helix

提示Picture a twisted ladder, or a spiral staircase with two handrails.

为什么The two strands of DNA coil around each other like a twisted rope ladder. The shape was worked out in 1953 by Watson and Crick, using X-ray evidence from Franklin and Wilkins.

Variation within a species

提示No two people in your class look exactly alike.

为什么Members of a species share the same basic body plan but differ in many details, such as height, colouring or blood group. Some of these differences are inherited, some are caused by the surroundings, and many are a mixture of both.

inherited; environmental

提示One kind arrives from your parents; the other from your surroundings and experiences.

为什么Inherited variation comes from the genes passed on by parents. Environmental variation comes from things like diet, climate, accidents and learning. Sorting a feature into one group or the other is the first step in explaining why individuals differ.

Both — genes set the potential and diet affects whether it is reached

提示Tall parents tend to have tall children, yet children who go hungry grow less.

为什么Many characteristics are shaped by both causes together. Genes give a possible range for height, and nutrition and health during childhood decide where in that range a person ends up. Body mass works in the same way.

That it was caused by the environment, not by genes

提示Rule out whichever cause the two of them share completely.

为什么Because identical twins share all their genes, any difference between them must come from their surroundings or the way they live. Scientists study twins for exactly this reason: it separates inherited causes from environmental ones.

No — the accident did not change his genes

提示Ask what is actually handed on to a child in the sperm and the egg.

为什么Only inherited variation can be passed on, because only genes travel in the egg and sperm. Changes made to the body during life, such as injuries, scars, pierced ears or bigger muscles, are environmental and stop with that person.

Discontinuous variation

提示The prefix means "not": the range is broken into distinct steps.

为什么A person has blood group A, B, AB or O and nothing in between, so the feature sorts everyone into a few clear groups. Characteristics like this are usually controlled by genes alone and are not changed by the environment.

discontinuous

提示Ask whether anyone could have a value halfway between two of the groups.

为什么Blood group has only a few possible values, so every person fits exactly one category. Height or mass, by contrast, can take any value within a range, which makes them continuous.

bar

提示Each category gets its own separate column.

为什么Separate categories are shown as separate bars with gaps between them. Continuous data such as height is grouped into ranges and drawn as a histogram, where the bars touch because the ranges run on from one another.

A bell shape, with most pupils near the middle

提示Very short and very tall people are both rare.

为什么Continuous variation usually gives a normal distribution: most individuals are close to the average and fewer and fewer appear towards either extreme. The same pattern shows up for mass, hand span and many other measurements.