Variation, Inheritance and Evolution

Why living things differ

No two people are exactly alike, and even members of the same species vary. Those differences are called variation, and they come from two sources. - Genetic variation is inherited, passed on in your genes. Eye colour and blood group are examples. - Environmental variation is caused by your surroundings. A scar, or the language you speak, are examples. - Many features, such as height, come from both at once. Inside almost every cell is a nucleus holding chromosomes, which are made of coiled DNA. A gene is a section of DNA that codes for one characteristic, and most genes come in different versions called alleles. An allele can be dominant, showing when only one copy is present, or recessive, needing two copies to show. The alleles you carry are your genotype; the way you actually look is your phenotype. This module follows variation into inheritance, and inheritance into evolution.

The molecules of inheritance

Five structural terms you need before you use them. Learn what each one means.

Match the inheritance word to its meaning

Genetic against environmental

Every difference between organisms comes from their genes, their surroundings, or a mix of the two. Telling them apart is a common exam task.

Read the genotype

A gene has a dominant allele B and a recessive allele b. An organism has the genotype bb. Will it show the dominant or the recessive characteristic?

• the recessive characteristic
• the dominant characteristic
• both at the same time
• neither characteristic

Spot the true genetics facts

Tap the TWO statements that are true.

• A gene is a section of DNA that codes for a characteristic
• A person with the sex chromosomes XY is male
• A scar is passed on to your children through your genes
• A recessive characteristic shows when only one recessive allele is present

How to use a genetic cross

A genetic diagram, or Punnett square, works out the possible offspring of two parents. The method is always the same. Write the alleles each parent can pass on along the top and down the side. A parent with the genotype Tt can pass on T or t. Fill each box by combining the allele from its row and its column, always writing the dominant allele first. So a cross of Tt with tt gives Tt, Tt, tt and tt. Then read off the ratio of the offspring. That Tt by tt cross gives two showing the dominant characteristic to two showing the recessive, a ratio of one to one. ⚠️ The square shows PROBABILITY across many offspring, not a guarantee for any single one.

Cross Bb by Bb

Both parents have the genotype Bb, so each can pass on B or b, shown along the top and down the side. Fill in the four offspring genotypes, writing the dominant allele first.

• BB
• Bb
• bb

Count the ratio

In the cross of Bb with Bb you just completed, the offspring are BB, Bb, Bb and bb. Out of every 4 offspring, how many are expected to show the recessive characteristic?

Natural selection at work

Here is how a species changes over time, worked through with an example. Imagine a population of beetles, some green and some brown, living on brown soil. There is variation: the beetles are not all the same colour to begin with. Some are better suited: birds spot the green beetles easily against the brown soil and eat more of them, while the brown beetles are camouflaged and survive. The survivors breed: the brown beetles that live go on to reproduce, and they pass their brown colouring to their young. Over many generations the population shifts: each generation has more brown beetles than the last, until almost the whole population is brown. This slow change, driven by which individuals survive to breed, is natural selection, the theory set out by Charles Darwin.

Order natural selection

Put the stages of natural selection into the order in which they happen.

• There is variation within a population
• Some individuals have characteristics that suit the environment better
• These individuals are more likely to survive and reproduce
• They pass the helpful characteristics on to their offspring
• Over many generations the species changes

Complete the inheritance facts

A version of a gene is an _____. An allele that shows even when only one copy is present is _____. The alleles an organism carries for a characteristic are its _____. A human body cell contains _____ pairs of chromosomes.

Build the genetics rule

Choose the words that complete this statement about a monohybrid cross.

Genetics in action

Read each case and choose the best answer, then think about why.

• Two plants have identical genes, but one is grown in bright light and grows tall while the other is grown in the dark and stays short. What kind of variation is the height difference?
• A couple are told their baby is female. Which pair of sex chromosomes does the baby have, and which parent determined it?
• A farmer keeps breeding only the cows that produce the most milk, generation after generation. What is this an example of?

Explain variation and evolution

A student in the year below is muddled about genes, inheritance and evolution. Explain the key ideas using what this module has covered.

• Explain the difference between genetic and environmental variation, with an example of each
• Explain what a gene and an allele are, and the difference between a dominant and a recessive allele
• Explain how a genetic cross of Bb with Bb gives a ratio of three to one
• Explain how natural selection changes a species over many generations
• Finish by explaining the difference between natural selection and selective breeding