Genetics and Inheritance

How traits pass on

Children resemble their parents because information is passed from one generation to the next. That information is carried by DNA, a long molecule found in the nucleus of a cell and packed into structures called chromosomes. A short section of DNA that codes for a feature is called a gene. Chromosomes come in pairs, so genes come in pairs too, and the two copies of a gene can be slightly different versions called alleles. You inherit one allele of each gene from each parent. Some alleles are strong and show whenever they are present; others are hidden unless both copies are the same. This module looks at genes, alleles, dominant and recessive features, and how a simple cross lets you predict what the offspring are likely to be.

Genetics words

Learn these terms before the crosses are worked. They name what is inherited and how it is described.

Match each genetics term to its meaning

Dominant against recessive

Two alleles of the same gene do not carry equal weight in the phenotype. Knowing which is which lets you predict the feature an organism will show.

Which allele shows in the offspring?

An offspring has the genotype Bb, where B is dominant for purple flowers and b is recessive for white flowers. What colour will its flowers be?

• Purple
• White
• A mixture of purple and white
• It cannot be known

Read the genotype, predict the phenotype

The genotype is the pair of alleles an organism carries; the phenotype is the feature you can actually see. To move from one to the other, ask a single question: is a dominant allele present? If yes, the dominant feature shows, whether the genotype is two dominants or one of each. If no dominant allele is present, so the genotype is two recessives, the recessive feature shows. That is the whole rule. When you meet a genotype in an exam, do not guess from the letters at random: read whether a capital letter is present, and let that decide the feature. This habit turns every genetics question into the same short check.

Pick the inherited features

Select every feature that is inherited through genes rather than caused only by the environment.

• Coat colour in an animal
• Flower colour set by alleles
• Seed shape in a plant
• A scar from an injury
• A skill learned by practice

Order the steps of a genetic cross

A genetic cross is worked in a set order. Put the steps in the order you would follow.

• Write the genotypes of both parents
• Work out the alleles each parent can pass on
• Draw a Punnett square with those alleles
• Fill in the offspring genotypes
• Read off the ratio of phenotypes

Complete the genetics facts

A gene is a section of _____. A version of a gene is an _____. An allele that shows only when two copies are present is _____. The pair of alleles an organism carries is its _____.

Work out the cross ratio

Two parents are each Bb, where B is dominant. Draw a Punnett square for this cross. Out of every four offspring, how many are expected to show the recessive feature?

A shuffled deck of cards

Picture each parent holding a pair of cards for one feature and passing just one of the two to each offspring, at random. If a parent holds one red card and one blue card, half its offspring get red and half get blue, and which one any single offspring receives is down to chance. Inheritance works the same way: a parent carries two versions of a feature and passes one to each offspring, chosen at random. That is why brothers and sisters can differ, and why a cross gives a predictable ratio across many offspring but never a guarantee for any single one. So treat a cross as a prediction of the odds, not a promise about one individual.

Tap the dominant-allele statements

Tap the TWO statements that are true of a dominant allele.

• it shows even when only one copy is present
• it is written as a capital letter
• it shows only when two copies are present
• it is always the more common allele in a population

Predict each genetic cross

Read each cross and choose the best answer.

• Both parents are Bb, where B is dominant purple and b is recessive white. What ratio of purple to white flowers is expected in the offspring?
• One parent is BB and the other is bb. What will all of the offspring look like?
• A recessive feature appears in an offspring whose parents both show the dominant feature. What must be true of the parents?

Build an inheritance sentence

Choose the word for each gap to complete one point about inheritance.

Explain how characteristics are inherited

A friend thinks children are simply a blend of their parents, like mixing paint. Using what you have learned, explain how characteristics are actually inherited.

• Explain what genes, alleles and chromosomes are
• Explain the difference between a dominant and a recessive allele
• Explain the difference between genotype and phenotype
• Explain how a genetic cross predicts offspring, using a Punnett square
• Explain why offspring are not simply a blend of their parents