A family tree diagram is a model of inheritance, showing whether a characteristic was passed from parents to offspring.

The genotype is the combination of alleles that an individual has for each gene.

The phenotype is the physical characteristics of an organism.

A carrier of a recessive genetic disorder does not express the disorder in their phenotype, but has one recessive allele in their genotype that can be passed to offspring.

Inheritance is the passing of genetic information from parents to offspring by reproduction.

Models of single-gene inheritance: family tree diagrams

Combined science

Pupils will learn about growth through cell division, mitosis as part of the cell cycle, cancer as the result of changes in cells that lead to uncontrolled growth and division, and the formation of gametes by meiosis.

Pupils learned about what the genome is, what it’s made of and where it’s stored, more detail about the chemical structure of DNA, and the relationship between an organism’s genome and its phenotype.

This unit covers key genetic terms, single-gene inheritance, and genetic crosses, including probability and ratios. It explores how most traits result from multiple genes and sex determination in humans. The unit also includes interpreting data and using models to solve problems.

Inheritance, genotype and phenotype

has the disorder but only has mild symptoms.

hasn’t got the disorder and who can’t pass it on.

hasn’t got the disorder, but can pass it on.

For a recessive genetic disorder, what does it mean when an individual is referred to as a “carrier”? It is someone who ...

Match the genotype to its correct description.

Here is a family tree. How many generations are shown?

Here is a family tree. Which individuals are brother and sister?

Here is a family tree. The shaded individuals have a genetic disorder caused by a recessive allele. Which two parents must be carriers?

Marfan syndrome is a genetic disorder caused by dominant allele **M**. A father is **Mm**.  The mother is **mm**. Their 1st child has Marfan. What is the probability that a 2nd child will have Marfan.

Starting with the smallest, put these in order of increasing size.

Which is the correct end for the sentence? All humans have the same ...

A single gene with two alleles controls the wetness of earwax. The **E** allele is dominant and causes wet ear wax. The **e** allele is recessive. What is the genotype of a person with dry earwax?

The shape of peas is controlled by a single gene with two alleles: **R** for round and **r** for wrinkled. **R** is dominant. Which of the numbered boxes in the Punnett square contains a mistake?

The probability that they will have a child with PKU is 0.

Only if they have four children, will one inherit PKU.

The probability that they will have a child who carries PKU is 0.75.

The probability that any child they have will inherit PKU is 0.25

PKU is a disorder in humans caused by a recessive allele, **p**. Two carriers of PKU are shown this Punnett square. What can they learn from the Punnett square?

A dominant allele **R** codes for red flower colour, and the allele **r** codes for white flowers. A **RR** plant is crossed with a **rr** plant. What colour flowers will their offspring have?

Starter Quiz