GCSE Biology: Genetics and Inheritance Guide

DNA and genes

DNA is a molecule found in the nucleus of cells that carries genetic information. A gene is a small section of DNA that codes for a particular characteristic or protein.

Alleles

Most characteristics are controlled by alleles, different versions of the same gene. An allele can be dominant (only one copy needed to show the characteristic) or recessive (two copies needed).

Punnett squares

A Punnett square predicts the possible genotypes and ratios of offspring from a genetic cross, using capital letters for dominant alleles and lowercase for recessive alleles.

Genetic disorders

Some inherited disorders are caused by specific alleles, such as cystic fibrosis (caused by a recessive allele) and Huntington's disease (caused by a dominant allele). Genetic testing and family trees can help predict the likelihood of a disorder being inherited.

Common mistakes

• Confusing dominant (shows with one copy) and recessive (needs two copies) alleles.
• Setting up a Punnett square incorrectly, mixing up rows and columns.
• Forgetting that having a recessive allele doesn't always mean showing the associated characteristic.