VMBO Year 3 · Inheritance and evolution · Section 3.5

Variation in genotypes

How does inherited variation arise?

During sexual reproduction, sex cells with different combinations of genetic information unite. Meiosis and fertilisation produce new allele combinations. This creates variation in genotypes, which can contribute to variation in phenotypes, such as differences in ladybird colour or markings.

Asexual reproduction

Asexual reproduction involves one parent. Offspring generally have the same genotype as that parent, apart from any new mutations. Their appearance can still differ because the environment affects phenotype. A plant grown from a cutting, for example, is usually genetically identical to its parent.

What is a mutation?

A mutation is a change in DNA. It can arise spontaneously, for example during DNA copying. External mutagens, such as ultraviolet radiation and substances in cigarette smoke, can increase the chance of mutations by damaging DNA.

Effects of mutations

A mutation in a sex cell, or in a cell producing sex cells, can be passed to offspring. Many mutations have no noticeable effect; some are harmful and a few are beneficial in a particular environment. An organism carrying a mutation may be called a mutant, whether or not the change is visible. Albinism, involving reduced or absent melanin production, is an example of a condition caused by particular inherited gene variants.

How can cancer develop?

Some mutations affect the control of cell division. A group of cells may then multiply abnormally and form a tumour. A benign tumour does not invade surrounding tissues or spread to distant sites. A malignant tumour can invade nearby tissues: this is cancer. Growth rate alone does not determine whether a tumour is benign or malignant.

Metastases

Cancer cells can detach and spread through blood or lymph to other parts of the body. They may establish new tumours called metastases, or secondary tumours.