Although we are all unique, there are often obvious similarities within families. Maybe you have the same nose as your brother or red hair like your mother? Family similarities occur because we inherit traits1 from our parents (in the form of the genes2 that contribute to the traits).
This passing of genes from one generation to the next is called heredity. Simple organisms pass on genes by duplicating their genetic3 information and then splitting to form an identical organism4. More complex organisms, including humans, produce specialised sex cells (gametes5) that carry half of the genetic information, then combine these to form new organisms. The process that produces gametes is called meiosis6.
Meiosis makes sperm and eggs
During meiosis in humans, 1 diploid7 cell8 (with 46 chromosomes9 or 23 pairs) undergoes 2 cycles of cell division but only 1 round of DNA replication10. The result is 4 haploid11 daughter cells12 known as gametes or egg and sperm cells (each with 23 chromosomes – 1 from each pair in the diploid cell).
At conception, an egg cell and a sperm cell combine to form a zygote13 (46 chromosomes or 23 pairs). This is the 1st cell of a new individual. The halving of the number of chromosomes in gametes ensures that zygotes have the same number of chromosomes from one generation to the next. This is critical for stable sexual reproduction14 through successive generations.
The phases of meiosis in humans
| Interphase | Replication15 of DNA16 in preparation for meiosis. After replication, each chromosome17 becomes a structure comprising 2 identical chromatids. |
| Prophase I | The chromosomes condense into visible X shaped structures that can be easily seen under a microscope18, and homologous chromosomes pair up. Recombination occurs as homologous chromosomes exchange DNA. At the end of this phase, the nuclear membrane19 dissolves. |
| Metaphase I | Paired chromosomes line up along the middle of the cell. |
| Anaphase I | The pairs of chromosomes separate and move to opposing poles. Either one of each pair can go to either pole. |
| Telophase I | Nuclear membranes reform. Cell divides and 2 daughter cells20 are formed, each with 23 chromosomes. |
| Prophase II | There are now 2 cells. DNA does not replicate21 again. |
| Metaphase II | Individual chromosomes line up along the middle of the cell. |
| Anaphase II | The chromosome copies (chromatids) separate and move to opposing poles. |
| Telophase II | Nuclear membranes reform. There are 4 new haploid daughter cells. In males, 4 sperm cells are produced. In females, 1 egg cell and 3 polar bodies are produced. Polar bodies do not function as sex cells. |
Genetic variation is increased by meiosis
During fertilisation22, 1 gamete23 from each parent combines to form a zygote. Because of recombination and independent assortment in meiosis, each gamete contains a different set of DNA. This produces a unique combination of genes in the resulting zygote.
Recombination or crossing over occurs during prophase I. Homologous chromosomes – 1 inherited from each parent – pair along their lengths, gene24 by gene. Breaks occur along the chromosomes, and they rejoin, trading some of their genes. The chromosomes now have genes in a unique combination.
Independent assortment is the process where the chromosomes move randomly to separate poles during meiosis. A gamete will end up with 23 chromosomes after meiosis, but independent assortment means that each gamete will have 1 of many different combinations of chromosomes.
This reshuffling of genes into unique combinations increases the genetic variation25 in a population26 and explains the variation we see between siblings with the same parents.
Useful link
Visit the Learn Genetics website to go on an animated tour of the basics. View the ‘What is inheritance?’ and ‘What is a trait’ segments to find out more about inheritance and variation.
- trait: Characteristic, usually a physical characteristic of a living organism, such as the height of a plant or the hair colour of a mammal.
- genes: A segment of a DNA molecule that carries the information needed to make a specific protein. Genes determine the traits (phenotype) of the individual.
- genetic: Of, relating to, or determined by genes.
- organism: A living thing.
- gamete: Male or female reproductive cells – a sperm or egg in animals, and pollen and ova in plants. Gametes contain only a single set of chromosomes.
- meiosis: A type of cell division that creates gametes (in humans, egg and sperm cells) for sexual reproduction.
- diploid: Having two copies of each chromosome. These cells have twice the haploid number of chromosomes as they inherit one set of chromosomes from each haploid gamete (egg and sperm).
- cell: 1. Building block of the body. A human is made of millions of cells, which are adapted for different functions and can reproduce themselves exactly. 2. A simple electrolytic device that enables chemical energy to be transformed into electrical energy.
- chromosome: A structure within the cell nucleus made of a single coiled piece of DNA that contains the genetic blueprint of an organism.
- DNA replication: The unwinding and copying of both strands of DNA prior to cell division.
- haploid: Having one copy of each chromosome, or having a single set of chromosomes. Gametes (egg and sperm cells) are haploid.
- daughter cell: One of two (or more) new cells that is formed when a cell divides.
- zygote: A fertilised egg (diploid cell) before cell division has begun.
- sexual reproduction: The formation of a new individual after the joining of male and female sex cells (gametes) from different parents. In some plants, sexual reproduction can involve gametes from the same parent.
- replication: The process of copying DNA within a cell; occurs before cell division.
- DNA: Deoxyribonucleic acid (DNA) is a molecule that contains the instructions needed for an organism to develop and function. These instructions are stored as a code made up of four chemical bases: adenine (A), guanine (G), cytosine (C) and thymine (T).
- chromosome: A structure within the cell nucleus made of a single coiled piece of DNA that contains the genetic blueprint of an organism.
- microscope: An instrument that uses a lens or a series of lenses to magnify small objects.
- nuclear membrane: Membrane that encloses the genetic material of eukaryotic cells.
- daughter cell: One of two (or more) new cells that is formed when a cell divides.
- replicate: Make an exact copy of.
- fertilisation: (Reproduction) The joining of male and female sex cells (gametes), resulting in combining genetic material.
- gamete: Male or female reproductive cells – a sperm or egg in animals, and pollen and ova in plants. Gametes contain only a single set of chromosomes.
- genes: A segment of a DNA molecule that carries the information needed to make a specific protein. Genes determine the traits (phenotype) of the individual.
- genetic variation: Slightly different sequences in the genetic code of different individuals from the same species. This is important for species adaptation to occur.
- population: In biology, a population is a group of organisms of a species that live in the same place at a same time and that can interbreed.
