Although we each have our own individual genetic code, we share a portion of this code with other species. Humans have in the genome traces of ancient genes stemming from the beginning of time that they share with every living organism. So, if humans have so much in common with other species, what is it that defines them as being human? What is it that turns human into the complex being capable of learning, speaking, thinking and feeling? What is it that makes an individual different from other human beings? What do humans have in common with a mouse or a worm? Humans share a surprising number of genes with other species. Although these genes don't all have the letters in the same order, their function is similar enough for them to be considered comparable. These genes likely stem from a common ancestor, one that lived 3.5 billion years ago. Scientists theorize that through evolution this ancestor's genome became the basis for every species that we know today.

% Common with Humans

Chimpanzee, Pan troglodytes
21 000 genes; Just like humans, chimpanzees have around 21 000 genes. But then why can't they speak? The difference could be in a single gene, FOXP2, which in the chimpanzee is missing certain sections.

98%

Mouse, Mus musculus
21 000 genes; Thanks to mice, researchers have been able to identify genes linked to skeletal development, obesity and Parkinson's disease, to name but a few.

90%

Zebrafish, Danio rerio
25 000 genes; 75% of the genes in these little fish are the same as in humans. Researchers use them to study the role of genes linked to blood disease, such as anemia falciforme and heart disease.

Fruit Fly, Drosophila melanogaster
13 600 genes; For the past 100 years, the fruit fly has been used to study the transmission of hereditary characteristics, the development of organisms, and, more recently, the study of changes in behaviour induced by the consumption of alcohol.

Thale cress, Arabidopsis thaliana
25 000 genes; This little plant, from the mustard family, is used as a model for the study of all flowering plants. Scientists use its genes to study hepatolenticular degeneration, a disease that causes copper to accumulate in the human liver.

Yeast, Saccharomyces cerevisiae
6 275 genes; Humans have certain genes in common with this organism that is used to make bread, beer and wine. Scientists use yeast to study the metabolism of sugars, the cell division process, and other diseases such as cancer.

Roundworm, Caenorhabditis elegans
19 000 genes; Just like humans, this worm possesses muscles, a nervous system, intestines and sexual organs. That is why the roundworm is used to study genes linked to aging, to neurological diseases such as Alzheimer's, to cancer and to kidney disease.

Bacterium, Escherichia coli
4 800 genes; The E. coli bacterium inhabits the intestines. Researchers study it to learn about basic cell functions, such as transcription and translation.

Heredity   

Similarities among family members have been recognized for thousands of years, but only in the last couple of centuries have we really started to understand how family traits get passed on. Genetics is the study of heredity and variation of organisms. Heredity is the passing on of traits from ancestors to their descendants through the genes. Because of heredity, offspring usually resemble their parents, so that cats have kittens and pumpkin seeds produce pumpkin plants. Genetic variation is when differences occur among individuals within the same population or species. Variation may result from mutation, or simply from the different combinations of the parents' genes. While humans are 99.9 per cent genetically similar, we are even more like members of our own family. We pass on a copy of our genome to our descendants and, as a result, all humans inherit half of their mother's and half of their father's genome. There is only one race - the human race. With the sequencing of the human genome, scientists have confirmed what they have known for a long time: there is no biological basis for race. Cultural and social differences are what distinguish one group from another. Indeed, genetic variations found within one racial group are often greater than those found between two different groups.

A neighbor-joining network of population similarities. The network was based on the frequencies of 100 Alu insertion polymorphisms (i.e. differences in the occurrence of so-called Alu DNA repeat sequences).

Sexual reproduction involves the merging of genetic material from different parents. It depends on reproductive cells (egg and sperm cells), each containing one half of the parent's genome. Such reproduction underlies evolution and genetic variation. Sexual reproduction is needed for some species like humans to survive, and so not surprisingly, it is one of the most powerful of all drives. For humans, each reproductive cell has 23 chromosomes (half the full set). Because these chromosomes come in pairs, when the cells combine genetic material, each chromosome from one parent must match up with the same chromosome from the other. But before this combination takes place, the male and female's genes each line-up on their respective sides in a process called meiosis. This process guarantees variation in the reproductive cells. The chromosomes unite to scramble the genetic information before dividing in two again. They then form four sex cells, each with different genetic compositions. This is the process that makes you different from your sister or brother.

Human sperm and egg

Every living thing is the product of the interaction that takes place between genes and the environment. So the genetic input from parents is partly responsible, but also the environment plays a role.  What are the proportions? Genomics may help us figure this out. For organisms that produce children through sexual reproduction (where different genes from each parent are combined), an offspring is never an exact copy of its parents. And in almost every case, no two children of the same parents are genetically identical to each other. The exception is identical multiples like monozygotic twins, who share the same genome. Thanks to genetic variations received from their parents, certain organisms are better adapted to their environment than others. For example, good camouflage means better chances of escaping from predators, finding food, and having many descendants. This can be seen in these different species.

Click here for an animation on Accumulating mutations

Click here for a movie on Radiation can cause DNA mutations

See also under Genetic variations: SNPs and CNVs.

Coquina Donax, Donax variabilis

Manus Island Tree Snails, Papustyla pulcherrima

Noble Scallops, Chlamys senatoria

  Bonsai tree

  Scotch pine, Pinus sylvestris

Certain variations do not have a genetic origin. When plants do not have enough water and food, they will not grow as well as those that do. This variation is not passed on to the next generation. For example, this bonsai tree is a Scotch pine that has been transformed by humans through control of its environment. If its seeds were planted outdoors, its descendants could grow to a height of more than 20 meters.