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Genetics explained:

A glossary of common genetic terminology

Put at its simplest, genetic inheritance means that the cells which make up any organism's body contain complex chemicals called DNA and RNA, which act as coded instructions to the cell to produce other, different chemicals which control how cells behave, and so how the organism develops. This genetic code is passed on from parent to offspring during reproduction.

Some primitive species reproduce by cloning, that is, by a single parent producing offspring which are copies of itself and which therefore have a genetic code identical to the parent's. In this case, the genetic material changes very slowly by "mutation", that is, by alterations in the genetic code caused by transcription errors or damage. In organisms which use sexual reproduction the offspring derive their genes from two parents, with roughly half their genetic material coming from each parent: this leads to a constant re-combination of different genes producing slightly different organisms each time. This means that the species can evolve much faster, because for any change in the environment there will be some individuals who are better at coping with it than the rest.

This glossary is set out in logical rather than alphabetical order: read straight through from start to finish it should provide a crash-course in basic genetics. The first section concentrates on the physical components of genetic inheritance; the second on the interactions of genes with each other and with the organism's observed characteristics; the third covers the way genes are re-assorted during reproduction and some of the ways this can go wrong, linkages between genes, and the various forms of twinning; and the fourth looks at some terms you may encounter in discussions of evolutionary genetics.

Physical aspects of genetic inheritance

genotypeAn organism's genetic make-up. phenotypeAn organism's structure and appearance - which is a result of a combination of genotype with environmental/developmental factors. proteinOne of a very large number of different complex molecules containing carbon and nitrogen. Most of the business of being a living organism is carried on via proteins of one sort or another. amino acidOne of a limited number of different chemical building-blocks from which proteins are constructed, and containing particular chemical combinations called acids and amines which we don't need to go into. nucleotide basesSmall chemical building-blocks which fit together in long chains to form DNA or RNA (see below). There are five different types of base, to which letters are assigned for convenience - adenine (A), thymine (T), guanine (G), cytosine (C) and Uracil (U). As well as stringing together in chains, each base has a surface shape which is able to lock like a jigsaw puzzle with a matching shape on one or two other bases, but no more. C and G lock only with each other. A can lock with T and with U, but T and U can each only lock with A, not with each other. So the possible pairs are CG, GC, TA, AT, UA and AU. DNADeoxyribonucleic acid. Long helical molecule built up from combinations of four of the nucleotide bases – A, T, G and C. These work like letters in a language: different combinations of the four "letters" cause the cell containing the DNA to manufacture different proteins, which in turn influence the organism's phenotype.

The molecule is normally found as a double helix – two strands coiled round each other, with their bases interlocking to zip the strands together. If there's a C on one strand, there has to be a G in the matching position on the other strand, and so on. RNARibonucleic acid. Single-helical molecule made from the same four bases as DNA, except that it has uracil (U) instead of thymine. It is used to transport the DNA's "code" to the manufacturing areas of a cell. The two strands of the DNA molecule partially unwind and "messenger" RNA (mRNA) then forms along one of them, matching up amino acids to their opposing pairs – so e.g. an A on the DNA is matched by a U on the RNA, a C by a G and so on. The RNA then detaches and is used as a template from which proteins are generated by bolting amino acids together in a particular sequence. These proteins then have various effects on the organism.

Other variants of RNA are used to e.g. transport amino acids to their correct position on the mRNA template. introns and exonsIn eukaryotes (organisms most of whose cells have a nucleus bounded by a membrane - which is most things that aren't bacteria) the raw mRNA which spools off from the DNA contains the code for a protein broken up into sections called exons which are interleaved with blocks of other code, called introns, which do not relate to that protein. These introns have to be snipped out before the messenger code is attached to an organelle called a ribosome, where the protein is assembled. The function of these introns is still being researched but they do seem to have some kind of regulatory rôle - they're not just junk. codonSequence of three bases, such as CUA, which together constitute a template against which a particular amino acid can be lined up. An mRNA template can be read as a series of three-base codons: amino acids line up against their particular codon and fuse to form a protein molecule, and the order of the codons, and hence of the amino acids, determines the type of protein being formed. chromosomeA very long string of super-coiled DNA. Higher animals have several different chromosomes of distinct shapes and sizes, and for each type of chromosome most cells in their body have two copies, one derived from the mother and one from the father.

The exception is the sex chromosomes, which are the main factor in determining gender: in mammals females inherit two copies of the X chromosome, one from each parent, but males inherit an X chromosome from the mother and a Y chromosome (different size and shape) from the father.

Note that there are plenty of things that can go wrong with the sex chromosomes, resulting in e.g. XXY males who are male and yet have two XXs. Since all mammalian embryos start off female-shaped and only develop male characteristics quite late on, it is also possible for e.g. hormonal problems in the womb to produce an individual who is female in hormones and external anatomy, and yet has male genes and male internal structures.

In both females and XXY males only one X chromosome per cell is active, the other being switched off. The switched-off X chromosome stains differently from the active one in microscope slide preparations, and is known as a Barr Body.

If the X-chromosomes from each parent carry different colour-genes, for certain colour-combinations this can result in a patchwork-coloured ("tortoiseshell") animal, with e.g. some cells expressing an active X with a red gene, giving rise to red patches, and some with an active X with a black gene producing black patches.

Once the spare Xs in a developing embryo switch off (and Barr bodies form), each cell gives rise to a spreading patch of daughter-cells each of which has the same X switched off as in the original. The exact stage of embryological development at which this happens varies. If Barr bodies form very early in embryological development, when there are only a few cells in the embryo, each of those early cells then gives rise to quite a large area of the animal's body all with the same Xs switched on and off - hence if it is tortoiseshell it will be covered in big solid patches of colour. If Barr bodies form a bit later, when there are thousands of cells, then each will result on only a small patch of cells with that particular on/off combination, and the animal, if tortoiseshell, will be covered in subtle flecks of colour rather than patches.

The non-sexual chromosomes are referred to as the "autosomal" chromasomes. chromatinWhile they are actually being used as a template from which to generate proteins, in between episodes of cell division, chromasomes uncoil and appear as a wooly, tangled mass called chromatin. organelleA discrete internal component of a cell, such as the nucleus and mitochondria. nucleusRegion in the centre of a cell, bounded by a membrane and containing the chromosomes. mitochondrion A little discrete sac of proteins etc., multiples of which which are found in the cells of most eukaryotic organisms (that is, organisms which have a cell membrane to contain their chromasomes, which is true of most things that aren't bacteria): they process oxygen and hence enable the organism to breathe and to metabolise fuel. They are believed to have started out as independent bacteria which became symbiotic with eukaryote cells very early in their evolution, and they still have their own cell-membrane and their own separate DNA.

Sperm cells do not contain mitochondria (they are too small) but egg-cells do: therefore, mitochondria are always inherited from one's mother. [In the case of cloned cells, where new genetic material has been inserted into an emptied ovum, they will be inherited from the female who donated the ovum, rather than the donor of the main genetic material.]

In addition to their own mitochondria, plants also have similar symbiotic bacteria called chloroplasts which likewise have their own separate DNA, and which carry out the photosynthesis which feeds the plant. karyotypeThe chromosome-complement of a cell, i.e. the number and type of chromosomes found in it. diploidHaving two of each chromosome - the normal condition for cells other than sex-cells (sperm and ova) and red blood cells, which have no nucleus haploidHaving only one of each chromosome - the normal condition for sex-cells. See meiosis and gamete. triploidHaving three of each chromosome – a rare chromosomal mutation (due to an error during meiosis) which is generally fatal in animals, although not necessarily so in plants. Animals may survive with abnormalities if they are triploid for only one chromosome or part of a chromosome, which is called trisomy: for example Down's Syndrome is due to a duplication of part of one particular chromosome, with the symptoms becoming more severe according to how large the duplication is.

Genes in action

locusA specific physical region on a particular chromosome, at which a gene for a specific characteristic is found: known loci are given a name and a code letter relating to the characteristic they control, such as C for Colour. Since there are two of each chromosome there are two "slots" for each locus, one to each chromosome.

The exception again is the sex chromosomes: the Y chromosome is smaller than the X and also has some loci unique to itself, so many of the loci on the X chromosome do not have a matching position on the Y, and there are a few on the Y which don't have a match on the X. Hence male mammals, who pair an X with a Y, have many unpaired loci and are more likely to express recessive genes on the X chromosome than females are. geneA sequence of DNA code which is found at a specific locus and produces a particular effect, usually by coding for the production of a particular protein. Excepting the sex chromosomes in males and some abnormalities, there are two genes (one per chromosome) present at each locus in any given individual, except in the sex-cells (i.e. sperm and ovum). allelesA series of two or more related but different genes which can occur at a given locus (only one at a time per chromosome). They will all have nearly the same DNA sequence, with small variations which may alter the finished organism in some way, and they are given codes which consist of the code for their locus, often plus a modifier, e.g. c^h is the Himalayan (Siamese) gene on the Colour locus - the small c indicates that the gene is recessive to the dominant C or normal-colour gene. homozygousHaving a matching pair of the same allele at a particular locus, one on each of a pair of chromosomes e.g. c^h and c^h. heterozygousHaving two different alleles at a particular locus, one on each of a pair of chromosomes, e.g. c^h and C. lethalA lethal gene is one which is only ever found in heterozygous form, because individuals with two copies die soon after - or even before - birth. dominant/recessiveIf a gene produces the same effect regardless of whether there are two copies of it (one at the same locus on each of a pair of chromosomes), or one copy plus some other allele on the other chromosome, that gene is said to be dominant to the other allele. The allele whose effect is masked by the dominant gene is said to be recessive to it, and an animal which has such an invisible recessive gene is said to be "carrying" it. Generally speaking, in genetic codes dominant genes are indicated by a capital letter and recessives by lower-case.

The mechanism of dominance is usually that the dominant allele produces a lot of a particular protein, while the recessive allele makes a weaker version of that protein, or less, or none. If we say that allele X codes for protein Y which has an effect Z on the phenotype of the organism, it often happens that allele X produces so much of protein Y that even a single copy of X is enough to trigger the effect Z, and so an organism with two X alleles and one which has one X allele and one modified allele x will develop just the same, even if the x allele produces little or no protein Y. Hence, it will not be possible to tell just by looking at the organism whether it has XX at that locus, or Xx, and X is said to be dominant and x recessive. Only if the organism has a double recessive xx pair, producing little or none of protein Y, will the phenotype be affected.

Where there are several possible alleles for a given locus, there is likely to be a hierarchy of dominance. There will be one gene which is dominant to all the rest, one which is recessive to all the rest and genes in the middle which are dominant to some of the others in the sequence and recessive to others. So for example c^h is recessive towards C but dominant to c^a (albinism). codominantIf two alleles in combination produce a phenotype which is different from the effect of either allele when homozygous, and which is intermediate between them, those alleles are codominant: for example in cattle one red and one white gene produces a pink cow (red roaned with white); and in rats animals with no rex genes have a smooth coat, two rex genes make a bald rat, and one rex and one smooth gene make a curly-coated one. incomplete dominanceIf two alleles in combination produce a phenotype which is a grainy mixture of the effects of either allele when homozygous, those alleles are incompletely dominant: for example in poultry a cross between black and white chickens produces a bird with a mixture of black and white feathers, rather than grey feathers. epistasisA gene which masks/suppresses the expression of another gene occurring at a different locus is said to be epistatic to the suppressed gene. One very common example is the albinism gene, which masks (is epistatic to) genes at other loci which would cause the animal to have colour and pattern if it were not albino. polygenicA polygenic characteristic is one which only occurs when several different genes at different loci are present. epigenomeThe epigenome is a group of chemical tags which can attach to the outer edge of a DNA molecule and affect how certain genes are expressed. These can be modified during life, not just in development, and the resulting epigenetic change can be passed on for several generations, so that e.g. a learned, conditioned response can become built into the genome in a quasi-heritable way.

Genetics of reproduction

chromatidsIn cells about to undergo cell reproduction, each chromosome partially splits into two side-by-side identical copies of itself, joined together - usually near the centre to form a rough x-shape. These sub-divided duplicates are called chromatids. Basically the DNA-strands in the original chromosome start to unwind from each other, and then new base molecules line up along each of the separated strands to form a new double helix. mitosisNon-sexual cell reproduction. Individual chromosomes sub-divide into pairs of identical chromatids, and then the whole cell divides and the chromatids separate and one of each pair goes to each new cell. Having separated, the chromatids become simply chromosomes: duplicate copies of the original and each other.

This produces two daughter cells which each contain two of each type of chromosome (except for the XY pair in males), and have a genetic make-up identical to that of each other and of the original cell. meiosisSexual cell reproduction. Matching pairs of chromosomes sub-divide into linked chromatids and then line up side by side, forming cross-links from a given locus on one chromosome to a matching locus on the other. Since each chromosome now consists of two identical chromatids, there are now four "slots" to each locus, two to each chromosome. Each chromatid forms its own unique set of connections: paired chromatids of the same chromosome don't have to form links at the same loci.

The chromosomes then separate again, breaking their chromatids apart at the cross-links in such a way that pieces of chromatid get mix-'n'-matched. You start with two chromosomes and four chromatids and you finish with two chromosomes and four chromatids, but the final chromatids often contain a mixture of material from both of the original chromosomes, and therefore from both of the organism's parents.

The cell then partially divides into two joined cells each of which normally contains only one of each type of paired chromosome, plus either an X or a Y sex chromosome. These cells are haploid (because they contain only one of each type of chromosome) but each of their chromosomes is still in the form of a pair of fused chromatids. The paired chromatids then part to form pairs of new chromosomes, and the cell then divides and separates these chromosomes into haploid cells again.

As a result one original diploid cell produces four haploid daughter cells, each containing one of each type of chromosome, with many or most of those chromosomes consisting of a mix of material from both of the chromosomes of that type which were in the original, diploid cell.

gameteA haploid sex cell (sperm or ovum, or a precursor thereof) formed during meiosis. gonadAn organ/gland in which gametes are produced, e.g. ovaries and testes. zygoteA diploid cell formed by the fusion of two gametes (sperm plus ovum): the first stage in forming an embryo. mutationAlteration of individual genes or of chromosome-structure to produce a cell with a new genetic makeup.

A somatic mutation is one affecting non-sexual cells within an organism. This can lead to e.g. cancers, but has no effect on (is not passed on to) the organism's offspring, except possibly in rare cases by RNA-mediated inheritance.

When mutation occurs in sex-cells it is passed on to any offspring which may form from those cells, resulting in an embryo containing genetic elements not found in either parent. Animals with major mutations often die in utero, and those which do live may be sickly and/or malformed (plants are more tolerant of distortion). But sometimes a mutant organism will have characteristics which are neutral or even useful, and which may then spread through the population and become an established, normal variation.

Also, in the case of minor mutations there are chemical buffers involved in embryological development which can often steer the organism towards a normal phenotype, even if it has some odd genes. These buffers tend to fail if the developing embryo is subjected to serious environmental stress, with the result that minor mutations which have been carried in a population for many generations may only start to affect phenotype when that population is exposed to environmental changes which make it a good idea to try out some new variants to see if they fit the altered situation.

Mutation of individual genes occurs when the DNA sequence of a gene is damaged/changed, sometimes because the organism is old and its cell-repair mechanisms are failing, and sometimes due to exposure to solar or other radiation, viral infection or to a "mutagenic" chemical.

Some loci are fracture-points at which mutation is especially likely to occur. For example in humans achondroplasia (a type of dwarfism) is dominant and yet normally occurs in children of normal-sized parents, the achondroplasia gene having occurred/recurred afresh in the sex-cells of one of the parents.

Chromosomal abnormalities normally occur when cell reproduction goes a bit wrong. During meiosis, chromatids may fail to separate symmetrically at the cross-linking stage, so that one chromatid ends up with a piece missing and one with a duplicate section: Down's Syndrome is due to such a duplication.

Alternatively the distribution of chromosomes between the daughter cells can go wrong, resulting in one cell with a whole chromosome missing and one with a duplicate chromosome. This happens during either mitosis or meiosis when chromosome pairs assort wrongly and both end up on the same side of the dividing-line, leaving one daughter-cell with three of a given chromosome and the other with one or, in meiosis, one daughter with two of a particular chromosome and the other with none.

A sex cell with a missing or duplicate chromosome will go on to fuse with another individual's sex cell, which itself will contain a copy of the same chromosome. The resulting zygote will then contain either one or three of that chromosome, and if it survives at all will end up as a very strange organism.

The exception to this is where it is the X chromosome which duplicates, and then fuses with a cell containing a Y. Having two X chromosomes is not harmful - females do it all the time - so the resulting XXY male will be healthy and not in any way obviously malformed; though he will be subfertile and with minor changes e.g. human XXY males tend to be tall and skinny, and may be slow to talk. XXX females are also viable (= able to live).

Note that the most drastic birth-defects are usually due either to gross chromosome abnormalities or to exposure to teratogens (substances which warp an embryo's development without altering its genes, so that the deformation is not heritable) rather than to actual genetic mutations.

Also note that new genetic material can sometimes be introduced not by classic mutation but as a result of viral infection. It can happen that a virus picks up part of the genetic code of its host and incorporates it into its own RNA, then carries that free-floating code to a subsequent host where it may become attached to the new host's genome. If this happens to a sex-cell then the new genetic material becomes heritable: the "blue" (actually a bluish-looking grey) coat-colour in mammals sometimes crops up in new species by this route. Hox genesHox (homoeotic complex) genes contain base-clusters called homeoboxes which bind to proteins, thereby switching other genes on and off. Homeoboxes are almost identical whichever gene, or animal, they are found in, but the rest of the Hox gene varies and determines where and when the homeobox will be applied. Hox genes control the head-to-tail order in which various structures appear during embryological development: they initiate e.g. a whole sequence of genes for forming a limb, in the manner of a program calling sub-routines.

Mutations of Hox genes in insects result in strange aberrations such as legs growing where antennae should be. Vertebrates with Hox gene mutations normally die before birth, but it has been suggested that mutations in Hox genes are involved in major changes such as the loss of the hind-limbs in whales, and that, very early on in animal evolution, Hox genes were responsible for the formation of different phyla. [A phylum is a group of animals with a body-plan very different from that of any other group of animals, such as molluscs, vertebrates etc.: the equivalent groups in plants are called divisions.]

It is thought that Hox mutations may sometimes result in the reappearance of ancestral features which had been switched off - such as a tail occurring in a normally tailless species. The characteristic is suppressed in the first place by a Hox mutation but the code for it is still present. This code is no longer being subjected to selection pressure to keep it accurate, and over the very long term it will become corrupted by random mutations - but it will probably remain readable, and capable of being reactivated by a reverse Hox mutation, for many millions of years. Mice with a Hoxa2 mutation have extra, non-functional jaw-bones - of a reptilian pattern. linked genesTwo loci which are very close together on the chromosome, so that when the chromosome breaks and recombines during meiosis they nearly always end up on the same piece, are said to be linked. Two genes on adjacent loci may travel around together for generations, before suddenly separating. Geneticists studying the effect of genes on phenotype can easily mistake such a linked pair for a single gene with far-reaching effects - and get a big surprise when the linked genes, and the characteristics which they control, suddenly head off in opposite directions. sex-linkedBecause not all the loci on the X-chromosome are duplicated on the Y, there are some loci for which a female has two genes and a male has only one. A recessive gene on one of these loci will still need to be homozygous to be visible in the female, but a single copy will suffice in the male: the effect of the gene is therefore seen twice as often in males. Well-known examples are haemophilia and the ginger or red colour in cats. sex-influencedSome genes may be expressed with equal frequency in both sexes but be far more noticeable in one sex than the other. An example is human pattern-baldness, which produces extreme hair-loss in males but only a slight thinning of the hair in females. sex-limitedSome phenotypic characteristics depend on having a pair of different genes on one of the loci which is present on the X but not the Y chromosome. Since it requires two X chromosomes to be present, the characteristic therefore can only occur in females or XXY males. The classic example is the tortoiseshell pattern in cats, which only occurs in females because it requires a red gene on one X chromosome and a black gene on the other; with each individual cell being either red or black according to which X-chromosome is active. Normal XY male cats can be red or black all over, but not both.

Genes can also be sex-limited if they affect a characteristic which is only found in one sex. A female may have a gene affecting the shape of the penis, or a male one regulating pregnancy, but they will have no effect and nothing to affect. Y-limitedThere are a small number of loci which are found only on the Y-chromosome: genes at these loci therefore can only occur in genetic males. RNA-mediated inheritanceIt has recently been discovered that in some cases messenger RNA can be caught up in a gamete and carry the code for a protein/characteristic from parent to offspring, even when the offspring has not actually inherited the DNA of the gene for that characteristic. See Scientific American: Mouse Finding Violates Laws of Heredity. So far as I know it has not yet been established whether or not this can persist through many generations, but it can certainly happen across two generations of descent, where the original parent has a particular DNA gene churning out mRNA, the offspring inherits the mRNA but not the DNA, and the offspring then passes the mRNA to its own offspring. fraternal twinsBabies born together but produced from different eggs and with differing genotypes - the normal situation in animals which produce litters. Genetically they are normal siblings and share roughly half of each other's genetic material. Each inherits half the mother's genes and half the father's, but some of those genes will be the same as the ones their sibling got, and some will be different. identical twinsTwo babies with identical genotype, produced from a single embryo which split early in its development. Under normal circumstances such twins will be identical in phenotype (appearance etc.) as well as in genotype: but there have been rare cases of human identical twins in whom either somatic mutation or external factors (probably occurring in the womb) caused them to end up clearly non-identical in appearance, e.g. one with brown and one with blonde hair. Characteristics such as coat-markings which have an element of chance will also differ slightly between identical twins. conjoined twinsConjoined twins, also called Siamese twins, occur when an embryo starts to split quite late in its development, and creates a pair of identical twins who are not properly separated, but still at least partially joined. The join can be anything from a band of skin joining two fully-formed individuals at the chest, to a single body with two heads, or even with one head and two faces. In milder cases it may be possible to separate the twins surgically and save both of them. semi-identical twinsThis type of twin was first reported in 2007 and is believed to be very rare. A single egg is fertilized by two sperm and the egg then splits to form two embryos, which have identical maternally-derived genes but differing paternal ones. That ought to make them about three-quarters identical but the position is more complex than that, because in the case which was reported in March 2007 the babies concerned were of different gender and were partial chimaeras (see below), so that although each child was predominately made up of cells which derived from one of the original sperm, each child also contained some cells which derived from the other sperm. One child ended up hermaphrodite as a result.

Presumably semi-identical twins could form conjoined twin pairs in the same manner as identical twins. It may also be that the cases mentioned above, where otherwise identical twins differed in some characteristic such as hair colour, were in fact unidentified cases of semi-identical twins. chimaeraA reverse twin - a single baby produced from the fusion of two embryos of different genotypes. If the fusion occurs very early in development the resulting animal will be made from a random mix of cells of the two genotypes: if late, it may be divided half and half between the two. It is possible to get a chimaera animal which is e.g. male on one side and female on the other, or curly-coated at the front and smooth at the back.

Even where a set of non-identical twins or triplets are all fully-formed and born alive, they may still have exchanged a few cells early in their development, and so be partial chimaeras. mosaicOne baby produced from a single embryo, but one in which a somatic mutation early in development has resulted in two distinct cell-lines. If this occurs very early in development it will result in major changes to the finished organism – e.g., if the mutation occurs when there are only four cells present, one quarter of the final organism will carry the mutation. If it occurs late in development it will result in only a small proportion of mutated cells, and may not be noticed unless it results in e.g. patchy colour, or sexual abnormalities.

It is unlikely that a mosaic mutation would result in multiple changes to the finished organism, unless it affects a very major controlling gene. If the organism only has e.g. odd patches of colour or of rexing, or a non-hormonal hermaphroditism (mixture of genders), it's probably a mosaiac: if it has two or more such mixed characteristics it's probably a chimaera.

It could be said that all female mammals are mosaics, because they are made from a mix of two cell-types according to which X-chromasome is switched off.

Genetics of evolution

mitochondrial DNAMitochondria reproduce asexually, which means that each mitochondrion is an exact copy of its parent, unless it has mutated due to exposure to mutagenic chemicals, cosmic rays etc. or random transcription errors. These mutations are believed to occur at a fairly steady rate, such that it is possible to work out how long ago two mitochondrial lines (and therefore lines of female animals) separated by counting the number of mutations they have accumulated since they diverged and dividing by the already established (or at least assumed) average number of mutations per year. This is called a "molecular clock". cladeAll the species and individuals which descend from a single common ancestor. The group of all primates plus all rodents, for example, is not a clade, because if you track back to the most recent common ancestor of rodents and primates, that ancestor had other descendants, as well as rodents and primates. The group of all primates (including non-flying lemurs), tree-shrews, colugos (flying lemurs), rodents and lagomorphs (rabbits, hares etc) is a clade, called the Euarchontoglires, because as far as we know the most recent common ancestor of the Euarchontoglires has and had no descendents outside that group. The science of identifying what groups go together to form a clade is called cladistics clineA cline is a situation in which a habitat changes gradually over distance, such as e.g. becoming colder and more mountainous as you travel north, or drier and more barren as you head towards a desert, and the species that live there develop a progression of subtly different races or sub-species to suit the changing habitat. So you might find e.g. that individuals from the colder end of a cline were hairier and more thickset than those from the warm end, whilst remaining identifiably the same species. ring speciesA ring species is a cline in which the sub-species at the far ends of the cline cannot interbreed with each other, even though each population at the ends of the cline can breed with their nearest neighbours on the side heading towards the centre of the cline, and there is a chain of interbreeding populations between them. closed ring speciesA ring species which curves round geographically so that the populations at the two ends of the cline are physically in contact and have the opportunity to interbreed if they were able to, but cannot.