Unit of information located on a given chromosome and consisting in a fragment of DNA. Each gene directs the production or several proteins and ensures the transmission and the expression of a given character.
A gene can take various values called alleles.
A locus is the place where genetic information is coded for a character; this information is made up of two alleles, one coming from the father, and the other coming from the mother.
It is the sum of all the genetic inheritance of an individual.
It is the sum of all the characters (physical or psychological) of an individual. It is different from the genotype.
When several alleles can be present on a locus, some are dominant compared to others; it means that they impose their phenotype.
Others are recessive; it means that they impose the phenotype only if they are present in two specimens.
Example of the locus coding the length of the hair: two alleles being able to take two values ("C" for the short hairs, and "l" for the log hairs).
Thus, for the character "length of hair", there will be four genotypes: "CC", "Cl", "lC" and "ll".
However, only two phenotypes can be observed: short hair and long hair.
This is due to the fact that the gene "C" is dominant compared to the gene "l": when "C" is present, it determines the phenotype (i.e. one sees only him).
The allele "l" being recessive, it determines the phenotype only if it is present twice.
One thus can draws up the table of the correspondences genotype-phenotype for the length of the hair:
|
Genotype |
Phenotype |
|
CC |
Short hair |
|
Cl |
Short hair |
|
lC |
Short hair |
|
Ll |
Long hair |
One says that there is an incomplete dominance when the character coded by the recessive allele appears partially in the heterozygous individuals.
In the example of the hair length, it is to say that the individuals having the "Cl" and "lC" genotypes will have half long hairs, or "breeches" (long hairs on the thighs).
The coefficient of inbreeding of an individual is equal to the probability that the two genes he has in a locus are identical by descent (it is to say that they are both inherited from a common ancestor).
The coefficient of inbreeding (expressed as a percentage or by a value between 0 and 1) measures the probability with which an individual has received, from both parental gametes, two copies of the same ancestor allel.
The inbreeding increase will statistically increase the homozygous percentage compared to the heterozygous. One can even quantify this increase, thanks to the law of Hardy-Weinberg:
At a given generation "N", if the frequency of an allele "A1" is "p", and the frequency of an allele "A2" is "q", here is the probability of having the various genotypes at the following generation:
|
Genotype |
Probability |
|
A1A1 |
p^2 + F*p*q |
|
A1A2 |
2*p*q*(1-F) |
|
A2A2 |
q^2 + F*p*q |