Affinity - Relationship by marriage, i.e. in-laws and step relatives.

Alkaptonuria - The name of this condition comes from excretion in the urine of alkapton or homogentisic acid, which turns black on exposure to air. It is due to a recessive gene which in the homozygous state prevents the formation of the enzyme homogentisic acid oxidase.

Albinism - Albinism is due to a recessive gene which blocks the metabolic pathway converting the amino acid tyrosine into 3,4 - dihydroxyphenylalanine and then to melanin. Absence of melanic pigment causes skin, hair and eyes to appear pink and hair white.

Allele - Abbreviation of allelomorph, meaning alternative form of a gene at the same locus resulting from mutation.

Ancestors - Predecessors in the pedigree with whom an individual is directly related.

Assortative mating - The tendency for like to mate with like. i.e. phenotypically alike.

Autosomal genes - Genes present on any chromosome other than the sex chromosomes.

Bilineal relationship - Collateral relatives exhibit bilineal relationship when they are connected through both their parents. Those with a direct relationship are said to be bilineally related when the descendant is connected through both parents to the ancestor and also the ancestor is able to pass both members of a pair of alleles at the same locus down to the descendant by different independent routes. The significance of this is that any variance due to dominance interaction between two alleles contributes to the phenotypic correlation between bilineal relatives but not between unilineals.

A further effect of bilineal relationship is that harmful genetic conditions caused by recessive genes may affect both relatives who are linked bilineally but not unilineally. When the harmful gene is dominant the method of transmission is irrelevant and the abnormality will appear under both forms. The above remarks only apply to autosomal genes; for sex-linked genes the situation is more complicated.

'Blood' relatives - Relatives that have a true genetic relationship, and are either related directly or through recent common ancestors.

Carriers - Individuals who are heterozygous with respect to some recessive trait.

Chromosome - A structure composed of DNA, found in the cell nucleus, on which genes are arranged in a linear order.

Clone - Two or more individuals with identical genotypes, e.g. identical twins.

Coefficient of correlation - Statistical measure of resemblance, at the population level, between two individuals for the same character, or the covariation between two characters in the same individual.

Coefficient of inbreeding - The probability that two alleles at a given locus are identical by descent. i.e. The extent to which an individual is more likely to be homozygous rather than heterozygous because of related parents.

Coefficient of relationship - Denotes the proportion of genes that are held in common by two individuals as a result of direct or Collateral relationship.

Collateral relationship - Individuals who are only related through common ancestors, e.g. cousins.

Consanguinity - Genetic or 'blood' relationship. The "degree of consanguinity" is a measure used by the legal profession to assess the closeness of relationship.

Conventional double relationships - Conventional denotes the more usual (or orthodox) types showing bilineal inheritance. This is in contrast to the less common variant forms whose inheritance is unilineal. It also differentiates between contemporary double cousins and the much rarer cross-generation doubles.

Covariance - Like correlation, covariance is a measure of the degree to which variables vary together or a measure of the intensity of association.

Deceased - Recently dead. Sometimes referred to as the decedent by the legal profession.

Descendants - Individuals who are directly descended from a particular ancestor.

Direct relationship - Two individuals are directly related when one passes genes to the other through a direct line of descent. Examples are: parent and child and between any ancestor and descendant.

Dizygotic twins - Derived from two separate ova (eggs) and therefor not identical. Also known as fraternal twins.

Dominance - Interaction between two alleles at the same locus where one partially or completely prevents the phenotypic expression of the other when present in the heterozygous state.

Epistasis - 1. A form of gene interaction whereby one gene prevents the phenotypic expression of another non-allelic gene. 2. In quantitative genetics epistasis refers to all non-allelic interactions.

Gene - Defined as a specific sequence of nucleotides which determines the sequence of amino acids in a specific polypeptide (enzyme or other protein).

Generation interval - The generation interval for an individual human family, class of livestock or species is the average age of the parents when the offspring are born (or hatched). The average generation interval for a particular direct line is the time difference between the birth dates of the first and last members of the line divided by the number of generations.

Genetics - The scientific study of heredity and variation.

Genome - The sum total of a species' genes as arranged on the chromosomes.

Genotype - The genetic constitution of an animal, as distinct from its physical appearance (phenotype).

Haemophilia - Haemophilia is caused by a sex-linked recessive gene which prevents fibrinogen, a soluble protein in the blood plasma, being converted into fibrin, the substance which forms blood clots. Sufferers can have severe haemorrhages from relatively small injuries..

Haploid - Refers to a single set of chromosomes, which is the usual constitution of gametes (sex cells) as opposed to the animal itself which has two sets of chromosomes and is diploid.

Hardy-Weinberg law - Defines the expected proportions of the 3 genotypes (AA, Aa and aa) in a random mating population, in the absence of migration, mutation and selection.

Hemizygous - Applies to sex-linked genes present on the single X chomosome in males. i.e. Females are either homozygous or heterozygous for these genes but males can be only hemizygous.

Heritability - For a given character, it is the fraction of the superiority of selected parents over the mean of their generation that is transmitted to their offspring. It shows how highly heritable a trait is.

Hermaphrodite - A hermaphrodite is an animal which possesses both male and female sex organs in the same individual.

Heterosis - The greater vigour in terms of growth, survival and reproductive performance of outcrosses, particularly crosses between highly inbred lines. Associated with increased heterozygosity.

Heterozygous - Carrying both the dominant and recessive members of a pair of alleles.

Homozygous - Having identical rather than different alleles in the corresponding loci of a pair of chromosomes and therefore breeding true.

Inbreeding - The mating of closely related plants or animals.

Inbreeding depression - A decrease in vigour or fitness, resulting from inbreeding.

In-laws - Related by marriage. Spouse's 'blood' relatives and 'blood' relatives' spouses (see here).

Intestacy - Dying without leaving a will.

Kindred - Archaic name for 'blood' relatives.

Locus - The position that a gene occupies on a chromosome.

Matrilateral - Used by anthropologists for relatives on the mother's side of the family. Genealogists tend to prefer the simpler term maternal.

Matrilineal - Inheritance down the female line where an individual, of either sex, is directly related to a female ancestor through female intermediates. Mitochondria are inherited in this way.

Meiosis - Two successive divisions of the cell nucleus preceding the formation of sex cells which result in halving the number of chromosomes.

Monoecious - A monoecious plant has both male and female sex organs (flowers) in the same individual, whereas a dioecious plant has male and female organs in different individuals.

Monozygotic twins - Derived from a single ovum and therefore identical.

Overdominance - Where a heterozygote is superior in performance to either of the two homozygotes.

Parity - The order in which children are born.

Parthenogenesis - The development of an individual from an egg without fertilization.

Patrilateral - The paternal or father's side of the family. (See matrilateral).

Patrilineal - Male line inheritance where a descendant, of either sex, is directly related to a male ancestor through male intermediates. Y chromosomes and family surnames follow this pattern.

Pedigree - Alternative name for family tree, i.e. record of ancestry and collateral relatives.

Phenotype - The observable properties of an organism, which may differ from the genotype because of dominance and epistasis. For many characters it is also influenced by the ambient environment.

Phenylketonuria - Condition resulting from the inability to produce the enzyme phenylalanine hydroxilase which converts the amino acid phenylalanine to tyrosine. It is caused by a recessive gene which in the homozygous state blocks enzyme synthesis. In the absence of the enzyme, phenylalanine is converted to phenylpyruvic acid which is toxic to the central nervous system and causes mental retardation.

Proband - Person forming starting point for study of family or pedigree. Also known as the nominated or root person. Sewell Wright uses the Latin term, Propositus.

Recessive - When two different alleles are present together at the same locus in a heterozygote, the allele not expressed in the phenotype is termed recessive and the one expressed is dominant.

Sex-linked genes - Genes which are present on the sex chromosomes.

Sibs or siblings - Progeny of the same parents. Brothers or sisters.

Spouse - Husband or wife.

Step relatives - Non-blood relatives brought in as a result of remarriage. See here for a fuller description of step relatives and in-laws.

Unilineal relationship - Collaterals are unilineally related when they are only connected through a single parent of one or both of them. For bilineal relationship, each must be related to the other through both of their respective parents. If A is related to B through both of his or her parents but B is only related to A through one parent, (e.g. uncle/nephew) then the relationship is unilineal. Direct relationships are unilineal when genes are transmitted from an ancestor to a descendant through only one of the descendant's parents. When there are several intermediate generations between the ancestor and descendant, the relationship is also unilineal if the ancestor is unable to pass both members of a pair of genes at the same locus to the descendant along different independent routes. This can happen if a single intervening path breaks the double connection at any stage (see Figs 43 and 45 of the Suppplement).
The genetic significance of the above is covered under Bilineal relationship.

Variant double relationships - Variant doubles are those less common forms which have a unilineal relationship. Variant unilineal doubles exist for both contemporary cousins and cross-generation relationships.

Wright's equilibrium law - Gives the expected proportions of the 3 genotypes (AA, Aa and aa) in an inbred population, or in the progeny of matings involving related individuals.