A pattern of inheritance of a characteristic (such as blue eye color) or abnormality (such as congenital absence of the vas deferens) in which two abnormal genes or alleles are needed to confer the characteristic or abnormality, in contrast to dominant inheritance, which requires just one abnormal gene. In the case of alleles found on X-chromosomes but not on the smaller Y-chromosomes, a recessive gene will be unopposed (and so will act as a dominant gene) in males, whereas female carriers of the allele will be unaffected except in the extremely unlikely event that they inherit (or gain by mutation) a second abnormal allele; this mode of inheritance is called sex-linked recessive inheritance.

Because genes come in pairs, one on each of the chromosomes that make up a chromosome pair, the two genes of the pair are not always identical. The manydifferent forms that a particular gene can take (and still function, for better or for worse, as a gene within that gene's job description) are referred to as its alleles. If the alleles are identical, you are homozygous for that gene; if they are not, you are heterozygous. Abnormal alleles cause genetic disease or disability: if one allele is enough to cause abnormality then the gene is dominant (inherited with dominant inheritance) and the abnormality is present in the heterozygous and the homozygous state; if two alleles are needed to cause abnormality, then the gene is recessive (inherited with recessive inheritance) and the abnormality is present in the homozygous state and in a state where the two alleles are different but both are harmful (called compound heterozygosity. In the case of alleles found on X-chromosomes (see sex chromosome) but not on Y-chromosomes, which are smaller, a recessive gene will be unopposed in males (and so will act as a dominant gene), whereas female carriers of the allele will be unaffected except in the extremely unlikely event that they inherit (or gain by mutation) a second abnormal allele; this mode of inheritance is called sex-linked recessive inheritance.

Remember that the ordinary cells of the body have 46 chromosomes in 23 pairs. The members of each of the pair don't contain the same genes (this is why you can carry a gene for recessive inheritance without being sick, but if you have both genes there's trouble). During the cell divisions that produce the germ cells (the process of meiosis), the 46 chromosomes first double to 92 before they end up with the 23 present in an egg or a sperm. Two of the four sibling chromosomes (we call them chromatids) then randomly exchange bits of themselves: there's a "cross-over". The points of junction where this happens are called chiasmata. Nature is jumbling up where the genes will end up so that in the long run you don't always have to inherit two particular genes together just because they live next door to each other on a chromosome. Cross-over does not mean that genes can end up in any chromosome they like: unless "translocated", they will remain in pairs (of alleles) within a particular pair of chromosomes. Chromosomal crossover is the origin of what geneticists call hybrid vigor.

Congenital (present from birth) absence of the two vasa deferentia, which conduct sperm from the testes to the ejaculate; hence a cause of obstructive azoospermia. Because the vas deferens is usually affected on both sides, the word "bilateral" is often inserted, as CBAVD. Infertility is inevitable, but can be overcome using in vitro fertilisation with testicular sperm extraction. The cause, however, is usually the presence among the man's genes of one of the serious alleles for the genetic disease cystic fibrosis (which, having recessive inheritance, results when there are two such alleles present); it can also come about when there are one or two of the less serious abnormal alleles for this condition. The more common of the abnormal alleles should therefore be screened for (using a specially set up polymerase chain reaction on white blood cells): should an abnormal allele be present (delta F508 is the most common seriously abnormal one), the woman should be screened too, to predict the chance of cystic fibrosis occurring in the offspring.

A serious genetic disease (with a recessive inheritance pattern) characterised by a major disturbance of the body's mucus secretions, and thus a cause of incapacitating disease of the lungs. Important in our context because the heterozygous (or carrier) state in men can manifest with azoospermia due to congenital absence of the vasa deferentia -- the infertility of which is nowadays able to be overcome with testicular sperm extraction and ICSI, thus risking inadvertent transmission.

A pattern of inheritance of a characteristic (such as brown eye color) or abnormality in which just one gene or allele is needed to confer the characteristic or abnormality, in contrast to recessive inheritance, which requires two abnormal genes.

An adjective to describe the genetic state when the pair of genes under consideration consists of two alleles that are the same. Disease or disability can follow if both alleles are seriously abnormal -- a condition known as recessive inheritance (and a good example of which is cystic fibrosis).