Autosomal Dominance

An autosomal dominant gene is one that occurs on an autosomal (non-sex determining) chromosome, and because it is dominant, the phenotype it gives will be expressed even if the gene is heterozygous (has a different allele). There is a 50-50 chance of inheritance, regardless of gender.

Chromosomes, Genes and Alleles

Humans have a total of 46 chromosomes, comprising one pair of allosomes and 22 pairs of autosomes. Allosomes are the sex determining chromosomes (the X chromosome and the Y chromosome). The autosomes are numbered according to size. Chromosome 1 is the longest and chromosome 22 is the shortest.

Cells that are used for reproduction (the egg cell or the sperm cell) only contain 23 chromosomes, 22 of them are autosomes while the remaining one is a sex chromosome (either X or Y chromosome).

DNA is made of molecules called nucleotides. Each nucleotide contains a phosphate group, a sugar group and a nitrogen base. There are four types of nitrogen base and they are called adenine (A), thymine (T), guanine (G) and cytosine (C). The order of these bases forms the genetic code that determines the DNA’s instructions. This four-letter alphabet forms words of three letters called codons.

Each chromosome contains a single double-stranded piece of DNA. A gene is a recognised section of the DNA that contains the instructions for a specific molecule, usually a protein. The average gene size in humans is between 10 and 15 thousand base pairs (10-15 kbp, kilo base pairs). Some genes are smaller, using just a few hundred base pairs. The total length of the human genome is over 3 billion base pairs, and contains 20-25,000 distinct protein-coding genes.

Different versions of the same gene are called alleles. For example a gene that determines height will have an allele for taller, and another for smaller, and probably a range of other alleles too. If an individual has different alleles for a gene pair, they are heterozygous for that gene. If they have the same allele from each parent for a gene pair, they are homozygous. Of course, there is no correct allele, but the one that is most prevalent is referred to as the wild type allele, and then all the others are referred to as the mutant alleles.

In a heterozygous individual (with different alleles), the effect of one allele may ‘cover up’ the effect of the other allele. When this happens it is called the dominant allele and the other is called the recessive allele. The traits an individual actually shows is called the phenotype. The traits that they carry, their genetic potential, is called the genotype.

Penetrance relates to the proportion of individuals having the same mutuant gene associated with a particular trait. Expressivity relates to the degree in which those traits affect the individual.

PROS1 - the gene for Protein S

The human genome contains two protein S genes, located on chromosome 3. One gene is active (PSα or PROS1), the other is a pseudogene (PSβ or PROS2).

The regulatory region of the gene is poorly defined but three frequent polymorphisms have been identified and one is in the coding region. Over 200 detrimental mutations have been indentified.

References

• Protein S Deficiency: A Database of Mutations - ISTH: Gandrille, Borgel, Sala et al, 27 Oct 2000
• PROS1 Gene - Genecards

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