Thursday, March 13, 2008

Genetics Definitions

A locus is the specific place on a chromosome where a gene is located.

An allele is any one of an alternative forms of a given gene. Blood group antigens are an example. Blood types A, B and O are each alleles or slightly different version of genes that code for a blood surface antigen.

A gene is the portion of DNA that usually encodes a protein. Genes confer inheritable traits.

A genotype describes the genetic constitution of an individual.

A phenotype is the observed appearance or quality of an individual. Blue eyes, blond hair, six fingers. It is the product of the genotype and the environment.

A dominent allele (gene) expresses its phenotype even in the presence of a recessive allele. Thus genotype AA (two dominent alleles - homozygous) and Aa (one dominent and one recessive allele - heterozygous) have the same phenotype.

A recessive allele (gene) is expressed only if two copies of the recessive allele occurs.

A homozygous gene pair is an identical pair of alleles at the locus of each chromosome - eg. AA or aa but NOT Aa which is heterozygous.

A heterozygous gene pair has different alleles at the locus of each chromosome - eg. Aa

A gene mutation is a permanent change in the DNA sequence that makes up a gene.

A point mutation is single nucleotide base change in the DNA.

For more basics about chromosomes take this tour of the basics.

For more basics about genes take this tour of the basics.

Genetics Refresher

First, refresh your memory about the cell and basic cell biology.

Then review the genome.

DNA is found in the nucleus

And in the mitochondria. Mitochondrial DNA is circular and carries the information for 37 genes. Almost all (99.9%) of your mitochondrial DNA is inherited from your mother.

DNA is transcribed into mRNA in the nucleus, and translated into protein in the cytoplasm.

A gene is the functional unit of heredity in an organism. A gene's nucleotide sequence encodes an RNA or polypeptide.

A gene mutation is a heritable change in the DNA sequence that makes up a gene.

A point mutation are those mutations that affect a single base pair of DNA. It occurs when 1 base on a DNA strand is replaced with a different one. There are four types of point mutations:

(1) missense mutation is where the point mutation results in a wrong codon and a wrong amino acid.

(2) nonsense mutation is where the point mutation results in a stop codon where the protein would be terminated at that point.

(3) silent (or sense) mutation is where the point mutation results in a new codon that still encodes the same amino acid

(4) frameshift mutation results from the addition or deletion of a base pair in a gene. Such additions or deletions shift the mRNA's reading frame by one base so that incorrect amino acids are added to the protein chain.

A chromosomal mutation occurs when a long segment of DNA is deleted, inserted, inverted or translocated.

Much more can be learned about the human genome at this Wellcome Trust website.

Immune System Disorders - Immune Deficiency

Disorders related to defective immunity can be classified as follows:

1) Inadequate response to exposure to foreign antigen (congenital immunodeficiency).

2) An excessive or inappropriate immune response in response to foreign antigen (allergy).

3) The loss of the capacity to distinguish self from non-self (autoimmunity).

4) Cancer of the immune cells.

In this post we will deal with #1 above.

PRIMARY IMMUNE DEFICIENCY is a condition that results if one or more essential components of the immune system fails or is missing due to a congenital genetic defect.

Some examples of primary immune deficiency diseases include:

Reticular dysgenesis

Severe combined immune deficiency disease

Ataxia Telangiectasia

Wiskott-Aldrich Syndrome

DiGeorge Syndrome (affects primarily T cell function)

Bruton's congenital agammaglobulinemia (affects B cell function)

X-linked Hyper-IgM Syndrome


Secondary immune deficiencies occur when damage is caused by an environmental factor.

Neoplastic disorder