Multiple alleles RICHELLE S. POSADAS BIO 202 Introduction Multiple alleles- When more than two different form of given gene exist in a species they are called as multiple allele Normally individual has two alleles During his study on Genetic Mendel also assumed only two alleles of one trait There can be large number of possible allelic forms in that same population. Features of Multiple Alleles Multiple alleles always occupy the same locus on the chromosome. Multiple alleles always influence the same character. No crossing over among the member alleles of the same multiple allelic series. Examples of Multiple Alleles a.) Blood group (ABO) in human. b.) Coat color in mice. c.) Coat color in rabbit. d.) Eye color in Drosophila. Blood group (ABO) in human Blood group (ABO) in human • Classical and most common example of multiple allele is ABO blood group of human being • Gene coding blood group has three alleles rather than two. • For the ABO gene, the three alleles are the IA, IB and i alleles. We typically call these alleles "A," "B," and "O," • “letter "I,“ stands for "immunoglobin." • The IA and IB show co-dominance - an individual who is heterozygous for these two alleles, the phenotypes of both alleles are completely expressed, thus producing blood type AB. • The i allele (the "O" allele) is recessive to both the IA and IB alleles (the "A" and "B" alleles). LETHAL GENES are alleles that cause the death of the organism that carries them. They are usually a result of mutations in genes that are essential for growth or development. Lethal alleles can cause death of an organism prenatally or any time after birth. Lethal alleles may be recessive or dominant. Lethal alleles fall into four categories. 1. Early onset- lethal alleles which result in death of an organism at early stage of life, for example, during embryogenesis. 2. Late onset- lethal allele which kills organism at their final stage of life 3. Conditional- lethal allele which kill an organism under certain environmental conditions only. e.g., some temperature sensitive alleles kill organisms only at high temperature. 4. Semi lethal- Lethal allele which kill only some individuals of the population but not all are know as semi lethal. Recessive Lethal Gene A pair of identical alleles that are both present in an organism that ultimately results in death Achondroplasia DOMINANT LETHAL GENES Dominant lethal allele kills both in homozygous and heterozygous states. Individuals with a dominant lethal allele die before they can leave progeny. Therefore, the mutant dominant lethal is removed from the population in the same generation in which it arose. Dominant lethal genes are rarely detected due to their rapid elimination from populations. One example of a disease caused by a dominant lethal allele is Huntington’s disease. HUNTINGTON’S DISEASE Huntington disease is a progressive brain disorder that causes uncontrolled movements, emotional problems and loss of thinking ability(cognitive). Symptoms usually begin between 30 and 50 years of age, but can start at any age. The disease may develop earlier in life in each successive generation. About eight percent of cases start before the age of 20 years and typically present with symptoms more similar to Parkinson's diseas e MODIFIER GENES A gene that work with other genes to control the expression of a particular trait. For example, one gene controls whether eye colour is blue or brown but other (modifier) genes can also influence the colour by affecting the amount or distribution of pigment in the iris.
