COMPLETE DOMINANCEA fully dominant allele will be able to express itselfonly when one copy is present ?as in heterozygous condition, whereas the alternativeallele will be fully recessive.When, in case of full dominance, the homozygousdominant cannot be differentiated from heterozygous dominant; that is at thephenotypic level, R/R=R/r.For example: The disease the Phenylketonuria is caseof recessive mutations. PKU is caused by a defective allele coding for the enzymePhenylalanine hydroxylase(PAH). In the absence of PAH, the phenylalanine in thefood is not broken down in the body and thus accumulates.
As a result of which phenylalanineis converted into phenyl pyruvic acid which reaches he blood through thebloodstream and there impedes normal development, leading to mental retardation.Why is defective allele recessive?One “dose” of the wild-type allele produces enough PAHto breakdown the phenylalanine entering the body. The PAH gene is also said to behaplosufficient.Hence, both P/p and P/P doses are enough to result innormal cellular chemistry. NULL MUTATIONAssume that 16 units of a gene’s product are neededfor normal chemistry and that each wild type allele can produce 10 units. Then twowild type alleles will produce 2 units.
But what will happen if one of themutation is a null mutation which produces non-functional protein. Then a nullmutation with a single type of wild allele will produce 10+0= 10 units.This is known as haplo insufficiency; the sameresponsible for DiGeorge syndrome in humans, a condition with cardiovascularand craniofacial abnormalities. DOMINANT NEGATIVE Polypeptides with this type of mutation act as ‘spoilers’or ‘rogues’.
In some cases, the gene product is a unit of homodimeric protein—composedof two units of the same type. In the heterozygote (+/M), the spoiler polypeptidebinds to the wild polypeptide and distorts it or interferes with its function. Example: in case of genes coding for collagen protein.Some mutations in this give rise to osteogenesis imperfecta-brittlebone disease. (Collagen protein is a trimer). INCOMPLETE DOMINANCEWhen a pure-breeding wild-type four-o’clock plant linehaving red petals is crossed with a pure line having white petals, the F1has pink petals.
If the F2 is produced by selfing the F1,the result is-¼ of the plants have red petals½ of the plant have pink petals¼ of the plant have white petalsThe occurrence of the intermediate character is aresult of incomplete dominance. CO DOMINANCEA classic example is the ABO blood grouping, wherethere is co dominance of antigen alleles. These three alleles interact inseveral ways to produce the four blood types of blood groups of the ABO system.The combinations result in six different genotypes: GENOTYPE BLOOD TYPEIA/IA, IA/I AIB/IB, IB/I BIA/IB ABi/i O The human disease sickle cell anaemia, the gene concernedencodes molecule haemoglobin which is responsible for transport of oxygen inblood vessels. The alleles HbA and HbS produce threepossible genotypes as follows:HbA/HbA :normal; red blood cells biconcave shapeHbS/HbS : severe. Fatal anaemia, abnormalhaemoglobin causes red blood cells to become sickle cell shaped HBA/HbS : no anaemia; red blood cells only sickle shapedunder low oxygen concentrations. RECESSIVE LETHAL ALLELES An allele thatis capable of causing death of an organism is called a lethal allele.
The diagnostic test for lethality- the test is well illustratedby one of the prototypic examples of a lethal allele, a coat-colour allele inmice. Normal wild-type mice have coats with a dark pigmentation overall. A mutationcalled yellow (coat colour) shows an interesting pattern. If any yellow mouseis mated with a homozygous wild-type mouse, a 1:1 ratio of yellow to wild typemice is always observed in the progeny. This suggests that a yellow mouse is always heterozygousfor the yellow allele and that the yellow allele is dominant over the wildtype. However, if any two yellow mice are crossed with each other, the resultis,Yellow x Yellow ? 2/3 yellow, 1/3 wild type The tailless Manx phenotype in cats, is produced by anallele that is lethal in the homozygous condition. A single dose of the Manxallele ML, interferes with the normal spinal development in such away, that the cat born are tailless in case of ML/M heterozygote.
Whereasthe ML/ML homozygotes do not survive the embryonic stage.Whether an allele is lethal or not, depends on theenvironment in which the organism has to survive. Human hereditary diseases, forexample, Sickle cell anaemia and Cystic fibrosis are deadly without treatment. TEMPERATURE SENSITIVE ALLELES There are certain alleles that are members of ageneral class of temperature-sensitivemutations. Their phenotype is wild type at the permissive temperature (often room temperature) but mutant at some higherrestrictive temperature.Temperature sensitive alleles are thought to be causedby twisting/ bending of protein prone shape to an inactive conformation at therestrictive temperature.