Multiple alleles and sex chromosomes - Assignment Example

? Biology Lecturer Introduction A major revolutionary discovery in the history of science was the discovery of the ABO blood group systems, which was discovered by Karl Landsteiner in 1900. This has significantly transformed the concept of blood transfusion from a risky and fatal system to a safe and secure process with accurate projections of possible outcomes. There is, therefore, a need to critically analyse the factors determining the blood group of a person. This paper explains the role of multiple alleles in the inheritance of the ABO blood group system. It also discusses the inheritance of sex chromosomes and sex linked characteristics. Role of multiple alleles in the inheritance of the ABO blood groups A person’s blood type is determined genetically. It is imperative that a thorough understanding of the DNA is ascertained for better grasping of the multiple alleles’ direct contribution to the inheritance of ABO blood systems. Histones allow the packaging of DNA and condense it into chromatin. Histones are highly alkaline proteins with a positive charge while DNA is negatively charged (Pollard and Earnshow, 2007). Accordingly, Histones and DNA will interact with the Histones acting as spool materials in which the DNA can attach itself. Histones and the DNA will form the nucleosomes, which on further packaging and separation by linker DNA’s, result in chromatins. Further condensation of the chromatins and other proteins will result in the chromosomes. Chromosomes can, therefore, be defined as a carrier of genetic information as it contains highly packed DNA and Histones (Pollard and Earnshow, 2007). DNA consists of nucleotide substances that are further comprised of deoxyribose, phosphate groups and the four bases. DNA is generally a linear sequence of ACTG bases that define the physical traits in an organism. DNA can be viewed as a double helix, which when separated produces two parallel linear strands of the nucleotide letters where each strand is complimentary to each other (Pollard and Earnshow, 2007). A interlocks with T while G interlocks with C. The exact order of the four bases along the molecule represents the coded genetic information. The complementary nature of the code is of great importance in understanding how information is passed on. Genomes are the genetic information defining each organism, which is determined by the genome sequences depending on how the four bases are aligned (Pollard and Earnshow, 2007). It is also important to note that apart from genes, a genome also contains other DNA sequences that do not encode genes. The human body has three billion pair of DNA, 28000-34000 pairs of genes and 23 pairs of chromosomes (Pollard and Earnshow, 2007). This translates into a myriad sequences which could explain the wide scope of genes. Genes are the basic and structural and functional units of genetics; therefore the basic unit of inheritance that is composed of DNA and RNA. They control the cells by directing the formation of proteins. An allele is an alternative form of a gene and it represents the different version of a similar gene (Pollard and Earnshow, 2007). A gene is comprised of two alleles and each individual carries only two alleles of each gene, which exhibit a dominant recessive relationship. The dominant allele will prevail over the recessive gene and is responsible for the resulting phenotype. Recessive alleles will, therefore, not affect the phenotype of the individual. A pair of allele resulting from the various combinations of alleles can be referred to as the genotype while the resulting physical trait as a result of the genotype is called the phenotype (Pollard and Earnshow, 2007). Multiple alleles arise where there are three or more different alleles of a particular gene in a gene pool. This results into polymorphism where two or more phenotypes exist in a given population (Pollard and Earnshow, 2007). The occurrence of multiple alleles can be attributed to difference in the non-coding DNA found between genes rather than the DNA sequence variation. Inheritance is, therefore, based on a non-mendelian pattern. An example is the ABO blood group and the coat colour of many species. The ABO blood group phenotype is determined by three alleles, IA, IB, and IO (Pollard and Earnshow, 2007). Bearing in mind that each individual organism will possess only two alleles with one allele being from each parent, three multiple alleles will, therefore, result in six possible genotypes. A homozygous individual will possess identical alleles while a heterozygous individual will possess different alleles (Pollard and Earnshow, 2007). The various possible genotypes are AA, AO, BB, BO, AB andOO. The alleles A and B are co-dominant since they both encode functional enzymes and will, therefore, be simultaneously expressed in a heterozygous individual. The genotype AB will, therefore, result in the blood group AB phenotype. Similarly, the genotypes AA and BB will automatically result in the blood group phenotype A and B respectively. This is, however, different for genotype AO and BO (Pollard and Earnshow, 2007). O is a recessive allele and will, therefore, be suppressed and superseded by any dominant allele present. Consequently, the genotype AO and BO will result in the blood group phenotype A and B respectively. The genotype OO will result in the blood group O as both alleles are recessive (Pollard and Earnshow, 2007). Various alleles control the formation of antigens. IA and IB will control the production and attachment of antigen A and antigen B on the red blood cells respectively while IO does not result in the production of antigens and it is, therefore, recessive (Pollard and Earnshow, 2007). Antigens are protein markers found in the surface of red blood cells. Antigens are an important part of the immune system in that immune-globins (antibodies) attach themselves to the antigen and help fight diseases. It is important to note that antibodies are found in the serum. Blood group A will have antibody B while blood B will have antibody A. Blood group AB produces no antibodies while blood group O will have antibodies A and B. Similar blood group antigens and antibodies will result in agglutination, this is the main reason why blood group O is a universal donor while blood group AB is a universal recipient. Inheritance of sex chromosomes and sex linked characteristics The sex of most organisms is determined genetically by the presence or absence of a Y or X chromosomes. Humans have 23 pairs of chromosomes, which consist of 22 pairs of homologous pairs of autosomes (non sex chromosomes) and one pair of sex chromosomes (Griffiths et al, 2000). Sex chromosomes are located on the somatic cells and are either homogametic or heterogametic chromosomes, which is the sex determination system in humans (Griffiths et al, 2000). Homogametic chromosomes are found in females as they have a pair of identical XX while heterogametic chromosomes are found in males since they possess X and Y chromosomes. The Y chromosome is among the smallest chromosomes. It contains the testis determining factor (TDF) in its short arm, which is the sole determining factor in male development because it is responsible for the development of the male characteristics (Griffiths et al, 2000). An individual will obtain each chromosome from each parent and the resulting genotype is either XX or XY and hence equal chances of being either male or female. There are other sex determination systems, such as the heterogametic females, which are mainly found in insects, birds and fish. The female has ZW chromosomes while the male has the ZZ chromosomes (Griffiths et al, 2000). Sex linked genes can be defined as genes that are unique to the sex chromosomes and are, therefore, carried in the sex chromosomes. Men are homozygous in that they have only one X chromosomes. Both men and women will inherit the X linked characteristics as they both inherit the X chromosomes while only men will inherit the Y linked chromosomes (Griffiths et al, 2000). This means that any X linked recessive trait present will be expressed in the male because the female has two X chromosomes and, therefore, the dominant X chromosome will mask the recessive genes. However, this is not the case in male; they have only one X chromosome hence a recessive trait in it will prevail. This is the main reason why females are carriers of X linked traits of disorders, such as haemophilia, red green colour blindness and baldness which are not expressed in their phenotypes but very prevalent in men (Griffiths et al, 2000). Bibliography Griffiths, A., Miller, J., Suzuki, D., Lewontin, R. & Gelbart, W. 2000. Patterns of Inheritance: An Introduction to Genetic Analysis, 7th edition. New York: W. H. Freeman. Pollard, T. & Earnshow, W. 2007. ‘Chromatins, Chromosomes and Cell Nucleus’. Cell Biology, 2nd edition. Section IV. Read More