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What is the relationship between genotype and phenotype?

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Introduction

Alice Helliwell What is the relationship between genotype and phenotype? Illustrate with a particular example of a genetic disease or disorder. Nearly all human cells (with the exception of red blood cells) contain DNA- deoxyribonucleic acid; complex right handed double helix polynucleotides, immortal hereditary molecules capable of semi-conservative self-replication, which enables it to pass its information from one generation of cells to the next. The famous physicist, Erwin Schiroger claimed that it was these tightly coiled molecules that truly hold the key to the nature of life- that there genes made life possible. Yet what are these infamous genes that seem to dominate contemporary genetic research and how are they related to the phenotype or physical attributes of an organism? A gene can be considered as a unit of hereditary information; a length of DNA which codes for the production of a polypeptide molecule. Together with introns, it is the linear sequence of nitrogen containing purine and pyramidine bases (thymine, adenosine, guanine and cytosine) which specify a gene and therefore the sequence of amino acids in the polypeptide that is synthesised from this DNA code. The Human Genome Project aims to identify all of the approximate 40, 000 genes1 in human DNA whilst considering the ethical, social and legal implications that this stored database may present to humanity. ...read more.

Middle

Consequently, these forms of mutations are described as 'silent' since the 1?, 2?, 3?and 4? structures of the protein coded for, hence phenotype, remain unaffected. Sufferers of sickle cell anaemia are not so fortunate in that the substitution of a cytosine to adenine base in the sequences CCT --> CAT, has significantly affected their phenotype. This change has caused the polar glutamine molecule present in one of the ?-polypeptide chains, which forms haemoglobin (?2?2) to be replaced by non-polar valine. Because of this new genotype the 1?, 2?, 3?and 4? structures of the functional globular protein alter in configuration so that the phenotype of the red blood cell is seriously altered. Thus, in low oxygen concentrations, these non-polar amino acids come out of solution and form sticky fibres on the inner membrane wall of the red blood cell. The red corpuscles become inflexible, brittle and less soluble, factors which increase the chances of venous stasis and generally prove far less efficient at transporting oxygen to respiring somatic cells. Yet despite this seemingly unfortunate relationship between genotype and phenotype, the influence of the environment does play a significant part in the total phenotype of an organism. ...read more.

Conclusion

Also, it does not provide a permanent cure, requiring permanent repetition as the targeted cells continually die. Gene therapy, at this point in time, cannot target the abnormally functioning cells either. Perhaps one sign of hope is the success of the integrase protein used in haemophilia trials, which is able to target therapeutic DNA at specific target sites in the genome. NOTES:- 1. The question of what constitutes a gene is still very hard to answer. A gene made of several sub-sections, interspersed with various introns, may be transcribed in many different combinations. Does this mean that all combinations (excluding introns) are deemed as the same gene or different genes? When transcribed to mRNA, the length of ribonucleic acid created from our length of DNA may then be further spliced. When finally translated to a polypeptide, the protein may then be subjected to post-secondary modifications of the genotype by the addition of polysaccharide chains, forming glycoproteins. So, the relationship between genotype to primary phenotype, and then to the total phenotype after subjection to external environmental factors is extremely complex. 2. Those with the genotype HbS HbA are affected phenotypically when they inhabit high ground, when, due to low oxygen concentrations, are fully affected by the symptoms of sickle cell anaemia. Away from lowland marshes- the ideal breeding grounds for the female anopheles mosquito, HbA HbA populations proliferate. ...read more.

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