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- Warda Abdulkadir Ahmed
In humans each nucleus contains 23 pairs of chromosomes, 23 each from a mother and father. Chromosomes are thread-like structures located inside the nucleus of animal and plant cells made of chromatids consisting of protein and a single molecule of deoxyribonucleic acid (DNA) which is in turn made up of four nucleotide bases, adenine, guanine, cytosine, and thymine (A, T, G, C) with sugar and phosphate bonds. The structure of chromosomes holds DNA compactly bound around histones, spool-like proteins. Chromosomes ensure DNA is correctly copied and disseminated ensuring the successful heredity of nucleotides coding for specific traits through genes. A gene can be described as a discrete unit of heredity assigned to a particular characteristic. A gene however is a distinct sequence of nucleotides within a chromosome determines the instruction of monomers within the actual polypeptide or nucleic acid molecule to be synthesised. Although genes can be described merely as a trait more accurately it’s the inherent coding instruction and sequencing of specific proteins. An allele of a gene denotes progeny as its genotype which is inheritable and not phenotype.
Gregor Mendel was an Austrian monk who conducted research in the field of genetics. Most notably known for his experiments with pea plants that expanded our understanding on heredity, so much so laws of inheritance are referred to as Mendelian. Mendel concentrated on the characteristics of the plants such as colour height and shape. Mendel discovered which traits were dominant and which were recessive. He deduced that genes although he did not know them as such at the time, come in pairs and are inherited as discrete units, one from each parent. Mendel traced the segregation of parental genes and their appearance in the offspring thus suggesting their hereditary nature. Mendel’s law of segregation describes how alleles segregate when two gametes form. Mendel’s second law is called the law of independent assortment stating hereditary traits in relation to peas for instance i.e. Colour and length are unrelated to each other in the order they recombine existing as discrete elements in genetic characteristic and how they are passed on. Mendel’s third law explores dominance stating that in every pair of alleles, one is more likely to be expressed than the other and states the probability of this eventuality.
The homozygous non tongue roller allele being identified as (hh) and the heterozygous more probably dominant roller being (Hh) when placed in a punnet square each parent could give either a (H) or a( h) in its gamete displaying a monohybrid cross.
Half of the children would have the ability to roll their tongue and half would not, there is a 50% probability of the gene that controls tongue rolling ability being passed on, or a ½ chance. This is because the dominant gene only requires one allele to exhibit the trait in the offspring.
The chemical PTC can be tasted by only some people this characteristic and its probability of arising when two parents with a heterozygous prevalence for heredity. 3/4 (or 75%) possible outcomes will have at least one dominant allele. This can also be expressed by saying if there were four offspring 1/4 (25%) has the genotype BB, 1/4 (25 %) has the genotype bb, while 1/2 (50 %) have the genotype Bb, this is also a monohybrid cross.
Although homozygous for both traits the female would still have an allele for passing on the trait so even though the male definitely is both heterozygous for both traits the female still has the ability to form a dihybrid cross with the male if they have children. Two heterozygous parents with both traits (dominant alleles) for PTC tasting and tongue rolling when having a child 1 concludes that two dihybrids having a child would produce four 4 possible gametes and 16 possible phenotypes having a phenotypic ratio of 9:3:3:1 phenotypic ratio. Having 16 children the phenotypic ratio would assort itself as 9:3:3:1, the Mendelian ratio for a dihybrid cross in which the alleles of two different genes group independently into gametes. Nine of the children would display both dominant traits. Three would display the first dominant and second recessive trait. A further three would display the first recessive trait and second dominant trait, the solitary offspring in the ratio represents the homozygote, displaying both recessive traits.
This would be a dihybrid cross also ,two heterozygous parents with both traits (dominant alleles) for PTC tasting and tongue rolling when having a child 1 concludes that two dihybrids having a child would produce four 4 possible gametes and 16 possible phenotypes having a phenotypic ratio of 9:3:3:1 phenotypic ratio. This presents hybridization as the two traits are distinct between mother and father. Having 16 children the phenotypic ratio would assort itself as 9:3:3:1, the Mendelian ratio for a dihybrid cross in which the alleles of two different genes group independently into gametes. Both dominant traits (BBEE, BBEe, BBee, BbEE, BbEe, Bbee, bbEE, bbEe, bbee) the 9 represents the proportion of individuals displaying both dominant traits. 3/16 would display the first dominant and second recessive trait. 3/16 would display the first recessive trait and second dominant trait, the solitary 1 in the ratio would be displaying both recessive traits.
Although one of Mendel’s principles stated independent assortment dictates alleles of different genes will segregate individually into gametes, the actuality of genetics doesn’t always allow this. Occasionally, alleles of certain genes are inherited in bundles, not undergoing independent assortment. An allele of one gene can couple with certain alleles of another gene; however this is essentially genetic linkage, an inheritance pattern in which two genes located in close proximity to each other on the same chromosome have a biased association between their alleles causing them to be inherited together in the same gene in contradiction to the Mendelian principle of independent assortment however there can be advantageous variation from this phenomenon. Heterozygous advantage where by evolution can be accelerated and genetic diversity increased providing a greater relative fitness than both the dominant and recessive genotypes. The linking of the two alleles provides a precedent for a higher probability of inheritance and their coupling makes them more resistant to be uncoupled and the discontinuation of said characteristic. On a human by human basis this can lead to a hybrid vigour effect or an over dominance effect resulting in dominance over deleterious recessive alleles. On a population scale within a particular genetic locus this sometimes provides immunity to diseases such as malaria and the adaptation to become more resistant to certain diseases.
Sex and gender are determined in humans by an XY system of sex chromosomes, gonosomes. Females have two of the same kind of sex chromosome (XX), and are called the homogametic sex. Males have two discrete sex chromosomes (XY), and are the heterogametic sex. Essentially the presence or lack of the Y chromosome is the determining factor yet it is the SRY gene present on the male Y chromosome is responsible for chemically signalling the process of virilisation that causes a fusing of gamete material to begin the development of becoming a male embryonic organism. While in female’s lionization is the process where one of the two X chromosomes is inactivated in the form of heterochromatin. Females inherit their X chromosomes from each parent, males receive their X chromosome from their mother and their Y chromosome from their father. The SRY gene of the Y chromosome initiates the development of the testes in males, and the ensuing production of hormones which causes the paramesonephric ducts to regress while in females, the mesonephric ducts are those that regress. Initially these ducts in both sexes are the equivalent internal structures before sex is truly determined and assigned.
The crossing over of chromosomes or genetic recombination plays an important role in genetic heredity. Recombination not unlike genetic linkage is the production of progeny with combinations of traits that differ from those found in either parent. Specifically In eukaryotes this process during meiosis can lead to a unique raft of genetic information’s. The pairing of homologous chromosomes can lead to information exchange between the chromosomes. This process is fundamental to the development of the human genetic code acting almost as a balance to the process of gene conversion which merely duplicates genetic information. Recombination allows adaptation in DNA structure and is crucial in the process of evolution both preserving genetic material through heredity the purpose of biological life but allows it to adjust to environment, mutate and become more diverse with variation which allows it to flourish. The communication between chromosomes also increases the frequency of deleterious cells improving and adding vigour to genetic material. Fundamentally recombination antitheses the stagnation of the gene pool these recombination’s provide a continual DNA homogenization benefiting the ecological stability of humans.
Examples of discontinuous variations are ones that are discrete in nature, with an inability to be recorded across a range. This data is categorical i.e. eye colour can be blue or brown, blood type could be A or O. This form of variation is controlled either by an allele of a single gene for instance or a combination as discussed already, or a small number of genes. The environment doesn’t necessarily have an effect on this form of variation although research into the field of epigenetics is burgeoning and shedding light on the ability for RNA and DNA to be influenced by environment post conception of a human or other organism and how more crucially these effects can then effect the progeny of a human or organism. Continuous variations are ones such as height which are more complex in nature and have a range of measurements effected by environmental as well as genetic factors.
A gene mutation is a permanent alteration in the DNA sequence that makes up a gene, ranging from a single base pair within DNA structure, to an entire segment of chromosome comprising several genes. Hereditary mutations are inherited and remain through the entire life in all cells of the body. Acquired or somatic mutations occur during a person’s life, present only in certain cells. Differing in that they can be caused by environmental factors i.e. Radiation or inadvertent gene replication. Acquired somatic cells can’t be passed on via gametes.
A de novo mutation is an alteration in a gene that is present originally in the subject not inherited from either parent. De novo essentially means new, the mutation can occur in the copying of genetic material or a mistake in processes of cell division. The defect in genetic code can also originate in the mutation of either an egg or sperm or within the fertilised egg itself or developing foetus, systematically effecting the genetic code carried by the gamete used to form the zygote. Germline de novo mutations can result in somatic mutant variants causing cancers. Autism can also develop through a de novo mutation affecting up to 1000 separate genes, these include CHD8, SCN2A, and KATNAL2.
Mosaicism denotes a specific type of mutation where presence of two or more populations of cells with different genotypes in one individual who has developed from a single fertilized egg. In other words one human being is comprised of more than one genetically distinct tissues. This mutation can result through various ways, including chromosome non-disjunction. This can happen either by failure of a pair of homologous chromosomes to separate in meiosis, failure of sister chromatids to separate during meiosis or the failure of sister chromatids to separate during mitosis. This is a form of anaphase lag. One form of mosaicism is chimerism, where two or more genotypes arise from the fusion of multiple fertilized zygotes in the early stages of embryonal development specifically the cleavage stage of embryonic transfer.
Polymorphism is the development of multiple distinctly different phenotypes within one single genus or species of organism. An example within humans would be difference in blood groups. Very often these adaptations arise in response to environmental stresses as well as random mutations that prove beneficial to survival. These resulting dimorphisms and often niche characteristics become fixed characteristics of a genus being dominant while original phenotypes co-exist within the same species. The genetic occurrence of polymorphism has to be categorised as multiple alleles at one locus, each with appreciable frequency as to not be a one off mutation the minimum frequency is typically taken as 1% i.e. the light morph and dark morph jaguar (6% of population).
Protein synthesis is DNA encoding for the production of amino acids and proteins, in other words the DNA’ and RNA nucleotide sequences which encompass code and instruction have to go through the actual production of the amino acid sequencing that will compose proteins of which everything is fundamentally made of. Protein synthesis takes place in the ribosome (transcription) and nucleus (translation) of cells and has stages in its process.
Transcription is the first stage before synthesis can occur taking place in the nucleus the gene coding for the protein untwists its double helix structure, the H-bonds between the strands breaking, free RNA nucleotides form corresponding base pairs with one strand of DNA bases. Weak hydrogen bonds form between base pairs to hold them in place while sugar phosphate bonds form between the RNA nucleotides an mRNA strand is then synthesized it breaks away from the DNA and moves out of the nucleus into the cytoplasm.
Translation occurs in the ribosome or rough endoplasmic reticulum. The mRNA strand synthesized during transcription attaches to a ribosome at the start of the codon (the triplet of bases on the DNA and mRNA) is recognized by the initiator tRNA which send amino acids to the ribosome. The ribosome enters elongation phase of synthesis, the anti-codons and codons match up and form complementary base pairs. Amino acids are added, translated into polypeptide sequences dictated by DNA and represented by mRNA. Release factor binds to the stop codon, terminating translation and releasing the complete polypeptide from the ribosome. TRNA is reused and collects another specific amino acid while mRNA may move to another ribosome to make a further protein or it can be broken down into free nucleotides to be reused.
The Human Body Book, Steve Parker, Edition 2, DK, (2013)
John Evans, Alison Lansley and Michael J. Sanderson, (2006), Biology A level
Richard Parsons (2010) AS/A2 Level Biology AQA Complete Revision & Practice
Clinical Genetics in Nursing Practice: Third Edition, Felissa R. Lashley, (2005)
Mendel and the Laws of Genetics: Primary sources of revolutionary scientific discoveries and theories Heather Hasan (2005)
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