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Gregor Mendel was one of the early pioneers in the field of genetic inheritance. Mendel used true breeding garden pea plants (Pisum sativum) to investigate how seven different phenotypes were represented in the offspring of parent plants across generations. These experiments led Mendel to propose that:
As discussed previously, Mendel's principles have needed slight modifications to still be valid in light of new experimental evidence. This is the case for the situation of genetic linkage.
Mendel stated that genes segregated independently of one another and this can be seen to be true if individual genes are on different chromosomes. However, if there is more than one gene per chromosome then can the inheritance of these two genes be linked? i.e. is it more likely that certain allelic combinations segregate together?
Experimental evidence suggested that this could be the case. Bateson and Punnet for example found that pollen shape and flower colour showed an inheritance pattern where parental genotypes are more likely. Thomas Hunt Morgan's Drosphila melanogaster studies of the inheritance of purple eyes (pr) and vestigial wings (vg) also indicated that linkage was a real phenomenon. Morgan suggested that the two genes were carried on the same pair of homologous chromosomes and pairing during meiosis lead to new genetic combinations, a process known as recombination. It was eventually conjectured that chiasmata formation between homologous chromosomes and crossing over lead to the exchange of genetic material.
Of course the human genome is a highly complex assemblage of genetic code. Genes can interact with both the external environment and each other in terms of their expression. The effect of gene interaction may be seen by considering the relationship between the genotype of an individual and its phenotype.
An interesting genetic interaction is epistasy. This is when the effect of one gene is masked by the effect of another meaning it does not find expression in the phenotype of an individual. An example of this is the eyegone gene in Drosphila which is clearly epistatic to any eye colour gene as eye formation itself does not occur! Another interesting genetic phenomenon is the "viability effect". Certain mutations may actually slow the growth of certain species in such a way that they become under-represented in the population. These effects are quite common in mutated genes in fungi such as Aspergillus nidulans.
Previously we met a situation where the mapping of a chromosome could be conducted by considering the recombination frequency of certain genetic markers in a cross. Now we consider a method where enzyme catalysed digestion of DNA can give a cruical insight into its structure.
A wide variety of bacteria produce restriction endonucleases in order to cleave foreign DNA and so destroy its coding capacity. This is a useful strategy against infecting bacteriophages. The cuts that a restriction endonuclease makes to a molecule of DNA may be flush or staggered, giving protruding sticky ends.
Why do you think that restriction enzymes don't digest the bacterium's own DNA?
Examples of restriction enzymes include EcoR I, Hind III and BamHI. Most recognize a specific DNA sequence that is 4-6 bases long and palindromic. For example the recognition sequence of EcoR I (produced by the bacterium E. coli) is 5' GAATTC 3'.
Many fungi and bacteria can be grown on synthetic media of a known chemical composition. This means single gene mutations introduced into organisms which affect a certain metabolic pathway can be characterised. This is because mutation can cause the requirement for a specific chemical for growth depending on which step in the pathway if affected.
Mutants which require a specific supplement are produced and can be genetically analysed to see how many genes are involved. Furthermore, the mutants are grown on media containing various intermediates in the metabolic pathway to investigate where in the pathway the mutation is acting. This approach is often sufficient to identify the intermediates and their order of formation.
In an experiment, a 1st year scientist replicates a master plate containing 26 colonies of the fungus Aspergillus to 5 plates containing different media. The plates have a nitrogen source that is an intermediate in the metabolic pathway that degrades hypoxanthine to NH$_4^+$.
The scientist wants to establish:
1) The growth responses of each of the strains
2) The order of intermediates in the pathway
3) The strains which are blocked in each step giving an idea of the genetics underlying the growth patterns
The table above shows the process by which the growth of the fungus is classified, where the "+" indicates growth of the strain on this medium and a "-" scoring means no growth.
Can you think what the purpose of the colony number 26 is, given it grows on all the media?
The order of intermediates a $\rightarrow$ can be determined.
If a mutation occurs affecting a step of a metabolic pathway close to the initial susbstrate, then if the species is plated onto media containing nutrients occuring downstream as intermediates in the pathway, then growth is still possible.
If mutation occurs affecting a step of the metabolic pathway close to the final product, then if a species is plated onto media containing nutrients occuring upstream as intermediates in the pathway, growth still won't be possible because of the mutation!
As all of the strains are capable of growth on media containing e, it must occur closest to NH$_4^+$ in the pathway. Using a similar analysis the order can be established as:
Hypoxanthine $\rightarrow$ d $\rightarrow$ a $\rightarrow$ c $\rightarrow$ b $\rightarrow$ e $\rightarrow$ NH$_4^+$
Can you determine which strains are blocked in which steps of the pathway? For example the step d $\rightarrow$ a is blocked by the mutants 1,3,6,9,15,17,18,20 and 25. What does this imply about the species genetically?
Such analysis is highly useful in the early investigation into the genetics behind metabolic pathways.
In this article, we've covered topics ranging from Mendelian inheritance, chromosome mapping, genetic interactions, the genetic analysis of metabolic pathways and restriction mapping. Hopefully over the course of this article you have gained an appreciation of some useful genetic concepts you will frequently encounter in your future studies.
Is the age of this very old man statistically believable?
bioNRICH is the area of the stemNRICH site devoted to the mathematics underlying the study of the biological sciences, designed to help develop the mathematics required to get the most from your study of biology at A-level and university.
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