There are many ways that variation can enter during the DNA copying process, and in a future post we will examine several of them.
At a certain very low frequency, inappropriate monomers are paired together. The arrow in the figure below shows one such mismatched pair, where a red monomer G on the bottom strand was incorrectly paired with a yellow monomer T when the top strand was made. When this set is replicated, both the top and bottom strands are copied, but now the correct partners for each monomer are found. The result is two different outcomes: one copy now has the original, correct C:G pair on the left , and the other has a new variant, with an A:T pair on the right.
The result is a new variant in the population. Taken together, the properties of DNA match what we observe in nature: faithful reproduction of form, but not perfect reproduction of form. At its base, constancy and heritable variation in biological populations trace back to how DNA functions.
While the properties of DNA make it a great hereditary molecule that nonetheless allows for variation to arise , DNA itself is not capable of doing the day-to-day functions that organisms need enzyme functions, structural functions, and so on.
For these functions, the vast structural diversity of proteins is required. Previously, we discussed how DNA replication is readily facilitated by its structure, since one half of the DNA double helix can serve as a template for making the other half. We also discussed how DNA, though well-suited for its hereditary role, is not at all suited to performing cellular functions—but that proteins fill these roles.
The first step in this discussion requires us to look into how chromosomes and genes work. Humans have 46 chromosomes in each of their cells, and they come in pairs. We receive one of each pair as a set of 23 chromosomes from each parent: eggs contain 22 non-sex chromosomes plus an X chromosome, and sperm contain 22 non-sex chromosomes plus either an X or a Y chromosome. Our largest chromosomes have about million base pairs, and our smallest about 50 million.
Taken together, the human genome has about 3 billion DNA base pairs in each set of 23 chromosomes, or a total of about 6 billion if you count both sets. Distributed on these 23 chromosome pairs are genes —the units of biological function encoded within our DNA. Humans have about 20, genes, and they can be quite spread out on chromosomes, with a lot of non-gene DNA in between them.
In this case, this is a gene that makes a protein product:. First off, we can see that the parts of the gene that are used to specify the protein amino acid sequence the blue boxes are only one part of the whole.
Other sequences such as those represented by the light blue lines and the red boxes are sequences that direct certain cell types to make this protein, and how much of it to make. With these details in mind, now consider how variation at the DNA level can affect chromosome structure. As we saw yesterday, when chromosomes are copied, DNA copying errors may occur. Not surprisingly, many types of mutation events can also impact the function of genes, and ultimately the characteristics of the organism:.
Single base pair mutations : mispairing of nucleic acids can lead to chromosome copies that differ from the original by one base pair as we saw yesterday.
Other changes may alter the amino acid sequence by substituting one amino acid for another, but still have no effect on the function of the protein since many protein functions can be accomplished by slightly different protein sequences.
Other changes might reduce or even remove protein function. Still other changes might improve protein function—give it better enzymatic activity, for example. Changes in regulatory DNA are also possible, and the effects of these changes can similarly be neutral, harmful or beneficial.
What is interesting about regulatory DNA is that small changes can have quite large effects on where and when a protein is made—and changes that alter key genes that function early in development can have significant downstream affects on the organism as a whole. Deletion events : sometimes, stretches of DNA can be lost during chromosome replication due to breakage and rejoining events.
Sometimes deletions affect only a few base pairs, but in some cases they can span thousands of base pairs. See below for an example of the similar but not equivalent " codominance " expression pattern. Example of incomplete dominance:. I think that using "W" instead of "r" for the "white" allele type emphasizes that neither "R" or "W" is dominant.
As we did for the case of simple dominance, here are the incomplete dominance pattern cross rules: Remember: RR,RW, and WW all produce different variations One variation x Same variation G.
Notice that you could determine the genotypes crossed depending on the results observed. For this reason, incomplete dominance is even simpler than simple dominance.
There is a one-to-one correspondence between genotypes and phenotypes. Another type of inheritance pattern is multiple alleles. An example is human blood types ABO blood typing. The multiple alleles case is just a mixture of the previous two cases. Polygenic trait is one determined my multiple gene pairs, acting together.
There are generally a large number of variations possible. An example is fingerprint patterns in humans. X-linkage humans 22 of our chromosome pairs are autosomes 1 pair are the sex chromosomes Sex chromosomes are different from autosomes because they are unmatched. In humans, an X chromosome is much larger than the Y chromosome. A human female got an X chromosome from mom and another X chromosome from dad. Sex chromosomes also have genes that produce other various traits. A female has inheritance patterns that follow the previous rules, depending on whether combinations of alleles follow simple dominant or incomplete dominance patterns.
A male may only have a single allele, because the corresponding allele from the Y chromosome is absent. If so, he is "hemizygous" for that trait. Red-green color blindness is an example of a sex-linked trait. If his mom is not colorblind, then she is heterozygous. If his mom is colorblind, then her father was as well. Was her mom? B Another kind of linkage occurs when separate genes are on the same chromosome. The following example is from Punnett's research on sweet peas Yes, the same Reginald Crundall Punnett for whom the Punnett Square is named.
Punnett and William Bateson, another earlier proponent of Mendelism worked with "Painted Lady" and "Duke of Westminster" varieties of sweet peas. Punnett Square Punnett called this a "chessboard" square; it was renamed in his honor in 's. Here are some other examples of features that show environmental variation:.
Some features vary because of a mixture of inherited causes and environmental causes. For example, identical twins inherit exactly the same features from their parents. However, if you take a pair of twins, and twin 'A' is given more to eat than twin 'B', twin 'A' is likely to end up heavier.
Weight and height are common examples of characteristics that are influenced by both genetic and environmental factors. Inherited and environmental variation Some variation within a species is inherited, and some variation is due to the environment.
Inherited causes of variation Variation in a characteristic that is a result of genetic information from the parents is called inherited variation.
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