Germ Line and Somatic Mutations
They may, for instance, occur by the substitution of some base pairs of the DNA. Point mutations - changes in one or a few nucleotide base pairs - may cause what are known as frame shifts. A mis-sense mutation may occur, where the sequence, say: AGACGACGATTTGTT becomes AGACGACGATCTGTT. In other words, at one point in the genome, the amino acid arginine (TCT) replaces lysine (TTT).
The result is that the protein it codes for may be less effective or completely non-functional. Mutations used to be the stuff of horror movies, but they are a regular occurrence. Their effects may be drastic, even lethal, but usually they are barely noticeable. The mere fact that a gene may have a number of allelles indicates that mutations have occurred in various generations. Without germ line mutations, the genetic variation in a species that produces Darwinian evolution could not occur.
Other errors during meiosis
However, errors also sometimes occur during meiosis where the DNA becomes fragmented. A chromosome may not separate, called non-disjunction. The egg or sperm therefore may have two copies, or it may have none. Sometimes fragments may be transposed or lost.
If non-disjunction occurs, the child will either have only one of the relevant chromosome, monosomy, which in humans, generally means that the embryo does not survive until birth, or it may have an extra chromosome, which is called trisomy. Only trisomies of the smaller autosomes frequently survive until birth. They include Downs Syndrome (chromosome 21), Edwards Syndrome (chromosome 18) and Patau syndrome (chromosome 13).
Yet monosomy of the sex chromosomes is tolerated fairly well, as in Turner's Syndrome (X0), where the individual has only one X chromosome and no Y. After all, the inactivation of the X chromosome in women can be thought as its equivalent. Trisomy is also well tolerated - XXX individuals are considered quite common, and some have been known to have many more.
One gene on the X chromosome is responsible for inactivation. Where there are extra X chromosomes, as in Klinefelter's Syndrome (XXY), one is inactivated. For children born as XXX, two are inactivated and so on. In fact there seems to be considerable tolerance to multiple X's - the people grow up without any problems and have genetically normal children. There is also Klinefelter's Syndrome (XXY) and XYY men. It is thought that the extra chromosomes are simply inactivated.
Somatic mutations can presumably occur at any time, but the majority are said to be phenotypically silent, if, for instance, the mutation is in a non-coding part of the genome, or a gene that is not expressed in the particular cell. Alternatively the cell may die.
Such a mutation could theoretically pass into the germ line if it occurred in a cell which would give rise to gametes. However, since germ-line cells are formed very early in development, it seems unlikely.
However, mutations that occur soon after fertilisation do pass on in the developing embryo, forming two separate cell lines, referred to as mosaicism.
Thus, if chromosomes are lost during the first few cell divisions, the individual may be X0/XX or X0/XY. They may range from female to male, though sometimes with elements of Turner's syndrome, or ambiguous genitalia. In addition, an XX/XY male fetus has been reported, in whom all the X chromosomes present in both cell lines were derived from a single maternal X chromosome.
A television documentary recently featured a brother and sister, who, naturally were expected to be fraternal twins. Worries about the sister, shortly after birth, prompted a genetic examination, when it was discovered that they were actually monozygotic, and would be expected to be both boys. It was thought that shortly after fertilisation, a Y chromosome had been lost in a cell during mitosis and, when the zygote split, the two different cell lines formed the two embryos. The little girl, in fact, had Turner's syndrome.
Bland, J., (2003) About Gender: Germ Line and Somatic Mutations
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Derby TV/TS Group. Text copyright Jed Bland.
08.04.98 Last amended 02.11.03, 01.05.14