DNA, genes and chromosomes — University of Leicester
For genes on the X chromosome women can be carriers of many This is a fold difference between the X and autosome in terms of copies. DNA is a molecule which stores genetic information - like the colour of your eyes, or whether you have earlobes or not. Every cell in your body contains the same. Non-chromosomal DNA matters for expression of traits complicating the relationship between an organism's genetic blueprint, or genotype.
The order of the bases along a strand is a code - a code for making proteins. Genes A gene is a length of DNA that codes for a specific protein. So, for example, one gene will code for the protein insulin, which is important role in helping your body to control the amount of sugar in your blood.
Genes are the basic unit of genetics. Human beings have 20, to 25, genes. These genes account for only about 3 per cent of our DNA. The function of the remaining 97 per cent is still not clear, although scientists think it may have something to do with controlling the genes.
Chromosomes If you took the DNA from all the cells in your body and lined it up, end to end, it would form a strand million miles long but very, very thin! To store this important material, DNA molecules are tightly packed around proteins called histones to make structures called chromosomes.
The packaging of DNA into chromosomes Human beings have 23 pairs of chromosomes in every cell, which makes 46 chromosomes in total. A photograph of a person's chromosomes, arranged according to size, is called a karyotype.
The different dynamics of memes vs. genes - Gene Expression
The sex chromosomes determine whether you are a boy XY or a girl XX. Concretely I just assumed that the only people in the gradual scenario who spoke the language of the source population would be the migrants themselves.
All of their children would speak the target population language.
More abstractly I am implying here that the change in language frequency has some non-linear response dynamics, at a minimum. By this, I mean that what language you speak is a function of the peer groups your parents place you in, as well as their own maintenance of their original language. In the shock scenario none of this may hold. The migrants are a huge proportion of the population in the initial generation and subsequent generation of migration.
The proportion of migrants need not be the majority, rather, the migrant culture simply needs to have critical mass in terms of coherency to seal itself off from native influence. Until then what you see is the proliferation of a particular genomic segment along with the increase in frequency of the favored gene which is embedded in that particular region. By straightforward logic when a whole segment with associated alleles starts to increase in frequency aggregate genetic diversity decreases, as variation is swept aside.
And yet evolution is not simply natural selection. There are two processes which have nothing to do with selection as such which might reduce genetic variation. The motor which both these phenomenon turn on is random genetic drift. As you increase the power of drift to fluctuate gene frequencies generation to generation you also increase its power to render alleles extinct as they are extinguished once they hit the zero frequency boundary condition.
This is why populations which have gone through population bottlenecks are so homogeneous; drift has squeezed most of the variation out of the gene pool by capriciously favoring some alleles and eliminating most of the rest.
The dynamics relevant to this specific case are differences in male and female effective population size, and large fluctuations in long term effective population size. For purposes of reduced X chromosomal diversity one would have to posit lower female effective population size than male effective population size.
So if females have lower effective population sizes than males the X chromosome is being buffeted by greater stochastic forces than the autosome. More generally, the X chromosome has a lower effective population even assuming sex balance because for every 4 copies of an autosomal chromosome there are 3 X chromosomes.
Because of this reduced effective population size the X would be more sensitive to bottlenecks and the like, one of the consequence of which is reduced genetic diversity.
All the above is important to keep in mind when reading a new report in Nature Genetics on the balance between selection and drift in reducing variation on the X chromosome and across populations.
The second refers to the fact that Africans seem to exhibit less relative reduction of variation on the X chromosome than non-Africans. The ratio of genetic diversity on chromosome X to that on the autosomes is sensitive to both natural selection and demography.
The genetic code
On the basis of whole-genome sequences of 69 females, we report that whereas this ratio increases with genetic distance from genes across populations, it is lower in Europeans than in West Africans independent of proximity to genes. This relative reduction is most parsimoniously explained by differences in demographic history without the need to invoke natural selection. These variants are especially informative, but they miss a lot of the genome. The takeaway is that the time is now nearly here when we can look at the genome at its most precise and fine-grained scale, rather than using approximations, whether it be one locus, or 1 million SNPs.
Is creative ability determined by our DNA? | Genetic Literacy Project
There are zones of the genome which are intergenicbetween genes. Natural selection generally targets functional regions, not intergenic ones. If natural selection is the primary dynamic effecting the pattern we see here then differences should manifest between genic and intergenic regions since selection plays a much larger role in the former than the latter, both in constraining variation and increasing the frequency of favored alleles.
The figure below has four panels.
- Gene Expression
- DNA, genes and chromosomes
Every panel has an x-axis defined by distance from a gene, left to right with increasing distance. So the leftmost point can be thought of as genic, and the rightmost point as intergenic.
The left panels define Europeans, and the right panels Africans. The top row shows the change in raw nucelotide diversities for autosomes and X, and the bottom row illustrates the change in ratio of diversity of the two genomic classes X vs.