Neanderthal mt DNA sequences have been compared to human. These sequences were amplified by PCR and were from samples 30,000 to 100,000 years old. The Neanderthal samples clustered with each other and distinct from modern humans. The level of diversity among Neanderthals and among humans is much lower than among chimps, or among gorillas. This suggests those apes may be "older species".
Genetics of complex traits is a field closely related to both classical genetics and population genetics. Classical genetics shares with it a common interest in the inheritance and transmission of traits. Population genetics shares a common concern with it in genetic change over time, and how a trait responds to either natural or artificial selection. Terminology: multifactorial trait refers to a trait influenced by both genetics and the environment, complex trait refers to the same, quantitative trait is one which can be expressed on a numerical scale. Complex traits may be either continuous or non-continuous. Continuous traits are measured on a continuous numerical scale and include height, weight and milk production. Non-continuous traits cannot be measured on a continuous numerical scale. Examples include meristic traits and threshold traits. Meristic traits are countable, e.g., vertebrae, scale counts in a fish. Threshold traits are all or none, but appear to be influenced by multiple genes and the environment. Exs: diabetes, schizophrenia.
Complex traits are most often expressed statistically by a mean (average) and a variance (s2) and are usually normally distributed. The mean is the sum of the trait values divided by the number of individuals studied. The variance is the sum of the trait value minus the mean trait value, divided by n-1 where n is the number of samples.
In trying to understand these traits, one approach is to partition the total variance (VP) into genotypic variance (VG) plus environmental variance (VE). One approach to do this is to rear genetically uniform inbred lines and examine the spread in values. This will be a direct expression of VE, the environmental variance. Genotype X environment interaction can be seen when trait values dramatically differ between environments, and specifically when trait values from the same genetic sources give different values (sometimes reversing their relative rank orders) in different environments. Twin studies are an approach to address this partitioning in humans. Identical and fraternal twins are compared and as a first approximation, the two are assumed to share the same environments. This comparison allows the calculation of H2, defined as the "broad sense" heritability, or VG/VP. The higher this value is on a sacle from 0 to 1, the greater the relative degree of genetic control of the trait. (refer to Table 14.2).
There is an interest in attempting to understand the types of genes associated with complex traits, and ultimately in identifying the genes involved. Two types of genes acting on quantitative traits are those acting in an additive manner, and those acting with dominance. Genes acting in an additive manner will show a consistent effect with the presence of particular alleles. An example would be a situation with genotypes A1A1 having a value of 1.0, A1A2 having a value of 1.5 and A2A2 having a value of 2.0. Here the allele A1 has a value of 0.5 and the allele A2 has a value of 1.0, and they are additive. Genes acting with dominance do not show predictable effects associated with each allele. For example, genotype B1B1 would have a value of 1.0, genotype B1B2 would have a value of 2.0 and genotype B2B2 would have a value of 2.0. Here the specific combination rather than the additive effect of particular alleles is what is critical.
The genotypic variance VG can be partitioned into two components corresponding to the above types of genes, VA for additive variance and VD for dominance variance. Additive variance can be estimated by the response to selection. The mean of progeny after selection, minus the population mean, divided by the mean of the selected parents, minus the population mean, will give a value known as the narrow-sense heritability, h2, VA/VP. This value is especially important to plant and animal breeders; the value predicts the probably success of artificial selection.
Narrow sense heritability = h2 = (progeny mean - population mean) / (Parental mean - population mean)
For example, if selected parents were 55, population mean was 40 and progeny mean was 45, the heritability would be 0.33
Another question in analyzing complex traits is: what genes are involved? QTL analysis is one way to address this question. QTL stands for quantitative trait locus. A locus refers to a spot or region of a chromosome. Lines differing for a trait are crossed to make an F1 hybrid. The hybrid is then either backcrossed to one of the parents or crossed to make F2 progeny. The F2 are then scored for DNA markers along the chromosomes (from either line) and the trait value. A statistical test is done to see if particular markers are associated with the trait values. A positive result would indicate that a gene or genes in the region was associated with the trait. A subsequent step could be to attempt to isolate the actual gene involved using "positional cloning" (discussed earlier). A related approach for testing for associations is to test for "candidate genes". The steps in this approach are as follows.