I. Chromosomes, Genes, and Human Variability
Genetic and environmental variation is so great that no two individuals who have lived or ever will live will be exactly alike. Observable phenotypic variation, therefore, comes from both our genes and our environment.
The genetic elements of variability reside in nuclear and environmental DNA. The double helix of nuclear DNA is thought to exist in a single strand in each chromosome.
Human chromosomes are 46 in number, appearing in 23 homologous pairs in the female, 22 in the male plus the XY heterologous pair.
The 22 non sex chromosomes pairs are called autosomes. The human chromosomes are classified on the basis of their shape, size, and position of their centromeres.
II. Mendelian and Molecular Genetics
Molecular genetics is the physical basis of Mendelian genetics. All is not neat and tidy, however. There have been some surprises. In differentiated cells, some genes are active and some are not. Only 5% of genes are active in some tissues, whilst in the brain, their number rises to 12%. Some portion of DNA has many repetitions of bases; other sections called interons are carried over into messenger RNA.
III. Simple and Complex Inheritance
In unit 5.1 we distinguished between Mendelian and non-Mendelian inheritance.
"Simple" Mendelian inheritance is particulate. Phenotypical it is either/or situation. Some literature describes Mendelian variation as discontinuous. An example is blood types. You are one or the other. Most Mendelian variations are observed at the biochemical or serological level. They are not blended in any way. Furthermore, Mendelian traits are NOT altered by the environment.
Nearly all morphological traits measured by anthropometry or paleontology are controlled by many genes. We call such traits, therefore, polygenic. Such traits when measured for a population are continuous and plot out as a bell curve. Sir Francis Galton was the first to develop statistical methods to treat population data in a manner we now call bio metrics.
IV. 'Independent Segregation' and Linkage
Genes exhibit truly independent segregation only when they are located on separate nonhomologous chromosomes. It was Mendel's good fortune to select traits with genes on separate chromosomes in his famous research.
Genes on the same chromosome tend to be linked together. Linked chromosomes may be both on one chromosome or on different chromosomes of a homologous pair. The position effect describes the expression of a gene is affected by its position on a chromosome or in its relations to neighboring genes.
Crossing over or recombination occurs in homologous pairs during meiosis.
Except in special cases, linkage cannot be detected just by looking for two or more character variants in a population.
An area where Mendelian family (pedigree) studies have been revealing of linkages is where genes have been on the sex chromosomes.
Crossing over releases variability within the genome. Mutation is something new. Mutation may change the number of chromosomes or rearrange the genes. Incidentally, mutation is a reversible process.
Translocation is a pathological condition in humans. It is where genetic material is exchanged between nonhomologous chromosomes.
VI. Rare versus Common Variation
Some types of variation are very common, such as the ABO blood types. In genetics, you would say that there are three alleles for a given locus. A population may contain three (a, b, & o), but any individual can only have one or two, never more.
In contrast, albinism is a rare variation; only a very few individuals have the allele for albinism.
Sometimes a gene affects many characters. This is called pleiotrophy (Gr pleion = more/greater).
When genes vary in their intensity of phenotypic expression, they are said to show variable penetration. This term is frequently encountered in dental anthropology.
VII. Quantitative Variation
A. The Gaussian Curve
Many characters show continuous variation. A population study of standing height, for example, shows continuous variation. Many measured traits distribute themselves on a bell curve, though sometimes skewed as in the case of measuring body fat with calipers.
When characters controlled by many genes are measured, they show a continuous variation. The terms polygenic or multifactorial are used for these characters. Linkage between a few and several genes tends to obscure the total number of genes involved.
The essence of heritability is this: how much of a phenotypic variation is due to genetic endowment or environmental influence. In humans, broad heritability is estimated from twin studies and narrow heritability from family studies.
C. Twin Studies
The difference between monozygotic twins is mainly environmental. Difference between dizygotic twins is both genetic and environmental; however, they both shared the same intrauterine environment.
The usual approach for estimating heritability is to express the difference in measures between dizygotic and monozygotic twins.
Twins are an infrequent occurrence; figures for the UK are about one birth in 90. Only a quarter of those are monozygotic twins. Monzygotic triplets do occur; one such person recently completed his orthodontic studies here.
VIII. Population Studies/Mating Systems
A. Random Mating
Individuals in populations share their genes through mating. This brief unit focuses on Mendelian genetics.
Mating between homozygous parents of one type can only produce children of the same (homosygous) type. Mating between homozygotes of differing types can only produce (heterozygous} children of differing types.
In a population, gene frequencies are usually expressed in decimals (as in the Hardy-Weinberg binomial expression). If a heterozygote is phenotypically different from both homozygotes, then the frequencies of both of the alleles can be calculated directly.
B. Inbreeding and Outbreeding
The most extreme inbreeding is a self-fertilization; this is something humans cannot do. Many plants have this capability. Corn is an example (see more on this in section D below). In the first generation, half of the progeny are heterozygotes and half are homozygotes. In each succeeding generation, the number of heterozygotes is halved. Ultimately, all offspring will be homozygotes.
Brother/sister matings would require many more successive generations than self-fertilization to eliminate heterozygosity.
First cousin marriage (consanguineal/blood-related) would require many more generations than sibling marriage to achieve homozygosity. The genetic relatedness between first cousins is 1/8th. This makes the likelihood of a harmful recessive gene pairing with another for a given gene a one in 16 chances. First cousin couples share one set of grandparents. Famous examples of first cousin marriage are Charles Darwin & Emma Wedgewood and Jacob who married both Rachel and Leah.
The coefficient of kinship in first cousins is 1/16th. It is 1/8th for aunt/uncle with nephew/niece matings. (Are you scandalized by this? Moses' father married his aunt.)
Small populations exhibit some practical problems in endogamy and exogamy. In a population founded by 50 unrelated men and 50 unrelated women, who maintained that number through, would result in individuals who would have 80% of the founder's genes after only eleven generations.
C. Assortive Mating
Mating is described as assortive when there is a correlation between partners in some character. This can be positive or negative. It is only true when the incidence is greater than chance. Sir Francis Galton, for example, observed that married couples tended to look alike. IF look-alikes in marriage can be demonstrated scientifically, THEN it is an illustration of assortive mating. Assortive mating is observed for social class, ethnicity, and economic status .
D. The Biological Effects of Mating Systems
Heterozygosity has many advantages. Strangely, however, there may be advantages to self fertilization. This is seen in certain plants, which are called 'selfers.' If a plant prospers in a given micro environment, it is to its advantage to have offspring with identical characteristics. Many domesticated grains are 'selfers.'
The population structure of humans has been one of small populations isolated for long time periods. Large populations and life in cities are a development of just the last 5000 years. Efficient transportation is even more recent. Some have suspected that the secular (generational) increase in height in Europe over the last 100 years is in part due to the breakdown of isolates.
IX. Population Genetics/Gene Frequency Changes
These notes correlate with article 6.3. Note that there are four factors that change gene frequencies: gene flow, mutation, random genetic drift, and natural selection.
A. Gene Flow
Gene flow is the exchange of genes between one population and another. It tends to make the genetic makeup of exchanging populations more similar.
The change in gene frequency in a population following intermixture with a second population clearly depends upon the differences in gene frequency between the two populations and the extent of the intermixture.
Movement up and down social classes can be thought of as a vertical flow since the classes form a hierarchy.
The sole source of a new gene is through mutations. For a given locus, it is a rare event. Most fully recessive conditions are deleterious. They are hidden unless they occur as a homozygote.
Dominant traits are easy to identify since they are expressed even in heterozygotes.
C. Genetic Drift
One way in which gene frequencies may change is by chance or genetic drift. In a closed system which is small, chance will lead to the fixation of one gene or the other. The founder principle can limit the number of alleles in a population because of sampling omission.
D. Natural Selection
Natural selection is differential viability and/or fertility according to genetic constitution. Sometimes, selection can maintain the status quo thus eliminating the unfavorable variability.
It can also operate in a directional manner subject to environmental pressures.
Since DNA displays a low rate of spontaneous mutation, some changes over time may be due to chance (random genetic drift).
Populations when separated over time tend to become different. This is called polytypism. Homo sapiens is a highly polytypic species. The patterns of genetic differentiation tend to follow broad geographical regions. We will explore these issues in future units on human variation.
..... CJ '99
Harrison, G., Tanner, J., D. Pilbeam, and P. Baker Human Biology 3rd ed. New York: Oxford University Press, 1993.