Download Genetic Variation

Anthropology 2105
HUMAN NATURE
Solving “Darwin’s Problem”; or
Ten Points That Explain Why and How Biological Variation is Constantly Produced in a Species
1. The expression of biological traits is controlled by genes (what Mendel called "particles" or "factors). Genes are
made up of DNA, a double-stranded molecule comprised of sugar, phosphate, and other chemical substances. DNA
carries the “genetic code” (mind what Marks says about this!) and manufactures all life-sustaining proteins.
2. Genes are found on chromosomes in the cells of all organisms. Chromosomes exist in homologous pairs-- humans
have 46 chromosomes, or 23 pairs of homologous chromosomes. The total number of genes possessed by a single
human is surprisingly small: around 25,000.
3. Each gene comes in two alternative forms called alleles. Humans have at least two alleles for each biological trait,
one allele at the same locus on each homologous chromosome. One allele was received from their female parent and
the other from their male parent.
4. For each Mendelian trait an individual can have either two copies of the same allele, or one copy of two different
alleles (for polygenic traits, the situation is more complex):
a. if two copies of the same allele: the individual is homozygous for that trait (i.e., has a homozygous
genotype for that trait).
b. if one copy of two different alleles: the individual is heterozygous for that trait (i.e., has a heterozygous
genotype).
5. In the heterozygous genotype for a Mendelian trait, only one allele is outwardly expressed as the phenotype. This is
the dominant allele. The other allele (the one which is "masked") is the recessive allele. The importance of the
masked allele is that it preserves genetic variation which can give a survival advantage to individuals should the
environment change (e.g., carriers of the sickle-cell allele discussed in your text and in class).
6. The inheritance of traits (i.e., the passing on of genes to offspring) follows Mendel's laws of segregation and
independent assortment:
a. law of segregation: each parent passes on, via their sex cells (sperm or ova), only 23 chromosomes, which
means they pass on only one allele for any given trait. It can be either a dominant or recessive allele.
b. law of independent assortment: the allele for any given trait is passed on independently of the allele for any
other trait, unless they are on the same chromosome, in which case they are passed on together.
7. Often, during the formation of sex cells, homologous chromosomes can exchange segments with each other, a
process known as crossing-over. This causes alleles originally located on the same chromosome to be separated and
passed on independently of each other.
8. As a result of segregation, independent assortment, and crossing over, a single human sex cell can vary in any one
of 8 million possible ways.
9. When sperm and ovum meet during reproduction to create a zygote (a new individual), brand new allele
combinations are produced. This process is called recombination. A single human mating could produce any one of
70 trillion different individuals.
10. These processes help explain why and how variation is continuously preserved in a species from one generation to
the next. Other important sources of new variation are mutation, gene flow, and genetic drift (all discussed in the
reading).
Together, these processes explain why natural selection always has variation to work on, thereby solving one
of the problems that confronted Darwin (if inheritance is “blending”, how could natural selection sort out superior
“fluids”?). They also explain why the unit of selection is the individual, and why the unit of evolution is the
population.