In Drosophila, fate maps showing what adult parts develop from specific embryonic regions have been developed. Mitotic crossing over and X chromosome nondisjunction have produced genetic mosaics, which have been used in developing these maps.
Substances stored in oocytes are important in early development. This is illustrated by maternal effect mutants in Drosophila and other animals, which are apparently defective in packaging of substances needed for development into the egg.
Both intracellular substances (internal signals) and external signals (communication between cells, and environmental signals) are important in programming development. Substances in the egg may be differentially distributed during cell division; this can lead to different pathways in those cells receiving the substance. Polar granules in nematodes and other animals are one example. Cells may influence the expression of adjoining cells through signals. (Examples from nematode).
Developmental regulatory genes are often conserved in evolution. Homeobox genes present in Drosophila are also present in many other animals, from worms to mammals. LIN-12 protein of C. elegans has elements associated with genes of yeast, Drosophila and mammals. Although the sequences of the genes may be conserved, the timing and tissue patterns of expression of these genes appears to vary dramatically. It is thought that changes in the timing and tissue pattern of gene expression in development may be more important in an evolutionary sense than changes in the coding regions of most genes involved in basic functions such as metabolism. (Allan Wilson, a WSU alum, was a pioneer in this area).
Developmental genes are often transcriptional activators, receptors or ligands. Examples: Homeobox genes as transcriptional activators, Lin-12 gene as a receptor, Lin-3 gene as a ligand.
Population genetics is the study of frequencies of alleles and genotypes in groups of individuals (populations). This area has much in common with classical genetics, but is now using DNA markers to address many questions. Hence, this is a good point in the course to address this area.
Genotype frequencies and allele frequencies are used to express genetic variation in populations. Populations are interbreeding groups of individuals. Codominant genetic markers such as blood types, protein and DNA variations are the typical markers studied. MN blood group is one example.
If a population had 90 M, 420 MN and 490 N individuals, the genotype frequencies would be .09, .42 and .49, respectively. The allele frequencies for M and N would be ((90 X 2) + 420 )/2000 = 600/2000 = 0.3 for the M allele, and ((490 X 2) =420)/2000 =1400/2000 =0.7 for the N allele. Note that they sum to one if there are only those two alleles present. A quick way to calculate allele frequencies is by summing the frequency of the homozygous type plus half the frequency of the heterozygous type. For the M allele, for example, this will be .09 + .42/2 = 0.3.
Under certain assumptions (to be discussed in more detail later) we can predict expected genotype frequencies, given the allele frequencies. This relationship is known as the Hardy-Weinberg principle. Essentially, this model assumes that alleles are being pulled from large pools (one for males and one for females) and come together at random to form the genotypes of the next generation. If the alleles are present in frequencies p and q, the genotype frequencies in the next generation can be expressed as p2 + 2pq + q2 = 1, where p2 and q2 are the frequencies of the homozygous genotypes and 2pq is the frequency of the heterozygote. For the example, where p = 0.3 and q = 0.7, the genotype frequencies expected will be .09, .42 and .49 respectively.
Allozymes are one of the types of genetic markers used in population genetic studies: variations detectable in proteins using electrophoresis. Variations detected are primarily in charges of the amino acids making up the protein, resulting in mobility changes. Study of allozyme variation showed that genetic variation was common in natural populations. This is expressed by the percent of polymorphic loci and by the heterozygosity (percent of loci that are heterozygous in an individual).