Without genetic variation there will be no evolution. Thus, characterizing the genetic variation in natural populations is fundamental to the study of evolution. What kinds of variation are there? Discrete polymorphisms e. Examples: the carrots in the Burpee Catologue; human height. Continuously varying traits will have both a genetic and environmental components. How much genetic variation is there? Historical debate: Classical school held that there was very little genetic variation, most individuals were homozygous for a "wild-type" allele.
Rare heterozygous loci due to recurrent mutation; natural selection purges populations of their "load" of mutations. Balance school held that many loci will be heterozygous in natural populations and heterozygotes maintained by "balancing selection" heterozygote advantage. Selection thus plays a role in maintaining variation. How do we measure variation?
To show that there is a genetic basis to a continuously varying character one can study 1 resemblance among relatives : look at the offspring of individuals from parents in different parts of the distribution; can estimate heritability more later. Took off in mid 60's Lewontin and Hubby, ; Harris, ; still used.
Grind up organism in buffer, apply homogenate to gel starch, acrylamide , apply electric field, proteins migrate in gel according to charge, stain gel with histochemical stain for enzyme activity, bands reveal variation.
The technique provides a minimum estimate because different amino acid sequences may migrate at the same rate in the gel. DNA variation. Measure the genetic material directly; sequencing is the most precise but the most laborious; restriction enzyme analysis faster but has less information. These techniques have revealed that there is even more genetic variation that what was revealed by protein electrophoresis.
How does DNA affect phenotype? Some mutations don't have any noticeable effect on the phenotype of an organism. This can happen in many situations: perhaps the mutation occurs in a stretch of DNA with no function, or perhaps the mutation occurs in a protein-coding region, but ends up not affecting the amino acid sequence of the protein. How many gametes can be formed from AaBb?
How many alleles per gene are found in haploid gametes? What is the difference between a gene and an allele? For example, the gene responsible for the hair color trait has many alleles: an allele for brown hair, an allele for blonde hair, an allele for red hair, and so on. A gene is a portion of DNA that determines a certain trait. An allele is a specific form of a gene. Genes are responsible for the expression of traits.
How many kinds of homozygotes are there? There are six different genotypes: the three homozygotes and three different types of heterozygotes. In this allelic series, the alleles IA and IB each determine a unique antigen, which is deposited on the surface of the red blood cells. These antigens are two different forms of a single protein. What are allele combinations? An allele is a variant form of a gene. Some genes have a variety of different forms, which are located at the same position, or genetic locus, on a chromosome.
Humans are called diploid organisms because they have two alleles at each genetic locus, with one allele inherited from each parent. Each pair of alleles represents the genotype of a specific gene. Genotypes are described as homozygous if there are two identical alleles at a particular locus and as heterozygous if the two alleles differ. Alleles contribute to the organism's phenotype, which is the outward appearance of the organism.
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