Biology Chapter 5: Variation Among Individuals

Variation Among Individuals

• Chapter 5 focuses on mutation and variation.
• Variation stems from three sources: genetic factors, environmental factors, and genotype-by-environment interactions.

Genetic Variation

• Phenylthiocarbamide (PTC) taste perception is linked to the TAS2R38 gene on chromosome 7.
• Two alleles, PAV and AVI, influence PTC taste sensitivity.
• PAV allele association with the ability to detect toxins.
• Charts display the percentage of subjects with different PTC genotypes (PAV/PAV, AVI/PAV, AVI/AVI) and their corresponding taste intensity.

Environmental Variation

• Environmental variation includes traits not genetically controlled; these traits are not heritable.
• Phenotypic plasticity: adaptability, including inducible defenses (e.g., in Daphnia).
• Hesse et al. (2012) research explores how organisms deal with multiple threats.

Genotype-by-Environment Interaction

• Aversion to cilantro is linked to multiple genes.
• Only a small percentage increase in cilantro aversion is tied to multiple copies of specific genes.
• Therefore, cilantro aversion is partially genetic but also potentially influenced by environment and genotype-environment interactions.
• An individual's response to various environments can be genetically controlled.
• Genotype-environment interactions mean the phenotype may vary depending on the environment.
• Reaction norm: A single genotype's expression across different environments.

Mutation

• The raw material for evolution is mutation.

• Without mutation, there are no new alleles, genes, or evolution.

• Studying mutations helps understand their role in evolution.

• DNA is made up of deoxyribose sugar, a phosphate group, and nitrogenous bases.

• The four bases are purines (adenine, guanine) and pyrimidines (thymine, cytosine).

• DNA replicates with the aid of DNA polymerase through complementary base pairing.

• Sometimes wrong bases pair, which aren't always corrected, resulting in point mutations.

• This process involves the unwinding of the DNA helix and the synthesis of new strands of DNA.

• DNA mutations in coding regions (exons) can be synonymous or nonsynonymous.

• Synonymous mutations don't change the amino acid sequence; they are also known as silent mutations.

• Nonsynonymous mutations cause changes in the amino acid sequence.

• Frameshift mutations occur when nucleotides are added or deleted, altering the reading frame.

• Nonsense mutations are caused by point mutations leading to stop codons.

• Various types of point mutations, like insertions and deletions, directly affect amino acid sequences.

Where New Genes Come From

• Gene duplication, often from unequal crossing over in meiosis, is a source of new genes.
• The extra gene copy can accumulate mutations without being immediately selected against.
• The globin gene family exemplifies this; the family includes a-like and B-like gene clusters, evolving to produce different chain forms during embryonic, fetal, and adult stages.
• Retroposition is another mechanism where processed mRNA is re-inserted into the genome.
• Chondrodysplasia in dogs illustrates retroposition and its effects.

Chromosomal Alterations

• Inversions: DNA segments are cleaved, reversed, and re-annealed.
• This can disrupt gene linkages and lead to dysfunctional gametes.
• Chromosomal inversions, a characteristic of Drosophil, are also found in several populations and are tied to temperature and body size.
• Polyploidy: Organisms may have extra sets of chromosomes, resulting in tetraploids, octoploids, hexaploids, and triploids.
• Polyploidy commonly arises in plants and sometimes appears in hermaphroditic animals that self-fertilize.
• Meiosis error can lead to diploid gametes.
• In species where diploid gametes fuse with other diploid gametes, polyploidy can lead to offspring.

Mutation Rate

• Data for mutation rates are predominantly from loss-of-function mutations.
• Loss-of-function mutations occur when a gene is inactivated, thus preventing the production of the associated protein.
• A loss-of-function mutation typically doesn't lead to evolution quickly but is typically studied to determine the rate of mutation.
• The mutation rates of genes vary across species and within species.
• Factors like DNA polymerase error rate, mismatch repair system efficiency, and environmental conditions influence the rates.

Measuring Genetic Variation

• Historically, genetic variation was perceived as low within populations, but current techniques have revealed incredibly high levels.
• Advancements in measuring genetic diversity rely on methods for directly measuring genomic and genetic variation.
• Determining genotypes: Phenotypes are used to determine some genotypes (e.g., achondroplasia). Protein and DNA sequences are required to identify other genotypes.

Calculating Allele Frequencies

• Estimating the frequency of alleles involves examining genotypes. Example data (Ashkenazi populations) demonstrates different alleles and their variation.
• The frequencies of alleles add up to one; this provides a useful check in calculating allele frequencies.

How Much Genetic Diversity Is Typical

• For the vast majority of organisms, between 15% and 33% of enzyme loci are polymorphic.
• Average individuals are typically heterozygous for 4% to 15% of loci; this is an average across different taxa.
• Heterozygosity, or mean heterozygosity, represents the average frequency of heterozygotes (individuals with two different alleles) based across loci and considers the fraction of loci in a population that have multiple alleles.
• Charts display heterozygosity frequency distributions for Vertebrates, Plants and Invertebrates across various species' genetic diversity.