Discontinuous Variation and Phenotypic Traits

• Discontinuous variation occurs when variations can be placed into discrete categories.
• A trait that exhibits discontinuous variation has a distinct number of phenotypes, not a continuous range.
• Blood type (A, B, O, AB) is an example of a trait with discontinuous variation.
• Tongue rolling is another trait that exhibits discontinuous variation; individuals can either roll their tongue or cannot.
• Discontinuous traits do not exhibit a normal distribution when plotted on a graph
• ABO blood groups, when graphed for a population, show a specific distribution (Figure 11.15).

Genetic and Environmental Influences on Phenotypic Variation

• Genetic variations contribute to phenotypic variations; e.g., three alleles for the ABO gene result in four blood group phenotypes.
• Different alleles cause variations in traits like height, eye color, and skin color.
• Environmental factors also contribute to phenotypic variation.
• Epigenetics studies the effect of the environment on genes.
• Sun exposure affects skin color, an environmental influence on a genetic trait.
• Bonsai trees exemplify environmental influence, as special cultivation leads to small size.
• Bonsai cultivation involves growing trees in small containers and pruning roots for stunted growth.
• Trees subject to bonsai techniques grow up to several meters in height in the normal environment
• Environmental factors in bonsai cultivation impact phenotype, but phenotypic variation from environmental factors is not inherited.
• Seeds from bonsai trees will grow into typical trees if grown without restrictions.
• Discontinuous phenotypic variation is controlled by genetic factors.
• Continuous phenotypic variation relies on both environmental and genetic factors.

Mutations

• A mutation is a permanent change in the genetic material of a cell, affecting one gene or a chromosome section.
• A gene mutation affects the DNA base-pair sequence of a gene.
• Mutations can result in new gene alleles by inducing differences in DNA base sequences.
• Mutations are a major source of genetic variation.
• Recombination and independent assortment during meiosis introduce genetic variation.
• Sexual reproduction (meiosis and fertilization) increases genetic variation.
• Fertilization allows for random fusion of male and female gametes, creating diverse offspring genotypes.
• Chromosome mutations affect chromosome structure or number.
• Down syndrome, involving an extra copy of chromosome 21, is an example of a chromosomal mutation.
• Individuals with Down syndrome have three copies of chromosome 21 in diploid cells.
• Mutations can occur spontaneously or by agents called mutagens, like ionizing radiation or heavy metals.
• Mutagens cause changes by interfering with DNA duplication (replication).
• Mutagens prevent correct copying of genetic material, leading to altered sequences.
• Mutations in gametes are inherited, while those in body cells are not.
• Mutations can be harmful (Down syndrome), beneficial (resistance to disease), or neutral.
• Some mutations in humans slow AIDS progression or provide HIV immunity.

Discontinuous variation involves traits with distinct categories and a limited number of phenotypes, like blood type or tongue rolling. Genetic variations, such as multiple alleles for a gene, contribute to phenotypic diversity. Environmental conditions and epigenetics also contribute to phenotypic variation by affecting genes.