Introduction to Population Genetics

• Population genetics involves the study of genetic variation within and between populations and how characteristics are passed down from generation to generation (inheritance), through genetics.

Basic Terminology

• A gene is the basic unit containing hereditary information.
• An allele is an alternative form/variation of a gene, responsible for inherited traits.
• Species are a group of individuals resembling one another that exchanges genes, interbreeds, and produces viable offspring.
• A population is a group of individual organisms of the same species living in a specific area.

Why Population Genetics?

• Understanding how and why allele and genotype frequencies change over time within and among populations is key.
• Looking at genetic variation helps determine disease susceptibility.
• Studying genetic variations can provide insights into species health, domestication, management, and conservation, and is crucial for conserving biodiversity, particularly for endangered species.

DNA

• DNA stores information about the development, structure, and metabolic activities of cells or organisms.
• DNA is stable, able to be replicated with high accuracy during cell division, and transmitted from generation to generation.
• Mutations, which are rare changes in DNA, introduce genetic variability needed for evolution.

More Terminology

• Each genetic trait is controlled by two alleles
• A homologous chromosome is a matching pair of chromosomes, one from each parent.
• Genotype refers to the alleles an individual receives at fertilization.
• Phenotype is the physical appearance of an individual.
• A dominant trait is expressed regardless of the second allele, represented by an uppercase letter.
• A recessive trait is only expressed when the second allele is the same, represented by a lowercase letter; two copies of the same allele are needed to express the trait.
• Homozygous dominant: genotype possesses identical, dominant alleles of a trait.
• Homozygous recessive: genotype possesses identical, recessive alleles of a trait.
• Individuals can be heterozygous: genotype possesses one dominant and one recessive allele of a trait, different alleles of the same trait.

Mendel and the Pea

• Pisum sativum, garden pea, was used because it is easy to cultivate and has a short generation time.
• Cross-pollination involves transferring pollen from the anther of one plant to the stigma of another.
• Mendel chose 22 true-breeding varieties that, when self-pollinated, produced offspring exactly like the parent plants.
• Traits chosen were easily observable, such as flower and seed color and shape. In his crosses, the offspring often resembled one of the parents in appearance.

Law of Segregation

• Original true-breeding parents consist of homozygous dominant (AA) and homozygous recessive (aa) for a trait. The first generation (F1) results from crossing these parents.
• When Mendel performed reciprocal crosses, all F1 offspring resembled the tall parent.
• Self-pollinating the F1 generation in the second procedure led to the F2 generation, where ¾ were tall and ¼ were short, a 3:1 ratio
• Inference: F1 plants contained the shortness characteristic despite its non-expression, concluding that tallness was dominant.
• Mendel developed a theory that a 3:1 ratio can be obtained if the F1 parents contained two separate alleles of each hereditary factor.
• Random fusion of all possible gametes happens with fertilization.
• The term "monohybrid cross" refers to a cross of a single trait between organisms hybrid for that trait.
• Assuming a simple dominant/recessive relationship, the expected phenotypic ratio is 3:1 in a monohybrid cross of heterozygotes.

Testcross

• Testcrosses determine if an individual expressing a dominant phenotype has either two dominant alleles (AA or BB) or one dominant and one recessive allele (Aa or Bb)
• By crossing with a known homozygous recessive individual, it is possible to determine if the genotype is homozygous or heterozygous for the dominant trait.
• Homozygous recessive parent: only gametes with length - t (short) are transferred to Punnett Square. Phenotypic ratio with heterozygous cross yields 1:1

Law of Independent Assortment

• Second series of crosses: involves true-breeding pea plants with two different traits
• E.g. tall plants green pods are crossed with short plants and yellow pods. Results show both dominant genetics characteristics
• When dominant alleles show up, and recessive is together: F2 plants show tall+green, tall+yellow, short+green, and short+yellow
• Independent assortment concludes law of heredity

Incomplete Dominance

• In incomplete dominance, one allele is not fully dominant over its partner, blending traits.
• Crossing two true-breeding snapdragons (red and white) reveals incomplete dominance: all F1 offspring have pink flowers
• Two mutated alleles (white) do not lead to pigment, therefore a "red" allele and a "yellow" allele appear pink
• Cross two pink F1 plants, results are red, pink and white flowers in F2 offspring, in a 1:2:1 ratio

Co-dominance

• Codominance: 2 non-identical alleles express fully – no dominant/recessive. Offspring show blending to both parents allele traits
• Human blood types are examples of multiple allele systems, where three or more alleles persist for a gene
• The A and B alleles are codominant. If only two alleles are paired, then two bloods exist.
• The "O" allele is recessive.

Epistasis

• Epistasis results in allele inhibiting/suppression effect on another
• results in phenotype being unexpected during genetic testing
• Dog coat colors occur by melanin, depending on the allele

Pleiotropy

• Pleiotropy: when one gene makes influences to multiple genes
• genes encode for parts used over the body
• Body shows elasticity, as well as proper performance over the body

Environmental Influences

• Multifactorial traits are influenced: traits controlled by polygenes subject to environmental influences
• multifactorial trait genes may respond differently to environmental factors-
• Temperature causes variation:
• Ex: White flowers grow at 32°C, but red flowers grow at 24°C.
• Coats on Himalayan rabbits are darker at the ears, nose, paws, and tail.

Sex-linked Traits

• Sex-linked traits: found from certain chromosomes determine gender of the body
• Female: XX
• Male: XY
• X-linked: what genes contain
• Y-linked: genes it may carry
• Humans have X-linked defects, like muscular dystrophy.

Continuous Variation

• Individuals in species commonly vary due to shared traits. Most appear in 2-3 forms, while range of smaller differences come from the continuous variation
• Product is polygenic inheritance that can show for eye color-
• Iris's color distributes with melanin.
• More melatonin = darker/less light shown.