Genetics and Population Genetics: Key Concepts and Applications

Genetics Fundamentals

Key Terms and Concepts

• Genotype: The genetic makeup of an organism; the combination of alleles present at specific loci. Example: AA, Aa, or aa for a single gene.

• Phenotype: The observable traits or characteristics of an organism, resulting from the interaction of its genotype with the environment. Example: Purple or white flower color in peas.

• Heterozygous: Having two different alleles at a gene locus (e.g., Aa).

• Homozygous: Having two identical alleles at a gene locus (e.g., AA or aa).

• Dominant: An allele that masks the effect of a recessive allele in heterozygotes. Example: The allele for purple flowers (P) is dominant over white (p) in peas.

• Recessive: An allele whose effect is masked by a dominant allele; only expressed in homozygous individuals (e.g., pp).

• Locus: The specific physical location of a gene on a chromosome.

• Barr Body: An inactivated X chromosome in female mammals, visible as a dense spot in the nucleus.

• SRY Region: The Sex-determining Region Y gene on the Y chromosome, responsible for initiating male sex determination in humans.

• Homologous Chromosomes: Chromosome pairs, one from each parent, that are similar in length, gene position, and centromere location.

• Karyotype: The number and visual appearance of chromosomes in the cell nuclei of an organism.

• Independent Assortment: The random distribution of homologous chromosome pairs during meiosis, leading to genetic variation.

Gamete and Offspring Possibilities

• To determine gamete possibilities, list all possible allele combinations that can be passed on from a parent genotype.

• To determine offspring possibilities, combine gametes from each parent using a Punnett square.

• Example: For a parent with genotype AaBb, possible gametes are AB, Ab, aB, ab (dihybrid cross).

Patterns of Inheritance

• Monohybrid Cross: Follows inheritance of a single gene (e.g., Aa x Aa).

• Dihybrid Cross: Follows inheritance of two genes (e.g., AaBb x AaBb).

• X-linked (Sex-linked) Inheritance: Genes located on sex chromosomes (usually X); inheritance patterns differ between sexes. Example: Hemophilia in humans.

• Codominance: Both alleles are fully expressed in heterozygotes. Example: AB blood type.

• Incomplete Dominance: Heterozygote phenotype is intermediate between the two homozygotes. Example: Pink flowers from red and white parents.

• Epistasis: One gene affects the expression of another gene. Example: Coat color in Labrador retrievers.

• Pleiotropy: One gene influences multiple phenotypic traits. Example: Marfan syndrome.

• Polygenic Inheritance: Multiple genes contribute to a single trait. Example: Human skin color.

Test Cross

• A test cross is used to determine the genotype of an individual with a dominant phenotype by crossing with a homozygous recessive individual.

Linked Genes

• Genes located close together on the same chromosome tend to be inherited together and are called linked genes.

• Linkage reduces the frequency of recombination between genes.

Chromosome Alterations

• Deletions: Loss of a chromosome segment.

• Insertions: Addition of extra chromosome segments.

• Polyploidy: More than two complete sets of chromosomes.

• Nondisjunction: Failure of chromosomes to separate properly during meiosis, leading to aneuploidy (e.g., Down syndrome).

Common Chromosomal Disorders

• Down Syndrome: Trisomy 21 (extra chromosome 21).

• Turner Syndrome: Monosomy X (XO).

• Klinefelter Syndrome: XXY genotype.

Population Genetics and Evolution

Key Terms and Concepts

• Population: A group of individuals of the same species living in the same area and interbreeding.

• Gene Pool: The total collection of genes and their alleles in a population.

• Locus: The specific location of a gene on a chromosome.

• Allele: Different forms of a gene found at the same locus.

• Genotype: The genetic constitution of an individual organism.

• Natural Selection: Differential survival and reproduction of individuals due to differences in phenotype.

• Sexual Selection: Selection for traits that increase mating success.

• Founder Effect: Genetic drift that occurs when a small group establishes a new population.

• Bottleneck Effect: A sharp reduction in population size due to environmental events, leading to loss of genetic diversity.

• Gene Flow: Movement of alleles between populations.

• Disruptive Selection: Favors individuals at both extremes of a trait.

• Directional Selection: Favors individuals at one extreme of a trait.

• Stabilizing Selection: Favors intermediate variants and reduces variation.

Effects on the Gene Pool

• Founder Effect: Can lead to reduced genetic variation and unique allele frequencies in new populations.

• Bottleneck Effect: Causes loss of genetic diversity and increased genetic drift.

• Gene Flow: Increases genetic similarity between populations and introduces new alleles.

Hardy-Weinberg Equilibrium

• The Hardy-Weinberg equilibrium describes a non-evolving population where allele and genotype frequencies remain constant from generation to generation.

• The equation is: $p^2 + 2pq + q^2 = 1$ Where:

  • $p^2$ = frequency of homozygous dominant genotype

  • $2pq$ = frequency of heterozygous genotype

  • $q^2$ = frequency of homozygous recessive genotype

  • $p$ = frequency of dominant allele

  • $q$ = frequency of recessive allele

• Allele frequencies: $p + q = 1$

Purpose and Application of Hardy-Weinberg

• Used as a null hypothesis to determine if evolution is occurring in a population.

• If observed genotype frequencies differ from expected, the population may be evolving.

• When a population is not evolving, allele and genotype frequencies match Hardy-Weinberg predictions.

Conditions for Hardy-Weinberg Equilibrium

• No mutations

• Random mating

• No natural selection

• Extremely large population size (no genetic drift)

• No gene flow (no migration)

Calculating Allele Frequencies

• If given one allele frequency, the other can be found using $p + q = 1$.

• Example: If $p = 0.7$, then $q = 0.3$.

Summary Table: Types of Selection

Type of Selection	Description	Effect on Population	Example
Directional	Favors one extreme phenotype	Shifts population mean	Antibiotic resistance in bacteria
Disruptive	Favors both extreme phenotypes	Increases variation, may lead to speciation	Beak size in African finches
Stabilizing	Favors intermediate phenotypes	Reduces variation	Human birth weight

Summary Table: Chromosomal Alterations

Alteration	Description	Possible Result	Example Disorder
Deletion	Loss of chromosome segment	Missing genes	Cri du chat syndrome
Duplication	Repeat of chromosome segment	Extra genes	Charcot-Marie-Tooth disease
Inversion	Reversal of segment within chromosome	Gene disruption	Some forms of hemophilia
Translocation	Segment moves to nonhomologous chromosome	Gene disruption	Chronic myelogenous leukemia
Nondisjunction	Failure of chromosomes to separate	Aneuploidy	Down syndrome

Additional info: These notes expand on the review topics by providing definitions, examples, and summary tables for key concepts in genetics and population genetics, suitable for exam preparation.