Basic Principles of Heredity and Evolutionary Genetics

Comprehensive vocabulary flashcards covering basic principles of heredity, Mendelian inheritance, sex determination, and extensions of genetic principles based on the provided lecture transcript.

TYRP1 gene

The tyrosinase-related protein 1 gene located on the short arm of chromosome 9, which determines blond hair in Solomon Islanders; a single base difference where thymine (T) replaces cytosine (C) characterizes the blond variant.

KITLG gene

A gene in Europeans associated with blond hair; mutations causing this trait occur in a DNA region affecting its expression rather than within the gene itself.

Gene

An inherited factor encoded in DNA that helps determine a specific characteristic.

Allele

One of two or more alternative forms of a gene.

Locus

The specific physical place on a chromosome occupied by an allele.

Genotype

The set of alleles possessed by an individual organism.

Homozygote

An individual organism possessing two identical alleles at a particular locus.

Heterozygote

An individual organism possessing two different alleles at a particular locus.

Phenotype

Also known as a trait; it is the physical appearance or manifestation of a characteristic.

Characteristic

A general attribute or feature possessed by an organism, such as seed shape or eye color.

Gregor Johann Mendel

An Augustinian priest known for discovering the basic principles of heredity through breeding experiments with pea plants ($Pisum\,sativum$).

SRY gene

The sex-determining region Y gene located on the Y chromosome that triggers the development of male characteristics in humans.

Androgen-insensitivity syndrome

A condition where individuals with XY chromosomes develop female external characteristics because of a defective androgen receptor that cannot respond to testosterone.

Monohybrid cross

A genetic cross between parents that differ in only a single characteristic.

Principle of Segregation

Mendel's first law, stating that each individual diploid organism possesses two alleles for any characteristic, which separate in equal proportions when gametes are formed.

Concept of Dominance

The principle that when two different alleles are present in a genotype, only the trait encoded by the dominant allele is observed in the phenotype.

Chromosome theory of heredity

The theory developed by Walter Sutton and Theodor Boveri stating that genes are located on chromosomes and that chromosome behavior during meiosis is the biological basis for inheritance.

Backcross

A cross between an $F_1$ genotype and either of the parental genotypes.

Punnett square

A shorthand grid method developed by Reginald C. Punnett used to predict the genotypic and phenotypic ratios of offspring from a genetic cross.

Multiplication rule

A probability rule stating that the likelihood of two or more independent events occurring together is calculated by multiplying their independent probabilities.

Addition rule

A probability rule stating that the likelihood of any of two or more mutually exclusive events occurring is calculated by adding their probabilities.

Conditional probability

Probability that is modified by additional information, such as knowing the phenotype of the progeny before determining its genotype.

Binomial expansion

A mathematical tool taking the form $(p + q)^n$ used to determine the probability of a specific combination of independent events.

Testcross

A cross in which an individual of unknown genotype is mated with a homozygous recessive individual to reveal the unknown genotype.

Wild type

The most common allele or phenotype for a characteristic found in nature, often symbolized by a plus sign ($+$).

Hemizygous

The possession of only a single allele at a locus; for example, human males are hemizygous for X-linked genes.

Nondisjunction

The failure of homologous chromosomes or sister chromatids to separate properly during meiosis.

Pseudoautosomal regions

Small homologous regions at the tips of the X and Y chromosomes that allow them to pair and segregate during meiosis.

Z-linked characteristics

Traits determined by genes on the Z chromosome in species with ZZ-ZW sex determination, where females are the heterogametic sex (ZW).

Holandric traits

Also known as Y-linked traits; characteristics present only in males and passed from father to all sons.

Dosage compensation

Mechanisms that equalize the amount of protein produced by X-linked genes in males and females.

Barr body

A condensed, darkly staining body in the nuclei of female cells that represents an inactivated X chromosome.

Lyon hypothesis

Proposed by Mary Lyon, it states that in female placental mammals, one of the two X chromosomes is randomly inactivated in every cell.

Xist

The X-inactive specific transcript gene that produces an RNA molecule to coat and inactivate an X chromosome.

Genetic maternal effect

An inheritance pattern where the phenotype of the offspring is determined by the genotype of the mother, often due to substances deposited in the egg cytoplasm.

Incomplete dominance

Occurs when the phenotype of the heterozygote is intermediate between the phenotypes of the two homozygotes.

Codominance

Occurs when the heterozygote simultaneously expresses the phenotypes of both homozygotes, such as in MN blood types.

Penetrance

The percentage of individuals with a specific genotype who express the expected phenotype.

Expressivity

The degree or intensity to which a particular trait is expressed in an individual.

Lethal allele

An allele that causes death at an early stage of development, often modifying phenotypic ratios to $2:1$ among surviving offspring.

Multiple alleles

The presence of more than two alleles at a single locus within a group of organisms, such as in the ABO blood group system.

Compound heterozygote

An individual who carries two different alleles at a locus that result in a recessive phenotype.

Historical Background of Mendel

Principles discovered by Gregor Johann Mendel (1822–1884), an Augustinian priest in Brno (now Czech Republic).

Historical Background of Mendel

Studied at the University of Vienna (1851–1853) in mathematics, physics, and botany providing the foundation for his scientific method.

Historical Background of Mendel

Experiments conducted between 1856 and 1863; findings published in 1866.

Historical Background of Mendel

Significance unrecognized until 1900 when Hugo de Vries, Erich Tschermak von Seysenegg, and Carl Correns independently reached similar conclusions.

Reasons for Mendel’s Success

Experimental Subject: The pea plant (Pisum sativum) is easy to cultivate, grows rapidly (one generation per season), and produces numerous offspring.

Reasons for Mendel’s Success

Discrete Characteristics: Focused on seven traits with two easily differentiated forms (e.g., round vs. wrinkled seeds).

Reasons for Mendel’s Success

Purity: Used genetically pure (homozygous) varieties.

Reasons for Mendel’s Success

Quantitative Method: Applied mathematics and formulated testable hypotheses rather than mere descriptions.

Seven Characteristics Studied by Mendel

Seed shape: Round vs. Wrinkled.

Seven Characteristics Studied by Mendel

Seed color: Yellow vs. Green.

Seven Characteristics Studied by Mendel

Seed coat color: Gray vs. White.

Seven Characteristics Studied by Mendel

Flower position: Axial vs. Terminal.

Seven Characteristics Studied by Mendel

Stem length: Tall vs. Short.

Seven Characteristics Studied by Mendel

Pod color: Yellow vs. Green.

Seven Characteristics Studied by Mendel

Pod shape: Inflated vs. Constricted.

The Monohybrid Cross

Crosses between parents that differ in a single characteristic.

The Monohybrid Cross

P Generation (Parental): Homozygous round (RR) x Homozygous wrinkled (rr).

The Monohybrid Cross

F1 Generation (First Filial): All progeny were round (Rr).

The Monohybrid Cross

Reciprocal Crosses yielded identical results (100% round in F1), indicating the trait is not sex-dependent.

The Monohybrid Cross

F2 Generation (Second Filial): Self-fertilization of F1 yielded a 3:1 phenotypic ratio (5474 round, 1850 wrinkled).

Mendel’s Derived Principles

Possession of Two Factors: Each plant possesses two genetic factors for a trait.

Mendel’s Derived Principles

The Principle of Segregation: Each individual diploid organism possesses two alleles for any characteristic, which segregate during gamete formation.

Mendel’s Derived Principles

Concept of Dominance: When two different alleles are present, only the trait of the dominant allele is observed.

Mendel’s Derived Principles

Equal Probability: Alleles separate with equal probability into gametes.

Molecular Basis of Wrinkled Seeds

The locus on pea chromosome 5 encodes the starch-branching enzyme isoform I (SBEI).

Molecular Basis of Wrinkled Seeds

R allele (Round): Encodes functional SBEI; converts linear starch to branched starch.

Molecular Basis of Wrinkled Seeds

r allele (Wrinkled): Contains a mutation producing an inactive enzyme, leading to sucrose accumulation and seed shriveling.

Punnett Square

Developed by Reginald C. Punnett in 1917; a grid used to predict genotypic and phenotypic ratios.

Multiplication Rule

The probability of two or more independent events occurring together is calculated by multiplying their independent probabilities.

Addition Rule

The probability of any of two or more mutually exclusive events is calculated by adding their probabilities.

Conditional Probability

Probability modified by additional information.

Binomial Expansion

Used for sets of events with the formula: (p+q)^n.

Testcross

Crossing an individual of unknown genotype with a homozygous recessive individual to reveal the unknown genotype.

Sex-Linked Characteristics

Thomas Hunt Morgan discovered a white-eyed male fly in a lab colony of red-eyed flies, showing eye color is X-linked.

Sex-Linked Characteristics

Hemizygosity: Males possess only one X chromosome and cannot be homozygous or heterozygous for X-linked traits.

Nondisjunction and Calvin Bridges

Found exceptional flies in crosses where they shouldn't occur, proving nondisjunction and supporting the Chromosome Theory of Heredity.

X-Linked Color Blindness (Humans)

Recessive trait caused by mutations in red/green pigment genes on the X chromosome.

Z-Linked Characteristics

In ZZ-ZW systems, males are homogametic (ZZ) and females are heterogametic (ZW).

Evolution of the Y Chromosome

Evolved from a pair of autosomes after acquiring a male-determining gene (SRY), leading to genetic degeneration.

Dosage Compensation

The problem arises because females have two X chromosomes, while males have one, potentially causing an imbalance in protein production.

The Lyon Hypothesis

One X chromosome in each female cell becomes inactivated, resulting in mosaicism.

Mechanism of Inactivation

The Xist gene produces a large RNA molecule that coats the X chromosome, silencing the genes.

Genetic Maternal Effect

Phenotype is determined by the mother's genotype, not its own.

Types of Dominance

Incomplete Dominance: Heterozygote phenotype is intermediate between homozygotes.

Types of Dominance

Codominance: Heterozygote expresses both homozygote phenotypes simultaneously.

Penetrance and Expressivity

Incomplete Penetrance: Genotype does not always produce the expected phenotype.

Lethal Alleles

Cause death early in development, altering progeny ratios (often resulting in a 2:1 ratio among survivors).

Multiple Alleles

More than two alleles exist at a single locus within a population.