Basic Principles of Heredity and Evolutionary Genetics

Genetic Basis of Blond Hair in the South Pacific

• Geographic and Cultural Context:

  • The Solomon Islands comprise an ancient chain of volcanic and coral islands located a thousand miles northeast of Australia.

  • Inhabited approximately $30,000$ years ago, contemporaneous with Neanderthals in northern Europe.

  • Populated largely by Melanesians; inhabitants predominantly have dark skin.

• Phenotypic Observation:

  • $5\%-10\%$ of the population possesses blond hair, the highest frequency of this trait outside of Europe.

• Historical Hypotheses regarding Origin:

  • Sun and salt-water bleaching of naturally dark hair.

  • Dietary influences.

  • Genetic legacy from early European explorers.

• Scientific Resolution (2012):

  • Geneticists Eimear Kenny, Sean Myles, and colleagues used a genome-wide association study (GWAS) to analyze saliva and DNA from over $1200$ islanders.

  • A strong statistical correlation was found between blond hair and a variant on the short arm of chromosome 9.

  • The TYRP1 Gene: This region contains the tyrosinase-related protein 1 ($TYRP1$) gene, which encodes an enzyme involved in melanin production and pigmentation.

  • Molecular Difference: Blonds possess a thymine ($T$) base instead of a cytosine ($C$) base at a specific location in the gene.

• Inheritance Patterns:

  • The trait is recessive: blonds must carry two copies of the mutant allele ($TT$).

  • Dark hair is dominant ($CT$ or $CC$).

  • Over $40\%$ of dark-haired islanders are heterozygous carriers ($CT$) of the blond gene.

  • The mutation is rare outside the South Pacific, suggesting an independent evolutionary origin from European blondness.

• Comparison to Europeans:

  • European blondness is associated with variations in at least eight different genes.

  • Example: The $KITLG$ gene in Europeans (mutation in a regulatory region affecting melanocyte development).

3.1 Gregor Mendel and the Discovery of Hereditary Principles

• Historical Background:

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

  • Studies at the University of Vienna ($1851$–$1853$) in mathematics, physics, and botany provided the foundation for his scientific method.

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

  • 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.

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

  • Purity: He used genetically pure (homozygous) varieties.

  • Quantitative Method: Mendel applied mathematics and formulated testable hypotheses rather than mere descriptions.

• Seven Characteristics Studied by Mendel:

  1. Seed shape: Round vs. Wrinkled.

  2. Seed color: Yellow vs. Green.

  3. Seed coat color: Gray vs. White.

  4. Flower position: Axial vs. Terminal.

  5. Stem length: Tall vs. Short.

  6. Pod color: Yellow vs. Green.

  7. Pod shape: Inflated vs. Constricted.

Important Genetic Terminology

• Gene: An inherited factor (encoded in DNA) that helps determine a characteristic.

• Allele: One of two or more alternative forms of a gene.

• Locus: A specific place on a chromosome occupied by an allele.

• Genotype: The set of alleles possessed by an individual organism.

• Homozygote: An individual possessing two of the same alleles at a locus.

• Heterozygote: An individual possessing two different alleles at a locus.

• Characteristic (Character): An attribute or feature (e.g., eye color).

• Phenotype (Trait): The appearance or manifestation of a characteristic (e.g., blue eyes).

  • Note: Only alleles (genotypes) are inherited; phenotypes result from the interaction of the genotype and the environment.

3.2 Monohybrid Crosses and the Principle of Segregation

• The Monohybrid Cross:

  • Crosses between parents that differ in a single characteristic.

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

  • $F_1$ Generation (First Filial): All progeny were round ($Rr$).

  • Reciprocal Crosses: Yielded identical results ($100\%$ round in $F_1$), indicating the trait is not sex-dependent.

  • $F_2$ Generation (Second Filial): Self-fertilization of $F_1$ yielded a $3:1$ phenotypic ratio ($5474$ round, $1850$ wrinkled).

• Mendel’s Derived Principles:

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

  2. The Principle of Segregation (Mendel’s First Law): Each individual diploid organism possesses two alleles for any characteristic. These alleles segregate during gamete formation, and one allele goes into each gamete in equal proportions.

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

  4. 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 ($SBEI$).

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

  • r allele (Wrinkled): Contains a mutation (an extra $800$ base pairs from a transposable element). This produces an inactive enzyme, leading to sucrose accumulation and high water absorption. On maturation, the seed loses water and shrivels.

Quantitative Tools: Punnett Squares and Probability

• 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. (Key indicator: "and").

  • Example: Probability of two fours on two rolls: $\frac{1}{6} \times \frac{1}{6} = \frac{1}{36}$.

• Addition Rule: The probability of any of two or more mutually exclusive events is calculated by adding their probabilities. (Key indicator: "either/or").

  • Example: Probability of rolling a three or a four: $\frac{1}{6} + \frac{1}{6} = \frac{2}{6} = \frac{1}{3}$.

• Conditional Probability: Probability modified by additional information.

  • Example: In a cross between $Tt \times Tt$, if a plant is known to be tall, the probability it is heterozygous is $\frac{2}{3}$ because the $tt$ (short) phenotype is excluded from the pool of possibilities.

• Binomial Expansion:

  • Used for sets of events. Formula: $(p+q)^n$.

  • Coefficient calculation: $\frac{n!}{s!t!}$ where $n$ is the total number of events, $s$ is the number of times event X occurs, and $t$ is the number of times event Y occurs.

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

  • If the unknown is $TT$, progeny are all tall.

  • If the unknown is $Tt$, progeny are $1:1$ tall to short.

4.2 Sex-Linked Characteristics

• Thomas Hunt Morgan and Drosophila:

  • Discovered a white-eyed male fly in a lab colony of red-eyed flies.

  • Crosses showed eye color is X-linked.

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

• Nondisjunction and Calvin Bridges:

  • Found exceptional flies (e.g., white-eyed females) in crosses where they shouldn't occur.

  • Hypothesized and proved that some flies had $XXY$ or $XO$ chromosomes due to failures in chromosome separation (nondisjunction).

  • This provided definitive proof for the Chromosome Theory of Heredity.

• X-Linked Color Blindness (Humans):

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

  • Affected mothers pass the trait to all sons. Affected fathers pass the trait to grandsons via carrier daughters.

• Z-Linked Characteristics:

  • In ZZ-ZW systems (birds, some fish), males are homogametic ($ZZ$) and females are heterogametic ($ZW$).

  • Example: Cameo phenotype in Indian blue peafowl ($Z^{ca}$ recessive to $Z^{Ca+}$).

• Evolution of the Y Chromosome:

  • Evolved from a pair of autosomes after acquiring a male-determining gene ($SRY$).

  • Lack of crossing over led to the accumulation of mutations and genetic degeneration.

  • Palindromes: The human Y chromosome contains eight massive palindromic sequences that allow internal recombination, helping to maintain gene stability.

  • Genetic Markers: Mutations on the Y chromosome are used to trace male ancestry (e.g., establishing the paternity of Jefferon's descendants with Sally Hemings).

4.3 Dosage Compensation

• The Problem: Females have two X chromosomes, while males have one, potentially causing an imbalance in protein production.

• The Lyon Hypothesis (Mary Lyon, 1961):

  • One X chromosome in each female cell becomes inactivated (condenses into a Barr Body).

  • Inactivation is random and occurs early in development.

  • Mosaicism: Females are functional mosaics. Heterozygous females express different alleles in different cell clusters (e.g., tortoiseshell and calico cats).

• Mechanism of Inactivation:

  • The Xist gene produces a large RNA molecule that coats the X chromosome, recruiting proteins to alter chromatin structure and silence the genes.

Chapter 5: Extensions and Modifications of Basic Principles

• Genetic Maternal Effect:

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

  • Example: Shell coiling (chirality) in Lymnaea peregra snails. Dextral (right-handed) is dominant to sinistral (left-handed). The direction is determined by substances deposited in the egg cytoplasm by the mother.

• Types of Dominance:

  • Incomplete Dominance: Heterozygote phenotype is intermediate between homozygotes (e.g., violet eggplant fruit from purple x white parents; 1:2:1 ratio).

  • Codominance: Heterozygote expresses both homozygote phenotypes simultaneously (e.g., MN blood types; $I^A I^B$ blood type).

• Penetrance and Expressivity:

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

  • Expressivity: The degree to which a trait is expressed (e.g., polydactyly can range from a skin tag to a functional finger).

• Lethal Alleles:

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

  • Example: Yellow coat color in mice ($Y$ is lethal in homozygotes).

• Multiple Alleles:

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

  • Formula for possible genotypes: $[n(n+1)/2]$.

  • Example: ABO Blood Group ($I^A$, $I^B$, $i$). $I^A$ and $I^B$ are codominant and both are dominant over $i$.

  • Compound Heterozygote: An individual with two different mutant alleles at a locus that result in a recessive phenotype (common in cystic fibrosis).", "title": "Basic Principles of Heredity and Extensions"}