Principles of Inheritance and Variation

Gregor Johann Mendel (1822–1884), hailed as the Father of Genetics, conducted hybridization experiments on the garden pea (Pisum sativum) between 1856 and 1863. Mendel succeeded where his predecessors failed because of his statistical methodology, large sample sizes, and choice of organism:

• 7 Pairs of Contrasting Traits: Easily distinguishable traits with clear dominant and recessive phenotypes:
  • Stem height: Tall (dominant, $T$) vs Dwarf (recessive, $t$)
  • Flower color: Violet ($W$ / $C$) vs White ($w$ / $c$)
  • Flower position: Axial ($A$) vs Terminal ($a$)
  • Pod shape: Inflated ($I$) vs Constricted ($i$)
  • Pod color: Green ($G$) vs Yellow ($g$)
  • Seed shape: Round ($R$) vs Wrinkled ($r$)
  • Seed color: Yellow ($Y$) vs Green ($y$)
• Bisexual & Self-Pollinating: Pea flowers are naturally cleistogamous/self-pollinated, guaranteeing true-breeding (homozygous) parental lines; easily cross-pollinated artificially via emasculation (removal of anthers) and bagging.
• Short Life Span & High Fecundity: Annual herbaceous plant with a life cycle of 3–4 months producing numerous seeds per pod, allowing multi-generational statistical analysis within a short timeframe.

A monohybrid cross examines the inheritance of a single trait (e.g., stem height). Crossing pure homozygous Tall ($TT$) with pure Dwarf ($tt$):

• $F_1$ Generation: All progeny are phenotypically Tall, but genotypically heterozygous ($Tt$). No blending is observed.
• $F_2$ Generation (Selfing $Tt imes Tt$):
  • Phenotypic Ratio: $3 ext{ Tall} : 1 ext{ Dwarf}$ ($3:1$)
  • Genotypic Ratio: $1 ext{ Homozygous Tall }(TT) : 2 ext{ Heterozygous Tall }(Tt) : 1 ext{ Homozygous Dwarf }(tt)$ ($1:2:1$)

A dihybrid cross traces the simultaneous inheritance of two independent gene pairs (e.g., Round Yellow $RRYY imes$ Wrinkled Green $rryy$). The $F_1$ dihybrid is $RrYy$ (Round Yellow). Selfing $RrYy imes RrYy$ produces 16 zygotic combinations in the Punnett Square:

• $F_2$ Phenotypic Ratio: $9 ext{ Round Yellow} : 3 ext{ Round Green} : 3 ext{ Wrinkled Yellow} : 1 ext{ Wrinkled Green}$ ($9:3:3:1$).
• $F_2$ Genotypic Ratio: $1:2:1:2:4:2:1:2:1$ (derived from $(1:2:1) imes (1:2:1)$).
• Law of Independent Assortment (Third Law): When two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair. Limitation: Holds true strictly when genes reside on different chromosomes or far apart on the same chromosome; violated by physical genetic linkage.

Occurs when neither allele is completely dominant over the other, resulting in an intermediate heterozygote phenotype. Discovered by Carl Correns in Mirabilis jalapa (Four O'clock plant) and also classic in Antirrhinum majus (Snapdragon / Dog flower):

• Pure Red ($RR$) $ imes$ Pure White ($rr$) $ o F_1$ all Pink ($Rr$).
• $F_2$ Generation (Selfing $Rr imes Rr$):
  • Phenotypic Ratio: $1 ext{ Red } (RR) : 2 ext{ Pink } (Rr) : 1 ext{ White } (rr)$ ($1:2:1$).
  • Genotypic Ratio: $1 ext{ } RR : 2 ext{ } Rr : 1 ext{ } rr$ ($1:2:1$).
  • Critical Observation: Phenotypic ratio is completely identical to the genotypic ratio ($1:2:1$), deviating from Mendel's $3:1$ ratio. The recessive white phenotype reappears unblended in $F_2$, proving alleles do not fuse.

In co-dominance, both alleles express their biochemical products equally and independently in the heterozygote without blending. Human ABO blood groups (discovered by Karl Landsteiner) exhibit both co-dominance and multiple allelism:

• Controlled by the gene $I$ (located on chromosome 9), encoding a glycosyltransferase enzyme that adds terminal sugar residues to the H-antigen lipid precursor on erythrocyte membranes.
• Multiple Alleles: Gene $I$ has 3 alleles ($I^A$, $I^B$, and $i$). Note: An individual diploid organism possesses only 2 alleles, but multiple alleles exist in the population pool.
• $I^A$ and $I^B$ produce N-acetylgalactosamine and galactose transferases respectively; allele $i$ produces no functional enzyme. $I^A$ and $I^B$ are completely dominant over $i$.
• When $I^A$ and $I^B$ are present together ($I^A I^B$), both A and B antigens are simultaneously expressed, manifesting as blood group AB (Co-dominance).

Walter Sutton and Theodor Boveri noted that the behavior of chromosomes during meiosis parallels the behavior of Mendelian factors (genes). They postulated the Chromosomal Theory of Inheritance:

• Chromosomes, like Mendelian factors, occur in homologous pairs in diploid somatic cells.
• Homologous chromosome pairs segregate during Anaphase I of Meiosis, ensuring each haploid gamete receives only one chromosome of a pair.
• Independent alignment of bivalents at the Metaphase I plate leads to independent assortment of non-homologous chromosomes.
• Fertilization restores the diploid paired condition of both chromosomes and genes.

Thomas Hunt Morgan (Nobel Laureate, 1933) provided the definitive experimental verification of the Chromosomal Theory using the fruit fly, Drosophila melanogaster. Scientific advantages of Drosophila:

• Easily cultured on simple synthetic cornmeal-agar medium in glass bottles in the laboratory.
• Extremely short life cycle: completes life cycle from egg to adult in about 2 weeks (10–14 days).
• High reproductive output: a single fertile mating produces hundreds of offspring flies.
• Distinct sexual dimorphism: males are noticeably smaller with specialized sex combs on the front legs; females are larger.
• Low chromosome number ($2n=8$, 4 pairs) and possesses numerous contrasting morphological variations observable under low-power dissecting microscopes.

Morgan crossed yellow-bodied, white-eyed females ($yw/yw$) with wild-type brown-bodied, red-eyed males ($y^+w^+/Y$). The $F_1$ dihybrids were intercrossed. Contrary to Mendel's expected $9:3:3:1$ ratio, the two genes did not assort independently:

• Linkage: The physical association of two or more genes located on the same chromosome. Linked genes tend to be inherited together as a unit into the same gamete, suppressing non-parental recombinant phenotypes.
• Recombination: The generation of non-parental gene combinations arising due to crossing over (reciprocal exchange of non-sister chromatid segments during Pachytene of Meiosis I).
• Morgan's Classic Dihybrid Cross Data:
  • Cross A (Yellow body $y$, White eye $w$): Tightly linked genes residing close to each other on the X-chromosome showed $98.7\%$ Parental types and only $1.3\%$ Recombinants.
  • Cross B (White eye $w$, Miniature wings $m$): Loosely linked genes located farther apart on the X-chromosome yielded $62.8\%$ Parental types and $37.2\%$ Recombinants.

Morgan's student, Alfred Sturtevant, utilized the frequency of recombination between gene pairs on the same chromosome as a linear measure of the physical distance separating them, constructing the world's first Genetic Map (Chromosome Map):

• $1\%$ Recombination Frequency = $1 ext{ Map Unit} = 1 ext{ Centimorgan (cM)}$.
• The greater the distance between two genes on a chromosome, the higher the probability of crossing over occurring between them (recombination frequency approaches a maximum asymptote of $50\%$).
• Genetic maps serve as the indispensable scaffolding for modern whole-genome sequencing projects (including the Human Genome Project).

In 1891, German cytologist Hermann Henking traced a specific nuclear structure through spermatogenesis in insects. He observed that $50\%$ of the sperms received this nuclear structure, while $50\%$ did not. Henking designated it the 'X-body'. Subsequent investigations established that Henking's X-body was a chromosome, hence named the X chromosome (Allosome), whereas regular non-sex chromosomes were designated Autosomes.

Honeybees, ants, and wasps exhibit a unique genetic mechanism known as Haplodiploidy, where sex is determined strictly by the number of chromosome sets received:

• Females (Queen & Workers): Diploid ($2n = 32$). Formed by the normal sexual union of a haploid egg ($n=16$) and a haploid sperm ($n=16$). Differential larval nutrition (Royal Jelly) dictates development into a fertile Queen or sterile Worker.
• Males (Drones): Haploid ($n = 16$). Developed from unfertilized haploid eggs ($n=16$) via Arrhenotokus Parthenogenesis.
• Cytogenetic & Genealogical Corollaries:
  1. Drones produce gametes (spermatozoa) exclusively by Mitosis, not meiosis (since they are already haploid).
  2. A male drone has no father (develops virgin from queen's egg) and cannot have sons (all fertilized eggs become female).
  3. However, a drone possesses a grandfather (his mother the queen had a father) and can have grandsons (his daughters produce drones).

A mutation is a sudden, discontinuous, heritable alteration in the genetic material (DNA nucleotide sequence or chromosome architecture):

• Point Mutation: A single base-pair substitution within a cistron. Classic paradigm: Sickle Cell Anemia, where a single nucleotide transversion ($A o T$) changes the 6th codon of the $eta$-globin gene from $GAG$ (encoding Glutamic acid) to $GUG$ (encoding Valine).
• Frameshift Mutation: Insertion or deletion of one or two nucleotide bases alters the downstream triplet translational reading frame completely, yielding a non-functional truncated polypeptide (e.g., Thalassemia).
• Chromosomal Aberrations: Structural reorganizations: Deletion (loss of short arm of chromosome 5 causes Cri-du-chat syndrome), Duplication, Inversion, and Reciprocal Translocation (e.g., Philadelphia chromosome $t(9;22)$ in Chronic Myeloid Leukemia).

Because controlled experimental crosses cannot be conducted in humans, medical geneticists analyze the inheritance of traits across successive ancestral generations using standard genealogical chart symbols (Pedigree Analysis):

• Square ($\square$): Normal Male | Circle ($igcirc$): Normal Female | Diamond ($\lozenge$): Sex unspecified.
• Solid/Shaded Symbol: Affected individual displaying mutant phenotype.
• Horizontal line connecting square and circle: Mating/Marriage; Double horizontal line: Consanguineous mating (mating between close blood relatives).
• Diagnostic Rules of Thumb:
  • Autosomal Dominant: Affected offspring has at least one affected parent; trait does not skip generations (e.g., Myotonic Dystrophy).
  • Autosomal Recessive: Trait skips generations; unaffected carrier parents ($Aa imes Aa$) produce affected offspring ($aa$) in $25\%$ of cases (e.g., Sickle Cell Anemia, PKU).
  • X-Linked Recessive: Shows distinctive Criss-Cross Inheritance; vastly higher frequency in males ($X^b Y$ hemizygous); affected father never transmits trait directly to his son.

X-linked recessive genes reside on the differential non-homologous segment of the X-chromosome. The transmission pattern goes from father $ o$ carrier daughter $ o$ grandson (skipping the daughter's generation phenotypically):

Aneuploidy is the gain or loss of one or more individual chromosomes resulting from non-disjunction (failure of homologous chromosomes or sister chromatids to separate properly during Meiotic Anaphase I or II). The three landmark chromosomal syndromes evaluated in board exams: