Biotechnology: Class 11 — Chapter 6: Basic Principles of Inheritance

The study of heredity and variation forms the basis of genetics, which is essential for biotechnological manipulation of living organisms.

• Heredity: The transmission of physical and physiological characters from one generation to the next, mediated by genes located on chromosomes.
• Variations: The structural or functional differences exhibited by offspring compared to their parents and each other.
• Genetics: The scientific study of the principles governing heredity and variation.

Gregor Johann Mendel (1822–1884), the 'father of genetics', established the laws of inheritance using the garden pea (Pisum sativum) as a model system due to its annual nature, bisexual flowers, and distinct contrasting traits.

A cross involving a single pair of contrasting characters to study the inheritance pattern of a single gene.

• Monohybrid Cross: Crossing pure tall ($TT$) and pure dwarf ($tt$) plants yields all tall ($Tt$) offspring in the $F_1$ generation. Selfing $F_1$ produces the $F_2$ generation with a phenotypic ratio of $3:1$ (tall to dwarf) and a genotypic ratio of $1:2:1$ ($TT : Tt : tt$).
• Punnett Square: A graphical checkerboard representation developed by Reginald C. Punnett to calculate the probability of all possible genotypes in offspring.
• Test Cross: Crossing an individual of dominant phenotype (genotype unknown, e.g., $TT$ or $Tt$) with a homozygous recessive parent ($tt$). If the progeny is 100% dominant, the parent is homozygous; if the progeny shows a $1:1$ ratio of dominant to recessive, the parent is heterozygous.
• Law of Dominance: In a heterozygous individual, one allele (dominant) masks the expression of the other allele (recessive) for the same character.
• Law of Segregation: Alleles do not blend or contaminate each other; they segregate or separate during gamete formation, ensuring each gamete receives only one of the two alleles.

Phenotypic expressions that deviate from typical Mendelian dominance-recessive relationships.

• Incomplete Dominance: Observed in the four-o'clock plant (Mirabilis jalapa). Crossing homozygous red ($RR$) with homozygous white ($rr$) flowers produces pink ($Rr$) $F_1$ hybrids. The $F_2$ generation yields a $1:2:1$ ratio of red, pink, and white flowers, showing a blending of parental traits.
• Codominance: A condition where both alleles in a heterozygote are fully and equally expressed. Examples include:
  • Coat Colour in Cattle: Crossing pure red ($RR$) with pure white ($WW$) yields roan ($RW$) offspring, which possess a mixture of individual red and white hairs.
  • MN Blood Group: Controlled by codominant alleles $L^M$ and $L^N$, resulting in three distinct phenotypes: M ($L^M L^M$), MN ($L^M L^N$), and N ($L^N L^N$).

A cross involving two pairs of contrasting characters to study the simultaneous inheritance of two genes.

• Dihybrid Cross: Crossing homozygous round yellow ($RRYY$) with wrinkled green ($rryy$) pea plants yields round yellow ($RrYy$) in $F_1$. Selfing $F_1$ results in an $F_2$ phenotypic ratio of $9:3:3:1$ (9 round-yellow, 3 wrinkled-yellow, 3 round-green, 1 wrinkled-green).
• Law of Independent Assortment: Alleles of two or more different genes assort independently of one another during gamete formation, creating new non-parental combinations in the offspring.

The physical association and co-inheritance of genes located on the same chromosome.

• Linkage: The tendency of genes present on the same chromosome to remain together during hereditary transmission.
• Linked Genes: Genes located on the same chromosome that do not assort independently. All the genes on a single chromosome constitute a linkage group.
• Bateson and Punnett Experiment: Conducted on sweet pea (Lathyrus odoratus) crossing red flowers/long pollen ($RL$) with white flowers/short pollen ($rl$). The $F_2$ generation showed a significant deviation from the expected $9:3:3:1$ ratio, with parental combinations appearing in much higher frequencies due to linkage.

The physical exchange of genetic material between non-sister chromatids of homologous chromosomes during meiosis.

• Crossing Over: Occurs during pachytene stage of prophase I in meiosis, leading to the separation of linked genes and the formation of recombinant chromatids.
• Morgan's Drosophila Experiments: Thomas Hunt Morgan (1910) crossed homozygous grey-bodied vestigial-winged ($BBvv$) Drosophila with black-bodied long-winged ($bbVV$) flies. The $F_1$ females ($BbVv$) test-crossed with double recessive males ($bbvv$) yielded 83% parental phenotypes (41.5% grey-vestigial, 41.5% black-long) and 17% recombinant phenotypes (8.5% grey-long, 8.5% black-vestigial), proving that linked genes can separate via crossing over.

The generation of non-parental gene combinations due to crossing over, used to map gene loci on chromosomes.

• Recombination Frequency: The percentage of recombinant offspring in a cross, which is directly proportional to the physical distance between the genes on a chromosome.
• Genetic Map Unit: $1 \text{ cM}$ (centimorgan) is defined as the distance between genes that exhibits $1\%$ recombination frequency.
• Creighton and McClintock Experiment (1931): Provided the first cytological proof of crossing over in maize (Zea mays) using chromosome 9 carrying physical markers (a heterochromatic knob at one end and a translocated piece of another chromosome at the other end) along with genetic markers for kernel colour ($C/c$) and texture ($Wx/wx$).

Transmission of characters controlled by genes located on the sex chromosomes (gonosomes).

• Sex-linked Genes: Genes located on sex chromosomes, most commonly the X chromosome.
• Haemophilia: A classic X-linked recessive bleeding disorder observed historically in the royal families of Europe.
• Morgan's White-Eye Cross: Morgan discovered a white-eyed male mutant ($w$) in a wild red-eyed ($W$) Drosophila culture. Crossing red-eyed females with white-eyed males produced all red-eyed $F_1$ flies. The $F_2$ generation showed a $3:1$ ratio, but all white-eyed flies were exclusively male, proving the gene for eye colour is located on the X chromosome.

Inheritance of traits controlled by non-nuclear DNA present in cytoplasmic organelles.

• Cytoplasmic Inheritance: Controlled by genes located in mitochondrial or plastid (chloroplast) genomes.
• Maternal Lineage: Since the zygote receives almost all of its cytoplasm from the egg cell (maternal parent) and none from the sperm, cytoplasmic traits are inherited exclusively from the mother (uniparental inheritance).
• Plastid Inheritance in Mirabilis jalapa: Leaves can be green, white, or variegated depending on the plastid DNA. Crosses always yield offspring matching the phenotype of the female (maternal) parent.

Variations in chromosome numbers that alter the genetic constitution of organisms.

• Polyploidy: The state of having more than two complete sets of chromosomes (e.g., triploid $3n$, tetraploid $4n$, hexaploid $6n$, octoploid $8n$). Highly common in plants, leading to larger cell size, larger leaves, and increased environmental tolerance.
• Aneuploidy: An abnormal chromosome number due to the loss or gain of one or more individual chromosomes (e.g., monosomy, trisomy) resulting from non-disjunction during meiotic division.

The classical genetic approach that starts with a visible phenotype and works backward to identify the underlying gene sequence.

Workflow: Identify phenotypic variation in a population $ ightarrow$ Perform genetic crosses to determine inheritance pattern $ ightarrow$ Map the gene to a chromosome $ ightarrow$ Sequence the DNA to identify the specific gene.

A modern molecular approach that starts with a known DNA/protein sequence of unknown function and works forward to determine its phenotypic effect.

Workflow: Select candidate gene sequence $ ightarrow$ Disrupt or alter the gene in vitro $ ightarrow$ Reinsert the altered gene into the host genome $ ightarrow$ Analyze the resulting phenotypic changes to deduce gene function.

• RNA Interference (RNAi): A biological process where double-stranded RNA molecules mediate the sequence-specific degradation of homologous mRNA, effectively silencing gene expression at the translational level.
• Targeted Gene Disruption: Knocking out a specific gene's function using homologous recombination to study the phenotypic consequences of its absence.