Biology: Class 12 — Chapter 4: Principles of Inheritance and Variation

During the mid-nineteenth century, Gregor Mendel conducted hybridization experiments on garden peas (Pisum sativum) for seven years (1856–1863). He applied statistical analysis and mathematical logic to biological problems, utilizing large sample sizes to establish general rules of inheritance.

• Mendel selected 14 true-breeding pea plant varieties, focusing on seven pairs of contrasting traits:
• Stem height: Tall vs. Dwarf
• Flower color: Violet vs. White
• Flower position: Axial vs. Terminal
• Pod shape: Inflated vs. Constricted
• Pod color: Green vs. Yellow
• Seed shape: Round vs. Wrinkled
• Seed color: Yellow vs. Green

A cross involving a single pair of contrasting traits is known as a monohybrid cross.

• F1 Generation (Filial 1): All progeny express only one of the parental traits (the dominant trait), with no blending observed.
• F2 Generation (Filial 2): Self-pollination of F1 plants results in the reappearance of the recessive trait. The phenotypic ratio is $3:1$ (dominant to recessive), and the genotypic ratio is $1:2:1$ (homozygous dominant : heterozygous : homozygous recessive).
• Mathematical Representation: The genotypic ratio can be expressed as a binomial expansion: $$(\frac{1}{2}T + \frac{1}{2}t)^2 = \frac{1}{4}TT + \frac{1}{2}Tt + \frac{1}{4}tt$$
• Test Cross: A method used to determine the genotype of an organism expressing a dominant phenotype by crossing it with a homozygous recessive parent.

1. Law of Dominance: Characters are controlled by discrete units called factors (now known as genes) that occur in pairs. In a heterozygous pair, one factor dominates (dominant) and the other is masked (recessive).
2. Law of Segregation: Alleles do not blend. During gamete formation, the two alleles of a gene segregate from each other so that each gamete receives only one allele. Homozygous parents produce identical gametes, while heterozygous parents produce two types of gametes in equal proportions.

A cross involving two pairs of contrasting traits (e.g., seed shape and seed color) is a dihybrid cross.

• Crossing round-yellow seeds ($RRYY$) with wrinkled-green seeds ($rryy$) yields an F1 generation of round-yellow seeds ($RrYy$).
• Selfing the F1 generation produces a phenotypic ratio of $9:3:3:1$ (Round-Yellow : Round-Green : Wrinkled-Yellow : Wrinkled-Green).
• Law of Independent Assortment: When two pairs of traits are combined in a hybrid, the segregation of one pair of characters is completely independent of the other pair during gamete formation.

A phenomenon where the F1 phenotype does not resemble either parent but is an intermediate blend of both.

• Example: Flower color in the snapdragon or dog flower (Antirrhinum majus).
• Crossing red flowers ($RR$) with white flowers ($rr$) produces pink flowers ($Rr$) in the F1 generation.
• The F2 generation yields a phenotypic and genotypic ratio of $1:2:1$ (1 Red : 2 Pink : 1 White).

A condition in which both alleles in a heterozygote are fully expressed simultaneously.

• Example: ABO blood grouping in humans, which is controlled by the gene $I$.
• The gene $I$ has three alleles: $I^A$, $I^B$, and $i$.
• Alleles $I^A$ and $I^B$ produce slightly different sugar polymers on red blood cells, while $i$ produces none.
• $I^A$ and $I^B$ are completely dominant over $i$. When $I^A$ and $I^B$ are present together, both express their respective sugars, demonstrating co-dominance.
• This system also serves as an example of multiple alleles, where more than two alleles govern a single character within a population.

• Polygenic Inheritance: Traits controlled by three or more genes, where the phenotype reflects the additive influence of each allele and can be affected by environmental factors (e.g., human skin color and height).
• Pleiotropy: A single gene influences multiple, seemingly unrelated phenotypic traits. This is often due to the gene's effect on metabolic pathways (e.g., Phenylketonuria, where a single gene mutation leads to mental retardation and reduced pigmentation).

Proposed independently by Walter Sutton and Theodore Boveri in 1902, this theory unites Mendelian laws with the behavior of chromosomes during cell division.

• Chromosomes and genes both occur in pairs, and alleles are located on homologous sites of homologous chromosomes.
• The pairing and separation of homologous chromosomes during meiosis lead to the segregation of the genetic factors they carry.

Thomas Hunt Morgan experimentally verified the chromosomal theory using the fruit fly (Drosophila melanogaster).

• Why Drosophila? They can be grown on simple synthetic media, have a short life cycle (about two weeks), produce large numbers of progeny, show clear sexual dimorphism, and possess easily observable hereditary variations.
• Linkage: The physical association of genes on the same chromosome. Genes located close together on a chromosome tend to be inherited together, deviating from the $9:3:3:1$ dihybrid ratio.
• Recombination: The generation of non-parental gene combinations due to crossing over during meiosis. Tight linkage results in low recombination, while loose linkage results in high recombination.
• Genetic Mapping: Alfred Sturtevant used recombination frequencies between gene pairs as a measure of physical distance to map gene positions on chromosomes.

An alteration in DNA sequences that results in changes to an organism's genotype and phenotype.

• Chromosomal Aberrations: Deletions, insertions, or duplications of large segments of DNA, commonly observed in cancer cells.
• Point Mutation: A change in a single base pair of DNA (e.g., Sickle-cell anemia).
• Mutagens: Physical or chemical agents that induce mutations (e.g., UV radiation).

The study of the inheritance patterns of specific traits or genetic abnormalities across multiple generations of a family, represented as a family tree using standardized symbols.

Disorders primarily determined by an alteration or mutation in a single gene, following Mendelian inheritance patterns.

• Colour Blindness: A sex-linked recessive disorder causing a defect in red or green eye cone cells, leading to an inability to distinguish these colors. It is more common in males because the causative genes reside on the X chromosome.
• Haemophilia: A sex-linked recessive disorder where a crucial blood-clotting protein is defective, leading to non-stop bleeding from minor cuts.
• Sickle-cell Anaemia: An autosomal recessive disorder caused by a point mutation in the $eta$-globin gene, substituting Glutamic acid (Glu) with Valine (Val) at the sixth position of the globin chain. Under low oxygen tension, mutant hemoglobin polymerizes, changing red blood cells from biconcave discs to sickle shapes.
• Phenylketonuria: An autosomal recessive inborn error of metabolism. The affected individual lacks the enzyme phenylalanine hydroxylase, causing phenylalanine to accumulate and convert into phenylpyruvic acid, leading to mental retardation.
• Thalassemia: An autosomal recessive quantitative blood disorder characterized by a reduced rate of synthesis of $eta$ or $eta$ globin chains, resulting in abnormal hemoglobin and anemia.

Disorders caused by the absence, excess, or abnormal arrangement of one or more chromosomes.

• Aneuploidy: The gain or loss of chromosomes due to failure of chromatid segregation during cell division.
• Polyploidy: An increase in a whole set of chromosomes due to failure of cytokinesis after telophase (common in plants).
• Down's Syndrome: Caused by the presence of an extra copy of chromosome 21 (Trisomy 21). Symptoms include short stature, a small round head, a furrowed tongue, a partially open mouth, a broad palm with a characteristic crease, and retarded physical and mental development.
• Klinefelter's Syndrome: Caused by an additional X chromosome in males, resulting in a karyotype of $47, XXY$. Individuals have overall masculine development but express feminine features (such as breast development, or gynaecomastia) and are sterile.
• Turner's Syndrome: Caused by the absence of one X chromosome in females, resulting in a karyotype of $45, XO$. Affected females are sterile with rudimentary ovaries and lack secondary sexual characteristics.