Principles of Inheritance and Variation

Mendel's Laws of Inheritance

Gregor Mendel conducted hybridization experiments on garden peas (Pisum sativum) and proposed the fundamental laws of inheritance. His work laid the foundation of modern genetics.

Mendel's Experimental Approach

Mendel selected pea plants because of several contrasting characters, easy cultivation, true-breeding varieties, and controlled self and cross-pollination capabilities.

Key Definitions

Genetics: The branch of biology that deals with the study of heredity and variations.

Heredity: The process by which characters are passed on from parents to progeny.

Variation: The degree by which progeny differ from their parents.

Mendel's Laws

Based on his crosses, Mendel formulated principles that are known as Mendel's Laws of Inheritance.

• Law of Dominance: Characters are controlled by discrete units called factors. Factors occur in pairs. In a dissimilar pair of factors, one member of the pair dominates (dominant) the other (recessive).
• Law of Segregation: The alleles do not show any blending and that both the characters are recovered as such in F2 generation though one of these is not seen at the F1 stage. The factors or alleles of a pair segregate from each other during gamete formation such that a gamete receives only one of the two factors.

Inheritance of One Gene

Monohybrid cross is a cross between two organisms differing in only one trait. It helps in understanding the inheritance of one gene.

Monohybrid Cross and F1 Generation

When Mendel crossed a true-breeding tall pea plant with a true-breeding dwarf pea plant, the F1 generation plants were all tall. When these F1 plants were self-pollinated, the F2 generation yielded both tall and dwarf plants in a 3:1 phenotypic ratio.

Test Cross

To determine the genotype of a tall plant in F2 generation, Mendel crossed the plant with a recessive dwarf plant. This is called a test cross. In a monohybrid test cross, the resulting ratio is 1:1.

Exam-Oriented Notes

• Phenotypic ratio of monohybrid cross in F2: 3:1
• Genotypic ratio of monohybrid cross in F2: 1:2:1 (Pure Tall : Hybrid Tall : Pure Dwarf)
• Test cross ratio: 1:1

Inheritance of Two Genes

A dihybrid cross involves the inheritance of two pairs of contrasting characters simultaneously.

Dihybrid Cross

Mendel crossed pea plants with yellow-round seeds with green-wrinkled seeds. The F1 plants produced yellow-round seeds. On selfing F1, the F2 generation produced four types of combinations in a phenotypic ratio of 9:3:3:1.

Law of Independent Assortment

Law of Independent Assortment: When two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters.

Formula Summary Table

Chromosomal Theory of Inheritance, Linkage and Recombination

Sutton and Boveri argued that the pairing and separation of a pair of chromosomes would lead to the segregation of a pair of factors they carried. This is the Chromosomal Theory of Inheritance.

Linkage and Recombination

Linked genes are located close to each other on the same chromosome and tend to be inherited together during meiosis. Recombination is the generation of non-parental gene combinations due to crossing over between homologous chromosomes during meiosis.

Important Observations

• T.H. Morgan coined the term linkage to describe the physical association of genes on a chromosome.
• Genes with tight linkage show very low recombination frequency, while loosely linked genes show higher recombination frequency.

Sex Determination

Sex determination mechanisms involve specific chromosomes or environmental factors that determine the sex of an offspring.

Mechanisms of Sex Determination

• XX-XY Type: Found in humans and Drosophila. Females have two X chromosomes (XX), and males have an X and a Y chromosome (XY).
• XX-XO Type: Found in insects like grasshoppers. Females have two X chromosomes, while males have only one X chromosome and no Y.
• ZZ-ZW Type: Found in birds. Males have two Z chromosomes (ZZ), and females have a Z and a W chromosome (ZW).

Mutation

Mutation is a phenomenon which results in alteration of DNA sequences and consequently results in changes in the genotype and the phenotype of an organism.

Types of Mutations

• Point Mutation: Change in a single base pair of DNA (e.g., Sickle-cell anemia).
• Chromosomal Mutation: Changes in structure or number of chromosomes.

Genetic Disorders

Genetic disorders are broadly grouped into two categories: Mendelian disorders and Chromosomal disorders.

Mendelian Disorders

Mendelian disorders are mainly determined by alteration or mutation in a single gene. These follow the principles of Mendelian inheritance.

• Hemophilia: A sex-linked recessive disease where a simple cut results in non-stop bleeding.
• Sickle-cell anemia: An autosome-linked recessive trait where mutant hemoglobin molecules undergo polymerization under low oxygen tension causing the change in shape of the red blood cell from biconcave disc to elongated sickle structure.
• Phenylketonuria: An inborn error of metabolism inherited as an autosomal recessive trait. The affected individual lacks an enzyme that converts the amino acid phenylalanine into tyrosine.

Chromosomal Disorders

Chromosomal disorders are caused due to absence or excess or abnormal arrangement of one or more chromosomes.

• Down's Syndrome: Caused by the presence of an additional copy of chromosome 21 (Trisomy 21). Characterized by retarded mental growth, furrowed tongue, and partially open mouth.
• Klinefelter's Syndrome: Caused by the presence of an additional X chromosome resulting in a karyotype of 47, XXY. Such individuals have overall masculine development, however, feminine development (gynaecomastia) is also expressed.
• Turner's Syndrome: Caused by the absence of one of the X chromosomes, i.e., 45 with XO. Such females are sterile as ovaries are rudimentary besides other secondary sexual characters being absent.

Genetic Disorders Comparison Table