Definitions [5]
Alternative forms of the same gene controlling a pair of contrasting traits are called alleles.
An organism having identical alleles for a character, such as TT or tt, is called homozygous.
An organism having unlike alleles for a character, such as Tt, is called heterozygous.
The trait expressed in a heterozygous condition is called dominant.
The trait that remains unexpressed in a heterozygous condition but appears in homozygous form is called recessive.
Theorems and Laws [1]
Law of segregation states that, when a pair of allelomorphs are brought together in the hybrid (F1), they remain together in the hybrid without blending but separate complete and pure during gamete formation.
- Each pair of alleles separates during gamete formation, with one going into each gamete.
- No blending occurs; alleles remain pure and distinct.
- Gametes fuse randomly during fertilisation to form a zygote.
or
When hybrid (F₁) forms gametes, the alleles segregate from each other and enter in different gametes.
Key Points
- Gregor Mendel is known as the Father of Genetics; he worked on pea plants (1856–1863).
- He used true-breeding pea plants and studied inheritance using cross-pollination experiments.
- Mendel selected 7 pairs of contrasting traits (e.g., tall/dwarf, round/wrinkled, yellow/green).
- He introduced the concepts of dominant and recessive traits.
- His experiments had a large sample size and statistical analysis, making the results reliable.
- Mendel’s work formed the basic laws of inheritance, explaining how traits pass from parents to offspring.
- His findings were confirmed by repeated experiments across generations.
| Chromosomal Disorder | Chromosomal Abnormality | Karyotype | Major Features | Remarks |
|---|---|---|---|---|
| Down’s Syndrome | Extra copy of chromosome 21 (Trisomy 21) | 47, +21 | Short stature, small round head, furrowed tongue, partially open mouth, broad palm with single crease, mental and physical retardation | Most common autosomal aneuploidy |
| Klinefelter’s Syndrome | Extra X chromosome in males | 47, XXY | Masculine body with feminine traits (gynecomastia), underdeveloped testes, sterile | Sex-chromosome aneuploidy |
| Turner’s Syndrome | Absence of one X chromosome | 45, XO | Short stature, rudimentary ovaries, absence of secondary sexual characters, sterile female | Only monosomy compatible with life |
| Aneuploidy | Gain or loss of one or more chromosomes due to non-disjunction | 2n ± 1 | Severe developmental abnormalities | Includes trisomy and monosomy |
| Polyploidy | Increase in whole set of chromosomes due to failure of cytokinesis | 3n, 4n, etc. | Common in plants; increases size and vigor | Rare and usually lethal in animals |
Concepts [11]
- Introduction of Principles of Inheritance and Variation
- Terminology Related to Mendelism
- Mendel's Experiments on Inheritance
- Extensions of Mendelian Genetics (Deviation from Mendelism)
- Intragenic Interactions - Dominance
- Historical Development of Chromosome Theory
- Comparison Between Gene and Chromosome Behaviour
- Mendel's Laws > The Law of Segregation (Law of Purity of Gametes)
- Sex Determination in Some Insects
- Mendelian Genetics
- Chromosomal Disorders or Abnormalities
