Revision: Heredity and Variation Science SSC (English Medium) 9th Standard Maharashtra State Board

The nucleus contains most of the cell's DNA, which is organised into discrete units called chromosomes.

After duplication, a chromosome has two identical sections. A chromosome is formed during cell division by the union of two chromatids.

Aneuploidy is the addition or deletion of one or two chromosomes in a diploid chromosomal pair.

Aneuploidy refers to the chromosomal variation due to a loss or a gain of one or more chromosomes deviating from the normal genome number for that species due to nondisjunction of the homologous chromosome.

One vertical half of a duplicated chromosome is called a chromatid.

A pair of corresponding chromosomes of the same shape and size, one obtained from each parent.

Each chromosome in its condensed form as visible during the start of cell division, consists of two sister chromatids joined at some point along the length. This point of attachment is called centromere, and it appears as a small constricted region.

Two identical chromatids that are joined by a centromere are called sister chromatids which eventually get separated during anaphase. 

The nucleus contains most of the cell's DNA which is organized into discrete units called chromosomes.

or

Chromosomes are highly coiled, ribbon-like structures formed by the condensation of chromatin fibres during cell division. 

Each chromosome contains one long DNA molecule associated with many proteins. This complex of DNA and proteins is called the chromatin.

Mendelism refers to the principles of inheritance proposed by Gregor Mendel based on his experiments with pea plants.​ These principles explain that traits are inherited in a predictable manner through discrete hereditary units.

Mendel's first experiments were with the varieties of garden pea that differed in only one visible character. These are known as monohybrid experiments.

• Mendel investigated not only those crosses in which the parent differed in a single pair of characters but also others in which the parent differed in two pairs. Such a cross, which involves two pairs of contrasting characters simultaneously, is called a dihybrid cross.
• A genetic cross involving two pairs of contrasting characters simultaneously is called a dihybrid cross.

Non-disjunction occurs when chromosomes fail to split during cell division, resulting in aberrant chromosomal combinations.

• Chromosomes are thread-like (filamentous) structures present in the nucleus, made of DNA and proteins, and carry genetic information (genes). They were first observed by Walther Flemming (1882).
• During cell division, chromatin condenses to form visible chromosomes, ensuring equal distribution of genetic material to daughter cells.
• Each chromosome consists of two sister chromatids joined at a centromere, which has a kinetochore for attachment to spindle fibres.
• Important parts include telomeres (protect chromosome ends), chromonemata (coiled DNA fibres), and satellites (small segments).
• Chromosome number is constant for each species and is best observed at the metaphase stage of cell division.
• Ploidy refers to the number of chromosome sets: euploidy (exact multiples like haploid, diploid, polyploid) and aneuploidy (abnormal numbers like monosomy and trisomy).
• Chromosomes play a key role in heredity, gene expression, and maintaining genetic stability across generations.

• Chromosomes are classified into four types based on centromere position: metacentric, sub-metacentric, acrocentric, and telocentric.
• Metacentric chromosomes have a centromere in the middle, forming a V-shape with equal arms.
• Sub-metacentric chromosomes have a centromere slightly off-centre, forming an L-shape with unequal arms.
• Acrocentric chromosomes have a centromere near one end, forming a J-shape with one very short arm.
• Telocentric chromosomes have a centromere at the end, forming an I-shape with only one arm.
• Chromosomes occur in pairs: homologous (similar) and heterologous (dissimilar); sex chromosomes determine sex, while others are autosomes.

• DNA was established as the primary genetic material and formally modelled as a double helix by Watson and Crick in 1953.
• The structural blueprint relied heavily on Erwin Chargaff’s chemical base equivalence rules and Rosalind Franklin’s X-ray diffraction data.
• The fundamental building block of DNA is a nucleotide, which comprises a five-carbon deoxyribose sugar, a phosphate group, and a nitrogenous base.
• A nucleotide is distinct from a nucleoside, as a nucleoside contains only the nitrogenous base and pentose sugar without the attached phosphate group.
• The double helix is formed by two antiparallel polynucleotide chains running in opposite directions (5′→3′ and 3′→5′) coiled in a clockwise, right-handed fashion.
• The physical architecture places the hydrophilic sugar-phosphate backbone on the exterior, while the information-carrying nitrogenous bases stack flat on the interior.
• Structural stability is maintained horizontally by complementary base pairing (A = T via two hydrogen bonds; G = C via three hydrogen bonds) and vertically by strong covalent phosphodiester bonds.

• RNA is a nucleic acid that is usually single-stranded and made of ribonucleotides linked by phosphodiester bonds.
• RNA contains ribose sugar and the bases adenine, guanine, cytosine, and uracil, with uracil present instead of thymine.
• The backbone of RNA consists of alternating ribose and phosphate groups, forming a flexible single-stranded chain.
• RNA plays a central role in gene expression by acting as an intermediate between DNA and proteins.
• Major types of RNA are mRNA, tRNA, and rRNA, each with specific roles in protein synthesis.

• Gregor Johann Mendel (1822–1884), an Austrian monk, is known as the Father of Genetics for his pioneering work on heredity.
• He studied science and mathematics at the University of Vienna, which helped him apply a quantitative approach to biological problems.
• Mendel conducted systematic hybridization experiments on garden pea (Pisum sativum) from 1856 to 1863.
• From these experiments, he formulated the fundamental Laws of Inheritance, explaining how traits are transmitted across generations.
• Although his work was ignored during his lifetime, it was rediscovered in 1900, leading to widespread recognition and the foundation of modern genetics.

• Meaning: Genetic disorders are diseases caused by abnormalities in genes or chromosomes.
• Types: They are broadly classified into Mendelian disorders and chromosomal disorders.
• Mendelian Disorders: Caused by a mutation in a single gene; examples include thalassemia, sickle-cell anaemia, colour blindness, haemophilia, and phenylketonuria.
• Chromosomal Disorders: Caused by the absence or excess of chromosomes or structural abnormalities; examples include Down syndrome, Turner’s syndrome, and Klinefelter’s syndrome.
• Examples of Effects: Down’s syndrome causes mental retardation; Turner’s syndrome leads to sterile females; Klinefelter’s syndrome causes sterility in males; thalassemia affects haemoglobin production.