Revision: Inheritance and Variation Biology HSC Science (General) 12th Standard Board Exam Maharashtra State Board

The term heredity may be defined as "transmission of genetically based characteristics from parents to offspring".

or

The transmission of characters from the parents to their offsprings is called heredity. 

GENOME is the full complement of DNA (including all genes and the intergenic regions) of an organism.

Genes are specific sequences of nucleotides on a chromosome that encode particular proteins which are expressed in the form of some particular feature of the body. They are the units of heredity which are transferred from parents to offspring and are responsible for some specific characteristics of the offspring.

Genetics is the study of heredity i.e. transmission of body features (both similarities and differences) from parents to offspring and the laws relating to such transmission. 

Genes are specific sequences of nucleotides on a chromosome, that encode particular proteins which are expressed in the form of some particular feature of the body.

or

GENES are the specific parts (DNA segments) of a chromosome, which determine the hereditary characteristics.

A cross between parents differing in two heritable traits is called a dihybrid cross. e.g., a cross of a pure, tall, round seeded plant with a dwarf, wrinkled-seeded plant.

A test cross is defined as a genetic cross between an individual showing a dominant phenotype with unknown genotype and a homozygous recessive individual to determine the genotype of the dominant individual.

A back cross is defined as a genetic cross between an F₁ hybrid and either of its parental forms (dominant or recessive) to study inheritance of traits.

Homologous chromosomes are chromosome pairs that are similar in length, gene position and centromere location.

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.

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

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. 

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

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

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

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

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

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.

The tendency of the genes on the same chromosome to inherit together is called linkage.

Test cross is a cross between an organism with an unknown genotype and a recessive parent. It is used to determine whether the individual is homozygous or heterozygous for a trait.

The biological mechanism by which the sex (male or female) of an individual is established based on genetic or chromosomal factors, is called sex determination.

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

Multiple alleles are the three or more alternative forms of the same gene that occupy the same locus on homologous chromosomes and control the same character in a population, though only two alleles occur together in an individual.

The degree or strength with which two genes remain associated on the same chromosome during inheritance is called linkage value.

All the genes present on a single chromosome that are inherited together as a unit are called a linkage group.

The tendency of two or more genes located on the same chromosome to be inherited together and not assort independently during inheritance is called linkage.

The condition in which genes located very close together on the same chromosome are inherited together without separation due to absence of crossing over is called complete linkage.

Mutual exchange of blocks of homologous genes between a pair of homologous chromosomes is known as crossing over.

The transmission of characters from parents to offspring through genes present on autosomes is called autosomal inheritance.

• Mendel is the father of modern genetics; he discovered the basic principles of heredity.
• Hugo de Vries (1901) proposed the mutation theory explaining sudden genetic changes.
• Walter Sutton (1902) linked chromosomes with heredity through his study on grasshoppers.
• Avery, McCarty & McLeod (1944) proved DNA is the genetic material in living organisms.
• Jacob & Monod (1961) developed a model of protein synthesis, leading to recombinant DNA technology.

1. Discovered by Walther Flemming in 1882 during his study of cell division in salamanders.
2. Chromatin (DNA + proteins) condenses to form chromosomes during cell division.
3. Chromosomes are ribbon-like, highly coiled structures that become visible when stained.
4. Each chromosome has two identical sister chromatids joined at a point called the centromere.
5. The centromere attaches to spindle fibres and helps separate sister chromatids during cell division.
6. After division, chromatids decondense into chromatin fibres in the nucleus.
7. Chromosomes ensure the equal distribution of genetic material to daughter cells.
8. Each chromosome contains one long DNA molecule that carries hereditary information.

• Down’s syndrome is a chromosomal disorder caused by trisomy of chromosome 21.
• Affected individuals have 47 chromosomes instead of the normal 46 due to non-disjunction.
• It is characterised by mental retardation, distinctive facial features, and short stature.
• Congenital heart defects and low muscle tone are commonly associated features.
• The risk increases with advanced maternal age (above 35 years) and can be detected by amniocentesis.

• Klinefelter’s syndrome is a sex chromosomal disorder caused by the presence of an extra X chromosome, giving the genotype 44 + XXY due to non-disjunction during meiosis.
• Affected individuals are phenotypically male but show feminized features, including gynaecomastia, underdeveloped testes, and absence of spermatogenesis, leading to sterility.
• Common characteristics include tall stature with long arms, harsh-pitched voice, and overall masculine appearance with some feminine traits.

• Turner’s syndrome is a sex chromosomal disorder caused due to non-disjunction, resulting in XO karyotype (44 autosomes + XO).
• Affected individuals are phenotypically female but show poor development of ovaries and breasts, leading to sterility.
• Common features include short stature, webbed neck, broad chest, low posterior hairline, and reduced intelligence.

• Thalassemia is an autosomal recessive inherited blood disorder caused by defective synthesis of haemoglobin chains.
• It occurs due to mutation or deletion of genes controlling globin chains—alpha chains (HBA1, HBA2 on chromosome 16) or beta chain (HBB on chromosome 11).
• Based on the affected chain, thalassemia is classified into alpha-thalassemia and beta-thalassemia.
• It leads to symptoms like anaemia, pale skin, abnormal RBCs, slow growth, and often requires repeated blood transfusions for management.