Revision: Basic Biology >> Genetics: Some Basic Fundamentals Biology (English Medium) ICSE Class 10 CISCE

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.

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

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

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

or

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 organised into discrete units called chromosomes.

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 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.

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

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

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

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.

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

Sex chromosomes (also called allosomes) are the kind of chromosomes that determine the sex of an organism. Every human has only l pair of sex chromosomes.

Autosomes are the kind of chromosomes which determine general body features like complexion, height, seed colour, etc. Humans have 22 pairs of autosomes.

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.

Every gene has two alternative forms for a character producing different effects. These alternative forms are called the alleles.

The kind of allele which expresses itself regardless of the presence of another allele/s for a given gene. e.g., The dominant allele for height in garden pea is tall. 

The kind of allele whose expression is suppressed in presence of a dominant allele for a given gene. A recessive allele can only express itself in presence of the same recessive allele. e.g. The recessive allele for height in garden pea is dwarf.

The expressed shown character is called phenotype (phene: to show). 

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The observable characteristics which are genetically controlled.

The genetic constitutions (pertaining to the kinds of genes possessed) are called genotype.

or

The set of genes present in the cells of an organism.

Sex-linked inheritance is the appearance of a trait which is due to the presence of an allele exclusively either on the X chromosome or on the Y chromosome.

Inheritance of X-linked genes as in colour blindness and haemophilia is also called criss-cross inheritance.

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

• 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.

• 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.

1. Alleles are alternative forms of a gene.
2. One allele is dominant, the other is recessive.
3. Dominant allele expresses even if single (e.g., tall pea = "T").
4. Recessive allele expresses only if both are recessive (e.g., dwarf = "tt").
5. Homozygous = same alleles (AA/aa); Heterozygous = different alleles (Aa).