HEREDITY Heredity is a biological process through which parents pass their characteristics to their children. Every living organism, whether a plant, animal, or human, inherits traits from its previous generations. It explains why we share similarities with our family — such as facial features, height, voice, or even certain habits. Heredity is controlled by genes, which are tiny units of information present inside our cells. Because of heredity, the continuity of characteristics is maintained in a species, meaning that the next generation shares many common characteristics with the previous one. VARIATION The process of heredity also brings variations, so even though we resemble our parents, we are not exact copies of them. Variation means the differences in traits or characteristics among individuals of the same species. *Importance of Variation Variations are essential because they: TRAITS Traits are the specific characteristics or features of an organism. They decide how an organism looks, behaves, or functions. Each trait is determined by a pair of genes. Examples: Eye colour, height, hair type, skin colour, blood group, etc. *Difference Between Inherited Traits and Acquired Traits Basis Inherited Traits Acquired Traits Meaning Traits that are passed from parents to offspring through genes. Traits that an organism develops during its lifetime due to environment or experience. Cause Caused by genetic information (DNA). Caused by environment, habits, lifestyle, training, etc. Present From Birth Yes, inherited traits are present from birth. No, they develop after birth. Transfer to Next Generation Yes, inherited traits can be passed to offspring. No, acquired traits are not passed to offspring. Examples Eye colour, hair type, height tendency, blood group. Learning to swim, bodybuilding, scars, tanning of skin. Role in Evolution Play a major role in evolution as they are heritable. Play little or no role in evolution because they aren’t inherited. Based On Genes and alleles. Environment and personal experience. Controlled By Genes on chromosomes. External factors and lifestyle. SOME IMPORTANT TERMS DNA DNA stands for Deoxyribonucleic Acid. DNA is the chemical substance present in the nucleus of every cell. It is a double stranded thread like structure. DNA carries all genetic information that decides the traits of an organism. DNA is made up of – Sugar, Phosphate, and Nitrogenous Bases. It passes from one generation to another. Chromatin and Chromosomes When DNA is in scattered form – this structure is called Chromatin. At the time of Cell Division, Chromatin get condensed. This structure of DNA is called Chromosomes. Basis Chromatin Chromosomes Definition Long, thin, thread-like structure made of DNA and proteins. Condensed, thick, rod-like structure formed from chromatin. Appearance Looks like loose threads. Looks like thick rods. Visibility Not visible clearly under a microscope. Clearly visible during cell division. State of Cell Present in non-dividing cells. Present during cell division (mitosis/meiosis). Structure Uncoiled and extended. Highly coiled and condensed. Function Allows gene expression, DNA replication, and packaging. Ensures accurate distribution of DNA to daughter cells. Composition DNA + proteins (histones). Tightly packed chromatin fibres. Genes Genes are present but loosely arranged. Genes are arranged in a fixed order along chromosome arms. *Chromosome Number – Chromosome number helps in maintaining genetic stability across generations. – Humans have 46 chromosomes (arranged in 23 pairs) – Out of these: – All Cells of Human body are Diploid except Gametes. – Gametes (sperm cell and egg cell) are Haploid Cells. HAPLOID AND DIPLOID A living organism’s cells have a set number of chromosomes. This number determines whether a cell is haploid (n) or diploid (2n). *Haploid *Diploid GENES A gene is a unit of heredity composed of DNA that regulates the expression of a specific trait in an organism. Genes are hereditary units on chromosomes that contain coded information (DNA instructions) for protein production, which determines an organism’s traits and characteristics. In simple words, Genes are small segments of DNA that carry information for a specific trait and are passed from parents to their children. A gene is basically a set of instructions that tells the body how to acquire a particular trait (such as eye color, height, hair type, and so on). Genes are found inside the nucleus of every cell and sit on chromosomes in a fixed, specific position called a locus. Humans have 20,000 to 25,000 genes approximately. These are spread across 23 pairs of chromosomes. ALLELES In genetics, every trait in our body is controlled by two types of a gene called alleles—one inherited from the mother and the other from the father. Allele is an alternative version or form of a gene. These are different variants of a gene that share the same location (locus) on homologous chromosomes. They are paired versions of a gene inherited from parents, with each allele carrying slightly different information, resulting in the formation of distinct features such as eye color, height, or hair type. We have alleles because we inherit one chromosome from our mother and one from our father, and the genes on them are also paired. Genes are the traits, whereas Alleles are Variations of the trait. Difference Between Genes and Alleles Basis Genes Alleles Meaning A gene is a unit of heredity that controls a specific trait. Alleles are different forms or versions of the same gene. Role Determines a particular character (like eye colour). Determines the different expressions of that character (brown/blue eyes). Count Each trait has one gene. Each gene exists in two allelic forms (one from each parent). Location Found at a fixed position on chromosomes. Found at the same position (locus) on homologous chromosomes. Examples Gene for eye colour, gene for height. B & b (brown and blue eye alleles), T & t (tall and short alleles). Nature General information-carrying unit. Specific variations of that information. Function Controls basic development of traits. Controls which form of the trait will be expressed. Dominant Allele Dominant Allele is the stronger one from the two Alleles. It is the stronger form of a gene that expresses its trait even if only one copy is present. It is always written…
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