Genetics (ICSE Class X)

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### Introduction to Genetics - **Genetics:** Study of heredity and variation. - **Heredity:** Transmission of characteristics from parents to offspring. - **Variation:** Differences among individuals of the same species. - **Gregor Johann Mendel:** "Father of Genetics", worked on garden pea (Pisum sativum). ### Basic Genetic Terms - **Gene:** A unit of heredity, a segment of DNA carrying information for a specific trait. - **Allele:** Alternative forms of a gene (e.g., T for tall, t for dwarf). - **Dominant Allele:** An allele that expresses itself even in the presence of an alternative allele (represented by capital letter, e.g., T). - **Recessive Allele:** An allele that can only express itself in the absence of a dominant allele (represented by small letter, e.g., t). - **Homozygous:** Having two identical alleles for a particular trait (e.g., TT or tt). Also called pure breed. - **Heterozygous:** Having two different alleles for a particular trait (e.g., Tt). Also called hybrid. - **Genotype:** The genetic constitution of an organism (e.g., TT, Tt, tt). - **Phenotype:** The observable characteristic of an organism (e.g., Tall, Dwarf). - **F1 Generation (First Filial):** Offspring produced from a cross between parental generation. - **F2 Generation (Second Filial):** Offspring produced from a cross between F1 generation individuals. - **Monohybrid Cross:** A cross involving a single pair of contrasting characters. - **Dihybrid Cross:** A cross involving two pairs of contrasting characters. - **Punnett Square:** A graphical representation to calculate the probability of all possible genotypes and phenotypes of offspring in a genetic cross. ### Mendel's Laws of Inheritance #### 1. Law of Dominance - In a cross between two pure breeding parents with contrasting traits, only one form of the trait (dominant) appears in the F1 generation. The other form (recessive) remains hidden. - **Example (Monohybrid Cross - Tall vs Dwarf Pea Plants):** - Parents: TT (Tall) x tt (Dwarf) - Gametes: T, t - F1 Generation: Tt (All Tall) #### 2. Law of Segregation (Law of Purity of Gametes) - During gamete formation, the two alleles for a trait separate or segregate from each other such that each gamete receives only one allele. - These alleles do not blend or contaminate each other. - **Example (F1 self-pollination from above):** - F1 x F1: Tt x Tt - Gametes: T, t (from each parent) - F2 Generation (Phenotypic Ratio): 3 Tall : 1 Dwarf - F2 Generation (Genotypic Ratio): 1 TT : 2 Tt : 1 tt #### 3. Law of Independent Assortment - When two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the segregation of the other pair of characters. - **Example (Dihybrid Cross - Round Yellow (RRYY) x Wrinkled Green (rryy)):** - Parents: RRYY x rryy - F1 Generation: RrYy (All Round Yellow) - F2 Generation (Phenotypic Ratio): 9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green ### Sex Determination in Humans - Humans have 23 pairs of chromosomes. - **Autosomes:** 22 pairs of non-sex chromosomes. - **Sex Chromosomes:** 1 pair (23rd pair) that determines sex. - **Females:** XX (Homogametic) - Produce only X gametes. - **Males:** XY (Heterogametic) - Produce X and Y gametes in equal proportion. - **Mechanism:** - If an X-carrying sperm fertilizes an egg (X), the offspring is XX (female). - If a Y-carrying sperm fertilizes an egg (X), the offspring is XY (male). - The father determines the sex of the child. ### Genetic Disorders - Diseases caused by abnormalities in an individual's DNA. - Can be inherited or result from new mutations. #### 1. Colour Blindness (Red-Green) - **Nature:** X-linked recessive disorder. - **Cause:** Defect in genes on the X chromosome responsible for producing red or green cone cells in the retina. - **Inheritance:** - Males (XY) are more commonly affected as they have only one X chromosome. If it carries the defective gene, they are colorblind. - Females (XX) are usually carriers; they need two defective X chromosomes to be colorblind, which is rare. - **Symptoms:** Inability to distinguish between red and green colors. #### 2. Thalassemia - **Nature:** Autosomal recessive blood disorder. - **Cause:** Defect in genes responsible for the production of globin chains of hemoglobin, leading to abnormal hemoglobin. - **Symptoms:** Anemia, fatigue, pale skin, bone deformities. - **Inheritance:** Both parents must be carriers for a child to inherit the disorder. #### 3. Haemophilia - **Nature:** X-linked recessive disorder. - **Cause:** Defect in genes coding for blood clotting factors (Factor VIII or Factor IX). - **Symptoms:** Prolonged bleeding from minor cuts, internal bleeding, joint damage. - **Inheritance:** Similar to colour blindness, males are predominantly affected, while females are typically carriers.