Heredity Class 10 Notes PDF

# Introduction - Genetics deals with the study of Heredity and Variation. - The transmission of characters/traits from one generation to the next is called Heredity. - The differences in the characters/traits between the parent and offspring is called Variation. # Types of Variations - Variation...

# Introduction - Genetics deals with the study of Heredity and Variation. - The transmission of characters/traits from one generation to the next is called Heredity. - The differences in the characters/traits between the parent and offspring is called Variation. # Types of Variations - Variation are of two types: - Somatic Variation - Gametic Variation ## Somatic Variation - It takes place in the body cell. - It is neither inherited nor transmitted. - It is also known as acquired traits. - Examples: cutting of tails in dogs, boring of pinna etc. ## Gametic Variation - Takes place in the gametes/Reproductive cells. - Inherited as well as transmitted. - Also known as inherited traits. - Example: human height, skin colour. # Accumulation of Variation during Reproduction - Variation occurs during reproduction whether organisms multiply sexually or asexually. ## Variations in Asexual Reproduction - Variations are fewer. - Occurs due to small inaccuracies in DNA copying. (Mutation) ## Variations in Sexual Reproduction - Variations are large. - Occurs due to crossing over, separation of chromosomes, mutation. # Importance of Variation - Depending upon the nature of variations different individuals would have different kinds of advantage. - Example, Bacteria that can withstand heat will survive better in a heat wave. - Main advantage of variation to species is that it increases the chances of its survival in a changing environment. - Free ear lobes and attached ear lobes are two variants found in human populations. # Mendel and His Work on Inheritance - Gregor Johann Mendel (1822 & 1884) started his experiments on plant breeding and hybridisation. He proposed the laws of inheritance in living organisms. - Mendel was known as Father of Genetics. - Plant selected by Mendel: Pisum sativum (garden pea). He used a number of contrasting characters for garden pea. ## Seven pairs of contrasting characters in Garden Pea | Character Trait | Dominant Trait | Recessive Trait | |---|---|---| | Flower colour | Violet | White | | Flower position | Axial | Terminal | | Seed colour | Yellow | Green | | Seed shape | Round | Wrinkled | | Pod shape | Inflated | Constricted | | Pod colour | Green | Yellow | | Height of plant | Tall | Dwarf/Short | # Mendel's Experimental Material - He chose Garden Pea (Pisum sativum) as his experiment material because of: - Availability of detectable contrasting traits of several characters. - Short life span of the plant. - Normally allows self-fertilisation but cross-fertilisation can also be carried out. - Large no. of seeds produced. - Mendel's Experiments: Mendel conducted a series of experiments in which he crossed the pollinated plants to study one character (at a time). # Mendel's Experiment - Mendel started his experiment on the pea plants. He conducted first monohybrid and then dihybrid crosses. ## Monohybrid Cross - The cross in which Mendel showed inheritance of dominant and recessive characters is monohybrid cross. To observe inheritance of single pair of contrasting characters - Pure Tall Plants (TT) - Pure Dwarf Plants (tt) - Parents - T - t - Gametes - T - t - (Genotype) Tt - F₁generation - All Tall Plants (phenotype) - Self pollination - Gametes (Male/Female) - T - t - T - TT - Tt - t - Tt - tt - he took pure tall (genotype TT) and pure dwarf (genotype tt) pea plants and cross pollinated them to obtain first generation or first filial generation. In this figuration (F1 generation) he obtained only tall plants. This meant that only one of the parental traits was seen, not the mixture of the two. The plants of F generation or progeny are then self pollinated to obtain F2 generation or progeny. Now all plants were not tall. He obtained 75% tall plants and 25% dwarf plants i.e. the phenotypic ratio was 3:1. This indicates that in the F₁ generation both tall and dwarf traits were inherited but tallness expressed it self. Tallness is a dominant trait and dwarfness is a recessive trait. F2 generation has a genotypic ratio of 1 : 2 : 1 of three types of plants represented by TT, Tt and tt as shown in the cross. ## Conclusion: - Phenotypic ratio-Tall : Dwarf 3:1 - Genotype ratio-Pure Tall : Hybrid Tall : Pure Dwarf 1 : 2:1 # Law of Dominance - When parents having pure contrasting characters are crossed then only one character expresses itself in the Ft generation. This character is the dominant character and the character/factor which cannot express itself is called the recessive character. # Dihybrid Cross - Mendel also carried out experiments to observe inheritance of two pairs of contrasting characters, which is called dihybrid cross. He cross breed pea plants bearing round green seed with plants bearing wrinkled and yellow seeds. In the Fx generation he obtained all round and yellow seeds it means round and yellow traits of seeds are dominant features while wrinkled and green are recessive. He self-pollinated the plants of F₁ generation to obtain F2 generation, he obtained four different types of seeds round yellow, round green, wrinkled yellow and wrinkled green in the ratio of 9:3:3:1. He concluded that traits are independently inherited ## Conclusion - Round and yellow seeds-9. - Round and green seeds-3. - Wrinkled and yellow seeds-3. - Wrinkled and green seeds-1. # How do traits get expressed? - Cellular DNA is the information source for making proteins in the cell. - A part of DNA that provides information for one particular protein is called a gene for that protein for example; the height of a plant depends upon the growth hormone which is in turn controlled by the gene. If the gene is efficient and more growth hormone is secreted the plant will grow tall. If the gene for that particular protein gets altered and less of it is secreted when the plant will remain short. - Both the parents contribute equally to the DNA of next generation during sexual reproduction. They actually contribute a copy of the same gene for example; when tall plant is crossed with short plant the gametes will have single gene either for tallness or for shortness. F1 generation will get one gene for tallness and other for shortness also - Round and Green (RRyy) - Wrinkled and Yellow (rrYY) - Parents - Ry - ry - Gametes - Cross Pollination - F1 generation All Round Yellow seeds (RrYy) - Self pollination - Gametes - RY - Ry - rY - ry - RY - RRYY - RRYy - RrYY - RrYy - Ry - RRYy - RRyy - RrYy - Rryy - rY - RrYY - RrYy - rrYY - rrYy - ry - RrYy - Rryy - rrYy - rryy # How do germ cells i.e. gametes get single set of genes from parents who have two copies in them? - Each gene set is present, not as a single long thread of DNA, but as separate independent pieces each called a chromosome. Each cell gets two copies of the chromosome, one from each parent. Each germ cell or gamete has one copy of it because there is reductional division in the sex organs at the time of formation of gametes. When fertilization takes place normal number of chromosomes is restored in the progeny ensuring the stability of DNA of the species. # How is the sex of a newborn individual determined? - It is the process by which sex of a newborn can be determined. - Different species use different strategies for this : - In some animals the temperature at which fertilized eggs are kept determines whether the developing animals will be males or females. - Some animals like snails can change sex indicating that sex is not genetically determined. - In human beings sex of the individual is determined genetically; means genes inherited from the parents decide the sex of the offspring. # Sex determination in human beings: - In human beings, all chromosomes are not paired. 22 chromosomes are paired but one pair called sex chromosome is odd in not having a perfect pair in males. Females have a perfect pair both represented by XX. On the other hand males have a normal sized X but the other is short called Y so it is shown as XY. - All gametes or ova formed by the homogenetic female are similar i.e. have X chromosome. Males heterogenetic form two types of sperms i.e. half with X chromosome and the other half with Y chromosome. Sex of the baby will depend on fertilization. There are two possibilities : - Sex determination in human being (flow chart) - Female (XX) - Male (XY) - Parents - Ovum (X,X) - Sperms (X,Y) - Gametes - Zygote (XX or XY) - Off springs (Female 50% or Male 50%) # Autosomes: - Those chromosomes which do not play any role in sex determination. # Sex chromosomes: - Those chromosomes which play a role in determining sex of the newborn. - If the sperm having X chromosome fertilizes with ovum with X chromosome then the baby will have XX chromosome and it will be female. - If the sperm having Y chromosome fertilizes with ovum with X chromosome then the baby will have XY chromosomes and it will be male. # Factors responsible for Sex Determination - Environmental and Genetic factors are responsible for sex determination. ## Environmental - In some animals, the temperature at which the fertilized eggs are kept decides the gender. Example: Turtle ## Genetic - In some animals like humans gender or individual is determined by a pair of chromosomes called sex chromosome. - XX - Female - XY – Male # Sex Chromosomes - In human beings, there are 23 pairs of chromosome. - Out of these 22 chromosomes pairs are called autosomes and the last pair of chromosome that help in deciding gender of that individual is called sex chromosome. # Some important terms 1. Chromosomes are long thread-like structures present in the nucleus of a cell which contain hereditary information of the cell in the form of genes. 2. DNA is a chemical in the chromosome which carries the traits in a coded form. 3. Gene is the part of a chromosome which controls a specific biological function. 4. Contrasting characters: A pair of visible charactes such as tall and dwarf, white and violet flowers, round and wrinkled seeds, green and yellow seeds etc. 5. Dominant trait: The character which expresses itself in a (Ft) generation is dominant trait. Example : Tallness is a dominant character in pea plant. 6. Recessive trait: The character which does not express itself but is present in a generation is recessive trait. Ex. dwarfism in the pea plant. 7. Homozygous: A condition in which both the genes of same type are present for example; an organism has both the genes for tallness it is expressed as TT and genes for dwarfness are written as tt. 8. Heterozygous: A condition in which both the genes are of different types for example; an organism has genes Tt it means it has a gene for tallness and the other for dwarfness only tall character is expressed. 9. Genotype: It is genetic make up of an individual for example; A pure tall plant is expressed as TT and hybrid tall as Tt. 10. Phenotype: It is external appearance of the organism for example; a plant having Tt composition will appear tall although it has gene for dwarfness. 11. Homologous pair of characters are those in which one member is contributed by the father and the other member by the mother and both have genes for the same character at the same position.