Notes
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Topics to be learn :
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Introduction :
Heredity : Heredity is defined as the transmission of genetic characters from parents to offspring.
- Offspring are always identical to their parents but one individual is always different from other.
- Variation is the degree by which offsprings differ from their parents.
- The science which deals with heredity and variations is Genetics.
- Variations result in organisms belonging to a species being different from each other.
Accumulation of variation during reproduction :
Survival of the fittest : Some of the variations may be helpful to the organisms in the prevailing natural conditions and thus the organisms having such positive variations would survive. Whereas organisms with variations not suited to the prevailing natural conditions may not survive. This concept is known as the “survival of the fittest”.
- Depending upon the nature of variations different individuals would have different kinds of advantages. Environmental factors act on these variants and the variant most suited to the existing environment would survive.
- For example, Bacteria variants which can tolerate heat will survive better in a heat wave than variant which cannot withstand heat wave. It proves that creation of variations in a species promote survival.
Variations in asexually reproducing organisms :
- In asexually reproducing organisms a single individual reproduces and variations occur due to errors in DNA copying at the time of nuclear division.
Example :
- One bacterium divides, it will give rise to two bacteria.
- These daughter bacteria would be similar in body design, but with minor differences (see below fig)
- The resultant bacteria divide again, and each bacterium will give rise to two bacteria in the next generation i.e., there would be four individuals as shown in Fig. (iii).
- In the bottom row i.e., Fig. (iii) the four individuals will be different from each other.
- They may have differences of two types (a) some of the differences are of unique type and (b) other differences are inherited from their respective parents, who were different from each other.
Heredity :
The process by which traits and characteristics are reliably inherited or transmitted from the parents to the offsprings is called heredity.
Gene : Gene is the unit of inheritance and contains information that is required to express a particular trait in an organism.
- It is a functional segment of DNA (Deoxyribonucleic acid) on a chromosome occupying specific position which carries out a specific biological function.
Inherited Traits :
- The traits which are inherited from the parents (father and mother) to the offsprings (progeny) are called inherited traits.
- These traits are due to genetic make up of the progeny.
- An individual's life experiences are stored in the somatic cells of the body. That is, in tissues that cannot be handed down to future generations. As a result, they cannot directly contribute to evolution.
- Traits must be transmitted down across generations and involved in direct evolution if they are to be inherited. DNA features are handed on to offspring and can also have a direct influence in evolution.
Acquired traits: The traits which are acquired by an organism during its lifetime are called acquired traits. These traits are not inherited as these traits develop as a result of changes in the non-reproductive tissues of the organism. Only the variations in the reproductive cells can be passed on to the next generation,
Examples of acquired and inherited traits:
- Traits like eye colour or height cannot be acquired or modified during a person's lifetime since they are determined by genes that a person gets from his or her parents. As a result, these features are passed down from one generation to the next through genetic inheritance.
- The ability to swim, read, lift heavy weight are developed over the course of a lifetime with practise. Such acquired traits do not alter germ cell DNA. Since they affect tissues not used for reproduction, they cannot be passed on to offspring. They are not inherited genetically.
Genetics : Genetics is the science of heredity and variations which includes study of the transmission of traits from parents to the offsprings and the occurrence of differences among the individuals.
Genes control traits: A gene codes for the formation of a particular protein controlling a specific characteristic of the organism. For example, in pea, the gene ‘T’ is responsible for the tallness of the plant. This gene ‘T’ gives instructions to the plant cells to make a lot of plant growth hormones. Due to formation of excess plant growth hormones, the plant will show much growth and so becomes tall. Thus, “genes control traits”.
Gene → Hormone (Protein) \(\underrightarrow{control\,of\,trait}\) Trait
Contribution of Mendel: Mendel did his experiments on garden pea (Pisum sativum) and discovered the scientific principles, which govern patterns of inheritance i.e., the principles of inheritance. He explained that contrasting visible characters are controlled by units which he called ‘Factors’. Today, these factors are called ‘genes’. Mendel is known as “Father of Genetics”.
Rules for the Inheritance of Traits – Mendel’s Contributions :
Mendel’s laws of inheritance are:
Law of Paired Characters or factors : Each diploid organism (e g man pea etc.) possess two factors or genes of each character. This is called law of paired haracters or factors.
Law of Dominance: When two homozygous individuals with one or more sets of
contrasting characters are crossed the characters that express in the F1 hybrids are dominant characters and the others which do not show their effects are recessive characters.
Law of Segregation (Mendel’s First Law): Contrasting characters brought together in a hybrid remain together without being blended and when gametes are formed from the hybrid, the two traits or characters separate out from each other and only one enters each gamete. It is also known as law of purity of gametes by some workers.
Law of Independent Assortment (Mendel’s Second Law): In inheritance of more than one pair of contrasting characters simultaneously, the factors for each pair of characters assort or separate independently of other pair/s and they get randomly rearranged in the offsprings.
Mendel’s experiments :
Mendel studied 7 pairs of contrasting characters in garden pea plant. These are
| Characters | Dominant factor | Recessive factor |
| 1-Plant height
2-Flower colour 3-Flower position 4-Seed colour 5-Seed texture 6-Pod colour 7-Pod texture |
Tall (T)
Coloured (C) Axillary (A) Yellow (Y) Round (R) Green (G) Inflated (I) |
Dwarf (t)
White (c) Terminal (a) Green (y) Wrinkled (r) Yellow (g) Constricted (i) |
Mendel selected garden pea for his experiments due to the following reasons:
- Garden pea flowers are normally self-pollinated but can be easily cross-pollinated.
- Many varieties with distinguished contrasting characters e.g., smooth seed coat, wrinkled seed coat are available.
- Pure breeding varieties of pea are available.
- Its flowers can be easily handled for experimentation.
- Many seeds are produced per pea plant.
- It is very easy to grow pea plant.
- It is an annual plant and results of experiment can be obtained in a year’s time span.
- No aftercare of pea plant is needed except at the time of pollination.
Mendel’s Experiment and conclusion :
(i) Mendel took Dihybrid (two pairs of alternative expression of two traits) of a pea plant and crossed them.
- The F1 progeny showed only the dominant characteristics among each pair.
- The F2 progeny had phenotypes similar to parents but also new phenotypes that did not exist in the parents.
- This indicates that pair of alternate characteristics behave independently of the other pair and are thus inherited independent of each other.
(ii) In the below figure, a monohybrid cross is depicted. Tallness is represented by allele T and dwarfness by allele t. Pure tall plant is represented by TT and pure dwarf pea plant is represented by tt.
- Cross pollination between. these two plants result in F1 All individuals of F1 generation hybrid tall plants with genotype Tt.
- When Mendel crossed pollinated pure tall pea plants with pure dwarf pea plant, he got tall plants in F1 generation.
- When Mendel self pollinated F1 tall plants, he got both tall and dwarf plants in the ratio of 3 : 1.
- This shows that dwarf character was present in F1 tall plant but it was not expressed and only trait of tallness was expressed.
- From this we conclude that traits may be dominant or recessive.
- The trait which gets expressed in the presence of its contrasting form is called dominant trait and the trait which remains unexpressed in the presence of its contrasting form is called recessive trait.
(iii) A Mendelian experiment consisted of breeding pea plants bearing violet flowers with pea plants bearing white flowers.
According to Mendel’s experiment, Monohybrid cross between :
All plants are bearing violet flowers, in F1 generation, because as per Mendel’s rule or Law of “Law of dominance” all the offsprings in F1 generation are having dominant traits.
How do these Traits get Expressed? :
- A trait in an organism results from the action of protein or proteins which is manufactured by a gene. Each protein is made by information from the gene/s for that protein.
- For example in pea plant, there are two genes for plant height—‘T’ and ‘t’. ‘T’ is dominant over ‘t’. In a heterozygous plant (Tt), only the gene ‘T’ will be able to make proteins as it is dominant over the ‘t’ gene. This protein would be for the growth hormone which will result in the plant being tall
Mendel’s Experiment : Below is the experiment carried out by Mendel.
He is carried out this experiment to study inheritance of two traits in garden pea, to show independent inheritance of traits or to prove law of independent assortment.
Terms :
F1 and F2 generations : When two parents cross to produce progeny, then their progeny is galled first filial generation or F1 generation. When the first generation progeny cross among themselves or selfed to produce second progeny then this progeny is called second filial generation or F2 generation.
Mendelian Factor or Gene: Mendelian factor or gene is a unit of inheritance and contains information that is required to express a particular trait in an organism. It consists of a linear segment of DNA or chromosome and controls the expression of a character usually along with another factor present at the same locus on the homologous chromosome.
Alleles (Allelomorphs): Alleles are genes which code for a pair of contrasting traits. They are mendelian factors which occur at the same locus on the homologous chromosomes and control the expression of the same character. The term ‘alleles’ was formerly used for only those factors which represent the alternate forms of a character, e.g., Y (yellow) and y (green) for seed colour in Pea.
Homozygote (= Homozygous): It is an organism which possesses similar or identical factors of a character (e.g., TT for tallness or tt for dwarfness). Homozygous individual is also called pure for that character since it produces an offspring similar to it in case of self-fertilization or self-breeding.
Heterozygote (= Heterozygous): It is an organism which has two different or contrasting factors or alleles of a character. Heterozygote or heterozygous individual is also called hybrid, e.g., hybrid tall pea plant (Tt). It is not pure. It does not breed true on self breeding.
Genotype: It is the genetic constitution or gene complement of an individual with
regard to one or more characters irrespective of whether the genes are expressed or not. Thus genotype for pure tall is TT, for hybrid tall Tt and for dwarf tt.
Phenotype: It is the observable structural or functional characteristic of an individual with regard to one or more characters. It is the external manifestation of a genotype.
Offspring : Offspring is an individual formed as a result of sexual reproduction involving the formation and fusion of two gametes. The genotype of an offspring is different from either of the parents due to segregation of chromosomes and their genes.
Reciprocal crosses : Ans. Reciprocal crosses are two types of crosses involving two groups of individuals where the male of one group is crossed with the female of the other and vice versa.
Monohybrid cross: Monohybrid cross is that cross which is made to study the inheritance of a single pair of genes or factors of a character.
Dihybrid cross: It is a cross which is made to study the inheritance of two pairs of genes or characters.
Monohybrid ratio: It is the ratio which is obtained in the F2 generation when a monohybrid cross is studied. It is usually 3 : 1 (phenotypic ratio) or 1 : 2 : 1 (genotypic ratio).
Dihybrid ratio: It is the ratio which is obtained in F2 generation when a dihybrid cross is studied. It is usually 9 : 3 : 3 : 1 (phenotypic ratio).
Hybrid: It is an organism derived from a cross between two genetically different organisms. It has dominant traits of both the parents and is used to improve the quality of economically important plants and animals.
Test cross: It is a cross to know whether an individual is homozygous or heterozygous for dominant characters. The organism is crossed with another individual as parent which is recessive for that character. If the organism is homozygous dominant the offspring will be 100% dominant and if it is 50% dominant and 50% recessive, the organism is heterozygous dominant. In dihybrid cross e.g., RrYy, test cross with rryy will give 1 : 1 : 1 : 1 dihybrid ratio.
Back cross: It is a cross made between hybrid and one of its parents in order to increase the number of traits of the parent during plant breeding experiments.
Sex Determination :
Sex determination in animals :
- Different species use very different strategies for determination of sex of newborn individual.
- Some animals rely on environmental cues. Thus, in some animals, the temperature at which the fertilized eggs are kept determines the sex of the newborn (e.g., crocodiles).
- In other animals, such as snails, individuals can change sex, indicating that sex is not genetically determined.
- In human beings sex is determined by the genes inherited from the parents so it is genetically determined.
Sex chromosomes : The chromosomes which determine sex of a person are called sex chromosomes.
- In human, there are two types of sex chromosomes, one is called X chromosome and the other is called Y chromosome.
- A male has one X chromosome and one Y chromosome. A female has two X chromosomes.
- If a sperm carrying Y chromosome fertilises an egg which carries X chromosome, then the child born will be a male.
Sex determination of the child in human beings :
- In human, 23 pairs (46) of chromosomes are present in each cell. Out of these 22 pairs of chromosomes which determine the characters other than sex are called autosomes.
- The 23rd pair of chromosomes is called sex chromosome.
- Women have perfect pair of sex chromosomes, both called XX chromosomes. Men have a mismatched pair of chromosomes—one is a normal size X chromosome and a short one called Y chromosome.
- So, women have XX sex chromosomes and men have XY sex chromosomes.
- All children will inherit X chromosome from their mother regardless of whether they are boys or girls. It means that the sex of the children will be determined by what sex chromosome they inherit from their father.
- A child who inherits an X chromosome from father will be a girl, and one who inherits a Y chromosome from father will be a boy.
