CBSE Class 10 Science Heredity And Evolution Notes Set 06

Heredity

TOPICS

• Accumulation of Variation During Reproduction
• Heredity

QUICK RECAP

Genetics : The science primarily concerned with precise understanding of biological properties which are transmitted from parent to offspring is called genetics. The term ‘genetics’ was coined by W. Bateson in 1905.

Heredity : A recognisable feature of a human being (or any other organism) like height, complexion, shape of hair, colour of eyes and shape of nose and chin, etc., is called ‘character’ or ‘trait’.

• The transmission of characters (or traits) from the parents to their offspring is called heredity. In most simple terms, heredity means continuity of features from one generation to the next.
• The hereditary information is present in the sex cells (or gametes) of the parents. Thus, gametes constitute the link between one generation and the next, and pass on the paternal (father’s) and maternal (mother’s) characters or traits to the offspring.

Variations : The offspring are never a true copy of their parents. In fact, no two individuals are exactly alike and the members of any one species differ from one another in some characters (or traits) or the other. These differences are known as variations, i.e., the differences in the characters (or traits) among the individuals of a species is called variation.

Accumulation of variations

Variations appear during reproduction whether asexual or sexual.

• Minor variations may arise during asexual reproduction due to small inaccuracies in DNA copying.
• Sexual reproduction generates even greater diversity. This is so because sexual reproduction involves two parents (father and mother) and every offspring receives some characters of father and some of mother. Different offsprings receive different combinations of characters of their parents and show distinct variations among themselves as well as from their parents.

During sexual reproduction, variations arise due to -

• Chance separation of chromosomes during gamete formation (gametogenesis)
• Crossing over during meiosis
• Chance coming together of chromosomes during fertilisation
• Mutations, i.e., alterations in the genetic material

• The significance of a variation shows up only if it continues to be inherited by the offspring for several generations.
• The great advantage of variation to a species is that it increases the chances of its survival in a changing environment.

Characters that are seen in an individual (e.g., height, facial features, body colour, etc.) are called traits. They are of two types:

• Inherited traits : Traits that are transferred from one generation to another are called inherited traits. These traits are determined by germplasm or, specifically by genes of an individual (e.g., body colour, eye colour, etc.).
• Acquired traits : These traits are simply acquired during the lifetime of an individual and are not transferred to future generations as they have not affected genes (e.g., loss of an organ in an accident, experiences gained in life).

Gene as unit of heredity

Chromosome is a thread-like structure in the nucleus of a cell and is formed of DNA which carries the genes.

• A gene is a unit of DNA on a chromosome which governs the synthesis of one protein that controls a specific characteristic (or trait) of an organism. However Mendel called them factor. Genes work in pairs and are represented by letters. Genes controlling the same characteristics are given the same letters. For example, the gene for tallness is represented by the letter T whereas the gene for dwarfness is represented by the letter t.
• Genes for controlling the same characteristic of an organism can be of two types : dominant or recessive. The gene which decides the appearance of an organism even in the presence of an alternative gene is known as dominant gene and the dominant gene is represented by a capital letter. The gene which can decide the appearance of an organism only in the presence of another identical gene is called as recessive gene. The corresponding recessive gene is represented by the corresponding small letter. Genotype is the description of genes present in an organism.

Mendel’s experiments

Mendel selected garden pea (Pisum sativum) for series of hybridisation experiments because it has some special features.

Special features of garden pea plant are:

• It is easy to grow.
• It has a short life cycle and therefore, it is possible to study number of generations in less time.
• Garden pea plant has distinct, easily detectable contrasting variants of features. Mendel, in fact, noted seven pairs of such contrasting characters in garden pea plant.
• The plant has bisexual flowers wherein artificial cross fertilisation could be easily achieved.
• Each pea plant produces many seeds in one generation.

The characters which always appear in two opposing conditions are called contrasting characters. There were seven contrasting characters noted by Mendel in garden pea. These are listed in the given table:

1. Plant height: Tall (Dominant), Dwarf (Recessive)
2. Position of flower on the stem: Axial (Dominant), Terminal (Recessive)
3. Colour of unripe pod: Green (Dominant), Yellow (Recessive)
4. Shape of pod: Inflated (Dominant), Constricted (Recessive)
5. Shape of seed: Round (smooth) (Dominant), Wrinkled (Recessive)
6. Colour of seed: Yellow (Dominant), Green (Recessive)
7. Colour of flower: Violet (Dominant), White (Recessive)

Mendel conducted breeding experiments in three steps:

• Selection of pure parent plants (i.e., plants producing similar traits in every generation).
• Production of first generation of plants by cross breeding (hybridisation).
• Raising of second and subsequent generations by self-fertilisation of hybrids.

• Homozygous organism: An organism which contains identical alleles of a character on homologous chromosomes is said to be homozygous or genetically pure for that character. Homozygous organisms give rise to offsprings having the same traits on self breeding. For example, pea plants with TT or tt alleles are homozygous for height.
• Heterozygous organism: An organism which contains two different alleles for a character on its homologous chromosomes is called heterozygous or hybrid for that particular character. These organisms on self breeding produce offsprings in the ratio of \( 3 : 1 \). For example, pea plants with alleles Tt are heterozygous for height.
• Genotype: is the genetic constitution of an organism, e.g., genotype of a hybrid tall is Tt, pure tall is TT and dwarf is tt with regard to the character of height.
• Phenotype: Phenotype of an individual refers to the expressed or observable, structural and functional traits produced by the interaction of genes and environment, for example, height of a plant, colour of a flower, colour and shape of a seed, etc.

Monohybrid inheritance and law of segregation

A breeding experiment dealing with a single character is called monohybrid cross.

• Mendel first crossed pure-bred tall pea plants with pure-bred dwarf pea plants and found that only tall pea plants were produced in the first generation or \( F_1 \) generation.
• Mendel then crossed the tall pea plants of the first generation (\( F_1 \) generation) and found that tall plants and dwarf plants were obtained in the second generation (or \( F_2 \) generation) in the ratio of \( 3 : 1 \) (monohybrid ratio).

• Mendel said that the trait of dwarfness of one of the parent pea plant had not been lost, it was merely concealed or suppressed in the first generation to re-emerge in the second generation. He called the repressed trait of dwarfness as recessive trait and expressed trait of tallness as the dominant trait.
• This phenomenon of appearance of only one of two contrasting traits in \( F_1 \) generation, is termed as dominance.
• Mendel also noted that all the pea plants produced from the hybrid tall parents of \( F_1 \) generation, were either tall or dwarf. There were no plants with intermediate height (or medium height) in-between the tall and dwarf plants.
• This is because there is no mixing or blending of traits and they are inherited independently.
• Mendel formulated his first law of inheritance, which is called the law of segregation. According to this law, the characteristics (or traits) of an organism are determined by internal ‘factors’ which occur in pairs. Only one of a pair of such factors can be present in a single gamete.

Dihybrid inheritance and law of independent assortment

A breeding experiment dealing with two characters at the same time is called a dihybrid cross.

• Mendel considered shape as well as colour of the seeds simultaneously. He selected pure line plants and then cross pollinated flowers raised from seeds of round shape and yellow colour with those from wrinkled seeds and green colour. He observed that in \( F_1 \) generation all seeds had the features of only one parental type, i.e., round shape and yellow colour. He raised plants from \( F_1 \) generation seeds and allowed the flowers to self pollinate to produce the seeds of \( F_2 \) generation.
• In \( F_2 \) generation, Mendel observed the appearance of four types of combinations. These included two parental types (round and yellow seeds, and wrinkled and green seeds) and two new combinations (round and green seeds and wrinkled and yellow seeds).
• Based on dihybrid cross, Mendel postulated that inheritance of factors controlling a particular trait in an organism is independent of the other. This is called law of independent assortment. Hence, at the time of reproduction, two pairs of factors of each of the two traits in a dihybrid cross segregated independently during gamete formation and randomly formed combinations in the \( F_2 \) generation.

\( F_2 \) ratio : Round yellow = 9, Round green = 3, Wrinkled yellow = 3, Wrinkled green = 1. So, the \( F_2 \) phenotypic ratio is \( 9 : 3 : 3 : 1 \).

Mechanism of heredity

Deoxyribonucleic acid (DNA) present in the chromosomes of cell is the hereditary material. It is the information source in the cell for making proteins.

• DNA is a macromolecule or polymer made up of very large number of ‘nucleotide’ units and hence is termed polynucleotide. Each nucleotide unit in a DNA molecule is made up of three components :
  • Deoxyribose sugar : A pentose sugar
  • Nitrogenous base : In a DNA molecule, nitrogenous bases are of two types: Purines in a DNA molecule are – Adenine (A) and Guanine (G). Pyrimidines in a DNA molecule are – Cytosine (C) and Thymine (T).
  • Phosphate group : It contains one phosphorus atom and four specifically linked oxygen atoms.

How do proteins control the characteristics

The height of a plant depends upon the amount of growth hormone. Gene for tallness carries the information for synthesis of more efficient enzyme which in turn produces more amount of hormone which leads to greater height of a plant. On the other hand if plant has both alleles for dwarfness then less efficient enzymes will be produced which in turn will synthesize less amount of hormone and plant will remain dwarf. In this way genes control characteristics or traits of organisms.

Basic features of the mechanism of inheritance

• Characters (traits) are controlled by genes which are present on chromosomes.
• Each gene controls one character.
• There may be two or more forms (alleles) of the gene.
• One form may be dominant over the other form.
• An individual has two (similar or dissimilar) forms of the gene.
• The two forms of the gene separate at the time of gamete formation so that each gamete has only one form of the gene.
• The two forms of the gene are brought together in the zygote after fusion of male and female gametes.

Sex determination

The process by which the sex of a person is determined is called sex determination.

• Human beings have genetic or chromosomal sex determination.
• Humans have separate sexes where a specific pair of chromosomes in each diploid cell determines the sex of the individual. They are called sex chromosomes.
• All other chromosomes are termed autosomes as these have genes which control the somatic (body) characters.
• A male individual contains one X chromosome and one Y chromosome, i.e., XY. Male produces two different kinds of gametes (sperms); half of the sperms have X chromosome and other half have Y chromosome. Therefore, male is called heterogametic.
• A female individual contains two X chromosomes, i.e., XX. Female, therefore, produces only one type of gamete (ova). So, female is called homogametic.
• The sex of the child is determined at the time of fertilisation when male and female gametes fuse to form zygote. If a sperm carrying Y chromosome fertilises an egg or ovum which has X chromosome, then the offspring will be a boy (male). This is because the offspring will have XY combination of sex chromosomes. If a sperm carrying X chromosome fertilises an ovum having X chromosome, then offspring will be a girl (female).
• In grasshoppers and some other insects, the male has one sex chromosome (XO) whereas the female has two homomorphic sex chromosomes (XX). This type of sex determination mechanism is called XX-XO mechanism.
• In some of the animals, sex determination is also controlled by the environmental factors. For example, in some reptiles, the temperature at which the fertilised egg is incubated before hatching, plays an important role in determining the sex of the offspring. E.g., In most turtle species, only females are produced at \( 30-35^\circ\text{C} \) and only males at \( 23-28^\circ\text{C} \).