"HERIDITY and Evolution" NCERT CLASS X - "SIMPLIFIED EXAPLANATION"

 CHAPTER 8: "HEREDITY& EVOLUTION"—SIMPLE NOTES

WHAT IS HEREDITY

Heredity can be explained in simple terms—a process that transfers certain mental or physical traits to children from their parents or ancestors is known as heredity. It is done by the genes. Therefore, it is referred to as the fundamental unit or part of heredity. A gene contains the instructions from the parent cell that are transferred to the new body.

Gene 

A gene is a part of the DNA on a chromosome. It carries special instructions for making protein to form a functional product. It determines the quality that is to be copied to the next cell of the specific organ in the next generation. 

Example: Eye color, hair color, height, and exposure to certain diseases like breast cancer or color blindness. 

Traits

A unique feature of the organism that can be identified is known as a trait. For example, the eye color, hair type, and height. It is influenced by the gene, which is said to be an inherited trait, and the features visible due to the environment are said to be acquired traits.

A person can make their body by performing exercises and following special diets. It is an acquired trait; this character will not be passed on to his child.

How variation is adopted during reproduction

In the process of reproduction, the DNA changes several times, and genes recombine during meiosis (meiosis is a specialized type of cell division in sexually reproducing organisms that reduces the chromosome number in a parent cell by half, producing four genetically unique daughter cells called gametes (sperm or egg cells)). Independent assortment of chromosomes (during meiosis, the alleles (versions) of different genes segregate independently of each other into gametes) and the random fusion of gametes during fertilization in sexual reproduction. This process generates a unique genetic combination that causes differences among the children/offspring and parents. 

 

Mutation

 Spontaneous and permanent changes in the DNA structure. Most of the mutations are harmful. 

Recombination during meiosis

Homologous genes exchange genetic material in the crossover process during sexual reproduction. 

Independent assortment of chromosomes

 Fusion of egg and sperm is a random event. The specific combinations from both sides (male and female) contribute to the makeup of offspring.

Random fusion of gametes

During the process of sexual reproduction, a male gamete can fuse with any one of the zygotes, creating a variation in the zygote, as each has a unique combination of alleles (It is a variant form of a gene that occupies the same position on the chromosome and determines a specific trait). Example: a gene for eye color might have a brown eye color allele and a blue eye color allele, with an individual inheriting one color character from each parent.

This zygote grows and becomes a living structure (body). 

Rules for inheritance

Mendel worked on the pea plant, as it has a short life span. It has a life span of 6 months. So, experimenting and getting a conclusion from the experiment was easier. 

He gave the three fundamental laws:

1. Law of Segregation

2. The law of dominance

3. Law of independent assortment

1. Law of segregation

The two alleles, which are forms of genes, for each trait get separated from each other during gamete formation. 

One allele comes from each parent for each gene for their offspring. 

2. Law of Dominance

According to Mendel's Law of Dominance, "only one of a pair's contrasting characters appears in the first generation when homozygous organisms cross."

The first law of heredity to be proposed from Mendel's work is the law of dominance. According to the legislation, each character in an individual is governed by unique units known as factors that appear in pairs.
One of the components predominates in heterozygous couples, which can be either homozygous or heterozygous.

Dominant characters are those that dominate, while recessive characters are those that are not expressed.
Similar to how the dominant feature is passed down to the offspring, the recessive character is also latent.
Only when the child carries two copies of the same allele—a trait known as homozygosity—does the recessive feature manifest.
During fertilization, the two alleles that determine a character are combined; one allele originates from the maternal gamete, while the other comes from the paternal gamete.
Individual phenotype is not represented by the concept of dominance, which is completely limited to genotypic characteristics.

3. Law of Independent Assortment

Mendel's law of independent assortment states that genes do not influence each other in the sorting of alleles into gametes. This can be illustrated by the dihybrid cross, where two true-breeding parents express different traits for two characteristics. For example, a green, wrinkled pea plant has yr gametes, while a yellow, round plant has its. The F1 generation of offspring is YyRr. For the F2 generation, each gamete receives either an R or an r allele, along with either a Y or a y allele. The law of independent assortment states that a gamete with an r allele is equally likely to contain either a Y or a y allele. This results in 16 equally likely genotypic combinations, with a phenotypic ratio of 9 round/yellow round/green wrinkled/yellow wrinkled/green.

Exam questions

Question 1. The exchange of genetic material takes place in
(a) budding 
(b) asexual reproduction
(c) sexual reproduction
(d) vegetative reproduction

Ans(c) Other options are the modes of asexual reproduction. 

Question 2. A crossing between the tall plant (TT) and the short pea plant (tt) resulted in a progeny that was all tall plants because

(a) Tallness is the dominant trait

(b) The height of the pea plant is not governed by the gene ‘T’ or ‘t’

(c) Tallness is the recessive trait

(d) Shortness is the dominant trait

Ans (a) Tallness is a dominant character

Board Exam paper questions (2025)

1. Mendel used many visible contrasting characters of pea plants (tall/short plants, round/wrinkled seeds, violet/white flowers, etc.) for his experiments and worked out the rules of heredity. He conducted several experiments by crossing one or two pairs of visible, contrasting characters in pea plants. Based on his observations, Mendel provided some interpretations that are helpful in understanding the mechanism of inheritance of traits in different organisms. 

 (a) When Mendel crossed pure tall pea plants (TT) with pure short pea plants (tt) to obtain F1 progeny plants, which two characteristics did he observe in F1 progeny plants? 1

 (b) Differentiate between dominant and recessive traits. 1

 (c) (i) Consider the following cross with two pairs of contrasting characters: RRYY  rryy (Round yellow) (Wrinkled green).

In this cross, Mendel observed four different combinations of pea seeds in the F2 generation pea plants. State the method used by Mendel to obtain F2 progeny. Write the new traits found in F2 progeny. State the conclusion that was drawn from this experiment. 

(c) (ii) “It is possible that a trait is inherited but may not be expressed.” Justify this statement.

Answer (a)

(a) When Mendel crossed pure tall pea plants (TT) with pure short pea plants (tt), all the plants in the F1 progeny were tall (Tt). 

The two characteristics he observed were that all the plants showed the dominant trait (tallness) and that they were heterozygous

(b)

Dominant and recessive traits differ primarily in how they manifest. A dominant trait is something that is inherited. Even when the allele is present in only one copy, it is always manifested in the phenotype. 

On the other hand, a recessive trait only manifests when a person receives two copies of the allele—one from each parent. 

The effect of a recessive allele is masked by the presence of a dominant allele. 

For example, in pea plants, tallness is dominant and shortness is recessive. A pea plant will be tall if it has at least one dominant allele for tallness (TT or Tt), but it will only be short if it has two recessive alleles for shortness (tt).

(c) (i) To obtain the F2 progeny, Mendel used the method of self-pollination on the F1 generation plants (RrYy).

The new traits found in the F2 progeny were round green seeds and wrinkled yellow seeds. These combinations were not present in the parental generation (RRYY and rryy).

Mendel came to the Law of Independent Assortment as a result of this dihybrid cross experiment. According to this law, during gamete formation, the alleles for distinct traits—such as seed shape and color—segregate independently of one another. 

(ii) The dominance principle can cause a trait to be inherited but not manifested. Every gene has two alleles that an organism inherits, one from each parent. Both dominant and recessive alleles are possible. If a dominant allele is present, the phenotype (the observable trait) will always express it. A recessive allele, on the other hand, will only manifest itself if the organism receives two copies of it, one from each parent.