Heredity | Class X

Heredity is the process of transmission of traits/characters from parents to the offspring or progeny

  • A trait is any characteristic that is transferred from parent to offspring.
  • It is the continuity of features from the prior generation to the next which is present in the fertilized egg or zygote.
  • That zygote develops into an organism of a particular type.

Traits transferred from the parents to the offspring:

  • The genetic information is carried within chromosomes, the chromosomes are found in the nucleus of a cell.
  • Long coiled molecules of DNA make up chromosomes.
  • The genes are contained in the DNA; they are the units of heredity and are responsible for inheritance.
  • The expression of characteristics is contained in genes.
  • Different traits are formed by different genes.
  • The gene carries the genetic code for a particular characteristic.
  • The height of a person, the facial features such as nose or jawline are all controlled by different genes.

The inheritance of characteristics in an organism is contributed to equally by the mother and father. The mother and father provide equal amounts of genetic material to their offspring. Therefore, each trait has two factors, one that comes from the mother and the other that comes from the father.

Most of the genes have more than two variations, they are called alleles. E.g. there are two alleles for the height gene, one is short and one is tall. The offspring may inherit the same alleles from both parents or two different alleles from them. When the offspring inherits two different alleles then it interacts in a particular way.

There is a dominant trait and a recessive trait.

The dominant trait comes from the gene that decides the trait in the presence of the other identical gene. The trait that does not get to express itself is called the recessing trait. The dominant trait is denoted using capital letters while the recessive trait is denoted using small letters.

Variation and its importance
Variation is the term given to the difference in the characters or traits that exist among the individuals of a species. Variations may occur during reproduction, it can happen due to an error in DNA copying or due to sexual reproduction. Evolution happens thanks to variation.

The various causes and variations are that there could be some positive gene mutation, an adaptation of the genes to the environmental changes or a different combination of the genetic material.

It is important to note though that variation occurs both in sexual as well as asexual reproduction. Although, the variation that takes place in sexual reproduction is far more than the changes brought about by asexual reproduction.

Genotype is the genetic makeup of an organism. Genotypes are represented by letters. Each letter stands for a trait that is inherited from each parent (as a result of sexual reproduction). One code of the trait comes from the father and the other comes from the mother.

Therefore there are always three possibilities with genotypes. For eg. For the height of an offspring, it can be HH, Hh or hh.

Phenotype is the expression of genotype. It is actually the result of how the genes interact with the environment. The characters that one can observe in an organism are known as the phenotype. Colour of the eyes, the hair etc.

Acquired Traits: Traits; which are acquired due to interaction with the environment; are called acquired traits. Acquired traits are not inheritable. For example; if a boxer develops bulging biceps, it does not mean that his son would be borne with bulging biceps.

Inheritable Traits: Traits; which can be expressed in subsequent generations; are called inheritable traits. Such traits bring a change in the genotype of the organism and hence become inheritable.


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