8th Class Science Human Reproduction Sexual Reproduction

      Sexual Reproduction

The method of reproduction, in which both male and female are involved, is called sexual reproduction. The male and female mate with each other, and their egg fertilized to produce the new offspring. Each of the two parents contributes half of the offspring genetic makeup by creating the haploid gametes. In these anisogamous species, the male produces sperm and female produces ova. In isogamous species, the gametes are similar, but they have separable properties. For example, the green algae, chlamydomonas, reinhardtii. There are many animals and plants, which reproduce sexually. The new offspring inherit the trait of both the parents. In plants, Bryophytes reproduces sexually; but they are normally haploid which produce gametes.

            Allogamy

In this method, the ovum of one individual fertilized with the spermatoza of another individual.

           Autogamy

It is also known as self fertilization, which occurs in hermaphrodite organisms. In this organisms, both the male and female sex are present in the same individual.

             Mitosis and Meiosis

There are the two methods of cell division, mitosis and meiosis. The mitosis occurs in somatic cells, where as meiosis occurs in reproductive cell orgametes. In mitosis, the number of cell after the cell division is doubled, but the number of chromosomes remains the same as in the parent cell. In meiosis, the number of cell becomes four times, but the number of chromosomes is reduced to half. This process occurs in two phase, meiosis I and meiosis II.

        There are different methods of cell division in the multicellular organisms, which helps in its growth and development. The method of cell division, which takes place in the reproductive cell for reproductions is?

(a) Mitosis

(b) Meiosis

(c) Prophase

(d) Anaphase

(e) None of these

Answer: (b)

Explanation

The cell division in the reproductive cell is by the method of meiosis and cell division in rest of the body parts is through mitosis.

        Some organisms have both the sex, while, most of the other multicellular organisms have male and female on different organisms. The organisms, in which have both male and female sex are present is called hermaphrodite. Which one of the following organisms given below is hermaphrodite?

(a) Leach

(b) Earthworm

(c) Tapeworm

(d) Silkworm

(e) None of these

Answer: (b)

         Asexual vs. Sexual Reproduction

The organisms, which reproduces sexually tend to grow exponentially in numbers, but rely on mutation for variations in DNA. They have similar vulnerabilities. The organisms that reproduce sexually, yield less number of offspring. They show large amount of variations in their genetic make up. It makes them less susceptible to disease. There are many organisms that can reproduce both sexually and asexually. For example, aphids, slime molds, sea anemones, some species of starfish and many species of plants. Asexual reproduction is employed when the environmental conditions, such as, abundant food supply, adequate shelter, favorable climate, etc is suitable for survival. When these conditions get depleted, the organism switch on to sexual mode of reproductions. The variations found in the offspring are more, that reproduce through sexual mode. The variations in the offspring help them to survive better in the changing climatic conditions. In addition/sexual reproduction usually results in the formation of life stage that endures the conditions, which threatens the offspring of an asexual parent. Thus, the seeds, spores, eggs, pupae, cysts or other "over-wintering" stages of sexual reproduction ensure the survival during unfavorable conditions. The organism can "wait out" adverse conditions until a swing back to suitability occurs.

            Why to Choose Asexual Reproduction

The sexual reproduction is more efficient way to reproduce more organisms. Sexual reproductions requires both male and female for the process to be completed. Two general explanations for the overwhelming prevalence of sexually-reproducing species over asexual ones are:

(i) It provides a mechanism to weed out harmful mutations, that arise in the population reducing its fitness.

(ii) Perhaps it is the ability to adapt quickly to a changing environment, which has caused sex to remain the method of choice for most living things.

        The population of the organisms, which reproduces by the sexual method is large, as compared to the population of the organisms reproduce by the sexual method. In asexual method, large number of new organisms is produced at a time. Thus, we can say that the number of population in asexual reproduction increases_______.

(a) Proportionally

(b) Inversely  

(c) Exponentially

(d) Linearly

(e) None of these

Answer: (c)

Explanation

The population in asexual method increases exponentially.

       Which of the following statement is correct?

Statement 1: In asexual reproduction, the organisms rely on mutation for variation in DNA.

Statement 2: In asexual reproduction the organism can reproduce in different condition.

(a) Both the statement 1 and 2 are correct

(b) Statement 1 is correct and 2 is incorrect

(c) Statement 1 is incorrect and 2 is correct

(d) Both statement 1 and 2 are incorrect

(e) None of these

Answer: (a)

Explanation

Since both the statement I and II regarding the asexual reproduction is correct.