What is Sexual Reproduction?

Biology is the study of living organisms. It includes various processes occurring in these organisms, like digestion, reproduction, and excretion. Reproduction is a process of producing offspring or new organisms by single or two parents. When reproduction occurs through one parent, it is called asexual reproduction, but when two parents are usually of the opposite gender, the reproduction is called sexual reproduction.

Characteristics of Sexual Reproduction

Sexual reproduction involves a process in which a gamete (sexual cells) having a single set of chromosomes (haploid) fuses with another haploid gamete to produce a double set of chromosomes (diploid). This process is common in eukaryotes with multicellularity like fungi, animals, and plants. It is not observed in prokaryotes due to the presence of a less complex metabolic system. The prokaryotes have other modes of reproduction, such as conjugation, transduction, and transformation, which are quite similar to advanced sexual reproduction methods.

The branch of biology which includes the study of both sexual and asexual reproduction is called reproductive biology. The asexual mode will pass down the whole exact genome from the parent to the new organism.

"Sexual reproduction in humans"
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Types of Sexual Reproduction

  1. Allogamy: The female gamete of an organism is fertilized by the male gamete of another organism called allogamy. The female gamete is called ovum or egg, and the male gamete is referred to as sperm. Each of the gametes has 23 chromosomes. When the two gametes fuse, they form a diploid that is 46 chromosomes. This diploid cell will undergo mitosis to grow into an organism. Mitosis occurs by the division of one cell into two. It occurs after the DNA (deoxyribonucleic acid) has replicated or duplicated in the nucleus.
  2. Hermaphroditism: In this fertilization, a single parent has both the gametes of male and female. The hermaphrodites such as slugs and snails self-mate and reproduce.

Sexual Selection

It takes place only when there are two mates, male and female. The physical appearance, combat, extreme features lead to the positive feedback of attraction between animals. Dimorphism is observed in both genders as the differences in the phenotypic traits, secondary sex characters, and sex organs. The strength, body size, biological orientation, sexual fitness, and shape also differ. These features create competition between organisms to attract and reproduce with their mate. After each mating, genes are passed down to the next generation; the survival of these genes will depend on natural selection. The natural selection theory says that the best traits that can overcome extreme conditions can only survive; thus, sexual selection and natural selection go side by side. Genetic variance is a term that explains the phenotypic differences due to different interactions between alleles. Genetic variability is important for biodiversity.

Internal Fertilization

When the fertilization of male and female eggs takes place inside the body of one of the parents, it is internal fertilization. This type of fertilization usually takes place in the female body. One exception is a seahorse, in which females implant the eggs into a male seahorse, and the male body supports the development of the fetus.

In some cases, the embryos develop in the form of eggs in the parent's body, and when they are ready to hatch, live birth occurs. This process is called ovoviviparity.

External Fertilization

When the fertilization takes place outside the body, it is called external fertilization. The amphibians who live both on land and water perform external fertilization. They produce hundreds of gametes in proximity. This quick release of gametes is called spawning. These sex cells generally have flagella so that they can independently move for fertilization.

Stages of Sexual Fertilization

1. Pre-fertilization

This stage includes gamete formation or gametogenesis, followed by the transfer of the gametes. Female gametes are immobile not to move, but the male gametes travel to the female eggs for fertilization. Sexual intercourse (in animals) or pollination (in plants) takes place during this stage.

2. Fertilization

During this stage, the male and female gametes fuse to produce a diploid cell which divides to give offspring. The zygote is the first cell formed after the fusion of gametes.

3. Post-fertilization

This stage includes forming a diploid zygote that divides mitotically to develop into an embryo, a process called embryogenesis. During embryogenesis, the cells differentiate according to their functions. Based on the location of zygote development, the animals are viviparous and oviparous. Mitosis is the cell division process that results in the formation of two daughter cells having the same kind and number of chromosomes as the parent.

"Stages of sexual reproduction in humans"
CC BY SA 4.0 | Image credits https://pressbooks.bccampus.ca

Advantages of Sexual Reproduction

  • It avoids the passing down of deleterious mutations from the parents to the offspring. Due to natural selection, the most relevant genes are only passed down to the next generation. 
  • The increase in genetic diversity in a population improves an organism's ability to live according to the changes in the environment. 
  • Sometimes a mutation in a gene that is of no use may become useful to the next offspring. This process is called speciation.
  • Formation of unique offspring due to fusion of two different gene pools.
  • The risk of a genetic disease is reduced because not all the genes are transferred; mutational genes are rarely passed down to the offspring.

Disadvantages of Sexual Reproduction

  • Due to the high diversity of organisms, only 40%-50% can reproduce sexually. Females have a specific structure called the uterus to hold the fetus. Certain hormonal or genetic imbalances in females affect the structure and function of the uterus.
  • Finding a mate is difficult as sexual reproduction must need two parents. The unavailability of a mate ceases reproduction. Moreover, these sexually reproducing organisms cannot switch to asexual mode.
  • The environmental factors affect the fate of the developing offspring in the womb as the mother's physical and mental state directly affects the fetus.
  • The probability of success of sexual reproduction is very less as the menstrual cycle, which usually prevails in sexually reproducing organisms, does not favor fertilization. If either of the parents is infertile, it can also contribute to the failure of reproduction.
  • The best genetics is not sure to be passed down to a new organism. The probability is always 50%. Genetic disorders are sometimes observed in an organism after 5 or 6 generations. The genetic pool of the offspring is not the same as that of a parent. It is different in both sexual mode and the asexual mode. Thus, the new genetic makeup introduces new traits into the offspring.

Common Mistakes

  • The genetic pool changes in the sexual mode of reproduction, not in the asexual mode.
  • Few organisms on earth follow the sexual reproduction method.

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for

  • Bachelor of Science in Biology
  • Master of Science in Zoology
  • Bachelor of Medicine and Bachelor of Surgery
  1. Autogamy
  2. Asexual reproduction
  3. Pollination

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