Fertilization: Definition, Types, and Examples

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Fertilization Definition

The act of creating new creatures by mixing the genetic information of two individuals of different sexes is known as sexual reproduction. The genetic information is stored on chromosomes within the nucleus of gametes, which are specialised sex cells.

In men, these gametes are known as sperm, whereas in females, they are known as eggs. During sexual reproduction, the two gametes fuse in a process known as fertilisation to form a zygote, which is the predecessor to an embryo child, absorbing half of its DNA from both parents.

A zygote in humans has 46 chromosomes: 23 from the mother and 23 from the father. When these chromosomes are combined, the result is a child that looks like both its mother and father but is not identical to either.

During sexual reproduction, phenotype features, such as physical adaptations to an organism’s environment, and genotype traits, such as disease resistance, are handed down from each parent.

Natural selection promotes evolution by allowing individuals with advantageous environmental adaptations to live and reproduce effectively. Natural selection can choose the people most adapted to a given environment because sexual reproduction increases the diversity of genotypes and phenotypes within a population.

Asexual reproduction requires only one parent, whereas sexual reproduction requires both parents. Because there is no fusing of gametes in asexual reproduction, unlike sexual reproduction, the children are genetically identical to their parents and hence clones.

Asexual reproduction does occur in certain species, albeit it is uncommon. Bacteria, fungus, starfish, corals, hydras (jellyfish), and some flowering plants, such as strawberries, are examples of asexual reproduction.

Types of Sexual Reproduction

Allogamy

When the gametes that fuse together during fertilization come from two separate people, it’s called allogamy. The female gamete generally takes the shape of an egg or ovum, while the male gamete is a sperm.

Both the egg and the sperm are cells that are specialised for reproduction; each sex cell has just 23 chromosomes (called haploid cells) rather than the typical 46 chromosomes found in other cells of the body.

The two haploid cells combine to produce a diploid cell, which then goes through mitosis to develop and form an individual creature. After the DNA has been duplicated within the nucleus, mitosis is the division of one cell into two.

Individuals are compelled to pick partners based on their capacity to create kids likely to survive and reproduce themselves, because genes are handed down through sexual reproduction and gene survival is regulated by natural selection.

It is thus in each individual’s best interests to select a partner who possesses traits such as excellent health, aggressiveness, speed and agility to survive conflicts, and qualities that will aid the progeny in attracting future mates.

Individuals can pick partners depending on the greatest presentation of these qualities due to the genetic and phenotypic variety created by sexual reproduction. The options available to sexually reproducing organisms create competition among individuals, resulting in only those individuals that exhibit desired characteristics being able to pass on their genes. Sexual selection is the term for this.

Sexual dimorphism occurs when men and females of the same species have drastically different appearances. Females are frequently disguised with drab hues to protect their young after birth, in contrast to males, who are often camouflaged with brilliant colours and exaggerated bodily features (such as large horns or antlers).

These sexually chosen traits frequently clash with the animals’ capacity to survive. Bright hues that attract females, for example, may also attract predators. Nonetheless, because there is a strong urge to mate, these traits endure and grow within populations.

Internal Fertilization

Internal fertilisation is the fertilisation of an egg by sperm within one of the parents’ bodies, generally through sexual contact. After the male implants sperm, internal fertilisation takes place within the female body.

There are a few exceptions, such as seahorses (Sygnathidae), where the female implants her eggs in the male and the zygote develops inside the male’s body. The species determines the next phase in internal fertilization.

Birds, insects, and reptiles, for example, lay an egg containing the mitosis-inducing cells as well as a reserve of yolk to nourish and maintain the embryo’s development.

A fully developed individual will emerge from the egg after a period of time (typically after being incubated). This is referred to as oviparity. Most mammal embryos develop and mature within their mother’s body, resulting in the live delivery of a fully developed offspring: this is known as viviparity.

The placenta supports embryos by providing nutrition absorption, waste disposal, and thermoregulation in placental species (most mammals). Alternatively, after a brief gestation time, marsupial babies (such as koalas and kangaroos) are taken from inside the mother’s body and develop in an external pouch on the front of the mother’s body.

Ovoviviparity is a third kind of development in which embryos develop in eggs kept in the parent’s body until they are ready to hatch, giving the impression of a live birth.

External Fertilization

When a sperm cell and an egg cell meet outside of the body, it is called external fertilisation. External fertilisation is used by most amphibians and fish, as well as many invertebrates, to produce hundreds to billions of gametes at a time in close proximity.

Spawning is defined as the rapid release of gametes into aquatic habitats. Females will occasionally lay eggs on a specific substrate, which are then fertilised by males.

Externally fertilised animals’ sex cells frequently contain specific adaptations for mobility, such as the insertion of robust flagella for autonomous movement.

Autogamy

Autogamy is the fusion of male and female gametes generated by a single person, often known as self-fertilization or self-pollination. Hermaphrodites are species capable of producing both male and female gametes.

Although autogamy is similar to asexual reproduction in that it does not require a mate, the recombination of chromosomes between male and female gametes results in children with slightly changed genetic information, which might differ phenotypically from their parents.

Autogamy is used by most plants and earthworms to reproduce. Hermaphrodites can procreate with other hermaphrodites on rare occasions. In this situation, the population’s genetic diversity increases.

Sexual Reproduction's Benefits

Through cell mitosis, harmful mutations accumulate in DNA throughout time. Organisms that reproduce asexually simply transmit these mutations on to their children, whereas organisms that reproduce sexually pass just a fraction of their harmful mutations on to their offspring, boosting their chances of survival.

Natural selection amplifies this impact, as individuals with particularly damaging mutations are unable to pass on their genes through sexual reproduction. Natural selection can increase an organism’s ability to adapt to environmental changes by increasing gene variety within a population.

Under changing environmental stresses, random mutations that are of no benefit to one generation may become critical to subsequent generations’ survival. Speciation is propelled by this force.

Fertilization FAQ

Asexual reproduction is a kind of reproduction in which the progeny of a single organism are produced.

Chromosomes are the proteins and nucleotides that make up DNA and are found within the nucleus of a cell.

An embryo is a eukaryotic organism’s unborn developmental stage.

Sexual intercourse is a sexual interaction between people that can result in cell exchange or transfer, and therefore, sexual reproduction.

Fertilization Citations
  • Sperm acrosome reaction: its site and role in fertilization. Biol Reprod . 2018 Jul 1;99(1):127-133.
  • Sperm competition and fertilization mode in fishes. Philos Trans R Soc Lond B Biol Sci . 2020 Dec 7;375(1813):20200074.
  • Importance of sperm morphology during sperm transport and fertilization in mammals. Asian J Androl . Nov-Dec 2016;18(6):844-850.
  • Sperm-egg interaction and fertilization: past, present, and future. Biol Reprod . 2018 Jul 1;99(1):134-146.
  • Fertilization. Adv Exp Med Biol . 2013;757:321-50.
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