6.2 Sexual Reproduction

Sexual reproduction in animals involves the fusion of male and female gametes (reproductive cells) to produce offspring. This process ensures genetic diversity, which is essential for the survival and evolution of species. In sexual reproduction, two parents contribute to the genetic makeup of the offspring, resulting in a combination of traits from both parents.

There are two main types of sexual reproduction in animals:

• External Fertilization: In external fertilization, the male and female gametes fuse outside the body of the female. This is commonly observed in aquatic animals like fish and amphibians, where eggs and sperm are released into the water. The fertilization takes place outside the female’s body, in the environment.
• Internal Fertilization: In internal fertilization, the male gamete (sperm) is deposited inside the female reproductive system, where fertilization occurs. This type of fertilization is observed in terrestrial animals like humans, mammals, birds, and reptiles.

The process of sexual reproduction in animals involves several steps:

• Gamete Formation: The male and female animals produce specialized reproductive cells, called gametes. In males, the gametes are sperm, and in females, the gametes are eggs (ova). These gametes are formed through a type of cell division called meiosis, which ensures that the gametes contain half the number of chromosomes compared to the parent organism.
• Fertilization: Fertilization is the process in which the male gamete (sperm) and the female gamete (egg) fuse to form a zygote. The zygote contains a complete set of chromosomes, half from each parent. The fusion of gametes restores the diploid number of chromosomes in the offspring.
• Development of the Embryo: After fertilization, the zygote begins to divide by mitosis to form a multicellular structure called the embryo. The embryo develops into a fetus inside the mother’s body in mammals or in an egg outside the body in species that lay eggs.

During fertilization, the following can occur:

• Chromosomal Combination: The sperm cell contributes one set of chromosomes, and the egg contributes another set. The fusion of the sperm and egg restores the diploid number of chromosomes in the zygote. If an organism has $2n$ chromosomes, after fertilization, the zygote will have $2n$ chromosomes, with $n$ chromosomes from each parent.
• Sex Determination: In many animals, the sex of the offspring is determined by the type of sex chromosomes that are present. In humans and many other animals, males have XY sex chromosomes and females have XX. When a sperm carrying the Y chromosome fertilizes the egg, the offspring will be male (XY); if the sperm carries an X chromosome, the offspring will be female (XX).

In mammals, after fertilization, the fertilized egg (zygote) travels down the fallopian tube and embeds itself in the uterus, where it develops into an embryo. The embryo is nourished by the mother through the placenta, an organ that allows the exchange of nutrients and waste between the mother and the developing fetus.

Reproduction in animals can vary in complexity, and some species exhibit a wide range of reproductive strategies. For instance, some animals may exhibit parental care, where one or both parents protect and care for the offspring after birth or hatching. In contrast, other species may not provide any parental care and rely solely on the offspring's survival instincts.