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Sexual and Asexual Reproduction Animal


Sexual and Asexual Reproduction Animal
u Asexual reproduction is producing progeny with only one parent and without any specialized sexual organs. This means that individual animals leave clones behind. These animals are genetically identical to their parents except in rare cases of genetic mutation.

u Amoebas and some other single-celled organisms reproduce through asexual reproduction, such as binary fission, which means equal cell division, or budding, which means unequal cell division. Binary fission only occurs in single-celled organisms, not in higher animals. Budding occurs in single-celled organisms as well as some plants and a small number of aquatic animals.
u There are other forms of asexual reproduction in animals that are more complex. For example, a hermaphrodite is an animal that has both male and female reproductive organs in the same individual. Thus, these creatures can potentially mate with every individual they meet from the same species. Most hermaphrodites in the animal kingdom are invertebrates. There are only a few hermaphrodites among thousands of species of insects. But there are many hermaphroditic worms.
u Another method of reproduction is parthenogenesis, which is a modified form of sexual reproduction for which males and females are present in the population, but females can develop unfertilized gametes, or eggs, into living offspring without a contribution from the male. This form of reproduction happens in bees, wasps, some lizards, and other animals that we consider not as complex as birds and mammals, and there are several types of parthenogenesis in the animal kingdom.
u Eggs and sperm are formed by the process of meiosis. In mitosis, which is ordinary cell division, all the chromosomes in the cell’s nucleus are doubled and then the cell splits in 2. But in meiosis, the chromosomes are doubled and then the cell splits in 4.

Sexual and Asexual Reproduction Animal

u The result is that each of the 4 cells—the gametes, which is the general name for eggs and sperm—has 1 set of chromosomes instead of 2, like the rest of the cells in your body.
u Cells with only a set of chromosomes are called haploid. If they have 2, they’re diploid.
u In sexual reproduction, an egg and sperm fuse and restore the diploid state. You have half of your mother’s DNA and half of your father’s DNA. Two sets of chromosomes fuse, and your cells become diploid.
u But in parthenogenesis, we only have eggs. And eggs are haploid.
u In one form of parthenogenesis known as ameiotic parthenogenesis, females are capable of producing diploid eggs. Females produce offspring by spontaneously activating a diploid egg, which is followed by normal embryonic development. In these cases, the mother’s chromosomal complement is wholly passed on to the offspring, so the offspring can be considered her clones.
u Haplodiploidy is a combination of parthenogenesis and sexual reproduction. In honeybees, we have proper meiotic egg production and proper meiotic parthenogenesis. A queen bee produces haploid eggs, but she has a few different ways she can handle them.
u First, she can lay unfertilized eggs. These eggs become haploid male offspring, which are called drones. They have one function in life: to fertilize a queen’s eggs. Second, if the queen is carrying sperm from a drone, she can choose to fertilize some of the eggs she lays. The offspring that hatch from the fertilized eggs are female diploid bees, which become worker bees or new queens.
u These aren’t the only options for parthenogenesis. In fish, we sometimes have gynogenesis, when a diploid egg is hormonally stimulated to develop by the presence of sperm, even though the sperm don’t contribute genetically to the offspring.
u In some insects and flatworms, sometimes haploidy spontaneously corrects itself and the offspring becomes diploid even though it came from a haploid egg, or 2 eggs will fuse to create a diploid individual, a form of self-fertilization, or autogamy.
u Variants on parthenogenesis appear because they confer a survival advantage to a particular species in its particular environment.
u Asexual reproduction offers a species 2 big advantages, and they’re both about population numbers.
      If you are a self-fertilizing hermaphrodite or a parthenogenetic female, then you only need to produce 1 surviving baby per generation to ensure the survival of your species. In a sexually reproducing species, you need at least 2 individuals, 1 male and 1 female.
      Single asexual individuals can reproduce more quickly than a member of a sexually reproducing species. In honeybees, the queen can lay 2000 eggs in one day, and perhaps a million in her lifetime. The complex, sexually reproducing mammals get nowhere near that rate.
u This means that zoologists working in conservation practice can quickly replace more of a critically endangered asexually reproducing organism than they can a sexually reproducing one. The disadvantage is that there will be no genetic variation among the organism’s descendants.
u This may be an advantage if the environment is stable and just right, but climate change is making environments less stable all over the world. As a result, asexually reproducing animals may be more susceptible to new diseases and changes in temperature than those
u Because animals that sexually reproduce seem to have the upper hand in terms of adaptability, that may be why the parthenogenetic form of reproduction is comparatively uncommon. Sexual reproduction is therefore an advantage when genetically robust reproduction is preferable to fast reproduction.
u In sexually reproducing organisms, there are 2 sexes: individual males and females in the species. Because there is at least some genetic variation between the 2 parents, the recombination of their genes creates variation between the parents and their offspring, and even between siblings.
u Unlike some of the parthogenetically produced animals, each of us is always genetically diploid: Each of our cells has 2 complete sets of chromosomes. And this happens because 2 haploid gametes—1 egg and 1 sperm—fuse, forming diploid cells once again.
u The first of these cells following fertilization is called the egg and sperm cell fusing. These diploid zygotes have copies of half of each parent’s DNA, which allows a little extra diversity between individuals within each species.
u In addition to this, the process of meiosis—the process of creating gametes—fosters even more genetic variation through the independent assortment of traits. The offspring created through this process are not only different from their parent because they have only half the parent’s