The male gamete, or sperm, and the female gamete, the egg or ovum, meet in the female's reproductive system. When sperm fertilizes meets an egg, this fertilized egg is called a zygote pronounced: ZYE-goat. The zygote goes through a process of becoming an embryo and developing into a fetus. The male reproductive system and the female reproductive system both are needed for reproduction. Humans, like other organisms, pass some characteristics of themselves to the next generation.
We do this through our genes , the special carriers of human traits. The genes that parents pass along are what make their children similar to others in their family, but also what make each child unique. These genes come from the male's sperm and the female's egg. The external part of the female reproductive organs is called the vulva , which means covering. Located between the legs, the vulva covers the opening to the vagina and other reproductive organs inside the body.
The fleshy area located just above the top of the vaginal opening is called the mons pubis. Two pairs of skin flaps called the labia which means lips surround the vaginal opening. The clitoris , a small sensory organ, is located toward the front of the vulva where the folds of the labia join. Between the labia are openings to the urethra the canal that carries pee from the bladder to the outside of the body and vagina. When girls become sexually mature, the outer labia and the mons pubis are covered by pubic hair.
The vagina is a muscular, hollow tube that extends from the vaginal opening to the uterus. Because it has muscular walls, the vagina can expand and contract. This ability to become wider or narrower allows the vagina to accommodate something as slim as a tampon and as wide as a baby. The vagina's muscular walls are lined with mucous membranes, which keep it protected and moist. A very thin piece of skin-like tissue called the hymen partly covers the opening of the vagina.
Hymens are often different from female to female. Most women find their hymens have stretched or torn after their first sexual experience, and the hymen may bleed a little this usually causes little, if any, pain.
Some women who have had sex don't have much of a change in their hymens, though. And some women's hymens have already stretched even before they have sex. The vagina connects with the uterus , or womb, at the cervix which means neck. The cervix has strong, thick walls. These methods of asexual reproduction do not require another organism of the opposite sex.
Indeed, some organisms that lead a solitary lifestyle have retained the ability to reproduce asexually. In addition, in asexual populations, every individual is capable of reproduction.
In sexual populations, the males are not producing the offspring themselves. In theory, an asexual population could grow twice as fast. Nevertheless, multicellular organisms that exclusively depend on asexual reproduction are exceedingly rare. Why is sexuality and meiosis so common? This is one of the important unanswered questions in biology and has been the focus of much research beginning in the latter half of the twentieth century.
There are several possible explanations, one of which is that the variation that sexual reproduction creates among offspring is very important to the survival and reproduction of the population. Thus, on average, a sexually-reproducing population will leave more descendants than an otherwise similar asexually-reproducing population. The only source of variation in asexual organisms is mutation. This is the ultimate source of variation in sexual organisms, but, in addition, those different mutations are continually reshuffled from one generation to the next when different parents combine their unique genomes and the genes are mixed into different combinations by the process of meiosis.
Meiosis is the division of the contents of the nucleus, dividing the chromosomes among gametes. The process of meiosis produces unique reproductive cells called gametes, which have half the number of chromosomes as the parent cell. Fertilization, the fusion of haploid gametes from two individuals, restores the diploid condition. Thus, sexually-reproducing organisms alternate between haploid and diploid stages. However, the ways in which reproductive cells are produced and the timing between meiosis and fertilization vary greatly.
There are three main categories of sexual life cycles: diploid-dominant, demonstrated by most animals; haploid-dominant, demonstrated by all fungi and some algae; and the alternation of generations, demonstrated by plants and some algae. The Sexual Life Cycle : In animals, sexually-reproducing adults form haploid gametes from diploid germ cells. Fusion of the gametes gives rise to a fertilized egg cell, or zygote.
The zygote will undergo multiple rounds of mitosis to produce a multicellular offspring. It is not in dispute that sexual reproduction provides evolutionary advantages to organisms that employ this mechanism to produce offspring.
But why, even in the face of fairly stable conditions, does sexual reproduction persist when it is more difficult and costly for individual organisms? Variation is the outcome of sexual reproduction, but why are ongoing variations necessary?
Possible answers to these questions are explained in the Red Queen hypothesis, first proposed by Leigh Van Valen in All species co-evolve with other organisms; for example, predators evolve with their prey and parasites evolve with their hosts.
This tutorial is a review of plant mitosis, meiosis, and alternation of generations. Developmental biology is a biological science that is primarily concerned with how a living thing grows and attains maturity. The tutorials included here focuses on human growth and development.
Thus, one can expect to learn about human zygote developing and maturing into adulthood after these tutorials. Also included here are informative guides on dietary sources.. Malaria : Plasmodium togetherness a strategy for breeding success. Developmental Biology. Skip to content Main Navigation Search. Dictionary Articles Tutorials Biology Forum.
Human Reproduction and Fertilization For human species to obviate extinction, reproductive mature adults should be producing viable offspring in order to continue the existence of the species and pass on genetic information from generation to generation.
Human Reproduction Humans are capable of only one mode of reproduction, i. Bryophytes Bryophytes nonvascular plants are a plant group characterized by lacking vascular tissues. Asexual reproduction in such a population preserves this variation bottom left , but sexual reproduction with random mating brings the population back into Hardy-Weinberg proportions and reduces variation bottom right. This example illustrates the fact that sex does not always increase variation. Figure Detail.
This example is overly simplified, but it serves to illustrate a general point: Selection can build more variation than one would expect in a population in which genes are well mixed. In such cases, sex reduces variation by mixing together genes from different parents. This problem arises in the case of a single gene whenever heterozygotes are less fit, on average, than homozygotes.
In this case, the heterozygote need not have the lowest fitness ; rather, its fitness must only be close to that of the least-fit homozygote. In general, mathematical models have confirmed that selection builds more variation than expected from randomly combined genes whenever fitness surfaces are positively curved, with intermediate genotypes having lower-than-expected fitness.
In such cases, sexual reproduction and recombination destroy the genetic associations that selection has built and therefore result in decreased rather than increased variation among offspring. The term " epistasis " is used to describe such gene interactions, and cases in which the intermediate genotypes are less fit than expected based on the fitness of the more extreme genotypes are said to exhibit "positive epistasis.
Interestingly, even when sex does restore genetic variation , producing more variable offspring does not necessarily promote the evolution of sex.
Again, this reality refutes one of the arguments often raised in the attempt to explain the relationship between sex and evolution. To understand how this operates, consider another simple case involving a single gene, but this time, assume that heterozygotes rather than homozygotes are fittest.
The gene responsible for sickle-cell anemia provides a great real-life example. Here, people who are heterozygous for the sickle-cell allele genotype Ss are less susceptible to malarial infection yet have a sufficient number of healthy red blood cells; on the other hand, SS homozygotes are more susceptible to malaria, while ss homozygotes are more susceptible to anemia.
Thus, in areas infested with the protozoans that cause malaria, adults who have survived to reproduce are more likely to have the Ss genotype than would be expected based on Hardy-Weinberg proportions.
In such populations in which heterozygotes are in excess, sexual reproduction regenerates homozygotes from crosses among heterozygotes. Although this indeed results in greater genetic variation among offspring, the variation consists largely of homozygotes with low fitness. Yet again, this simple example illustrates a more general point: Parents that have survived to reproduce tend to have genomes that are fairly well adapted to their environments.
Mixing two genomes through sex and genetic recombination tends to produce offspring that are less fit, simply because a mixture of genes from both parents has no guarantee of functioning as well as the parents' original gene sets. In fact, mathematical models have confirmed that when selection builds associations among genes, destroying these associations through sex and recombination tends to reduce offspring fitness.
This reduction in fitness caused by sex and recombination is referred to as the "recombination load" or the " segregation load" when referring specifically to segregation at a single diploid gene. The reason that the recombination load is a problem for the evolution of sex is better appreciated by looking at evolution at the level of the gene.
Imagine a gene that promotes sexual reproduction, such as by making it more likely that a plant will reproduce via sexually produced seeds as opposed to some asexual process e. Carriers of this gene will tend to produce less fit offspring because sexual reproduction and recombination break apart the genetic associations that have been built by past selection. The gene promoting sex will fail to spread if the offspring die at too high a high rate, even if the offspring are more variable.
Indeed, theoretical models developed in the s and s demonstrate that genes promoting sex and recombination increase in frequency only when all of the following conditions hold true:. Unfortunately, empirical data have not indicated that fitness surfaces curve in just the right way for these models to work in real-life situations.
To make matters worse, sexual reproduction often entails costs beyond the recombination load described earlier. To reproduce sexually, an individual must take the time and energy to switch from mitosis to meiosis this step is especially relevant in single-celled organisms ; it must find a willing mate; and it must risk contracting sexually transmitted diseases.
This last cost is often called the "twofold cost of sex. These are substantial costs—so substantial that many species have evolved mechanisms to ensure that sex occurs only when it is least costly. For instance, organisms including aphids and daphnia reproduce asexually when resources are abundant and switch to sex only at the end of the season, when the potential for asexual reproduction is limited and when potential mates are more available.
Similarly, many single-celled organisms have sex only when starved, which minimizes the time cost of switching to meiosis because mitotic growth has already ceased. Although various mechanisms might reduce the costs of sex, it is still commonly assumed that sex is more costly than asexual reproduction, raising yet another obstacle for the evolution of sex. The aforementioned points might lead one to conclude that sex is a losing enterprise. However, sex is incredibly common. Furthermore, even though asexual lineages do arise, they rarely persist for long periods of evolutionary time.
Among flowering plants, for example, predominantly asexual lineages have arisen over times, yet none of these lineages is very old. Furthermore, many species can reproduce both sexually and asexually, without the frequency of asexuality increasing and eliminating sexual reproduction altogether. What, then, prevents the spread of asexual reproduction?
The first generation of mathematical models examining the evolution of sex made several simplifying assumptions—namely, that selection is constant over time and space, that all individuals engage in sex at the same rate, and that populations are infinitely large. With such simplifying assumptions, selection remains the main evolutionary force at work, and sex and recombination serve mainly to break down the genetic associations built up by selection.
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