Sexual Reproduction

Lesson Content

Inquire: Sexual Reproduction


Nearly all eukaryotes undergo sexual reproduction. The variation introduced into the reproductive cells by meiosis provides important advantages that have made sexual reproduction evolutionarily successful. Meiosis and fertilization alternate in sexual life cycles. The process of meiosis produces unique reproductive cells called gametes, which have half the number of chromosomes as the parent cell. When two haploid gametes fuse, it restores the diploid condition in the new zygote. Thus, most 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.


Big Question

What are the three different life cycles among sexual multicellular organisms, and what are their commonalities?

Watch: Sexual Reproduction and Inherited Disorders

Read: Sexual Reproduction


Sexual reproduction was an early evolutionary innovation after the appearance of eukaryotic cells. The fact that most eukaryotes reproduce sexually is evidence of its evolutionary success. In many animals, it is the only mode of reproduction. And yet, scientists recognize some real disadvantages to sexual reproduction. On the surface, offspring that are genetically identical to the parent may appear to be more advantageous; if the parent organism is successfully occupying a habitat, offspring with the same traits would be similarly successful. There is also the obvious benefit to an organism that can produce offspring by asexual budding, fragmentation, or asexual eggs. These methods of reproduction do not require another organism of the opposite sex. There is no need to expend energy finding or attracting a mate. Instead, that energy can be spent on producing more offspring. Indeed, some organisms that lead a solitary lifestyle have retained the ability to reproduce asexually. In addition, asexual populations only have female individuals, so every individual is capable of reproduction. In contrast, the males in sexual populations (half of the population) are not producing offspring themselves. Because of this, an asexual population can grow twice as fast as a sexual population in theory. This means that in competition, the asexual population would have the advantage. All of these advantages to asexual reproduction, which are also disadvantages to sexual reproduction, should mean that the number of species that reproduce asexually should be more common.

DecorativeMulticellular organisms that exclusively depend on asexual reproduction are exceedingly rare. Why is sexual reproduction so common? This is one of the important questions in biology and has been the focus of much research from the latter half of the 20th century until now. A likely explanation is that the variation that sexual reproduction creates among offspring is very important to the survival and reproduction of those offspring. The only source of variation in asexual organisms, mutation, is the ultimate source of variation. 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 that divides the chromosomes among gametes. Variation is introduced during meiosis, as well as when the gametes combine in fertilization.

Life Cycles of Sexually Reproducing Organisms

Fertilization and meiosis alternate in sexual life cycles. What happens between these two events depends on the organism. The process of meiosis reduces the resulting gamete’s chromosome number by half. Fertilization, the joining of two haploid (having a single set of chromosomes) gametes, restores the diploid (having a complete set of chromosome) condition. There are three main categories of life cycles in multicellular organisms: diploid-dominant, in which the multicellular diploid stage is the most obvious life stage (and there is no multicellular haploid stage), as with most animals including humans; haploid-dominant, in which the multicellular haploid stage is the most obvious life stage (and there is no multicellular diploid stage), as with all fungi and some algae; and alternation of generations, in which the two stages, haploid and diploid, are apparent to one degree or another depending on the group, as with plants and some algae.

Nearly all animals employ a diploid-dominant life-cycle strategy in which the only haploid cells produced by the organism are the gametes. The gametes are produced from diploid germ cells, a special cell line that only produces gametes. Once the haploid gametes are formed, they lose the ability to divide again. There is no multicellular haploid life stage. Fertilization occurs with the fusion of two gametes, usually from different individuals, restoring the diploid state.

Most fungi and algae employ a life-cycle strategy in which the multicellular “body” of the organism is haploid. During sexual reproduction, specialized haploid cells from two individuals join to form a diploid zygote. The zygote immediately undergoes meiosis to form four haploid cells called spores.

DecorativeThe third life-cycle type, employed by some algae and all plants, is called alternation of generations. These species have both haploid and diploid multicellular organisms as part of their life cycle. The haploid multicellular plants are called gametophytes because they produce gametes. Meiosis is not involved in the production of gametes in this case, as the organism that produces gametes is already haploid. Fertilization between the gametes forms a diploid zygote. The zygote will undergo many rounds of mitosis and give rise to a diploid multicellular plant called a sporophyte. Specialized cells of the sporophyte will undergo meiosis and produce haploid spores. The spores will develop into the gametophytes.

Reflect: Being a Geneticist


If you were a geneticist, which disorder would you want to study?

Expand: Sex Chromosome Nondisjunction in Humans


Disorders of chromosome number include the duplication or loss of entire chromosomes, as well as changes in the number of complete sets of chromosomes. They are caused by nondisjunction, which occurs when pairs of homologous chromosomes or sister chromatids fail to separate during meiosis. Nondisjunction can occur during either meiosis I or II, with differing results.

Humans display dramatic deleterious effects with autosomal trisomies and monosomies. Therefore, it may seem counterintuitive that human females and males can function normally, despite carrying different numbers of the X chromosome. Rather than a gain or loss of autosomes, variations in the number of sex chromosomes occur with relatively mild effects. In part, this happens because of the molecular process, X inactivation. Early in development, when female mammalian embryos consist of just a few thousand cells, one X chromosome in each cell inactivates by tightly condensing into a quiescent (dormant) structure, or a Barr body. The chance that an X chromosome inactivates in each cell is random, but once this occurs, all cells derived from that one will have the same inactive X chromosome or Barr body. By this process, females compensate for their double genetic dose of X chromosome. In so-called “tortoiseshell” cats, we observe embryonic X inactivation as color variegation. Females that are heterozygous for an X-linked coat color gene will express one of two different coat colors over different regions of their body, corresponding to whichever X chromosome inactivates in each region’s embryonic cell progenitor.

DecorativeAn individual carrying an abnormal number of X chromosomes will inactivate all but one X chromosome in each of her cells. However, even inactivated X chromosome continues to express a few genes, and X chromosomes must reactivate for the proper maturation of female ovaries. As a result, X-chromosomal abnormalities typically occur with mild mental and physical defects, as well as sterility. If the X chromosome is absent altogether, the individual will not develop in utero.

Duplications and Deletions

A chromosomal segment may duplicate or lose itself. Duplications and deletions often produce offspring that survive but exhibit physical and mental abnormalities. Duplicated chromosomal segments may fuse to existing chromosomes or may be free in the nucleus. Cri-du-chat (French for “cry of the cat”) is a syndrome that occurs with nervous system abnormalities and identifiable physical features that result from a deletion of most 5p (the small arm of chromosome 5). Infants with this genotype emit a characteristic high-pitched cry on which the disorder’s name is based.

Chromosomal Structural Rearrangements

Cytologists have characterized numerous structural rearrangements in chromosomes, but chromosome inversions and translocations are the most common. We can identify both during meiosis by the adaptive pairing of rearranged chromosomes with their former homologs to maintain appropriate gene alignment. If the genes on two homologs are not oriented correctly, a recombination event could result in losing genes from one chromosome and gaining genes on the other. This would produce aneuploid gametes.

Chromosome Inversions

A chromosome inversion is the detachment and reinsertion of part of a chromosome. Inversions may occur in nature as a result of mechanical shear, or from transposable elements’ action. Unless they disrupt a gene sequence, inversions only change gene orientation and are likely to have more mild effects than aneuploid errors. However, altered gene orientation can result in functional changes because regulators of gene expression could move out of position with respect to their targets, causing aberrant levels of gene products.

An inversion can be pericentric and include the centromere, or paracentric and occur outside the centromere. A pericentric inversion that is asymmetric about the centromere can change the chromosome arms’ relative lengths, making these inversions easily identifiable.


DecorativeA translocation occurs when a chromosome segment dissociates and reattaches to a different, nonhomologous chromosome. Translocations can be benign or have devastating effects depending on how the positions of genes are altered with respect to regulatory sequences. Notably, specific translocations have occurred with several cancers and with schizophrenia. Reciprocal translocations result from exchanging chromosome segments between two nonhomologous chromosomes such that there is no genetic information gain or loss.

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Additional Resources and Readings

Formation of gametes

Learn about spermatogenesis, oogenesis, and nondisjunction in this video!

Sexual Life Cycles

A video teaching the different sexual life cycles and giving examples of organisms which use them

Haploid vs Diploid

A video explaining the difference between haploid and diploid cells

Lesson Glossary


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  • alternation of generations
    life-cycle type in which the diploid and haploid stages alternate
  • chromosome inversion
    detachment, 180° rotation, and chromosome arm reinsertion
  • diploid-dominant
    life-cycle type in which the multicellular diploid stage is the most obvious life stage and there is no multicellular haploid stage, as with most animals including humans
  • gametophytes
    a multicellular haploid life-cycle stage that produces gametes
  • germ cells
    specialized cell line that produces gametes, such as eggs or sperm
  • haploid-dominant
    life-cycle type in which the multicellular haploid stage is the most obvious life stage and there is no multicellular diploid stage, as with all fungi and some algae
  • life cycles
    the sequence of events in the development of an organism and the production of cells that produce offspring
  • meiosis
    a type of cell division that results in four daughter cells each with half the number of chromosomes of the parent cell, as in the production of gametes and plant spores
  • nondisjunction
    failure of synapsed homologs to completely separate and migrate to separate poles during the meiosis' first cell division
  • paracentric
    inversion that occurs outside the centromere
  • pericentric
    inversion that involves the centromere
  • sporophyte
    a multicellular diploid life-cycle stage that produces haploid spores by meiosis
  • translocation
    process by which one chromosome segment dissociates and reattaches to a different, nonhomologous chromosome
  • X inactivation
    condensing X chromosomes into Barr bodies during embryonic development in females to compensate for the double genetic dose

License and Citations

Content License

Lesson Content:

Authored and curated by Jill Carson for The TEL Library. CC BY NC SA 4.0

Adapted Content:

Title: Biology – 11.2 Sexual Reproduction – Life Cycles of Sexually Reproducing Organisms. OpenStax CNX. License: CC BY 4.0

Title: Biology – 13.2 Chromosomal Basis of Inherited Disorders – Sex Chromosome Nondisjunction in Humans: Rice University, OpenStax CNX. License: CC BY 4.0

Media Sources

DecorativeEukaryoteUnknownWikimedia CommonsPublic Domain
DecorativeTranslocationVariousWikimedia CommonsPublic Domain
DecorativeTortoiseshell cat Cindy in a boxCalicocindyWikimedia CommonsPublic Domain
DecorativeMushrooms in late summerHardyplantsWikimedia CommonsPublic Domain
DecorativeParazoanthus axinellae (Schmidt, 1862) 3Parent GéryWikimedia Commons Public Domain