What is sex?

Learning Objectives

  1. Explain the advantages and disadvantages of sexual vs asexual reproduction
  2. Define ploidy, recognize that meiosis results in haploid cells, and know that gametes (germ cells) must be haploid so the resulting zygote will have a diploid chromosome count
  3. Know the three life cycles of sexual diploid organisms and give examples of each.
  4. Differentiate between mechanisms of sex determination.


Why reproduce sexually?

Sexual reproduction is the combination of (usually haploid) reproductive cells from two individuals to form a third (usually diploid) unique offspring (more on ploidy below…read on!). Compared to asexual reproduction, sexual reproduction has a couple of big disadvantages: it requires the time and energy to find a mate, and only half of the populations (females) can actually make offspring.  Because every member of an asexually-reproducing population can generate offspring, this means that, with all else equal, an asexually reproducing population will rapidly outcompete a population reproducing sexually.


This diagram illustrates the twofold cost of sex. If each individual were to contribute to the same number of offspring (two), (a) the sexual population remains the same size each generation, where the (b) asexual population doubles in size each generation. CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=1599721

However, asexually-reproducing individuals can only make clones, meaning that every one of their offspring are genetically identical (except in the case of mutations). This can be a disadvantage if conditions suddenly change, and the individuals are no longer well-adapted to the conditions. In contrast, sexual reproduction produces offspring with novel combinations of genes, which can be an advantage in unstable or unpredictable environments.

The information below was adapted from Wikipedia.

A water flea, Daphnia pulex (Photo credit: Paul Hebert. From PLoS Biology Vol. 3/6/2005, e219. doi:10.1371/journal.pbio.0030219.)

Species that can switch between sexual and asexual reproduction, such as the water flea Daphnia pulex (see image), highlight the relative advantages of asexual and sexual reproduction.  Daphnia live in various aquatic environments, including ponds, lakes, streams, and rivers. Early in the growth season, when food is abundant, diploid Daphnia females reproduce asexually via parthenogenesis. Almost all Daphnia in these populations are female and genetically identical to their mothers (via parthenogenic reproduction). Toward the end of the growing season, when conditions are not as stable or ideal, the Daphnia females alter their reproductive strategies to produce male as well as female offspring, as illustrated in the graph below. Instead of reproducing parthenogenic, diploid daughters, they start producing haploid eggs which are then fertilized by the males. These special haploid eggs are capable of surviving extreme conditions such as cold or drought, and they will hatch into diploid individuals once environmental conditions improve.This video discusses some of the above as it address the question of “why sex?”:

Gametes (sperm and eggs) are haploid, and made by meiosis

Most eukaryotic cells are diploid (2N), indicating that they contain two full sets of chromosomes, one set from each of the organism’s parents. “Ploidy” is the number of full sets of chromosomes that a cell has, and “di” means two, so diploid = two copies.  When diploid organisms reproduce sexually, they produce new offspring by the fusion of two haploid gametes (sperm and eggs).  Haploid (1N) cells have a single copy of each chromosome. When the two haploid gametes fuse, they merge their genetic information, resulting in diploid offspring. If gametes were not haploid, then fusion of diploid gametes would result in tetraploid (4N) offspring, who would have twice as many chromosomes as they were supposed to have, and twice as many chromosomes as other members of their own species.

Meiosis is a special cell division process that produces haploid gametes from diploid cells. Just like mitosis, meiosis starts with a diploid cell that has undergone DNA replication. But unlike mitosis, meiosis has two sequential divisions (meiosis I and meiosis II) and results in 4 haploid cells. By comparison, mitosis has just one division and results in 2 diploid cells. Meiosis is basically a mechanism to reduce ploidy and allocate the DNA equitably into the gamete cells.

Meiosis sets the stage for how inheritance works (which is the topic of our next class). Here’s what happens during meiosis, which is divided into two cell divisions, numbered I and II:

  • Meiosis I (aka “reduction division”):
    1. Homologous chromosomes pair up and align end-to-end (synapsis), and crossing over (swapping DNA) occurs between homologous chromosomes (Prophase I). 
    2. Pairs of homologous chromosomes line up along the metaphase plate (metaphase I).
    3. The homologous chromosomes separate, with one going to each side of the dividing cell; this separation creates haploid (1N cells); the separation of each pair of homologous chromosomes occurs independently, so all possible combinations of maternal and paternal chromosomes are possible in the two daughter cells (anaphase I). 
    4. The nuclear envelope reforms around the separated homologous chromosomes; cytokinesis occurs after telophase I to produce two, haploid cells (telophase I). 
  • Meiosis II is essentially exactly the same as mitosis, except the cells are haploid instead of diploid. Cells enter meiosis II with haploid cells that contain two identical copies (sister chromatids) of each chromosome, and those are separated into two new cells by the end of meiosis II. 

The take-home is that meiosis results in the production of haploid cells. The cells are actually haploid after meiosis I, which seems pretty confusing, but it’s true!

Here are two simplified diagrams illustrating the overall process and products of meiosis (top) vs mitosis (bottom). Compare the final results of mitosis with the products of meiosis I, and you can see that things look pretty different even after the first meiotic division:

Meiosis Overview. In prophase I, homologous chromosomes pair and separate in the first division (Meiosis I). In Meiosis II, sister chromatids separate. Image credit: from Wikipedia by Rdbickel. 

File:Major events in mitosis.svg

Mitosis overview. Image credit: Wikimedia Commons (https://commons.wikimedia.org/wiki/File:Major_events_in_mitosis.svg)


And this video provides an overview of meiosis:


The chromosome number, N, in eukaryotes, refers to the number of chromosomes in a haploid cell, or gamete (sperm or egg cell). Diploid cells (all the cells in our body except our gametes) have 2N chromosomes, because a diploid organism is created by union of 2 gametes each containing 1N chromosomes. In terms of chromosome number (ploidy), it’s useful to think of chromosomes as packages of genetic information. A pair of sister chromatids (the result of DNA replication during S phase) is actually just one chromosome because it has genetic information (alleles) inherited from only one parent. A pair of homologous chromosomes, each consisting of a single chromatid in a daughter cell at the end of mitosis, has alleles from the father and from the mother, and counts as 2 chromosomes.

The video below presents a helpful way for recognizing how many chromosomes are present in a cell (and thus the ploidy level of that cell):

Sexual reproduction life cycles

Fertilization and meiosis alternate in sexual life cycles. What happens between these two events depends on the organism. The process of meiosis reduces the chromosome number by half. Fertilization, the joining of two haploid gametes, restores the diploid condition. Different organisms accomplish these steps in different ways and at different times in their life cycles (note that a change in ploidy is always required). All life cycles involve a haploid (1 complete set of chromosomes) and diploid (2 complete sets of chromosomes) stage, but they vary in how and when in the life cycle these stages occur. There are three main categories of life cycles in multicellular organisms:

  • Diploid-dominant, in which the mature, multicellular organism (the most “obvious” stage of life) is diploid. Most animals (including humans) are diploid-dominant.

    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. The germ cells are generated early in the development of the zygote.

  • Haploid-dominant, in which the mature, multicellular organism is haploid. All fungi and some algae are haploid-dominant.

    Fungi, such as black bread mold (Rhizopus nigricans), have haploid-dominant life cycles. The haploid multicellular stage produces specialized haploid cells by mitosis that fuse to form a diploid zygote. The zygote undergoes meiosis to produce haploid spores. Each spore gives rise to a multicellular haploid organism by mitosis. (credit “zygomycota” micrograph: modification of work by “Fanaberka”/Wikimedia Commons)

  • Alternation of generations, in which there are two mature, multicellular organisms: one haploid, and one diploid. All plants and some algae have an alternation of generations life cycle.

    Plants have a life cycle that alternates between a multicellular haploid organism and a multicellular diploid organism. In some plants, such as ferns, both the haploid and diploid plant stages are free-living. The diploid plant is called a sporophyte because it produces haploid spores by meiosis. The spores develop into multicellular, haploid plants called gametophytes because they produce gametes. The gametes of two individuals will fuse to form a diploid zygote that becomes the sporophyte. (credit “fern”: modification of work by Cory Zanker; credit “sporangia”: modification of work by “Obsidian Soul”/Wikimedia Commons; credit “gametophyte and sporophyte”: modification of work by “Vlmastra”/Wikimedia Commons)

Sex determination – How are sexes determined?

In humans, individuals with two copies of the X chromosome (“homozygous” for X) are biologically female, and individuals with one X and one Y chromosome (“heterozygous”) are biologically male.  (X and Y are just the names of the chromosomes.) Because this is how sex is determined in humans, we tend to think of it as “normal,” however, there are many different mechanisms for determining biological gender:

Some species determine biological gender (sex) based on the types of chromosomes they have, and the sex-determining chromosomes are called “sex chromosomes”. The phenomenon where chromosomes determine biological gender is called chromosomal sex determinism:

  • Mammalian sex determination is determined genetically by the presence of X and Y chromosomes. Individuals who are homozygous for X (XX) are female and heterozygous individuals (XY) are male. The presence of a Y chromosome causes the development of male characteristics and its absence results in female characteristics. The XY system is also found in some insects and plants. [OpenStax Biology]
  • Avian (bird) sex determination is essentially the opposite of how mammalian sex determination works, with chromosomes that are named Z and W.  In birds, individuals who are homozygous for Z (ZZ) are male and heterozygous (ZW) results in a female. The W appears to be essential in determining the sex of the individual, similar to the Y chromosome in mammals. Some fish, crustaceans, insects (such as butterflies and moths), and reptiles use this system. [OpenStax Biology]

In simplified terms, the two systems above seem like chromosomal sex determinism, and some may truly be, with multiple genes on the sex chromosome that encode sex-specific traits. However, in humans, we now believe that one genetic region in particular encodes male traits: the sex-determining region Y (SRY), located on the Y chromosome, encodes a protein during development that differentiates its bearer into a male human.

Sex determinism can also be based on ploidy, in a phenomenon called haplodiploidy:

  • In some species, notably in most social insects (bees, wasps, yellow jackets (!)), females are diploid, with two copies of every chromosome, while males are haploid, with only one copy of each chromosome. If Buzz has one set of chromosomes, he’s a male; if two sets, a female. Female yellow jackets have ovipositors. The queen’s ovipositor develops so that she can lay eggs, but the workers do not lay eggs. Their ovipositors develop into stingers that can deliver a venom to organisms that threaten the worker. According to the GT logo and the life size mascot that runs around performing antics on campus, Buzz has a stinger, making her female. (Here’s an optional link to a fuller explanation of this at Ask An Entomologist.)

The sex of some species is not determined by genetics at all but by some aspect of the environment, called environmental sex determinism:

  • Sex determination in some crocodiles and turtles, for example, is often dependent on the temperature during critical periods of egg development. This is a general example of environmental sex determination, specifically called temperature-dependent sex determination. In many turtles, cooler temperatures during egg incubation produce males and warm temperatures produce females. In some crocodiles, moderate temperatures produce males and both warm and cool temperatures produce females. In some species, sex determination is both genetic- and temperature-dependent.

And in some species, biological gender isn’t as simple as male or female, or it may even change during the course of an individual’s life!

  • In some species, genders aren’t distinctly male or female, with some individuals being hermaphroditic (capable of producing both eggs and sperm). In nudibranchs, commonly called sea slugs, all individuals are hermaphrodites but cannot fertilize themselves.  In some species of nematode worms, most notably C. elegans, there are both hermaphrodites and males. Hermaphrodites can either mate with males or can use their own sperm to fertilize their eggs.
  • Individuals of some species change their sex during their lives, alternating between male and female. If the individual is female first, it is termed protogyny or “first female,” if it is male first, its termed protandry or “first male.” Oysters, for example, are born male, grow, and become female and lay eggs; some oyster species change sex multiple times. Wrasse fish live in groups of females with a single male, and the largest female will become male if the existing male dies. In some species, such as the water flea Daphnia pulex, individuals can even switch between sexual and asexual (parthenogenic) reproduction.