The male (XY) and female (XX) offspring in the above table are in a 50-50 ratio with an equal number of boys and girls. Therefore, the chance of having a boy is 1/2 or 50% and the chance of a girl is also 1/2 or 50%. This ratio can be demonstrated by tossing a coin many times and keeping track of the number of heads and tails. If enough tosses are made, the number of heads and tails should be very close to 50-50.

Unfortunately in biology, sex ratios in humans are not that easily explained. In the United States, there is a slightly better chance of having a boy, about 105 males to 100 females. There are a number of hypotheses (tentative explanations) for this unequal birth ratio, most of which are probably not accurate. If this unequal birth ratio is the result of a greater number of male conceptions, then perhaps the Y-sperm has a slightly better advantage in reaching the egg or penetrating the barrier of follicle cells around the egg, possibly by the smaller size of its head and faster speed. According to some references, muscular contractions and ciliary currents within the female reproductive tract are primarily responsible for transporting the sperm to the egg. In vitro, sperm can swim about 3 mm per minute, but within the vagina and oviduct (in vivo) they can travel about 5 mm per minute. Since the speed is additive, a faster Y-sperm could potentially still win the race to the egg. It has been demonstrated that Y-bearing sperm do not live as long as X-bearing sperm, so the time of ovulation and sexual intercourse could be a factor in determining the sex of a child. By the time the sperm reach the upper part of the fallopian tube where fertilization occurs, most of the Y-sperm may have already died. In this case when the egg is released from the ovary, the odds would be favor fertilization by an X-bearing sperm resulting in a girl. Another hypothesis for unequal sex ratios is an unequal number of X-bearing and Y-bearing sperm in the man's semen. Some references state that this condition might be hereditary, but it is not clearly explained in more scholarly texts.

A more plausible explanation for unequal birth ratios may involve the rejection of an embryo by the mother's antibodies. Conceptions may be approximately equal, but a greater loss of female embryos early in the gestation period may account for slightly more male births. The mother has two X-chromosomes, one from her mother and one from her father. Perhaps the X-chromosome from her father carries a gene that sensitizes her against the female fetus. In other words she develops antibodies against proteins of the female fetus but not the male fetus. Remember that these are only hypotheses at this time and should not be accepted as the final answer. When more testing is done, perhaps one or more of these hypotheses will become a widely-accepted theory.

[1] Parthenogenesis (agamogenesis): A haploid or diploid egg cell develops into an embryo. Contrary to some authors, parthenogenesis does not always result in genetically identical clones. If the haploid cells are formed by normal meiosis (as in the queen honeybee), crossing over during Prophase I of meiosis may result in some genetic variability. Crossing over is discussed later on this page. If the unfertilized eggs develop from mitotic oögenesis (without the reduction division of normal meiosis), then their offspring will be identical clones of each other.

[2] Agamospermy in plants: An embryo arises from tissue surrounding the embryo sac. If this involves cells of the nucellus or inner integument it is called a nucellar embryo. Nucellar embryos are chromosomally identical to the sporophyte parent. They are essentially clones of a fruit tree. Apomictic seeds allow propagation of choice cultivars without the transmission of viruses through cuttings. Mangosteen seeds (Garcinia mangostana) typically contain nucellar embryos and are used to propagate clones of this delicious tropical fruit tree.

Thickets of jumping cholla (Opuntia bigelovii) covering entire hillsides or alluvial fans in the Colorado Desert develop from fragmented stem segments that became rooted in the desert soil. Although jumping cholla produces flowers, the seeds of most populations are typically sterile and reproduction is accomplished without sexual reproduction (technically referred to as apomixis). You could say that jumping cholla is a master in the art of hitchhiking and cloning itself.

The virgin whiptail lizard (Cnemidophorus neomexicanus) of the western United States has only females in its population. One female mounts and clasps another female, presumably to induce ovulation. Because the genetic information has already been recombined in meiosis, the offspring are not identical clones of each other. In a PBS TV broadcast about honeybees, the narrator referred to drone bees as "clones" of each other. Since clones are usually defined as genetically identical individuals (usually derived asexually), Wayne's Word strongly disagrees with the accuracy of this statement. Although the haploid drone comes from an unfertilized egg with only one set of maternal chromosomes, they are certainly not all genetically identical. The diploid queen bee undergoes normal meiosis (oögenesis) producing haploid eggs. During this meiotic process her 16 pairs of homologous chromosomes become altered by crossing over and reshuffled through random assortment, resulting in haploid eggs that are not chromosomally identical. In fact, with 16 pairs of homologous chromosomes, there are 2¹⁶ or 65,536 different chromosomal combinations possible. Haploid drone bees produce haploid sperm in their testes through a mitotic spermatogenesis without a meiotic reduction division.

Another species with only females in the population is Fuller's rose weevil (Naupactus cervinus = N. godmanni = Asynonychus godmanni), a small flightless weevil introduced into California in the late 1800s. This beetle feeds on many cultivated plants and is especially troublesome in citrus groves where the adults eat new growth on young trees. The larvae feed on roots and make furrows in the bark. Eggs are laid on citrus fruits under the green calyx, and are transmitted during the shipment of infected fruits. Males have never been found in this species, so the females must produce viable eggs without fertilization (parthenogenesis).

Drone honey bee larvae develop within larger cells of the hive in order to accommodate the larger adult male. Worker bees develop in smaller cells from fertilized eggs and are essentially sterile females. The size of the cells is determined by workers who construct and mold the hexagonal wax cells with their mandibles; however, honeycomb cell size in sex determination is contradicted by K. Sasaki and Y. Obara (Zoological Science Volume 16, 1999) who reported fertilized eggs laid by queens in open areas outside honeycomb cells. Certain diploid worker larvae develop into fertile queens in much larger cells only if they are fed a special hormonal-nutrient mixture known as "royal jelly." The additional random combination of gametes during fertilization insures that worker bees are more genetically diverse than drones. With 2¹⁶ or 65,536 different chromosomal combinations in the gametes, there are (2¹⁶)² or 4,294,967,296 different ways for the egg and sperm to combine. Since drone bees possess only maternal genes, a sister worker bee cannot share any paternal genes with her brother. Worker bees can inherit paternal genes from one set of drone chromosomes and maternal genes from two sets of queen chromosomes. Since the available gene pool is much greater for worker bees compared with drones, their genetic variability is greater. Therefore, the female workers contribute more to the Darwinian fitness of the species through natural selection. The social relationship of honeybees in a colony is similar to the Borg collective in Star Trek. Whole castes of sterile females devote their entire existence to the welfare of the queen and the colony.

Entomologist Robert Page and his research team at the University of California, Davis have discovered the sex-determining "gender gene" (csd gene) in honeybees. Haploid drones have only one set of this gene per cell, while females have two sets. There are at least a dozen different forms of this gene (e.g. cds1, cds2, cds3, etc). Queen bees mate with many males, perhaps to insure that they get a good mix of different forms of the csd genes. If a queen mates with a male carrying her identical version of the csd gene, half of her fertilized eggs will develop into phenotypic sterile males, even though they are genetically diploid females with two sets of the same csd gene. These larvae are destroyed by worker bees. Apparently, two sets of identical csd genes will not function together.

Fire ants that are currently invading the southern United States also have a similar sex-determining (csd) gene. They suffer from high levels of sterility, probably because they brought only a few versions of csd genes with them from South America.

For beekeepers, inbreeding honeybees is a method of selecting docile insects that produce copious honey and reproduce rapidly; however, inbreeding also results in a high percentage of sterile males that are destroyed by workers. Pinpointing the precise "gender gene" in the sperm of males will enable beekeepers to produce disease-resistant, female honeybees containing different combinations of csd genes.

Parthenocarpy can be induced by growth hormones such as gibberellic acid (GA₃) in which the ovaries mature without fertilization. Grape cultivars such as 'Thompson Seedless' are treated with gibberellic acid to order to produce larger fruits with longer internodes. The bunches have wider spaces between the grapes and better air circulation, reducing their susceptibility to fungal diseases and rotting within the bunch. Contrary to some references, 'Thompson Seedless' grapes are not parthenocarpic because fertilization does occur, but the ovules fail to develop into seeds within the maturing fruit.

In cultivated figs, parthenocarpy generally refers to the development of the ovaries of female flowers within the syconium into drupelets without fertilization. The syconium is the structure that you typically associate with an edible fig fruit; however, it is really a flask-shaped structure lined on the inside with numerous unisexual flowers. The actual botanical fruits (called drupelets) develop within the syconium. Since the entire syconium enlarges and ripens into a juicy, sweet morsel, it is often referred to as a fruit. The female flowers are pollinated by a tiny female fig wasp that enters the syconium through a pore called the ostiole. According to W.B. Storey (Advances in Fruit Breeding, 1975), there are 2 genetically determined forms of parthenocarpy: stimulative and vegetative. Stimulative parthenocarpy involves the insertion of the wasp's ovipositor down the stylar canal into the ovary of short style flowers. It can also be induced by blowing air into the syconium, or by spraying the syconium with a plant growth regulator. The mature drupelets may contain a wasp (if an egg was laid in the ovary) or it may be empty. Vegetative parthenocarpy involves the formation of drupelets without any external stimulation, and is responsible for the hollow drupelets inside common figs such as "black mission," "kadota," and "brown turkey." [Some authors use the term parthenocarpy to describe the ripening of seedless fig syconia on the tree without any pollination or fertilization.]

The number of different possible gametes in diploid organisms due to independent assortment of the chromosomes during meiosis can be calculated from the simple formula 2ⁿ where n = the haploid number. For a human male with 23 pairs of chromosomes (2n = 46), this number would be 2²³ or 8,388,608 genetically different sperm. If crossing over occurs an average of three times per meiosis (spermatogenesis), this number can be multiplied by 2³ resulting in a total number of 2²⁶ or 67,108,864 genetically different sperm. The following table shows random combination of the gametes resulting in additional chromosome combinations and genetic variability.

In the following diagram, normal spermatogenesis is compared with spermatogenesis with nondisjunction at meiosis I (anaphase I) and nondisjunction at meiosis II (anaphase II). If the doubled X and Y chromosomes move to the same cell at meiosis I, the resulting gametes will each contain single X and Y chromosomes. If meiosis I proceeds normally and nondisjunction occurs at meiosis II when the chromatids separate, it is possible to get gametes containing two single X chromosomes and gametes containing two single Y chromosomes: