Genetics of Sex

Department of Biological and Environmental Sciences: Samford University

The Genetics of Sex
Text: pp. 175-192, 90-91

There may be some things better than sex, and some things worse, but nothing is exactly like it. W. C. Fields Sex Determination: Sex determination occurs by a variety of mechanisms. Monoecious and Hermaphroditic Organisms: Some organisms do not have separate sexes but produce both gametes, usually from separate male and female reproductive organs. Male and female tissue have the same genes, so the difference is not genetic. Most plants are monoecious. Many animals, like the earthworm, Lumbricus and the snail, Helix, are hermaphroditic. The earthworm has separate testes and ovaries while Helix has a single gonad that makes both egg and sperm. Environmentally Determined Sex: In some organisms, sex is determined by the "environment." In the marine worm Bonellia (Phylum: Echiura), if the larva settles to the bottom where there are no other females around, it will develop into a female. If, however, it lands on a female's proboscis, it develops into a male and lives in the cloaca of the female. It is tiny and has no digestive system. Other examples of environmentally determined sex include some nematode parasites where high population density in the host results in males, while low density in the host results in females. Among the reptiles, sex determination in some turtles, lizards, and crocodilians is dependent on the temperature at which the egg is incubated. (Also seen in some birds). Among plants, sex determination in some orchids is such that plants in bright sunlight develop into females and those in shade develop into males. (Reference for these is here)
Genetically Determined Sex: Sex is determined genetically in most organisms. Single-Gene Systems: In organisms like the the green alga Chlamydomonas and the fungi Saccharomyces and Neurospora, a single gene determines the mating type. In Saccharomyces, there are two mating types: a and α determined by a single gene. A similar case exists between the two mating types in Neurospora (A and a) and the two mating in Chlamydomonas types (mt⁺ and mt^-).
Euploid Systems: In some hymenopterans, sex is determined by the ploidy of the individual. If a queen honey bee's eggs are fertilized (2N) they become females but if not, they develop parthenogenetically into males (1N). Females produce eggs by meiosis but males produce sperm by mitosis or altered meiosis. (Queen eats royal jelly and becomes fertile.)
Sex Chromosomes: Many animals and some plants have a sex-chromosome mechanisms for sex determination. XY or XO Systems: The female is homogametic and the male is heterogametic. In grasshoppers, males have one X and females have two (there is no Y). Balance Theory of Sex Determination: In Drosophila, sex is determined by the ratio of X chromosome sets to autosome sets but the Y is necessary for fertility. (X/A≥1 is female, X/A≤0.5 is male, others are intersex.) There is a master gene (Sxl, Sex-lethal) which controls sex determination and is controlled by the X:A ratio. The mRNA of this gene can be alternatively spliced, producing a functional protein in females and a non-functional protien in males.
Y Determination in Mammals: In mammals, sex is determined by the presence or absence of the Y chromosome More specifically, maleness is determined by the presence of the gene SRY on the Y (sex-determining region Y). This gene codes for a protein transcription factor called testis-determining factor (TDF). The presence of TDF causes embryonic changes which result in the undifferentiated future gonad to develop into a testis. (Many other Y genes having to do with fertility.)
ZW Systems: In birds, moths and butterflies, and some others, the male is homogametic (ZZ) and the female (ZW) is heterogametic. Here's a weird one: A small non-coding RNA determining sex in a moth.
Dosage Compensation: Since XX females have one more X chromosome than males, there is a mechanisms to equalize the dose of X-linked genes between males and females.
Lyonization (X Inactivation): In mammals, dosage compensation is accomplished by Lyonization: the random (?) inactivation of one X chromosome early in development. (Barr bodies and tortoise-shell or calico cats.) This inactive X can be seen as a Barr body in females (heterochromatinized X chromosome). Inactivation mechanism: Inactivation of a specific X chromosome involves a region on the X called the XIC (X inactivation center). The XIST (X-inactive specific transcript) gene is part of this center. The XIST transcript (RNA) is made by this gene and this RNA coats and inactivates the inactive X chromosome. (XIST is not translated into protein.) Some areas of the inactivated X escape inactivation. Gene expression is not dose dependent after S phase of the cell cycle in yeast due to acetylation of histone H3.
Other Mechanisms: In Drosophila, dosage compensation is by another mechanism. The same Sxl gene involved in sex determination causes increased transcription of the genes on the single X of males so that it is the same as that on the two Xs of females.
Sex-Linked Traits versus Sex-Limited Traits versus Sex-Influenced Traits: Each of these phenomena can result in differences between the results of reciprocal crosses. Sex-linked traits involve genes on a sex chromosome. Sex-limited traits involve autosomal genes expressed genes that are only expressed in one sex (Cock feathers in fowl). Sex-influenced traits involve autosomal genes that are expressed when heterozygous in one sex but not in the other sex (male pattern baldness in humans used to be believed to be due to a sex-influenced gene. One type is now known to be be involved with an X-linked gene that is a variant of an androgen receptor: see this info from "23andMe").
Things I Learned at the Movies: All bombs are fitted with electronic timing devices with large red readouts so you know exactly when they're going to go off.