Zoology · Ch 4 — Principles of Inheritance and Variation
Sex Determination
Sex Determination
Sex determination is simply the mechanism by which a species establishes and fixes the distinction between male and female individuals. In dioecious (separate-sexed) organisms this job usually falls to a dedicated pair of sex chromosomes; every other chromosome in the set — which does not differ between the sexes — is called an autosome by contrast. Sex chromosomes can be homomorphic (structurally alike) in one sex, in which case that sex produces only one kind of gamete and is called homogametic, or heteromorphic (structurally different) in the other sex, which then produces two kinds of gametes and is called heterogametic. This section catalogues every combination animals actually use.
Where males are the heterogametic sex, two systems exist. In the XX-XO system (bugs, cockroaches, grasshoppers), females carry two X chromosomes (XX, homogametic) while males carry only a single, unpaired X (XO, heterogametic) — such males produce two sperm classes, one carrying the X and one carrying no sex chromosome at all, and whichever sperm fertilises the egg fixes the offspring's sex (Fig. 4.2). In the more familiar XX-XY system (humans, Drosophila), females are again XX-homogametic, but males instead carry one X and one distinct Y chromosome, producing X-bearing and Y-bearing sperm in roughly equal numbers (Fig. 4.3): an X-bearing sperm gives a female, a Y-bearing sperm gives a male.
Where females are instead the heterogametic sex — certain insects and vertebrates such as fishes, reptiles and birds — the letters Z and W are used in place of X and Y to avoid confusing the two systems. In the ZO-ZZ system (certain moths, butterflies, domestic chickens), the female is heterogametic with a single Z (ZO) and produces two egg classes, while the male is homogametic (ZZ) and produces one sperm class (Fig. 4.4). In the ZW-ZZ system (gypsy moth; fishes, reptiles, birds generally), the female instead carries one Z and one W chromosome (ZW, heterogametic, producing two egg classes) while the male remains homogametic ZZ (Fig. 4.5). …
What this figure shows. A simple cross diagram (as in bugs, cockroaches and grasshoppers) with the parental (P1) generation written as AAXX (homogametic female, two X chromosomes plus autosomes 'A') crossed with AAXO (heterogametic male, a single unpaired X chromosome plus autosomes, no second sex chromosome at all). Below it, the gametes line is shown as AX from the female and, from the male, two gamete classes AX and AO (sperm with the X and sperm with no sex chromosome). The F1 row shows the two resulting offspring classes, AAXX (female) and AAXO (male), illustrating that which sperm fertilises …
What this figure shows. The same style of cross diagram for the human/Drosophila pattern: parental generation AAXX (homogametic female) crossed with AAXY (heterogametic male). The gametes line shows AX from the female and two sperm classes, AX and AY, from the male. The F1 row shows AAXX (female) resulting from an X-bearing sperm and AAXY (male) resulting from a Y-bearing sperm, showing sex is fixed at fertilisation by which sperm type fuse …
What this figure shows. A cross diagram for the pattern seen in certain moths, butterflies and domestic chickens: parental generation AAZO (heterogametic female, a single unpaired Z chromosome) crossed with AAZZ (homogametic male, two Z chromosomes). The gametes line shows two egg classes from the female, AZ and AO, and a single sperm class AZ from the male. The F1 row shows the resulting AAZZ (male) and AAZO (female) offspring, the mirror image of the XX-XO sys …
What this figure shows. A cross diagram for the pattern in gypsy moth and vertebrates such as fishes, reptiles and birds: parental generation AAZW (heterogametic female, one Z and one W chromosome) crossed with AAZZ (homogametic male, two Z chromosomes). The gametes line shows two egg classes from the female, AZ and AW, and a single sperm class AZ from the male. The F1 row shows the resulting AAZZ (ma …
What this figure shows. A branching pedigree-style diagram of the human XX/XY cross. The parents are labelled Male (Heterogametic), 44A+XY, and Female (Homogametic), 44A+XX. Below them the gametes are split into sperms — one class 22A+X, one class 22A+Y — and ova, a single class 22A+X. The offspring/progeny row shows the four resulting combinations, 44A+XX and 44A+XY arising on the sperm side and the same two combinations arising on the ova side, each labelled Female or Male, illustrating that a normal human female karyotype is 44 autosomes plus XX and a normal male karyotype is 44 autosomes plus XY, and th …