Sex-Determining Mechanisms Help (page 2)

By — McGraw-Hill Professional
Updated on Apr 25, 2014

Heterogametic Females

This method of sex determination is found in a comparatively large group of insects including the butterflies, moths, caddis flies, and silkworms, and in some birds and fishes. The 1-X and 2-X condition in these species determines femaleness and maleness, respectively. The females of some species (e.g., domestic chickens) have a chromosome similar to that of the Y in humans. In these cases, the chromosomes are sometimes labeled Z and W instead of X and Y, respectively, in order to call attention to the fact that the female (ZW) is the heterogametic sex and the male (ZZ) is the homogametic sex. The females of other species have no homologue to the single sex chromosome as in the case of the XO mechanism discussed previously. To point out this difference, the symbols ZZ and ZO may be used to designate males and females, respectively. A 1 : 1 sex ratio is expected in either case.

Sex Chromosome Mechanisms

The W chromosome of the chicken is not a strong female-determining element. Recent studies indicate that sex determination in chickens, and probably birds in general, is similar to that of Drosophila, i.e., it is dependent upon the ratio between the Z chromosomes and the number of autosomal sets of chromosomes (see next section, Genic Balance).

Genic Balance

The presence of the Y chromosome in Drosophila, although essential for male fertility, is not involved in the determination of sex. Instead, the factors for maleness residing in all of the autosomes are "weighed" against the factors for femaleness residing on the X chromosome(s). In fact, it is the ratio of X chromosomes to haploid sets of autosomes that determines sex in Drosophila. Using the letter A to represent a haploid set of chromosomes, a normal female (2X : 2A) has an X : A ratio of 2 : 2, or 1.0, and therefore the balance is in favor of femaleness. When only one X chromosome is present in a normal male (XY : 2A), the ratio is 1 : 2 or 0.5. Several abnormal combinations of chromosomes have confirmed this hypothesis. For example, an individual with three sets of autosomes and two X chromosomes (2X: 3A) has a ratio of 2 : 3 or 0.67, in between the ratios for normal maleness and femaleness. This kind of fly, called intersex, is sterile and has sexual characteristics intermediate between the male and female. Ratios above 1.0 produce sterile metafemales (previously called super-females) and ratios below 0.5 produce sterile metamales.

The ratio determines sex by activating sex-specific gene expression of several genes, such as Sex-lethal (Sxl), transformer (tra), and doublesex (dsx). Simply stated, in females, the Sxl gene is active and leads to the production of an active tra gene product. This results in the further production of a female-specific DSX protein and development of female flies. The X:A chromosome ratio in normal males does not result in production of the SXL protein; thus, the tra protein is not produced and the default, or male developmental pathway, is followed. A recessive mutation in the tra+ gene, when homozygous, can transform a diploid female into a sterile male, since the absense of tra+ leads to maleness. The X/X, tra/tra individuals resemble normal males in external and internal morphology with the exception that the testes are reduced in size. This mutation has no effect in normal males. The presence of this mutation can considerably alter the sex ratio.

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