Single gene control of sex

Sex Determination, Sex Differentiation, Dosage Compensation and Genetic Imprinting
Chromosome Theory of Sex Determination 
Balance Theory of Sex Determination X/A ratio in Drosophila
Triploid intersexes in Drosophila and genie balance theory
X/A ratio and gynandromorphs in Drosophila
X/A ratio in Coenorhabditis elegans (a free living nematode)
Balance Between Male and Female Factors
- Diploid intersexes in gypsy moth (Lymantria)
- X/A ratio and multiple numerator elements (Drosophila and Coenorhabditis)
Sex Determination in Plants
Methods for determining heterogametic sex in plants
Sex determination in Coccinia and Melandrium
Sex determination in other dioecious plants
Sex Chromosomes in Mammals Including Humans (Homo sapiens)
TDF, ZFY and SRY genes in humans
H-Y antigen and male development in mammals
Single gene control of sex
Sex determination in Asparagus
Tassel seed (ts) and silkless (sk) genes in maize
Transformer gene (tra)in Drosophila
Haploid males in Hymenoptera
Hormonal control of sex
Environmental Sex Determination in Reptiles
Dosage Compensation in Organisms with Heterogametic Males
X-chromosome inactivation in mammals
Position effect variegation
Hyperactivity of X-chromosome in male Drosophila
Lack of Dosage Compensation in Organisms with Heterogametic Females
Genetic imprinting
In examples discussed in previous sections of this section, sex was determined by individual sex chromosomes (X or Y) or by a complete set of-autosomes. It is also known that, in such cases, whole chromosome having large number of genes, rather than single gene, controlled the sex. However, there are other instances where individual single genes were found to be responsible for determination or expression of sex. Three such cases will be briefly described in this section.

Sex determination in Asparagus
Asparagus is a dioecious form. However, rarely female flowers bear rudimentary anthers and male flowers bear rudimentary pistils. Thus, rare male flowers having poorly developed pistils may set seeds. In one such case, when seeds obtained from a rare male flower, were raised into plants, male and female plants were found to be present in 3 : 1 ratio. When male plants raised thus were used to pollinate female flowers on female plants, only two-third of them showed segregation indicating that sex is controlled by a single gene. In this case, maleness should be dominant over femaleness and male plants should ordinarily be heterozygous. These results are diagrammatically represented in Figure 17.21.
Segregation for sex in seed obtained from a rare bisexual flower in Asparagus showing monogenic control.
Fig. 17.21. Segregation for sex in seed obtained from a rare bisexual flower in Asparagus showing monogenic control.

Tassel seed (ts) and silkless (sk) genes in maize
Maize is monoecious with male inflorescence (tassel) and female inflorescence (silk) located on the same plant. A gene tassel seed (ts) is known, which will convert the tassel into seed bearing inflorescence. Another gene silkless (sk) is responsible for the absence of silks. Therefore, a plant sk/sk will be effectively a male plant and a plant, ts/ts will be effectively a female plant. By using these genes it has been possible to convert maize from a monoecious to a dioecious form, where plants will bear only male or only female inflorescences.

For instance, if we use females with genotype ts ts sk sk (seeds on tassels, silkless) and males with genotype Ts ts sk sk (normal tassel and silkless) on crossing, they will give males and females (having same genotypes as parents) in 1 : 1 ratio. This will be stable dioecious situation.

Transformer gene (tra)in Drosophila
In Drosophila, a transformer gene (tra) has been identified, which if present in homozygous condition (tra/tra) converts a female into a sterile male, but does not act upon normal male individual.