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In the XY sex-determination system, the female-provided ovum contributes an X chromosome and the male-provided sperm contributes either an X chromosome or a Y chromosome, resulting in female (XX) or male (XY) offspring, respectively. Hormone levels in the male parent affect the sex ratio of sperm in humans. [25]
Sexual differentiation is the process of development of the sex differences between males and females from an undifferentiated zygote. [1] [2] Sex determination is often distinct from sex differentiation; sex determination is the designation for the development stage towards either male or female, while sex differentiation is the pathway towards the development of the phenotype.
The sex-determination system of zebrafish is polygenic. Juvenile zebrafishes (0–30 days after hatching) have both ovary-like tissue to testis tissue. They then develop into male or female adults, with the determination based on a complex interaction genes on multiple chromosomes, but not affected by environmental variations. [41] [42]
Depending on the mechanism of sex determination for a species, decisions about sex allocation may be carried out in different ways. In haplodiploid species, like bees and wasps, females control the sex of offspring by deciding whether or not to fertilize each egg. If she fertilizes the egg, it will become diploid and develop as a female.
An offspring formed from the union of a sperm and an egg develops as a female, and an unfertilized egg develops as a male. This means that the males have half the number of chromosomes that a female has, and are haploid. The haplodiploid sex-determination system has a number of peculiarities.
Determination of sex is a process by which scientists and medical professionals determine the biological sex of a person or other animal using genetics and biological sexual traits. The term sex assignment may be used in reference to humans.
The female-biased sexual size dimorphism observed in many taxa evolved despite intense male-male competition for mates. [28] In Osmia rufa, for example, the female is larger/broader than males, with males being 8–10 mm in size and females being 10–12 mm in size. [29] In the hackberry emperor females are similarly larger than males. [30]
In many fishes, female fecundity increases continuously with age, while in other species larger males have a selective advantage (such as in harems), so it is hypothesized that the mating system can determine whether it is more selectively advantageous to be a male or female when an organism's body is larger.