Tuesday, February 28, 2017

On sexual orientation


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Part I

It may disturb some and it may even shock many. Homosexuality, bisexuality and asexuality have in the past few years been recorded in a wide variety of animals in the wild, particularly birds and mammals. Is it a matter of the animals' choices, or, a perfectly natural phenomenon? Many seem to look at the rest of life from a human perspective, very often failing to see the diversity of sexual reproduction and the diversity of ways in which it is manifested.

Perhaps it might be helpful to examine sex and sexual reproduction from the biological perspective of reproduction of both plants and animals. While life forms may reproduce themselves asexually, most forms reproduce sexually. What do the terms mean? The word "asexual'' simply means "without sex'', so that it is important to understand clearly the meaning of the term "sex'' in biology.

Classically the meaning of the word "sex'' is simply the sum of the diverse characteristics, structures or functions that enable one to determine that a plant or animal is either male or female. Of course there are many plants and animals in which it is often impossible by simple direct observation to determine gender. It is therefore necessary to look into the heart of sexual reproduction which is at the cellular level and a process that originated, or if you will, was created, over a billion years ago, and not at the diverse behaviours that are involved in facilitating the process. In a nutshell, sexual reproduction is about the union of male and female sex cells or gametes that are produced by two parents.

In most life forms the body cells are seen to have a discrete nucleus in which are the paired chromosomes that contain the DNA or genetic material contributed by both parents and are said to be diploid.

To compound the general picture is the phenomenon of polyploidy as may be seen in certain plants where chromosomes may be in multiples of the diploid state. In the production of the gametes the number of chromosomes is reduced by half. They are said to be the haploid state. In the process of the reduction of the number there may be a crossing over of fragments of individual chromosomes to form new combinations of DNA material from both parents.

In addition, as is well known in humans, there may be an unlike pair of sex chromosomes, the X and the Y chromosomes. Gametes may therefore have either an X or a Y chromosome and when the gametes fuse and the chromosomes become paired up, the combinations of these may be XX and XY, the former becoming a female and the latter a male. And just to confound us further the process of sex cell formation or gametogenesis abnormalities may develop with say an XYY or simple XO combinations of chromosomes. Also some genes may be carried only on one of the two sex chromosomes. The medical condition of haemophilia or bleeding is carried on the X chromosome, making females carriers and their male offspring victims.

There is much variability in the form of the gametes, some may be of similar shape and form, while others may be strikingly different. Some may be filled with nutrients for facilitating growth while others may be small and motile, being no more than small packets of DNA with some form of a tail for propulsion.

In sexual reproduction haploid chromosomes of the gametes recombine and restore the characteristic diploid state of a new cell or zygote that then grows into a new organism. Growth is of course not a sexual process for as cells divide and differentiate into different forms division of the cell nuclei is by a different process and the diploid state is maintained throughout life of that individual. This is all the way in passing to the equally important questions of maleness and femaleness.

In many animals and most plants it is quite impossible to determine maleness or femaleness in terms of physical structures or behaviours. Many plants, for example, do not display any difference in form between male and female principally because they are hermaphroditic, that is producing both male and female gametes from the same plant. Some, for example, like the common pumpkin, produces separate male and female flowers. Others like the paw paw, however, may produce separate male and female plants. In many primitive animals hermaphroditism is the norm while in many advanced animals the sexes are separate, often with marked differences in form, and some even abandon maleness and all are female.

Sexual reproduction ranges from a simple shedding of the gametes to the wind or waters, while in others there may be complex mechanisms that have evolved to enhance the possibilities of contact between gametes, the employment of pollinators in many plants, or the transfer of sperm in some form of physical contact.

To be continued

Julian Kenny is a biologist and natural history author. Heis a former UWI Professor of Zoology and

Independent senator