Of Birds and Bees

If you are like me - and I'm not accusing anyone - then you are infinitely curious about the progress of life science with respect to genetics. The joke about it is this: in reviewing information about human reproduction on an educational C.D. meant for junior high school this morning, I had the dizzying experience of finding out I had some of the basics all wrong. Spermatozoids develop as such and travel in the male anatomy only to be later mixed with liquids from a small sac and from the prostrate gland (prostrate cancer is very serious). On the female side it turns out that the timing of ovulation is totally unpredictable. Temperature rises when it does occur and two days later the female is no longer fertile because the yellow liquid within the ovule which is the basis of pregnancy hormones has leaked out. Thus conception is always possible before temperature rise and never after. ( Be sure to let me know if this doesn't work out). Personally I went through adult life thinking pretty much the contrary: that the safe time was after one's period. Ah progress!

Theoretical genetics is becoming quite sophisticated even though practices are what "works", millenia of evolution having taken care of the details, as it were. A karyotype - that well-known view of 23 pairs of Xs representing chromosomes - is a human cisor and paper construct. A human cell is diploid - has a double of every chromosome (22) while the sexual pair is different in the male. Each member of a pair has been contributed by one of the two parents. In view of reproduction the cell must become haploid, that is form gametes which are singles. The first step in doing so is for each chromosome to double itself, forming that famous X shape, two sticks joined together at the centermere which is actually a connection mode that allows things to be moved around during cell division( a bit like handles in a graphics program) and go through two rounds of division. Each arm of the X is meant to mirror the other, coding for the same gene at the same place but in fact there are often small differences between one arm and the other known as alleles. Thus the X pairs are identical in sequence, known as homologous, but the mirror arms on each X are instances of genetic variability, slightly different versions of the same gene code. The process by which variations are created in the formation of not quite identical copies is known as crossing-over, a process which we meet again when gametes get together during fertilization.

I never knew his: there are actually two moments when diversity is created. It feels really good to understand this process a bit better.

A really interesting exercise is the creation of a table showing the possibility of passing on a disease or condition between one generation and the next: that is so because some diseases are dominant while others are recessive. A and B blood types are a very rare co-dominant: this is why some individuals will have AB blood type, expressing both. Women ovulate from one or the other ovary every month; identical twins occur when she ovulates from both ovaries and each ovule gets fertilized. Neat!

So one has a genotype, that is the nuclei of our cells contain double versions of the genetic code and this is the stuff from which we pass on traits to the next generation, but that particular version which one expresses oneself is called a phenotype. Traits are characteristic of a species, attributes are particular to the individual. Voilà!

SOURCE: ADI Sciences, Coktel, 2000. A few readings from Wikipedia.