Wednesday, December 9, 2009

PerMutations and Combinations

Today I have this overwhelming urge to write about Genetics. I don’t know who/what triggered this line of thought but I dreamt of dominant and recessive genes and my biology teacher, Mrs.Pramila Francis. I can never forget her – her neatly pressed cotton sari, her one-rupee coin sized bindi, her perfect enunciation and her involvement in the subject; one of my favorite teachers from school.

Coming back to Genetics, we have heard of/seen irrational (and at times funny) arguments about the how the child has a fair complexion when both the parents are dark skinned. To understand this, one simply has to go the level of Genes and Chromosomes.

Genes are arranged as long chains of DNA sequences, called chromosomes. Although DNA strands are extremely long and super computers apparently work for hours or days to decode a gene sequence, we can still get to the simple basics. XX chromosome indicates a girl and XY chromosome indicates a boy. While the female body produces only X chromosome, the male can produce both X and Y. So essentially it is the chromosome from the male that determines the sex of the foetus. I am tempted to say, en iniya gramathu/nagarathu makkale, note this point and stop blaming mothers for the sex of the children but why should anyone worry about the sex of a new-born child anyway? I like to believe we have grown beyond that stage (Note: Comments with female infanticide stats will not be published, like I said let me believe in the goodness of people!)

Dominant genes, as the name states, are more common than recessive genes. For example, w.r.t Asians straight hair is a dominant gene while curly hair is recessive.Lets take the scenario of a desi guy (D) married to a Chinese girl (C), both with straight hair. Capitals indicate dominant genes, and small recessive. DD and Dd will show the same trait in a person since the dominant gene suppresses the recessive trait. So for the recessive quality to be exhibited, the structure has to dd or cc in this example. The desi guy and Chinese girl have a girl (Cd) and a boy (CD) both with beautiful black straight hair.

The girl marries a desi guy (Dd) and they end up having two baby girls but one with confusing black curls ! Result: Everyone is surprised about the curls since no one has had such hair in generations....Yes, the world of Genes is a wonderful mystery and you unravel it at your own risk. So, lesson learnt: Recessive genes can skip generations before the trait shows up in an offspring. Coming back to the age-old complexion discussion, in case of Dravidians, fair skin is a recessive trait. So the child can have a fair complexion (dd) even if both parents are dark skinned (Cd) but the converse is not true (Fair skinned parents producing a dark skinned offspring is a very rare or exceptional case).

A permanent change can occur in the DNA sequence and the extent of the change can vary. These changes or gene mutations have baffled geneticists for generations because not all of them are hereditary. They can be caused by exposure to UV radiation (now you know why pregnant women are asked to be extra careful when working with radiation or asked to avoid an eclipse) or even during cell division itself....till date, there are only probabilities as to what can occur, but no pre-defined cases. Add to this mix, evolution, the natural change in the DNA sequence as the environment changes, to complete the puzzle....err...did I say complete ?!

In case of hereditary mutations, the off-spring inherits the defective gene but it can remain recessive, say, Dd - d is the mutated gene; the offspring is unaffected but is a carrier. In such a case, marrying someone from the same family, who is a carrier (say, Cd - d is the mutated gene) would increase the chances of the recessive genes showing up in the next generation (dd - trait or disorder shows up). This is why it is generally advisable not to marry someone in the immediate family. Namoorla maman magal, ponnumani madhri padam edukaravanga kitta idha yaaravadhu poi sollungappa....tholla thanga mudiyala....!

Then there are the gender-related or sex-linked genetic disorders. Lets say a particular mutation, affects only the X chromosome but not the Y. This would mean that for a woman to possess this trait, both the chromosomes should be affected but for a man, there is a 50% more likelihood since one affected chromosome would do the trick. A good example is the red-green color blindness that is more common in men than in women. In this case, women act as carriers and are rarely affected.

Genetic diseases or Chromosome Disorders are however characterized differently, by possession of more copies of chromosomes in place of just XX or XY. For example, a XYY is an alpha-male, someone who is very built, aggressive but is probably mentally retarded. Now you know why directors portray Gundas in movies as unbelievably dumb, you see, they understand Genetics ! A XXY male is usually sterile and has more female-like features, again, thanks to movies, everyone has hopelessly made fun of people with this genetic disease as well.

While genes explain how we are what we are, mutations raise numerous questions as to why we are what we are - is this the fine line between Science and God ?
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