[[Genetic]] Crossing

Genetics. So much fun to find out where you came from and who you will be producing. Luckily, its part of the A level course, so typing this can be classed as Biology revision. Or I can just tell myself that to make myself feel better. Ah, procrastination! It lurks in so many forms.

Sitting in Chemistry today, I was hugging the radiator (is it my fault that I sit right next to it?) listening to Dr Goode go through the test scores and I suddenly thought that I could work out the percentage chance that I would have a colourblind child. Thoughts like that constantly run through my mind in chemistry...occupational hazard of being a student.

My cricket-and-football-playing Dad is colourblind, so much so it ruined his dream of being a pilot. I decided that I wanted to know whether I would have to dash the dreams of my child should he or she take a mind to pilot an aeroplane, or perhaps a shuttle craft. Or a hot air balloon.

So I decided, sat in Chemistry being told that my for answer to question 3b) iii) I had used the wrong mole ratio and therefore would loose a mark, that when I got home I would draw out some genetic crosses and find out, thus satisfying my urge and revising at the same time. Half a day later, I am now sat infront of the computer (but only because my TV refuses to work so I cant watch the Apprentice) ready to work it out.

The colourblindness disease is sez-linked, so the alleles that I am using for this cross are:
XN = normal
Xn = colourblind
Y = normal (male)

First, I must work out my genotype.

My dad is colourblind, therefore he carries the gene for it, so his genotype is XnY and my mum knows that she is not a carrier, therefore her genotype is XNXN.

The genetic cross for my genotype (f1) is:

........Xn...... Y
XN XNXn XNY

XN XNXn XNY

As shown in red, my genotype is XNXn, which means that I am a carrier of colourblindness. Note that my brother does not carry the disease, as my dad must pass on the Y chromosome to him, which does not contain the allele for colourblindness, and the X chromosome he recives from my mother contains the normal allele, as she does not carry a copy of the colourblind allele.

If I was to marry and start a family with a man who was not colourblind, then the genetic cross would look like this. Note that men cannot carry the disease as it is only on the X chromosome, which they only have one copy of.

.........XN......Y
XN XNXN XNY

Xn XNXn XnY

There is a 25% chance that I would have a normal, non-carrying female and a 25% chance that I would have a normal male, while there is a 25% chance that I would have a carrying female and 25% chance that I would have a suffering male. Note that it is impossible for any female offspring to suffer from colourblindness.

If I married and started a family with a man who suffered from colourblindness, then the genetic cross would look like this:

........Xn.......Y ........
XN XNXn XNY

Xn XnXn XnY

There is a 25% chance that I would have a normal, carrying female and a 25% chance that I would have a normal male, while there is a 25% chance that I would have a female who suffered from colourblindness, and a 25% chance that I would have a male who suffered from colourblindness. Note that I would not have a female who did not carry the diesease.

This practise has told me that I would have to crush the flying dreams of 3/8 of my kids. I got depressed, I mean what Mother would want to do that to her own Children? And then I remembered that I am only 17, and by the time that I am ready to have children, scientists will have probably developed 'designer' children, so that you can pick the traits that your children have.

See. I told you genetics was intersting.