Genes, Chromosomes, and DNA
Sex Linkages
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Each cell in the human body (except eggs or sperm) contain 23 pairs of chromosomes
22 of those pairs are called AUTOSOMES, the remaining pair are the sex chromosomes
(autosomes are chromosomes that are not involved in determining the sex of an individual)
An egg always contains an X chromosome.
If the sperm is an X chromosome, the baby will be female. If the sperm s a Y, the baby will be a
male.
The X chromosome contains many genes, the Y chromosome contains only a few.
Traits controlled by the gene on the X chromosome are called X-linked
o A gene on the X chromosome in a male has no matching X chromosome on the Y.
o A gene on the X chromosome in the male is expressed whether it is dominant or
recessive.
The most common example of X-linked traits in humans are: haemophilia, colour blindness,
Muscular dystrophy, balding.
Often sex linked traits appear to skip a generation.
Ex. a man with normal vision marries a woman, who also has normal vision, but her father is colourblind. What is the probability that they will have a son who is colour-blind?
Structure of DNA
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Described as a double helix (looks like a spiral ladder)
Sugar and phosphate molecules form the backbone of the ladder
Nitrogenous bases from the rungs
The nitrogenous base from one spine pairs with the nitrogenous base from the other spine.
The bases are joined with a hydrogen bond (weak and easily broken)
Estimated that you have 3.5 billion nucleotides (runs on the ladder) and 30 000 genes on your 46
chromosomes
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Nitrogenous bases with a double ring are called purines (adenine and guanine)
Nitrogenous bases with a single ring are called pyrimidine (thymine and cytosine)
Adenine always pairs with Thymine
Guanine always pairs with cytosine
Replicating the Code
Every cell must duplicate its genetic information before mitosis or meiosis can proceed
1. The hydrogen bonds between the bases are broken. This is called unzipping
2. Free-floating nucleotides in the cell attach themselves to the complimentary strands of
DNA
When completed, two identify DNA helices (helix pl.) are produced. Each containing one of the original
strands. This method of replication is termed semiconservative.