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A Beginners' Guide to Molecular Biology



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Key concepts and definitions

A chromosome

is the visible state of genetic information during a phase of division of the cell (metaphaseask Dr Chromo!). Humans have 23 pairs of chromosomes, which makes the diploidask Dr Chromo! number 46. Their structure is made up of a compacted DNA molecule.

Two chromosomes of each.

Every individual has grown from a fertilised egg. This egg is the result of the fusion of two gametesask Dr Chromo!: one comes from the father, one from the mother. Therefore the fertilised egg contains information from both parents. It has two lots of chromosomes. That is why there is always an even number of chromosomes in a normal somaticask Dr Chromo! cell.

The genetic information is contained in DNA,

which is what chromosomes are mainly made of. This was shown by several researchers in the early years of molecular biology and genetics.



An individual is characterised by some traits,

(characters) i.e. colour of eyes, colour of hair, some genetic diseases, mood, shape of hands and so on. A trait like 'colour of the eyes' could be blue, green, brown. For each trait there will be at least one gene.ask Dr Chromo!

The trait is different from the allele

A trait could exist in different forms or allelesask Dr Chromo!. So green, blue, brown are different alleles of the same gene 'colour of the eyes'.

Dominant is different to recessiveask Dr Chromo! :

an allele can be dominant, recessive, or co-dominant with the others. This is roughly how the colour of the eyes is decided, for example. In most cases, we have two alleles of the same gene, or trait. If the two alleles are different, and one is dominant, then the character expressed is the one of the dominant allele. If both alleles are the same, then, the character is expressed, regardless of whether they are recessive or dominant. If they are co-dominant, then a bit of everything is expressed. For a recessive allele to be expressed, it has to be either alone (on the X chromosome of a male), or present on both chromosomes of a homologous pair (i.e. no dominant alleles are present).

NB: in a problem, or experiment, some conventions are used for writing and labelling dominant and recessive gene. The dominant gene has a capital letter and the recessive is named with the same letter in lower case. e.g., the gene 'colour of the eyes'; if brown is the dominant allele, then one can choose to call the gene B, and green, the recessive allele will be called b not g, or G.



In our illustration, the pair of alleles (a) code for the colour of the eyes. Both alleles are blue, so the colour of the eyes of the person is blue (although blue is recessive). The pair of alleles (b) is for the colour of the hair. In our example, one allele is red, the other one is black. Black is dominant, red is recessive so black is expressed. The third pair of alleles is the thickness of the lips. One allele is thin, and the other one thick. In our example, the alleles are co-dominant, and the thickness of the lips of our individual is an average of the two.

NOTE FROM THE AUTHOR, these are just examples : the genetics of the thickness of lips and colour of hair or eyes is much more complex.

Phenotypeask Dr Chromo! and genotypeask Dr Chromo!

The genotype is the genetic make up of an individual. The phenotype is what we see expressed in the individual, and is the result of the combination of the two alleles for each gene. So if an individual has brown eyes, the phenotype is 'brown eyes', but the genotype can be either 'brown/ brown', or 'brown/blue' , as blue is recessive to brown. If the individual has blue eyes, then its phenotype is blue eyes, and its genotype has to be blue / blue, as blue eyes, in our case is recessive and is expressed only if there are two copies of it (one from the father, and one from the mother).

These are just examples, as blue eyes and brown eyes are not exactly dominant and recessive, and one could sometimes see some brown patches in blue eyes...

Example of genotypes and phenotypes : blood groups

examples Mum Dad Children Conclusions
1 O A All children are A O is recessive, and A dominant. Mum's genotype is OO, and Dad's is AA. Dad can also be AO, but we can't prove it.
2 O A One child is O. Mum is OO, dad is AO. Child is OO
3 A B One child is O Mum is AO, Dad is BO, and child is OO
4 O AB One child is O The child has been adopted ...



Sex related genetics

All the chromosomes exist in pairs. All but one pair are homologues : both chromosomes of the pair are identical in shape, lengths and stripes (the chromosomes look striped under a microscope), but one pair is special. In males, the sex chromosome pair is made of a normal one (X) and a very small one, called Y. Males are XY. Females have two X chromosomes. They are XX.

The Y chromosome is actually like a broken X. It has some of the genes present in X, but lacks most of them. It can happen that some genes are only carried on Y, and not on the X, but it is very rare.

If a trait is carried on X, and is recessive, then it will be expressed in male carriers because they have only one copy. Some females could carry the trait, but not express it.

It is interesting to study family trees in order to spot sex related genetic diseases. Follow me on that road.



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