The blueprint for the development and appearance of each individual is contained within the chromosomes of our cells. All of this information is efficiently organized and tightly packed into pairs of chromosomes located inside the cell nucleus.
Chromosomes are made up of a molecule called DNA (deoxyribonucleic acid)
Scientists have found that there are two kinds of genes. One type of gene is called recessive and the other type is called dominant .
Brown eye colour is a dominant gene and blue eye colour is a recessive gene. If you inherited a blue eye colour gene from one parent and a brown eye colour gene from the other parent, you would have brown eyes.
Specific genes determine hereditary traits. A gene specifies a single inherited characteristic. There are genes for height, weight, eye colour, earlobe attachment and so on.
Each person carries two genes for a given characteristic. One gene comes from the mother’s egg and one from the father’s sperm.
One of the two genes is dominant over the other. The dominant allele will mask the other recessive allele. For example, if the mother provides a blue eye-colour allele and the father a brown eye-colour allele, the off spring will have brown eyes since brown eye-colour is dominant.
A punnett square assigns two alleles to each parent and predicts the outcome for each trait .
For example tallness.
Punnet Squares The mother is called homozygous since she has two alleles that are the same. The TT represents the two dominant alleles for tallness. The father is called heterozygous since he has two different alleles. The Tt represents the one dominant( T ) and one recessive( t ) allele for tallness.
Sex-linked traits are traits carried on sex chromosomes (X and Y).
The male determining chromosome (Y) has no corresponding alleles on the X chromosome to mask its effects. The presence of the Y chromosome causes maleness. The female determining chromosome (X) does not carry "male" genes of the Y chromosome.
The X and Y chromosomes carry genes that code for traits other than gender. Traits determined by genes on the X chromosome are called sex-linked. Some genes are also carried on the Y chromosome and they are called sex-limited traits. An example is hairy ears that many older men have.
Some sex-linked traits that show up as disorders are hemophilia and colour blindness.
Recall also the father contributes either an X or Y chromosome to the mother's X chromosome.
Females receive X chromosomes from both parents and therefore can inherit sex linked traits from either parent.
If a female is to show a sex-linked trait, she must have one dominant allele on an X chromosome or two recessive alleles on both X chromosomes.
If a female receives one recessive sex-linked allele from her mother or father she will not show the trait, but she is a carrier and there is a probability that she will pass the sex-linked trait one-half of her sons. The same probability exists for her daughters. If masked by a corresponding allele on their other x chromosome, they won't show the trait.
A pedigree is a diagram representing the phenotype history of a family. A phenotype history is used since we have traits only of ancestors to work with. Genotypes would not have been known for any ancestors.
Symbols can be used to represent female and male ancestors. If the symbols are empty, the trait is dominant; if the symbol is filled, the trait is recessive. Traits shown by the offspring help determine whether the trait in the parent is dominant or recessive.
The second type of pedigree is sex-linked. These pedigrees are ones that demonstrate a trait that is attached to a sex (X,Y) chromosome. A common sex-linked pedigree is the one for hemophilia in European aristocracy. Recall that sex-linked traits are inherited through the allele located on the X sex chromosome.