In genetics, a genotype is a single piece of information about a person's genetic makeup which is measurable by looking at the trait that is expressed. A phenotype on the other hand is the complete genetic information, made up of that which dominates what is expressed (dominant genes) and that which is dominated by other gene types when it comes to gene expression in a person's traits (recessive genes).
In humans, the DNA code found in all the cells in our body is made up of 23 pairs of chromosomes. One half of the pairs came from our mother and the other half from our father, where they each 'donate' at random from the pool of DNA material that they have. The material they donate however may not be copied exactly as changes can occur in transmission. The chromosomes contain alleles, which are essentially made up of groupings of the 'lego bricks' of the chromosome strands. These 'lego bricks' (base pairs) are simply chemicals. The paired information about an allele from the chromosome pairs is a gene. So a gene is made up of two halves called alleles. The genes are encoded information about traits we display in our physical appearance and personality. An individual chromosome could have some sections (alleles) that dominate over its twin chromosome's version of those alleles, but other sections that are dominated by its twin's version. The relation the two alleles have to each other determines the way the genetic information is expressed (genotype). The precise combination of alleles present in the gene is the phenotype.
The best way to see what all this means is to look at examples.
The example in the first sheet here is about expressed blood types. The possible blood (geno)types given are A, B, O and AB. The possible alleles for each chromosome (which when put with the same allele on that chromosome's twin gives the phenotype) are A, B, and O. These alleles are chemical compounds of the type seen in DNA molecules, but are given letters because the chemical compounds are complex. We are told A and B are co-dominant (meaning 'jointly dominant' - neither dominates over the other) and that both A and B dominate over O, which is then said to be recessive.
The question suggests using Punnett squares to work out all the possible combinations of allele pairs (that determine genes) the children of the described parents could have. A Punnett square combines the possible alleles the parents can pass on to their potential children. Each parent has two alleles so that there are four possible combinations (phenotypes) for potential offspring. The dominance relationship in the pairs of alleles (genes) in the offspring determines genotype. The question asks for percentages amongst potential offspring of the possible different (geno)types A, B, O and AB.
In the first table for example, we're told both the mother and father are blood type O. This is the simplest case as to be blood type O your pair of alleles relating to the trait of blood type must be OO (if A or B are present at all, the type would be something else, because O is recessive to everything - it's like A and B can shout very loud, but O has a quiet voice that can only be heard when type O's chat with each other). You write the table like this
father's alleles Possible genotypes of the offspring:
___|___O___|___O____ The only possibility for their children
mother's _O_|__OO__|___OO___ is to have the same blood type as
alleles _O_|__OO__|___OO___ both their parents. Therefore the offspring
will be 100% type O, ie type O every time.
Table 3 is probably the most complicated case here as the father is blood type A heterozygous (meaning his pair of alleles is AO - 'hetero' meaning 'different' and 'homo' meaning 'the same') and the mother is blood type B heterozygous so that her pair of alleles is BO. Writing the table in the same way as above we get
father's alleles Possible genotypes of offspring:
_____|___A___|___O____ 1/4 (25%) of the outcomes are type AB
mother's __B__|___BA__|___BO___ 1/4 (25%) of the outcomes are type A
alleles __O__|__ OA__|___OO___ 1/4 (25%) of the outcomes are type B
1/4 (25%) of the outcomes are type O