I need help with a Mendelian inheritance problem please?

A. Codominance rules say that two genes will be expressed.  For example in Roan cattle both red and white are expressed so the cattle have red hairs and white hairs which gives them an overall rust looking color called roan.  Mendel's law of dominance states that if a dominant gene is present it will be expressed.

B. Mendel stated that in a cross between two heterozygous individuals the genotypic ratio would be 1:2:1.  In the case of recessive lethal alleles that ration will not hold true because the recessive offspring will not make it therefore the  genotypic ratio is messed up.  

C. Mendel stated that in a dihybrid cross, characteristics are not linked - they are inherited independently (his low of Independent Assortment).  If there is linkage then the characteristics will not be inherited independently.

Some basic explanations which you can expand on:

A) Normal dominance produces 2 kinds of phenotypes, but codominance produces THREE phenotypes.  Consider the cross between blood-type genotypes AA x BB;  you'lll get A, AB, and B.

B) Instead of the normal 3:1 phenotypic  ratio observed in simple dominance, the 25% that are homozyogous recessive will die.  In the cross Bb xBb, the bb individuals will die.

C) Mendel saw a 1:1:1:1 ratio in a dihybrid cross when the two traits were NOT linked.  If they are, then there will not be an even distribution among the 4 phenotypes-- there will be more parental phenotypes than crossover phenotypes due to linkage.  If R and T are linked, then the cross of
RrTt x rrtt will produce more RT and rt phenotpes than Rt and rT phenotypes.