Assume that we have the dominant allele 'D' and the recessive allele 'd'. Lets make the example that the gene these encode for the fake disease called 'Fake Disease'. The recessive allele 'd' encodes for if you HAVE the disease. The dominant allele encodes for NOT having the disease. You get passed on a version of this allele from both your parents and hence any individual will have the genotype of 'XX' (basically your genotype will be a combination of the two alleles dependent on what you got from your parents). ALSO for a recessive allele to be expressed in the phenotype (definition below) you need to have two copies of it because other wise the dominant allele 'D' will 'hide' the occurance of the single 'd' allele.
Homozygous (the prefix 'homo' means SAME).
Heterozygous (the prefix 'hetero' means DIFFERENT).
Genotype = the genes you have in your genome
Phenotype = what trait is expressed due to what genes you have
Homozygous dominant = SAME of the Dominant allele
- Genotype: DD (the two alleles you have are the same and are both the dominant alleles)
- Phenotype: Does not have Fake Disease (as the recessive 'd' allele encodes for this and the individual does not have two of the 'd' the disease is not expressed in the genotype)
Heterozygous = DIFFERENT alleles
- Genotype: Dd (you get one copy of each allele but both are different from each other)
- Phenotype: Does not have Fake Disease (the dominant allele 'D' is what it is called 'dominant' it dominants over the recessive allele and will cover the expression of the recessive allele)
Homozygous Recessive = SAME of the Recessive Allele
- Genotype: dd
- Phenotype: Does have Fake Disease (having two recessive alleles of a trait means that it is expressed in the individual.
Hopefully this helps!!! (sorry if you already know some of the stuff I explained I don't know your level of knowledge on this topic and just wanted to make sure my answer made sense)