Let’s go through these choices one by one.

1. Codominant genes: Normally in a gene with two alleles, one is dominant and the other recessive. In heterozygotes, the dominant allele will be expressed, while the recessive one won’t. In codominant genes, neither allele is recessive, and both are expressed. A classical example is the human ABO blood group, where people with both the A and B alleles express both antigens on their red blood cells, rather than A “winning out” over B.
   
   
2. Pseudogenes: “False” (pseudo) genes. They are segments of DNA that look very similar to functional genes, but lack the regulatory sequences needed for them to be transcribed, so they never lead to a functional product.
   
   
3. Alleles: These are the two or more variants of any given gene.
   
   
4. QTL: Some traits are binary – like having black eyes or blue. Others, however, are quantitative, where there is a large, continuous variation in that trait in individuals. If we use human height as an example, it’s not as if there are only people of two height, 5 feet or 7 feet. We see a lot of variation in between those numbers. This happens because such a trait is usually affected by, or the result of, multiple genes – a polygenic trait.
   
   When we try to find out which areas of DNA are involved in such a trait, we often find that there are multiple SNPs (single-nucleotide polymorphisms) and AFLPs (amplified fragment length polymorphisms) that map to the same area on a chromosome. So we know that this region of DNA (locus) is involved in producing these quantitative traits – this is the Quantitative Trait Locus.