Final answer:
Polygenic inheritance involves multiple genes and their alleles contributing to a trait, such as eye color, leading to a wide range of possible phenotypes. Brown is a dominant eye color, but heterozygous parents with green alleles can have a green-eyed child. Eye color also correlates with melanin levels, and linked genes can influence traits that are often inherited together.
Step-by-step explanation:
Polygenic inheritance explains why there are so many different iris colors because it involves multiple genes each contributing to the phenotypic expression of a single characteristic - in this case, eye color. Multiple genes and their various alleles interact with each other, resulting in a wide range of possible colors. Unlike traits controlled by a single gene, polygenic traits like eye color do not follow the simple Mendelian inheritance pattern. Instead, they show a gradient of possible phenotypes.
For example, regarding the eye color inheritance, having two copies of one allele of the EYCL3 gene typically results in brown eyes, which is dominant over the blue eye allele. If both parents have a mixture of brown and green alleles, it's possible for them to produce a child with green eyes even if both parents have brown eyes, which indicates that both parents carry the gene for green eyes but it's not expressed in their phenotype.
Considering our options, the correct answer for how a couple with brown eyes can have a green-eyed child is that: both parents are heterozygous, having the green trait on the green-blue eye gene. Hence, option (b) is the correct one.
Eye color variations are also thought to be linked to melanin levels; lower melanin levels typically result in lighter eye colors such as blue, whereas higher melanin levels result in darker eye colors such as brown or black. Linkage between genes can result in certain traits, like hair and eye color, being inherited together frequently.