CBSE - X - Biology - Heredity and Evolution
Eye color is a polygenic trait and is determined by the amount and type of pigments in the eye's iris. Humans and animals have many phenotypic variations in eye color. In human eyes, these variations in color are attributed to varying ratios of eumelanin produced by melanocytes in the iris. OCA2 gene polymorphism, close to proximal 5′ regulatory region, explains most human eye-color variation.
Eye colors can range from the most common colour, brown, to the least common, green. Rare genetic specialties can even lead to unusual eye colours: black, red, and violet. Eye color is an inherited trait influenced more than one gene. The actual number of genes that contribute to eye color is currently unknown. There are modifier genes which may affect the amount of pigment in the iris, the tone of the pigment (which may be light yellow, dark brown, etc) or its distribution over the surface of the iris. On top of this genetic complexity there are other factors that can influence eye colour, for example chemical factors such as prostaglandins. To summarize, eye colour is now interpreted as an outcome of a developmental process involving many genetic interactions. In addition, eye colour can be modified by non-genetic factors such as prostaglandins.
About 60 to 80 percent of the difference in height between individuals is determined by genetic factors, whereas 20 to 40 percent can be attributed to environmental effects, mainly nutrition. In addition exercise, and living conditions also play an important role. When a given environment maximizes the genetic potential of a population for a given trait, this population tends to have a higher heritability for that trait, and vice versa. In developed countries, nutrition for childhood development is strong, which maximizes the genetic potential for height assuming no selection or new mutations.
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