Question

how do you know if a person has a dominant or recessive trait?

Answer 1

http://genetics.thetech.org/ask/ask428

Answer 2

This site is very helpful :)

Answer 3

By the parents and offspring's genotypes?
For example, if the parents have heterozygous genotypes, Yy and Yy (in simpler terms, the two letters are not of the same case, one is the upper case, the other is a lower case),
when you cross them, you end up with,
YY, Yy, Yy, yy

The cross between two heterozygous ALWAYS gives you the phenotypic ratio of 3 dominant is to 1 recessive.
So from the results, you can conclude that the upper case represents the dominant trait.

The difference between a dominant gene and recessive gene? The dominant one is always expressed. The ONLY way a recessive phenotype can be expressed is if BOTH are recessive. (E.g. aa, bb, cc, dd) Although you may see a recessive one, it is "overpowered" by the dominant one in a heterozygous allele. (E.g. Yy, Aa, Bb, Cc) That's why ONLY THE DOMINANT is expressed physically. And that's why in the earlier results, you ended up with 3 dominant is to 1 recessive. In YY, Yy and Yy, the upper case Y is dominant over the lower case y.
(BUT REMEMBER, UPPER CASED LETTERS DO NOT NECESSARILY MEAN IT REPRESENTS THE DOMINANT TRAIT. QUESTIONS MAY TRICK YOU! THIS IS ONLY AN EXAMPLE I'M GIVING.)

However, there are also cases whereby co-dominance and incomplete dominance occur.
For example, in blood types, blood type A and blood type B are of equal dominance, that's why people can end up with blood type AB. This is called co-dominance.

To compare side by side,
if a black and a white cat breed,
and a white cat with black patches is produced, this is called co-dominance.

However, if a GREY cat is produce, this is called incomplete dominance.