Dominant vs. Recessive Traits

What's is an allele? In the previous chapter, we talked about homologous chromosomes pair up during meiosis. You may be confused about the word gene loci. A gene is found on a chromosome. A chromosome contains many genes, and they occupy specific places. Genes are said to have the same locus if they occupy the same position on the homologous chromosomes. Since they occupy the same loci, the genes must be referring to the same trait. For example, if a gene on position 9 on homologous A codes for eye color, then the gene on position 9 of homologous B also codes for eye color. But one gene may code for black eye while the other gene may code for blue eye. In this case, the two genes are called alleles.  After reading this, you may be thinking what color will the eye be when two alleles are different. In fact, only one of the alleles will be expressed in this case. The gene of the unexpressed allele is not transcribed and translated.  The expressed gene is said to be dominant over the unexpressed gene, which is described as recessive. We usually assume that there are only two distinctive traits, in this case, the eye can either be black or blue, not any other color. If blue eyes is the dominant trait, the person will have blue eyes when both or either one of his alleles for eye color is blue, because the dominant allele will always be expressed instead of the recessive alleles. The person can only have black eyes when both of the alleles codes for black eye. You can thus calculate that the chance of this person having blue eye to black eye is 2:1. Relating Topics - Paternal and Maternal Chromosomes