DNA is the material responsible for inherited traits in human beings. Genes--specific segments of DNA that regulate the types of proteins assembled by cells--contain hereditary information. Genes, with various forms known as alleles, regulate the proteins responsible for various physical traits. Dominant alleles mask the effects of recessive alleles. Each individual inherits two alleles of a gene. If a single copy of a dominant allele is present, the person will have the trait of the dominant allele. For a person to express a recessive trait, both alleles must be recessive.
A straight hairline or a widow’s peak depends on genes. A widow’s peak is a dominant trait, and a straight hairline is recessive. A straight hairline requires that both the alleles for hairline be recessive. Even one dominant allele will result in a widow’s peak. For example, suppose W and w represent the dominant and recessive alleles, respectively. If you inherit two dominant alleles, so that your genotype (genetic make-up) is WW, or even one dominant allele for a genotype of Ww, you will have a widow’s peak. Only a person in whom both alleles for hairline are recessive (ww genotype) will have a straight hairline.
In some people, the ends of the earlobes are attached to the skin on the neck. In others, the earlobes curve inward distinctly, or are unattached. There is debate about whether the trait of attached or unattached earlobes is controlled by several genes or by a single gene with two alleles--the dominant allele causing unattached earlobes and the recessive allele causing attached earlobes.
The presence of dimples has a dominant pattern of inheritance, which means that offspring will have dimples if they receive dominant alleles from both parents or inherit a dominant allele from either parent. However, offspring who receive recessive alleles from both parents will not have dimples. Even if both parents have dimples, there is no gurantee that their children will also inherit them. If each parent has one dominant and one recessive allele, they can both pass on the recessive allele to their offspring , and the child will lack dimples.
The ability to taste the chemical phenylthiocarbamide (PTC) is an inherited trait. This trait is determined by a single gene located on chromosome 7. However, this gene has three variations, and each variation has two alleles. One allele, which is dominant, is responsible for perception of the bitter taste of PTC; the other, recessive allele makes an individual insensitive to its taste. The remaining alleles allow varying levels of sensitivity to the bitter taste of PTC. Every individual inherits two alleles of a single gene. Whether someone finds PTC very bitter, finds it somewhat bitter or can’t taste it all depends on which two PTC tasting alleles he has inherited.