It's much easier to understand genetics and heredity if you examine actual examples of dominant and recessive phenotypes that you possess.

What are Alleles?

Alleles are variations of a gene. A diploid organism gets one set of their alleles (genes) from one parent and the other set of alleles from the other parent.

Some of the traits that we have are based on simple inheritance where one version of a gene (dominant allele) masks the expression of the other version of that gene (recessive allele).

In writing, we represented dominant and recessive alleles with letters that distinguish the different types of alleles. A capital letter is used to represent the dominant allele and a lower-case letter is used to represent the recessive allele (example: dominant allele = P; recessive allele = p)

Homozygous and Heterozygous

When an organism has a pair of identical alleles for a character, they are said to be homozygous for that characteristic (PP, pp) When an organism has two different alleles for a gene (Pp) they are said to be heterozygous for that characteristic.

Dominant or Recessive Phenotypes

In cases if simple inheritance, where a characteristic is controlled by one pair of alleles and one allele is dominant over the other, this is called complete dominance. For that characteristic, an organism will have one of two phenotypes.

When an organism has the recessive phenotype, this means that both recessive alleles must be present (bb). With recessive traits, you know what the genotype is if the phenotype is recessive.

An organism showing the dominant phenotype can be homozygous or heterozygous for dominant allele (PP, Pp). It is more difficult to know genotype when dominant allele is involved.

Examples of Simple Inheritance of Traits that Show Complete Dominance

The characteristics listed below are products of simple inheritance. One can either possess the dominant phenotype or the recessive phenotype.

All of the following are dominant traits, meaning that if the trait described is present (mid-digital hair, hand clasping with left thumb on top & bent little finger) it is the dominant phenotype and the corresponding genotype would be either homozygous dominant or heterozygous. If the trait is absent (no hair on joint, hand clasping with right thumb on top, straight little finger) the corresponding genotype is recessive.

Mid-digital Hair

Some people have hair on the second (middle) joint of one or more of their fingers, while others don’t. Having any hair at all means that you have the dominant phenotype. Complete absence of hair is recessive.

Alleles: H, h

Dominant phenotype: hair on skin finger’s second joint

Dominant genotype: H –

Recessive genotype: hh

Hand Clasping

Clasp your hands together (without thinking about it!). Most people place their left thumb on top of their right and this happens to be the dominant phenotype. Now, for fun, try clasping your hands so that the opposite thumb is on top. Feels strange and unnatural, doesn’t it?

Alleles: L,l

Dominant phenotype: left thumb on top

Dominant genotype: L –

Recessive genotype: ll

Bent Little Finger

A dominant allele causes the last joint of the little finger to dramatically bend inward toward the 4th finger. Lay both hands flat on a table relax your muscles, and note whether your have a bent or straight little finger.

Alleles: B, b

Dominant phenotype: bent little finger

Dominant genotype: B –

Recessive genotype: bb

More Examples of Simple Inheritance of Traits the Show Complete Dominance

There are many interesting phenotypic traits that you can examine and see if you have the dominant or recessive phenotype. For more information on human genetics, see the website Science Prof Online or see the Suite 101 articles Genotype, Phenotype & Simple Inheritance, Complete Genetic Dominance (Melanin / Eye Color & Myopia) and Traits of Simple Genetic Inheritance (Facial Dimples, Chin Cleft & Free Earlobes).

Sources

Thorpe ed. (2007) Biology 120 Lab Manual. Grand Valley State University.

Doezema, B. (2006) Biology 101 Lab Manual. Grand Rapids Community College.