Homologous

Homologous Definition

“Homologous,” in biology, means a similarity in internal or chromosomal structures.

With internal structures, homology indicates organs that have similar positions, structures, or evolutionary origins. It’s important to note, however, that organs do not have to have the same function to be homologous.

When it comes to chromosomal structures, “homologous” is used to describe chromosomes that carry the same type of genetic material. Nonetheless, this genetic material does not have to be the same: one half comes from the mother and the other half from the father.

Examples of Homologous

As stated above, “homologous” can be used to describe two things:

  • Structures
  • Chromosomes

Climbers, Flyers, and Swimmers

What do squirrels, birds, and whales have in common? The obvious answer is that they breathe, have beating hearts, and use their upper appendages to move. Let’s explore this latter idea a bit more, using the squirrel’s ability to climb, the bird’s ability to fly, and the whale’s ability to swim as examples:

Examine the images below, focusing on the squirrel’s arm, the bird’s wing, and the whale’s fin. Note some similarities and differences.

Red squirrel skeleton

Red squirrel skeleton

X-ray of a bird wing

X-ray of a bird wing

Sperm whale skeleton

Sperm whale skeleton

One of the things these illustrations show is that each example consists of three main parts: the humerus, or the “upper arm,” the radius and ulna, which form the “forearm,” and the metacarpals, which form the “fingers.”

On the other hand, we can also identify differences. The whale’s humerus, for instance, tends to be shorter, wider, and flatter. In fact, some whales have a patella, or “shoulderblade,” instead of a humerus. Likewise, the bird has no fingers; its metacarpals taper into something that resembles a dagger.

Despite minor differences in form and major variation in function, these structures still qualify as homologous. In the simplest terms, the squirrel, the bird, and the whale all possess tripartite, or “three-parted,” upper appendages. This evidence allows us surmise that these animals might have evolved from a common ancestor, which used its tripartite limbs to move.

Because the wing, the fin, and the arm allow us to connect the bird, the squirrel, and the whale to a common ancestor, we can conclude that the appendages are homologous.

The Genetic Code

To quote the popular Darwinist phrase, you are 98% chimp. While technically true, this information can mislead beginning biologists who have yet to explore it further.

The genetic code of most animals contains four nucleotide bases, also called nucleobases, and marked as A, T, C, and G. In different combinations, they account for features such as the color and location of body hair, nose size, blood type, and even ear lobe attachment. More recent research even suggests that nucleobases also determine whether you will develop a psychological or personality disorder.

Nonetheless, most genetic expression, or the manner in which nucleobase combinations manifest themselves, is relatively benign. We don’t give much thought, for instance, to the nucleobase combinations that make our bones hard, our heart muscular, or our liver able to regenerate. In fact, geneticists estimate that only 0.1% (that’s one-tenth of a percent) of our genes actually account for the features we see. The other 99.9% rests dormant as “junk DNA,” or makes up features that we take for granted.

The four nearly-universal nucleobases in the genetic code, when paired with the fact that some human DNA remains dormant, permits us to understand the phrase “You are 98% chimp” more deeply. In short, however, humans and chimps have a homologous genetic code. The differences lie in how that code is expressed.

Your Mother’s Eyes, but Your Father’s Hands

If you know your birth parents, you have probably noticed that you have inherited some features from your mother, and some features from your father. You may have also had relatives and friends tell you resemble one or the other.

For better or for worse, you probably resemble both of your parents. This is because, during conception, you inherited a set of 23 chromosomes from your mother’s egg, and a set of 23 chromosomes from your father’s sperm. The same genetic information is stored on each set, in similar locations.

Because you can “match” one of your mother’s chromosomes with one of your father’s, they are homologous.

Homologous chromosome pairs

Homologous chromosome pairs

Nonetheless, the alleles, or modes of expression, of these genes may differ. This is why you may have inherited your mother’s brown eyes, a dominant allele, but not your father’s cleft chin, a recessive allele. Typically, however, the fact that you have colored eyes or a chin in the first place indicates that your parents’ chromosomes both carried the information necessary for creating them.

Related Biology Terms

  • Chromosome – A combination of genes and proteins, some inherited from the mother and some inherited from the father, located within the nucleus of the cell.
  • Nucleotide base, nucleobase – A protein that combines with another protein to form a gene.
  • Gene – A sequence of nucleobases that gives cells the “information” for certain physical features, or how the body should function.
  • Allele – An expression of a gene. Many alleles come from mutations.

Quiz

1. A, T, C, and G are parts of the genetic code called
A. Nuclear bombs
B. Nucleotide bases or nucleobases
C. Nucleotidal waves
D. Alleles

Answer to Question #1

2. Homologous structures do not have to have the same function.
A. True
B. False

Answer to Question #2

3. The genetic information you inherit from your parents is carried on:
A. Genes
B. Nucleotide bases or nucleobases
C. Chromosomes
D. Mitochondria

Answer to Question #3
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