Homologous: Definition, Characteristics & Structure Examples

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  • 0:05 Etymology
  • 1:18 Homologous Structures
  • 2:35 Homologous Chromosomes
  • 3:25 Homologous Organs
  • 3:51 Lesson Summary
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Lesson Transcript
Instructor: Joanne Abramson

Joanne has taught middle school and high school science for more than ten years and has a master's degree in education.

Discover the meaning and origin of the term 'homologous.' Learn how this expression is used in several branches of biology including evolutionary biology, genetics and developmental biology.

Etymology

A homophone is a word that sounds the same as another but has a different meaning. If a group is homogenous, all of its members are uniform or consistent. A homosexual is a person sexually attracted to a member of the same sex. Noticing a pattern? Words with the prefix 'homo' all have the idea of sameness in their meaning. We will be continuing with this etymology as we discuss the various definitions of 'homologous' that are found in biology.

The term homologous comes from the Greek 'homos' meaning same and 'logos' meaning relation. So, literally, it means having the same relation. To make that a little clearer, we can say it means having a similar relative position, structure, function or origin. However, that definition is still somewhat broad and ambiguous. The reason for this is because the term 'homologous' is used in several different fashions in biology. The individual definitions are more specific, but they are all derived from the idea of 'having the same relation.' In this lesson, we will discuss homologous structures, homologous chromosomes and homologous organs in males and females.

Homologous Structures

A homologous structure is an organ, system, or body part that shares a common ancestry in multiple organisms. This definition is found in evolutionary biology and uses the meaning of having a similar structure or origin.

Common examples of homologous structures are the bones in the forelimbs of various vertebrates, such as humans, dogs, birds and whales. Even though the functions of these limbs are different (a human hand is for grasping, a dog's foot is for walking, a bird's wing is for flying, and a whale's flipper is for swimming), they all have similar bone structures. In evolutionary biology, this is evidence for descent with modification from a common ancestor. For example, check out the forelimbs of four modern vertebrates. Compare these to the flipper bones of the three ancient, lobe-finned fishes, one of which could be the common ancestor of the modern vertebrates.

Ancient fish
flipper bones of three ancient fish

Another example of homologous structures can be found in plants. Pine needles in fir trees, spines on a cactus, the petals of the poinsettia and the trap of the Venus fly trap all have different functions. However, they are all modifications of a leaf derived from a common ancestor.

Homologous Chromosomes

In genetics, you will hear reference to the term homologous chromosomes. Humans have 46 chromosomes; we'll get half of them from our mother and half from our father. Homologous chromosomes refer to a pair of chromosomes that have genes for the same traits at corresponding chromosomal locations. One member of the pair is maternal and one is paternal. This uses homologous in the sense of having a similar position and function.

Human karyotype
Human Karyotype

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