Question

As related to sex determination, what is meant by (a) homomorphic and heteromorphic chromosomes; and (b) homogametic sex and heterogametic sex?

Short answer

Question: Define and differentiate between the following terms related to sex determination: (a) Homomorphic chromosomes and Heteromorphic chromosomes, (b) Homogametic sex and Heterogametic sex. Answer: (a) Homomorphic chromosomes are identical sex chromosomes in size, shape, and gene content, whereas heteromorphic chromosomes differ in these aspects. Homomorphic chromosomes are found in individuals from the same sex, while heteromorphic chromosomes are distinctly different between the two sexes, such as the human XY sex determination system. (b) Homogametic sex refers to the sex that produces gametes with identical sex chromosomes (e.g., human females with XX chromosomes). In contrast, heterogametic sex refers to the sex that produces gametes with different sex chromosomes (e.g., human males with XY chromosomes).

Step-by-step solution

Homomorphic chromosomes

Homomorphic chromosomes are chromosomes that are the same size and shape and have the same genes in the same locations. In the context of sex determination, they would refer to the sex chromosomes that appear identical in a particular sex.

Heteromorphic chromosomes

Heteromorphic chromosomes are chromosomes that differ in size, shape, or gene content. In terms of sex determination, they refer to the sex chromosomes that are distinctly different between the two sexes. An example of this is the human XY sex determination system, where the X and Y chromosomes are heteromorphic.

Homogametic sex

Homogametic sex refers to the sex that produces gametes with identical sex chromosomes. In other words, the sex that has two homomorphic sex chromosomes. For example, in humans and many other species, the female is the homogametic sex, as they possess two X chromosomes (XX).

Heterogametic sex

Heterogametic sex refers to the sex that produces gametes with different sex chromosomes (heteromorphic chromosomes). This means that the individuals of this sex carry two different sex chromosomes. For example, in humans and many other species, the male is the heterogametic sex, as they possess one X and one Y chromosome (XY).

Key concepts

Homomorphic Chromosomes

The term "homomorphic chromosomes" refers to pairs of chromosomes that are identical in size, structure, and gene arrangement. Think of these chromosomes as mirror images, as they are visually and functionally similar. In the context of sex determination, homomorphic chromosomes are often mentioned when discussing organisms where the sex chromosomes do not differ between the sexes. For example, in birds, females have two similar (or homomorphic) Z chromosomes, while males have one Z and one W chromosome.

• Identical in structural appearance.
• Same genes are located in the same positions along the chromosome.
• Commonly found in organisms where a pair appears similar but has different roles in determining sex.

Heteromorphic Chromosomes

Heteromorphic chromosomes are significantly different from each other in terms of size, shape, or genetic content. These differences are essential in systems where sex is determined by distinct sex chromosomes, like the XY system found in humans. In this system, females have two similar X chromosomes, while males have one X and one smaller, differently shaped Y chromosome. This difference is why they are termed heteromorphic.
Examples of heteromorphic sex chromosome systems include:

• Human males, who have an X and a Y chromosome.
• Bird males, who have a Z and a W chromosome.

This distinction in chromosome appearance often correlates with different functional roles in sex determination.

Homogametic Sex

The homogametic sex produces gametes (sperm or eggs) that carry identical sex chromosomes. This means that all the sex cells they produce are uniform in terms of the sex chromosome they carry. In many species, the female is the homogametic sex. For instance, in humans, females have two X chromosomes (XX), so the eggs they produce also carry an X chromosome.
Key features of homogametic sex include:

• Both sex chromosomes are homomorphic (XX in females).
• All gametes produced carry the same sex chromosome (only X).

This uniformity ensures that the offspring's sex is largely dependent on the type of sperm that fertilizes the egg.

Heterogametic Sex

In contrast to the homogametic sex, the heterogametic sex produces gametes with different sets of sex chromosomes. This is because individuals of this sex carry heteromorphic sex chromosomes, like XY in humans. Males in humans, for example, produce two types of sperm, one carrying an X chromosome and the other a Y chromosome. Hence, they determine the sex of their offspring based on which sperm fertilizes the egg.
Characteristics of heterogametic sex include:

• Possession of heteromorphic sex chromosomes (e.g., XY).
• Production of two types of gametes, differing in the sex chromosome they contain (X or Y).

This setup plays a crucial role in the diversity of sex determination mechanisms across different organisms.