Question

In exploring the evolution of sex determination systems, particularly the X/Y system used by humans and most mammals, some questions arise about its origin and potential connections to other species. Which of the following statements accurately reflects the current understanding of the evolution of the X/Y sex determination system in therian mammals?

A) The X/Y genes used in therian mammals, including humans, have a common evolutionary origin with the X/Y genes found in non-therian species like snakes and insects.
B) The term therian X/Y genes specifically refers to the X/Y genes found in mammals, distinct from those in other species like snakes or insects.
C) Monotremes, such as the platypus and Echidna, exhibit an X/Y system similar to that in therian mammals, suggesting a shared evolutionary ancestor.
D) There are no therian mammals that share a last common ancestor with humans and use the same X/Y genes for sex determination.
E) The similarity between monotremes' sex determination system and Z/W chromosomes in birds indicates a pre-mammal ancestor with a Z/W system, but the transition to autosomal chromosomes in therian mammals is not fully understood.

What do we know about the presence of autosomal chromosomes similar to human sex chromosomes in non-therian mammals, and what might this suggest about the evolution of the X/Y system?

A) Monotremes like the platypus and Echidna exhibit autosomal chromosomes similar to human sex chromosomes, indicating a direct evolutionary link.
B) Therian mammals share a common ancestor with non-therian mammals that used a Z/W sex determination system, which later evolved into autosomal chromosomes in therians.
C) The X/Y system in therian mammals has no evolutionary connection to the sex determination systems observed in non-therian mammals.
D) The transition from a Z/W system to autosomal chromosomes in therian mammals remains unknown, and no clear evolutionary evidence has been identified.

Answer 1

The X/Y sex determination system in therian mammals is distinct from other systems found in non-therian species, and while the transition from a Z/W system to an XY system is a subject of ongoing research, the evolutionary pathway remains unclear.

Step-by-step explanation:

The current understanding of the evolution of the X/Y sex determination system in therian mammals, including humans, is a complex and dynamic area of study. Statement B) correctly identifies that therian X/Y genes are specific to mammals and are distinct from sex-determining systems in other species like snakes or insects. In therian mammals, the presence of an XY chromosome pair typically denotes a male (heterozygous), and an XX chromosome pair denotes a female (homozygous), which is a contrast to the ZW system found in birds, where the female has the heterogametic sex chromosomes (ZW) and the male is homozygous (ZZ).

Moreover, while monotremes such as the platypus exhibit a sex determination system with multiple sex chromosomes that are somewhat similar to the X and Y of therian mammals, this does not necessarily indicate a direct evolutionary link, suggesting evolutionary divergence between their sex determination systems. The transition from a Z/W system to an XY system in therian mammals remains an area of ongoing research, hence statement D) rightly suggests that the evolutionary pathway of this transition is not fully understood, and more studies are needed to elucidate the details.