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The Ratio of X Chromosome to Autosomes02:45

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In most organisms, sex is determined by the ratio of X and Y chromosomes. However, in some organisms, such as Drosophila and C.elegans, sex is determined by the ratio of the number of X chromosomes to the number of sets of autosomes. The Y chromosome in Drosophila is active but does not determine sex. It contains genes responsible for the production of sperms in adult flies.  
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The Y chromosome is a sex chromosome found in several vertebrates and mammals, including humans. In addition to 22 pairs of autosomes, the human males have one X chromosome and one Y chromosome. In these organisms, the presence or absence of the Y chromosome determines the development of male traits.
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In animals, gender is determined by the number and type of sex chromosome. For example, human females have two X chromosomes, and males have one X and one Y chromosome, whereas C.elegans with one X chromosome is a male, and the one with two X chromosomes is a hermaphrodite.
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Among mammals, the gender of an organism is determined by the sex chromosomes. Humans have two sex chromosomes, X and Y. Every human diploid cell has 22 pairs of autosomes and one pair of sex chromosomes. A human female has two X chromosomes, while a male has one X chromosome and one Y chromosome.
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In most mammalian species, females have two X sex chromosomes and males have an X and Y. As a result, mutations on the X chromosome in females may be masked by the presence of a normal allele on the second X. In contrast, a mutation on the X chromosome in males more often causes observable biological defects, as there is no normal X to compensate. Trait variations arising from mutations on the X chromosome are called “X-linked”.
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One-factor sex determination evolves without linkage between feminizing and masculinizing mutations.

Michael F Scott1, Simone Immler1

  • 1School of Biological Sciences, University of East Anglia, Norwich Research Park , Norwich NR4 7TJ, UK.

Proceedings. Biological Sciences
|July 9, 2024
PubMed
Summary
This summary is machine-generated.

Separate sexes evolve through one-factor sex determination when double mutants develop into either males or females. This reveals alternative evolutionary pathways for sex determination systems, differing from classical two-factor models.

Keywords:
dioecyevolutionhermaphroditemodelsex chromosome

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Area of Science:

  • Evolutionary biology
  • Genetics
  • Developmental biology

Background:

  • The evolution of distinct sexes from hermaphroditism requires mutations for female and male development.
  • Classical models propose two-factor sex determination, where males and females differ at two genetic loci, assuming double mutants are sterile.
  • However, some species exhibit one-factor sex determination, with sexual development controlled by a single locus.

Purpose of the Study:

  • To investigate the evolutionary pathways leading to one-factor sex determination.
  • To determine the conditions under which one-factor sex determination evolves from cosexuality.
  • To explore how the viability of double mutants influences the evolution of sex determination systems.

Main Methods:

  • Theoretical modeling of allele fixation and sex determination evolution.
  • Analysis of genetic conditions for the spread of feminizing and masculinizing alleles.
  • Comparison of one-factor and two-factor sex determination models.

Main Results:

  • One-factor sex determination evolves when the double mutant is viable and develops as either male or female.
  • The feminizing allele fixes if the double mutant is male; the masculinizing allele fixes if the double mutant is female.
  • The other locus establishes XY or ZW systems based on allele dominance, with feminizers and masculinizers spreading under similar conditions as classical models, but linkage is not required.

Conclusions:

  • Viability of double mutants provides alternative routes for sex determination evolution.
  • One-factor sex determination can arise without linkage between the involved alleles.
  • Distinguishing between these evolutionary pathways requires empirical data on the genetics of sex determination.