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Related Concept Videos

X and Y Chromosomes02:32

X and Y Chromosomes

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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.
The germline cells such as egg and sperm cells carry only half the number of chromosomes, i.e., 22 autosomes and one sex chromosome. All eggs have an X chromosome, while sperm cells can carry an X or...
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The Y Chromosome Determines Maleness02:19

The Y Chromosome Determines Maleness

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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.
Evolution
Around 300 million years ago, the two sex chromosomes diverged from two identical autosomal chromosomes. Over time, the Y chromosome has lost most of its genes, shrinking in size....
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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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Dosage Compensation02:50

Dosage Compensation

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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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Nondisjunction

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During meiosis, chromosomes occasionally separate improperly. This occurs due to failure of homologous chromosome separation during meiosis I or failed sister chromatid separation during meiosis II. In some species, notably plants, nondisjunction can result in an organism with an entire additional set of chromosomes, which is called polyploidy. In humans, nondisjunction can occur during male or female gametogenesis and the resulting gametes possess one too many or one too few chromosomes.
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Nondisjunction01:21

Nondisjunction

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Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate correctly and move to the opposite poles of the cells. This produces daughter cells with abnormal chromosome numbers.  Nondisjunction is common during anaphase I or anaphase II of meiosis.  Mutations in synaptonemal complex proteins that attach homologous chromosomes increase the chances of nondisjunction in anaphase I of meiosis I. In contrast, mutations in topoisomerases and condensins that hold...
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Sex Chromosome Evolution: So Many Exceptions to the Rules.

Benjamin L S Furman1,2, David C H Metzger1,2, Iulia Darolti1,2

  • 1Beaty Biodiversity Research Centre, University of British Columbia, Vancouver, British Columbia, Canada.

Genome Biology and Evolution
|April 22, 2020
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Summary

Sex chromosome evolution shows remarkable diversity, challenging linear theories. Genomic studies reveal varied origins, recombination suppression, and dosage compensation mechanisms across species.

Keywords:
recombination suppression, sex determination, inversions, methylation, dosage compensation

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Area of Science:

  • Evolutionary biology
  • Genomics
  • Sex chromosome evolution

Background:

  • Classic theory posits sex chromosomes evolve linearly from autosomes with suppressed recombination.
  • This leads to Y chromosome degradation and dosage compensation needs in the heterogametic sex.

Purpose of the Study:

  • To explore the diversity of sex chromosome systems and their evolutionary pathways.
  • To highlight exceptions and variations in established sex chromosome evolution theories.

Main Methods:

  • Genomic analysis of diverse nonmodel species.
  • Comparative analysis of sex chromosome systems and evolutionary patterns.

Main Results:

  • Observed significant diversity in sex chromosome origins, not always from homologous autosomes.
  • Found varied mechanisms and causes for recombination suppression.
  • Documented diverse and variably effective dosage compensation strategies.
  • Highlighted rapid turnover and variable divergence rates of sex chromosomes.

Conclusions:

  • Sex chromosome evolution is more diverse and less linear than previously theorized.
  • Emerging data challenge assumptions about inevitable progression and linearity.
  • Further research is needed to understand the drivers of sex chromosome birth-death cycles.