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The Y Chromosome Determines Maleness

The Y Chromosome Determines Maleness

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. Today,...

The Ratio of X Chromosome to Autosomes

The Ratio of X Chromosome to Autosomes

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.
Normal male Drosophila has a ratio of one X chromosome to two sets of autosomes. In contrast, normal female Drosophila...

Dosage Compensation

Dosage Compensation

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.
In addition to sexual development, the X chromosome has genes involved in autosomal functions such as brain development and the immune system. Therefore, males and females with distinct numbers of X chromosomes will have...

Sex-linked Disorders

Sex-linked Disorders

Like autosomes, sex chromosomes contain a variety of genes necessary for normal body function. When a mutation in one of these genes results in biological deficits, the disorder is considered sex-linked.

Infertility in Males

Infertility in Males

Male infertility affects millions of couples worldwide, arising from various factors that impact different stages of the reproductive process. An endocrine imbalance resulting from conditions like hypogonadism, Klinefelter syndrome, or pituitary disorders can disrupt hormone levels and reduce sperm production. Testicular defects, such as tumors, cryptorchidism, atrophic testes, abnormal sperm morphology, and low sperm count or motility, may arise due to genetic factors, structural...

X-linked Traits

X-linked Traits

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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Related Experiment Video

Updated: Jun 5, 2026

Assessing Differences in Sperm Competitive Ability in Drosophila

Assessing Differences in Sperm Competitive Ability in Drosophila

Published on: August 22, 2013

Dwarf males.

¹Dept of Zoology, University of Aarhus, 8000 Aarhus C, Denmark; Dept of Zoology, Oxford University, Oxford, UK OX1 3PS.

Trends in Ecology & Evolution

|January 18, 2011

Summary

This summary is machine-generated.

Male dwarfing in species with sedentary females is common, where males provide sperm for resources. Natural selection, not sexual selection, is the likely driver of this reproductive strategy.

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Last Updated: Jun 5, 2026

Assessing Differences in Sperm Competitive Ability in Drosophila

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Published on: August 22, 2013

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Determination of Reproductive Competence by Confirming Pubertal Onset and Performing a Fertility Assay in Mice and Rats

Determination of Reproductive Competence by Confirming Pubertal Onset and Performing a Fertility Assay in Mice and Rats

Published on: October 13, 2018

Area of Science:

Evolutionary Biology
Reproductive Strategies

Background:

Dwarf males frequently occur in species exhibiting sedentary females.
Male dwarfing involves cohabitation, ranging from minimal to intimate.
This phenomenon differs from parasitism as males offer sperm in exchange for resources.

Purpose of the Study:

To investigate the evolutionary pressures driving male dwarfing in species with sedentary females.
To differentiate between sexual and natural selection as explanations for male dwarfing.

Main Methods:

Comparative analysis of species with dwarf males and sedentary females.
Review of existing literature on sexual selection and natural selection in dwarfing species.

Main Results:

Sexual selection is seldom a sufficient explanation for male dwarfing.
Natural selection provides a more consistent explanation for the evolution of male dwarfing.

Conclusions:

Male dwarfing is primarily driven by natural selection.
The symbiotic relationship benefits both sexes, with males providing reproductive services for sustenance.