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Recently Customer Asked “What Affects Sex Ratio of Guppy and Endler Fry?”

Recently a Customer Asked “What Affects Sex Ratio of Guppy and Endler Fry?”
In regard to what effects gender / sex ratio in guppies, this is a quite involved and multi-faceted question as well as there are many different opinions.Here is my information based on 50 + years experience of fish breeding and research on the topic.Both Environmental as well as genetic factors affect the ratio outcome.

Environmental factors:

Water Chemistry:

Softer water with low pH yields mostly females or sometimes all females.

Harder water with higher pH yields a more even gender ratio.


Colony Existing Gender Imbalance:

Overpopulation of male fish will cause the fry to produce more females.

If there is a shortage of males, the males will super develop so they can breed with the females at an earlier age.

Out of balance populations may release beneficial hormones for whichever sex is most needed to balance the gender ratio of the colony.


Water Temperature:

Note: Excessively High Water Temperatures will kill females more quickly than male fish.
Please refer to the first published article abstract below…


Genetic Factors:

Proportions of Sex Linked Genes related to the Physical Appearance of the Line:

Some of the characteristics of Strain Phenotypes have more Y Male Gene factors for male appearance and develop early and stay small contributing to the overall appearance, other male phenotypes have few factors contributing to the overall appearance of the strain and are late bloomers and are the largest of male specimens.

Rate of Sexual Development:
Both genders offer a X or Y linked variable genetically determined rate of sexual development.

Late males mated with an Early Females will result in mostly females.

Early developed males mated with a late developing female will result in mostly males.

Some Guppies are neither Late nor Early developing, but this is also a factor in the gender ratios you are getting.


Sex Linked Traits:

In many Guppy Strains Both Genders X and Y chromosomes in males offer contributions to the physical appearance of both the males and females Physical Appearance of a respective strain of Guppies.

So If for example a certain trait for a specific tail shape defines the physical appearance of a particular strain or line and this trait is a result of line breeding from only females contributing this recessive trait, more females may result in any of the batches of fry on a regular basis.


For More Information about environmental factors, here are 2 published known scientific articles on this topic:

Biased sex ratios in laboratory strains of guppies, Poecilia reticulata
First published:08 December 2005


In this study, the effect of temperature on survival and sex ratios of guppy Poecilia reticulata (Peters 1860) was investigated. Treatments of 19, 21 and 22.5°C low and 29, 32 and 35°C high water temperature were used on fry and gravid females for 11 days after parturitions and several days starting from the 16th day after first parturition until second parturition respectively. The high water temperature treatments caused the death of gravid females. Survival rates of heat‐treated fry were generally decreased with increasing and decreasing temperature. The sex ratios of the progenies from survived gravid females were not differed from a balanced sex ratio (P>0.05). The proportion of females increased gradually with decreasing temperature and the proportion of males increased gradually with increasing temperatures rather than having a threshold response in heat‐treated fry. However, mortality of heat‐treated fry was high; it is most likely that our results were biased by sex‐dependent mortality, which were also revealed by corrected sex ratios of the treatment groups and progeny testing of some individuals. These results suggest that a major gene linked to X‐chromosome could be responsible for resistance and sensitivity to both high and low temperature.

Effect of temperature on sex ratio in guppy Poecilia reticulata (Peters 1860)


Older laboratory strains of guppies, Poecilia reticulata, exhibit female-biased sex ratios. The results of interstrain crosses and crosses involving exceptional Y Y males showed that the sex ratio phenomenon is determined primarily by Y-linked genes which result either in decreased production of Y-bearing sperm, or in lowered ability of Y-bearing sperm to compete with X-bearing sperm for ova. Inbreeding and local mate competition cannot account for the evolution of the sex ratio deviations. It is suggested that the sex ratio effect is caused either by pleiotropic effects of Y-linked genes increasing the ability of males to compete for females, or by a genetic deterioration of the Y-chromosomes through accumulation of deleterious alleles. The latter suggestion is supported by the theoretical models of Nei (1970) and Charlesworth (1978) on the evolution of inert Y-chromosomes.