Preliminary analyses have demonstrated that some stallions have significantly more male offspring, while others sire more female offspring than expected from an equal sex ratio. During this project we aim to investigate distorted sex-ratios in Icelandic horses, and assess the heritability of distorted sex-ratios in Icelandic stallions. These aims were investigated using the Icelandic studbook, including 509.008 horses. Of these horses 43.7% was male, 55.7% female and 0.6% had an unknown sex. Significant individual were identified using chi-square tests.
For the further analyses we only included horses with eight or more offspring, since this is the minimal number of offspring needed for a significant chi-square test. Heritability was calculated using a regression analyses. In total 22490 horses were included in our analyses. Of the stallions, 16% had a significant distorted sex-ratios among their offspring. For mares skewing sex-ratios were less common, only 5.8% skewed. Female skewing in stallions was more common than male skewing, 13.9% compared to 2.1%. The heritability for having more female or male offspring was 0.29. This heritability seems to increase when father and son have more offspring.
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