Epsilon Archive for Student Projects

Abstract

The Swedish mountain cattle (Fjällko) has been selected for generations in order to maintain its characteristic phenotype of white colour coat with coloured pattern sides, unfortunately this led to the propagation of a recessive gene disorder, gonadal hypoplasia, characterized by underdeveloped gonads and reduced fertility. As established in previous studies both the colour-sided phenotype and the gonadal hypoplasia condition have been associated with the reciprocal translocation between chromosomes 6 and 29, denoted t(6;29). The Cs29 allele refers to the segment of chromosomes 29 that has been translocated to chromosome 6 as part of this rearrangement. However, what has been missing until now was a direct statistical association between homozygosity for the Cs29 allele (carrying two copies) and the hypoplasia phenotype in Swedish mountain cattle.
This is why this study aimed to validate and characterize the previously described molecular breakpoints of the t(6;29) translocation in the Swedish mountain cattle and across other Swedish cattle breeds genetically related to this breed and test for a statistical association between the Cs29 genotype and gonadal hypoplasia, this way providing additional evidence supporting the proposed genotype-phenotype relationship.
DNA from 29 individuals across four Swedish breeds (Fjällko, Väneko, Bohuskulla and SKB) was analysed using Sanger sequencing of three primer regions (A-E, alpha-D and C-beta) specific to the breakpoint junctions, and previously certified in other studies cited in this study. Samples from Väneko, Bohuskulla and SKB were originally collected for previous studies conducted in the same laboratory and were included in the present study for comparative sequence analysis only. Two Fisher’s exact tests were performed exclusively on Fjällko samples as hypoplasia status and fertility records were not available for the other breeds. The first test focused on 20 Fjällko samples with confirmed hypoplasia status while the second expanded test included the same 20 samples with 8 additional Fjällko samples with documented fertility problems but unknown hypoplasia status.
The sequence analysis revealed a complete conservation of the three targeted breakpoint regions across all four breeds, supporting a common ancestral origin. Statistical analysis demonstrated a strong significant association between homozygosity for the Cs29 allele and gonadal hypoplasia. Among animals with confirmed hypoplasia 5 out of 6 were homozygotes for the Cs29 allele while only 1 out of 14 non hypoplastic animals carried this genotype. When other animals with certified fertility problems but no certain hypoplasia condition were
included, the association strengthened. In contrast the Cs6 allele, the segment of chromosome 6 that has been translocated to chromosome 29, showed no association with hypoplasia indicating its neutrality or its involvement in other traits.
This study provides an additional statistical link between Cs29 homozygosity to gonadal hypoplasia in Swedish mountain cattle. These findings demonstrate an association but do not establish a causal mechanism, which would require further functional studies. However, the molecular characterization of the breakpoints confirms their conservation across breeds and provides the base for developing targeted genetic screening tools to enable informed breeding decisions.