Goats are among the first domesticated farm animals, due to their great adaptability to different environmental conditions, therefore they can be raised in arid, humid, tropical, cold, desert or mountain conditions (Kaliber et al., 2016). Goats are considered as very useful animals for their good productivity and easy to handle, moreover, they do not compete with man for food. In Egypt, there are five indigenous goat breeds: Baladi (primarily in the delta), Barki (in the west desert), Zaraibi (northeast of the delta), Sinaoy (in Siani peninsula) and Saidi (in Upper Egypt). Zaraibi goat is a dual-purpose animal and considered as being the most promising dairy goat among the local Egyptian breeds due to its high genetic potential for prolificacy and milk production (Dowidar et al., 2018).
Although milk production potential of goat dams is highly associated with their kids’ growth and survival, litter size (LS, numbers of kids born per doe) is also a very important factor determining the reproductive efficiency of the farm animals and has a highly significant influence on goat prolificacy (Tesema et al., 2020). For Zaraibi goat, litter size was ranged from 2.1 and 2.14 kids per doe (Abu El-ella et al., 2011 and Aboul-Naga et al., 2012), while total milk yield (TMY) was ranged from 249 to 363.15 kg/h (El-Saied et al., 2007, Abdelhamid et al., 2013).
As milk production and reproduction are complex traits (i.e. controlled by many genes and environmental factors), some nucleotides polymorphism might account for large amounts of genetic variation. Therefore, selection programs using specific genetic markers could be a good strategy for precise and improving genetic changes of these traits (Bhowmik et al., 2019). Genes attributed with different economic traits including growth, reproduction, meat and milk production traits as well as disease resistance trait is known as candidate genes (Supakorn, 2009).
Prolactin plays an important regulatory function in mammary gland development. It is an anterior pituitary peptide hormone and is essential for reproductive performance. The action of this hormone is mediated by its receptor encoded by prolactin receptor gene (PRLR), which is a member of growth hormone/prolactin receptor gene family (Ahlawat et al., 2015). In goats, PRLR gene was mapped on chromosome 20 and consists of five exons and four introns, encoding the 199-amino-acid for mature protein (Hayes et al., 1996). PRLR is considered as an excellent candidate for linkage analysis of quantitative trait loci (QTL) affecting milk production traits (Shi et al., 2011).
Zhang et al. (2007) sequenced 315 bp fragment of exon 10 of PRLR gene in Chinese goat breeds and revealed one mutation (143A→G), this mutation caused a change in amino acid sequence (Met→Val), and had a significant effect on litter size, where goats with genotype AG had more kids than those with genotype AA. Also, Wu et al. (2014) detected SNPs at exon 10 significantly associated with litter size in Lezhi black goats. In a study using SSCP technique, Ran et al. (2011) revealed significant association between three SNPs in intron 1 and 2 of PRLR gene and litter size in four Chinese goat breeds. In addition, Hou et al.  found two SNPs at exon 9 of PRLR gene which caused a change in amino acid sequence Ser485Asn and Val548 Met. These SNPs were significantly associated with litter size and milk production in Boer and Guanzhong goat breeds. Moreover, they detected SNPs in 3’ UTR that showed a significant effect on milk production in the same breed (Hou et al., 2014b).
Insulin-like growth factor 1 (IGF1) is considered as a strong candidate gene for reproductive traits (Gobikrushanth et al., 2018). In goats, IGF1 gene was mapped on chromosome 5 (Naicy et al., 2017), and consists of 1–6 exons in different species (Andrade et al., 2008), that produce a polypeptide of 70 amino acids that are highly conserved (Ge et al., 2001). According to polymorphism studies, Tahmoorespur et al. (2009) found a significant association between polymorphism in 5’ flanking region of ovine IGF-1 and average daily milk yield (ADMY). Moreover, Naicy et al. (2016) detected two SNPs (g.224A > G and g.227C > T) in low prolific Attappady Black and high prolific Malabari goat breeds, where three genotypes (PP, PQ and QR) have been observed. PQ genotype showed (A→G transition at 224th position), while QR genotype showed two SNPs, one SNP as PQ genotype and the other at 227bp (C→T transition). There was a significant association between the three IGF1 genotypes and litter size, but genotypes PQ and QR revealed a greater number of born kids than homozygous PP genotype. In local goat breeds (Zaraibi, Baladi and Barki), Othman et al. (2016) have detected a SNP (C/G) at position 282 bp that has a significant effect on growth trait. Recently, Lestari et al. (2020) identified SNP (g5752G→C) at intron 4 of IGF-I gene in Kejobong goats, where two genotypes GG and CC found to be significantly associated with body weight. In addition, Sebastiano et al. (2020) found two SNPs at 5’ UTR and exon 3 of IGF-1gene showed a significant association with reproductive traits and milk production in Sarda dairy sheep.
Leptin (LEP) is synthesized mainly by adipose tissue; it plays the main role in regulation and control of productive performance of animals (Kumar et al., 2020). LEP is synthesized as a pro hormone (premature and inactive form of leptin protein), it consists of 167 amino acids, but their mature functional polypeptides consist of 146 amino acids (Marwarha et al., 2012). It is located on chromosome 4 in the ovine genome and consists of three exons and two introns, two exons only are translated into protein (Wallace et al., 2014). Exon1 is a non-coding part (Buchanan et al., 2002).
Singh et al. (2009) found 5 haplotypes in exon 2 and 6 haplotypes in intron 2 of LEP gene in Indian goats. Maitra et al. (2014) found non-synonymous (g.1029T > C) mutation, where amino acid Valine has changed to Alanine in 7 breeds of Indian goat. Another study on exon 3 of LEP gene in Barki sheep found one SNP which has a significant association with milk production (Abousoliman et al., 2020). Therefore, this study was designed to detect SNPs of PRLR, IGF-1 and LEP genes in Zaraibi goat using single-strand conformational polymorphism (PCR–SSCP) analysis and sequencing technique to investigate the potential associations between these polymorphisms with milk production and litter size.