The R160W variant is a loss-of-function gene. Genetics is often far more complicated than a "simple" Mendelian model, and 4% of people who have one copy of R160W actually have red hair (Beaumont, K., Shekar, S. et. al. (2007). Receptor function, dominant negative activity and phenotype correlations for MC1R variant alleles. Human Molecular Genetics (Volume 16, Issue 18), pp. 2249–2260). R160W is considered a dominant negative variant, and interactions from other alleles will affect how much red pigment is produced. In short, there is not a 25% chance your child would have red hair with this genotype. The actual probability is unknown as we do not know the full genetic sequence of you and your partner's gene.