Final answer:
The yellow kitten from the litter where the other kittens are tortoiseshell must be male. This is because the tortoiseshell pattern only occurs in female cats due to their two X chromosomes enabling a Bb combination, while the male's single X chromosome with the b allele results in yellow fur.
Step-by-step explanation:
In cats, an X-linked pair of alleles controls the color of fur, where allele B is for black fur, allele b is for yellow fur, and the combination Bb results in a tortoise-shell pattern due to incomplete dominance. Since tortoiseshell cats represent a heterozygous condition and only female cats have two X chromosomes, this tortoiseshell phenotype can only occur in females. Therefore, a yellow cat can only be male if it expresses the yellow phenotype because males have only one X chromosome and no opportunity for a heterozygous combination.
Considering the situation where a yellow cat had a litter of two tortoiseshells and one yellow kitten, and knowing that tortoiseshell is indicative of female cats because of the need for two different X chromosome alleles (B and b), the yellow kitten must be male. This is because he would have only one X chromosome carrying the b allele causing the yellow fur color.