A human trait is influenced by a single pair of genes, with "S" representing the dominant allele and "s" representing the recessive allele. Tim, with genotype SS, and Leslie, with genotype ss, decide to have children. According to Mendelian genetics, all of their children will exhibit the trait.

In Mendelian genetics, traits are controlled by alleles which are the different forms of a gene. When Tim, with genotype SS, and Leslie, with genotype ss, decide to have children, their offspring receive one allele from each parent.

Tim has two dominant alleles (SS), and Leslie has two recessive alleles (ss). Here is how you determine the offspring's genotype:

1. Make a list of possible alleles that each parent can contribute. Tim can only contribute an 'S' allele because both of his alleles are dominant. Leslie can only contribute an 's' allele because both of her alleles are recessive.

2. Combine one allele from each parent to form the offspring's genotype. Since Tim contributes an 'S' and Leslie contributes an 's', each child will have the genotype 'Ss'.

3. According to Mendelian genetics, if an allele is dominant (like the 'S' allele in this case), only one copy of this allele is required for the trait to be expressed. Therefore, even though the children have one copy of the recessive allele ('s'), the dominant 'S' allele will mask the presence of the recessive allele.

As a result, all of their children will exhibit the dominant trait because they all have at least one 'S' allele. The genotype of all the children will be heterozygous (Ss), and since 'S' is dominant over 's', the trait associated with the dominant allele 'S' will be expressed.