In humans, neurofibromatosis type 1 (NF-1) is an inherited disorder in which tumours of the nerve tissue form - VCE - SSCE Biology - Question 28 - 2014 - Paper 1

To determine the probability of the next child having NF-1, we start by understanding that NF-1 is an autosomal dominant trait. This means that only one copy of the NF-1 allele is needed for the offspring to express the disorder.

1. Parents' Genotypes:

   • The father has NF-1, so his genotype is either NN (homozygous for the disorder) or Nn (heterozygous for the disorder).
   • The mother does not have the trait, so her genotype is nn.

2. Possible Combinations:

   • If the father is NN, all children will inherit an N and an n, giving a 100% chance of having NF-1.
   • If the father is Nn, then the possible combinations for each child are:
     • Nn (has NF-1)
     • nn (does not have NF-1) This gives a 1 in 2 chance of having NF-1 for the next child.

3. Conclusion:

   • Since the father is only stated to have one child without the trait, we can conclude that he is Nn. Therefore, the chance that the next child has NF-1 is one in two.

Thus, the correct answer is B. one in two.