1. correct answer: B) 0; 1/2, 1/16
2. correct answer: A) X chromosome
3. correct answer: A) Changes in the number of chromosomes per cell or in the structure of individual chromosomes can affect phenotype. Such alterations cause Down syndrome (usually due to trisomy of chromosome 21), certain cancers associated with chromosomal translocations, and various other human disorders.
4. correct answer: A) karyotype shows the full set of chromosomes of an individual arranged in sequence.
5. correct answer: C) Recessive; if the disorder were dominant, it would affect at least one parent of a child born with the disorder. The disorder's inheritance is sex-linked because it is seen only in boys. For a girl to have the disorder, she would have to inherit recessive alleles from both parents. This would be very rare, since males with the recessive allele on their X chromosome die in their early teens.
6. correct answer: A) The disorder would always be inherited from the mother.