Final answer:
To determine the probability of an unaffected sister having an albino child, a Punnett square is used to analyze the transmission of the relevant alleles. The sister probably has a 2/3 chance of being a carrier, and if mated with a non-albino man, there is a 1/4 chance of them having an albino child. Consequently, the overall probability of them having an albino child is 1/8.
Step-by-step explanation:
Understanding the likely genotype of an individual related to an albino child requires knowledge of genetics and how alleles are transmitted from parents to offspring. If albinism is caused by a recessive allele, and the unaffected sister has an albino child, she must be a carrier of the recessive allele. The first step in determining the unaffected sister's genotype would be to make a Punnett square for the albino's parents to ascertain the possible genotypes they could pass on.
For example, if both parents were carriers (heterozygous for the albino trait), their genotypes would be Aa, where 'A' is the dominant non-albino allele and 'a' is the recessive albino allele. The Punnett square would thus show a 3:1 ratio of non-albino to albino phenotypes in the offspring. Therefore, the probability of the sister being a carrier is 2/3 because we know she is not affected by albinism (therefore excluding the possibility of her being aa).
Next, we consider the mating of the unaffected sister (carrier) with the unrelated non-albino man. To have an albino child, the man must provide the recessive allele and the unaffected sister must also pass on the recessive allele. The probability of this event is 1/4, as shown by the Punnett square when crossing Aa (sister) with AA or Aa (man, assuming he could be heterozygous without knowing his exact genotype).
Finally, the probability that the couple will have an albino child is found by multiplying the chance of the sister being a carrier by the chance of both partners passing the recessive allele to their offspring, resulting in 1/2 * 1/4 = 1/8.