The accompanying pedigree shows the transmission of albinism (absence of skin pigment) in a human family.

One child of female I-3 has albinism. What is the probability that any of the other four children are carriers of the allele for albinism? 

Verified step by step guidance

1. Step 1: Understand the inheritance pattern of albinism. Albinism is typically inherited in an autosomal recessive manner, meaning that an individual must inherit two copies of the recessive allele (one from each parent) to express the trait. A carrier has one dominant allele and one recessive allele but does not express the trait.
2. Step 2: Analyze the pedigree. Female I-3 must be a carrier of the recessive allele for albinism because one of her children has albinism. This means that her genotype is heterozygous (Aa). The father of the children must also be heterozygous (Aa) or homozygous recessive (aa) for the child to inherit two recessive alleles.
3. Step 3: Determine the possible genotypes of the children. Using a Punnett square, cross the genotypes of the parents (Aa x Aa if both are heterozygous). The possible genotypes of the children are: AA (not a carrier), Aa (carrier), and aa (affected). The probabilities are 25% AA, 50% Aa, and 25% aa.
4. Step 4: Exclude the child with albinism from the calculation. Since one child is already known to have the genotype aa, focus on the remaining four children. For each of these children, the probability of being a carrier (Aa) is 50%, based on the Punnett square.
5. Step 5: Conclude that the probability of any of the other four children being carriers of the allele for albinism is 50%. This probability applies independently to each child, as genetic inheritance is random for each offspring.

Key Concepts

Pedigree Analysis

Pedigree analysis is a diagrammatic method used to trace the inheritance patterns of traits through generations in a family. It helps identify whether a trait is dominant or recessive, and can indicate the likelihood of individuals being carriers of specific alleles. In this case, understanding the pedigree is crucial to determine the inheritance of albinism.

Autosomal Recessive Inheritance

Albinism is typically inherited in an autosomal recessive manner, meaning that an individual must inherit two copies of the recessive allele (one from each parent) to express the trait. Carriers possess one copy of the recessive allele and one dominant allele, which does not manifest the trait. Recognizing this pattern is essential for calculating the probability of the other children being carriers.

Probability in Genetics

Probability in genetics involves calculating the likelihood of certain genotypes or phenotypes occurring in offspring based on parental genotypes. In this scenario, understanding the genetic makeup of the parents and the known child with albinism allows for the application of probability rules, such as the Punnett square, to estimate the chances that the other children are carriers of the albinism allele.