Question

Albinism in humans is inherited as a simple recessive trait. For the following families, determine the genotypes of the parents and offspring. (When two alternative genotypes are possible, list both.) (a) Two normal parents have five children, four normal and one albino. (b) A normal male and an albino female have six children, all normal. (c) A normal male and an albino female have six children, three normal and three albino. (d) Construct a pedigree of the families in (b) and (c). Assume that one of the normal children in (b) and one of the albino children in (c) become the parents of eight children. Add these children to the pedigree, predicting their phenotypes (normal or albino).

Short answer

Answer: The genotypes of the parents in the new family (d) are Aa (normal) and aa (albino). The genotypes of their offspring will be Aa (normal), Aa (normal), Aa (normal), Aa (normal), aa (albino), aa (albino), aa (albino), and aa (albino).

Step-by-step solution

Case (a)

Two normal parents have five children, four normal and one albino. This means that both parents are carriers for the albino trait (heterozygous Aa) since they have produced an albino offspring (aa). Aa x Aa mating can produce AA (normal), Aa (normal), or aa (albino) offspring. The genotypes of parents: Aa and Aa, and the genotypes of the offspring: AA (normal), Aa (normal), Aa (normal), Aa (normal), and aa (albino).

Case (b)

A normal male and an albino female have six children, all normal. The albino female must have the genotype aa. The normal male must have at least one dominant A allele, and since all the offspring are normal, he must not carry the recessive albino allele. Therefore, the genotypes of the parents are AA (normal male) and aa (albino female), while the offspring are all Aa (normal).

Case (c)

A normal male and an albino female have six children, three normal and three albino. The albino female must have the genotype aa for the recessive trait. The normal male must be heterozygous Aa as three albino offspring are produced (meaning he has passed on his recessive a gene to those children). Therefore, the genotypes of the parents are Aa (normal male) and aa (albino female) while the offspring have genotypes Aa (normal), Aa (normal), Aa (normal), aa (albino), aa (albino), and aa (albino).

Case (d) - Pedigree construction

Construct a pedigree for families in (b) and (c) by assuming that one of the normal children from (b) and one of the albino children from (c) become parents of eight children. Parents from (b): AA (normal male) x aa (albino female) - All of their children will be Aa (normal). Parents from (c): Aa (normal male) x aa (albino female) - 3 children with genotype Aa (normal) and 3 children with genotype aa (albino). Choose one normal child (Aa) from family (b) and one albino child (aa) from family (c) as the new couple. So, their genotypes will be Aa (normal) x aa (albino). Their possible offspring genotypes are: Aa (normal), Aa (normal), Aa (normal), Aa (normal), aa (albino), aa (albino), aa (albino), and aa (albino). The pedigree will show the following phenotypes: - Family (b) - Parents: Normal male and Albino female. Offspring: 6 Normal children. - Family (c) - Parents: Normal male and Albino female. Offspring: 3 Normal children and 3 Albino children. - New family - Parents: Normal child from family (b) and Albino child from family (c). Offspring: 4 Normal children and 4 Albino children.

Key concepts

Albinism

Albinism is a genetic condition characterized by the absence or reduction of melanin pigment in the skin, hair, and eyes. This lack of pigmentation results in very light skin and hair, and vision problems, as melanin is crucial for proper vision development. In humans, albinism is inherited as an autosomal recessive trait.

This means that a person must inherit two copies of the mutated gene, one from each parent, to exhibit the condition. People with only one copy of the mutation are carriers and typically show no signs of albinism.

• Albinism can affect people of all races.
• It is often associated with other conditions affecting vision such as nystagmus and photophobia.
• Individuals with albinism need special care to protect their skin and eyes from sun damage.

Understanding the genetic basis of albinism can help in early diagnosis and management of the associated health issues.

Recessive Trait

A recessive trait in genetics refers to a characteristic that only appears when two copies of the recessive allele are present in an organism's genotype. When an allele is recessive, its effects are masked by the presence of a dominant allele.

In the context of albinism, the alleles can be represented as "A" for the dominant normal pigment trait and "a" for the recessive albinism trait. Thus:

• Individuals with two dominant alleles (AA) will have normal pigmentation.
• Heterozygous individuals (Aa) will be carriers, displaying normal pigmentation but carrying the albinism allele.
• Only those with two recessive alleles (aa) will express albinism.

This clear distinction underscores why understanding dominant and recessive traits is crucial in genetics, especially when predicting genetic variations in offspring.

Pedigree Analysis

Pedigree analysis is a valuable tool used in genetics to study the inheritance patterns of traits in families. It is depicted through a diagram called a pedigree, which resembles a family tree but specifically lists genetic relationships and trait occurrences across generations.

Each individual is represented by a symbol: usually a square for males and a circle for females. Shading indicates individuals who exhibit the trait, while connected lines illustrate familial relationships. In the case of albinism, pedigree analysis helps identify carriers of the recessive gene and predict the likelihood of the condition appearing in future generations.

• It helps ascertain whether a trait is dominant or recessive.
• Determines the probability of offspring inheriting a particular trait.
• Useful in counseling families about genetic conditions.

By constructing a pedigree, geneticists and families alike can visualize and understand how genetic traits and disorders are passed down.