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: In the third generation, there is a 1:1 ratio of normal to albino phenotype among the children, with half having a normal phenotype (Aa genotype) and half having an albino phenotype (aa genotype).

Step-by-step solution

Understanding Dominant and Recessive Alleles

In a simple genetic trait, there are two alleles for a gene: dominant (A) and recessive (a). Normal phenotype (non-albino) is the dominant trait, and albino phenotype is the recessive trait. To have an albino phenotype, an individual must have two recessive alleles (aa), while a normal phenotype can have either two dominant alleles (AA) or one dominant and one recessive allele (Aa).

Determine the Parent Genotypes for Scenario (a)

Two normal parents have five children, four normal and one albino. Since albinism is a recessive trait, both parents must be carriers (Aa) to have a child with albinism. We can represent this using a Punnett square: A a A AA Aa a Aa aa The child with albinism has a genotype of aa, while the four children with normal phenotype could be AA or Aa.

Determine the Parent Genotypes for Scenario (b)

A normal male and an albino female have six children, all normal. This means the albino female has a genotype of aa, and since all children are normal, the male must have a genotype of AA. This can also be shown using a Punnett square: A A a Aa Aa All the children have an Aa genotype.

Determine the Parent Genotypes for Scenario (c)

A normal male and an albino female have six children, three normal and three albino. The albino female has a genotype of aa, and since half of the children are albino, the male must be a carrier (Aa). We can represent this using a Punnett square: A a a Aa aa Three of the children have a genotype of Aa (normal phenotype), and three have a genotype of aa (albino phenotype).

Construct a Pedigree for Scenarios (b) and (c)

We will construct a pedigree considering that one of the normal children in (b) and one of the albino children in (c) become parents of eight children. Pedigree Chart: - Generation 1: AA (male) x aa (female) [Scenario B], Aa (male) x aa (female) [Scenario C] - Generation 2: All Aa (from Scenario B), 3x Aa, and 3x aa (from Scenario C) - Generation 3: pair one child from B (Aa) and one child from C (aa), use a Punnett square to predict the phenotypes of their eight children. A a a Aa aa In this pedigree, half of the third-generation children will have an Aa genotype (normal phenotype), and half will have an aa genotype (albino phenotype).

Key concepts

Recessive and Dominant Alleles

In the world of genetics, understanding the difference between recessive and dominant alleles is crucial to deciphering inheritance patterns. Alleles are different forms of a gene that determine particular traits. In albinism, which is the trait under study, the allele for albinism is recessive, symbolized as 'a'. For a person to exhibit this trait, they must have two recessive alleles (aa).

Conversely, the allele for the normal pigmentation is dominant, represented by the letter 'A'. Having at least one 'A' allele (AA or Aa) will result in a normal phenotype, overshadowing the recessive allele if present. This is why two parents who appear normal but are carriers (Aa), can have an albino child (aa). Understanding this concept is pivotal for grasping how traits are passed down through generations.

Punnett Square Analysis

When predicting the potential genetic outcomes for offspring, a Punnett square analysis is an invaluable tool. It's a grid that allows us to visualize and calculate the probability of an offspring's genotypes based on the parents' genotypes. For example, in the case where two carriers (Aa) have children, a Punnett square outlines the possible allele combinations.

Sample Punnett Square for Two Carriers (Aa x Aa):

• 1/4 chance of child being AA (normal, non-carrier)
• 1/2 chance of child being Aa (normal, carrier)
• 1/4 chance of child being aa (albino)

By using this method, students can improve their understanding by picturing how alleles distribute among offspring. It also helps in resolving complex genetic scenarios and is essential when it comes to exercises involving inheritance patterns.

Pedigree Chart Construction

A pedigree chart is akin to a family tree, but it focuses on the inheritance of a specific genetic trait across different generations. Constructing a pedigree chart is a systematic way to track and visualize the genetic relationships within a family, determining who carries particular alleles and how they might affect the descendants.

In the exercise provided, mapping the families in scenarios (b) and (c), and their subsequent offspring, would involve noting individuals with the dominant phenotype (normal pigmentation) and the recessive phenotype (albinism). The chart would allow us to predict genetic outcomes in future generations and to see the inheritance pattern of albinism through a clear lineage. This visualization tool is fundamental for students studying genetics as it provides a comprehensive overview of how traits are transmitted and helps demystify complex inheritance questions.