Question

Albinism, lack of pigmentation in humans, results from an autosomal recessive gene (a). Two parents with normal pigmentation have an albino child. (a) What is the probability that their next child will be albino? (b) What is the probability that their next child will be an albino girl? (c) What is the probability that their next three children will be albino?

Short answer

Answer: Based on the provided analysis and solution, the probabilities are as follows: (a) The probability of their next child being albino is 25%. (b) The probability of their next child being an albino girl is 12.5%. (c) The probability of their next three children all being albino is approximately 1.56%.

Step-by-step solution

Understand the Genotype of Parents

Both parents must be carriers of the recessive gene (Aa) to have an albino child (aa). These parents do not express the albinism trait as they each have one copy of the dominant gene (A) and one copy of the recessive gene (a).

Create Punnett Square

A Punnett square is a chart that shows the possible combinations of alleles that can result from a genetic cross. Lay out a 2x2 Punnett square with each parent's genotype (Aa). The square will contain the following combinations: AA, Aa, aA, and aa. Each of these combinations has an equal chance of 1/4 (or 25%) to occur.

Find Probability of Albino Child

According to the Punnett square, the genotype 'aa' (which results in albinism) has a probability of 1/4 (or 25%).

Respond to Part (a)

The probability that their next child will be albino is 1/4 (or 25%).

Calculate Probability of Albino Girl

The gender of a child (male or female) is a separate event from inheriting the recessive gene for albinism. Since the probability a child is a girl is 1/2 (or 50%), and the probability that the child is albino is 1/4 (or 25%), the joint probability that a child is an albino girl is found by multiplying these two probabilities: (1/2) * (1/4) = 1/8 (or 12.5%).

Respond to Part (b)

The probability that their next child will be an albino girl is 1/8 (or 12.5%).

Calculate Probability of Three Albino Children

Each child's genotype is an independent event. So the probability of having three albino children in a row is calculated by multiplying the probabilities of each event: (1/4) * (1/4) * (1/4) = 1/64 (or about 1.56%).

Respond to Part (c)

The probability that their next three children will be albino is 1/64 (or about 1.56%).

Key concepts

Punnett Square

Understanding genetics can sometimes be tricky, but a Punnett square makes visualizing genetic crosses much simpler. It's a grid that helps predict the genotypes of offspring from two parents. Here's how it works: You place one parent's alleles along the top and the other parent's along the side.Each cell within the grid then shows a possible combination of alleles that their offspring could inherit. For example, when the parents are carriers of a recessive trait like albinism, with genotypes Aa each, their Punnett square would look like this:

• Top row: A a
• Side column: A a

Inside the grid, you'd pair the top and side alleles to get the possible genotypes: AA, Aa, aA, and aa.Each of these combinations represents a potential genotype for the offspring, with each having an equal chance, 1 in 4, to occur. The simplicity of this tool allows students to quickly understand the basics of genetic inheritance patterns, making it an indispensable part of learning genetics.

Autosomal Recessive Inheritance

Autosomal recessive inheritance is one of the key patterns for understanding genetic conditions like albinism. For a recessive trait to be displayed in an individual, they must inherit two copies of the recessive allele, one from each parent. If an individual has just one copy of the recessive allele and one dominant allele, they're known as carriers.In our example, since both parents have normal pigmentation and an albino child, it's clear that each parent has one recessive allele (a) responsible for albinism. They are heterozygous (Aa), which means although they carry the gene for albinism, the dominant allele (A) masks the expression of the recessive trait.When two carriers have offspring, there is a 25% chance that their child will inherit the recessive allele from both parents, resulting in albinism (aa). There's also a 25% chance of the child inheriting both dominant alleles (AA), and a 50% chance of inheriting one dominant and one recessive allele (Aa or aA), being a carrier like the parents. This concept provides a fundamental insight into how genetic traits are passed through generations.

Albinism Genetics

Albinism is a genetic condition characterized by a lack of pigmentation in the skin, hair, and eyes due to the body's inability to produce or distribute melanin adequately. The genetics behind albinism are associated with several genes, but for simplicity, we often focus on a single autosomal recessive gene. People with albinism have two recessive alleles (aa), which means both parents must be carriers or affected by albinism themselves.It's important to note that albinism does not affect the number of melanocytes (cells responsible for melanin production) in the skin. Instead, it affects the production of melanin within these cells. Albinism can be accompanied by a range of health issues, primarily affecting vision. Genetic counseling is recommended for individuals or couples who may be carriers, to understand the risks and implications for their offspring.Combining this genetic understanding with the probability exercises like the ones in the textbook helps students appreciate the real-world applications of genetics and deepen their knowledge of hereditary patterns. By mastering the concepts of Punnett squares, autosomal recessive inheritance, and albinism genetics, students are better equipped to tackle more complex genetic problems.