Question

In man, the gene (i) for ichthyosis is sex-linked and recessive to the gene (I) for normal skin. Ichthyosis is a disease which produces scaley skin. Diagram on chromosomes the genotypes of the following crosses and summarize the expected phenotypic ratios: (a) a normal man, and a woman with ichthyosis; (b) a man with ichthyosis and a normal heterozygous woman; (c) a man with ichthyosis and a normal homozygous woman.

Short answer

The expected phenotypic ratios for each cross are: (a) 1:1 normal male to male with ichthyosis & 1:1 normal heterozygous female to female with ichthyosis. (b) 1:1 male with ichthyosis to normal male & 1:1 female with ichthyosis to normal heterozygous female. (c) 1:1 normal male to male with ichthyosis (no females as the mother's genotype does not have any i alleles).

Step-by-step solution

Determine the genotypes of the parents for each cross

We must determine the genotypes of the parents for each cross. (a) a normal man would have the genotype X^IY, and a woman with ichthyosis would have the genotype X^ix^i. (b) a man with ichthyosis would have the genotype X^iY, while a normal heterozygous woman would have the genotype X^IX^i. (c) a man with ichthyosis would have the genotype X^iY, while a normal homozygous woman would have the genotype X^IX^I.

Perform each cross

Now, we will perform each cross and determine the genotypes of the offspring. (a) X^IY (normal man) x X^ix^i (woman with ichthyosis) Offspring genotypes: X^IY, X^iY, X^IX^i, && X^ix^i (b) X^iY (man with ichthyosis) x X^IX^i (normal heterozygous woman) Offspring genotypes: X^iY, X^IY, X^IX^i, && X^ix^i (c) X^iY (man with ichthyosis) x X^IX^I (normal homozygous woman) Offspring genotypes: X^IY, && X^iY

Determine phenotype ratios

Now, we will determine the expected phenotypic ratios for each cross using the genotypes of the offspring: (a) - X^IY: 1 normal male (X^IY) - X^iY: 1 male with ichthyosis (X^iY) - X^IX^i: 1 normal heterozygous female (X^IX^i) - X^ix^i: 1 female with ichthyosis (X^ix^i) Phenotype ratio: 1:1 normal male to male with ichthyosis & 1:1 normal heterozygous female to female with ichthyosis (b) - X^iY: 1 male with ichthyosis (X^iY) - X^IY: 1 normal male (X^IY) - X^IX^i: 1 normal heterozygous female (X^IX^i) - X^ix^i: 1 female with ichthyosis (X^ix^i) Phenotype ratio: 1:1 male with ichthyosis to normal male & 1:1 female with ichthyosis to normal heterozygous female (c) - X^IY: 1 normal male (X^IY) - X^iY: 1 male with ichthyosis (X^iY) Phenotype Ratio: 1:1 normal male to male with ichthyosis (no females as the mother's genotype does not have any i alleles)

Key concepts

Ichthyosis Genetics

Ichthyosis is a genetic condition characterized by dry, scaly skin and is associated with a variety of genetic alterations. In the context of the exercise, we are dealing with an X-linked recessive type of inheritance pattern for ichthyosis. This means that the mutated gene causing ichthyosis is located on the X chromosome and has to be recessive, meaning a person needs two copies of the mutated gene to express the disease if they are female (XX), but only one copy if they are male (XY) due to males having only one X chromosome.

Understanding the basics of ichthyosis genetics is crucial for students delving into genetics, as it provides a clear example of how sex-linked disorders are inherited and expressed differently in males versus females. It's essential to remember that females have two opportunities to inherit the mutant gene (one from each parent), whereas males have only one chance, given their single X chromosome inherited from their mothers.

Phenotypic Ratios

Phenotypic ratios are a cornerstone concept in genetics. They represent the ratio of different visible traits (phenotypes) that appear in offspring. Through the exercises provided, students are asked to predict phenotypic ratios from certain genetic crosses involving ichthyosis. To calculate these ratios, it's vital to understand that each parent contributes one allele for a trait to their offspring.

For instance, in a cross between a carrier mother (heterozygous for the ichthyosis gene) and a father with normal skin, the phenotypic ratios of their offspring are determined by the combination of alleles they inherit. Phenotypic ratios are significant for predicting the likelihood of an offspring inheriting a particular trait and provide insight into the patterns of inheritance that characterize genetic transmission. Breaking down the crosses and offspring phenotypes step by step helps clarify how these ratios are derived in practice.

X-linked Recessive Disorders

X-linked recessive disorders are conditions caused by mutations in a gene on the X chromosome. Because males have only one X chromosome, a single recessive mutated gene on this chromosome will result in the male exhibiting the disorder. Conversely, females, with two X chromosomes, must have the mutation on both copies of the X chromosome to express the disorder; otherwise, they are carriers. This is why X-linked recessive disorders are more commonly observed in males.

Examples of X-linked recessive disorders, aside from ichthyosis, include hemophilia and color blindness. When teaching this concept, it's helpful to employ pedigree charts and Punnett squares to illustrate how these conditions are passed down through generations. Additionally, discussing real-world implications, such as genetic counseling and testing options, can make the material more relatable and understandable for students grappling with the abstract nature of genetic inheritance.