The gene controlling the Xg blood group alleles (Xg⁺ and Xg⁻) and the gene controlling a newly described form of inherited recessive muscle weakness called episodic muscle weakness (EMWX) (Ryan et al., 1999) are closely linked on the X chromosome in humans at position Xp22.3 (the tip of the short arm). A male with EMWX who is Xg⁻ marries a woman who is Xg⁺ and they have eight daughters and one son, all of whom are normal for muscle function, the male being Xg⁺ and all the daughters being heterozygous at both the EMWX and Xg loci. Following is a table that lists three of the daughters with the phenotypes of their husbands and children. For each of the offspring, indicate whether or not a crossover was required to produce the phenotypes that are given.

Key Concepts

X-Linked Inheritance

X-linked inheritance refers to the pattern of inheritance for genes located on the X chromosome. Males have one X and one Y chromosome, while females have two X chromosomes. This means that X-linked traits can manifest differently in males and females, with males expressing the trait if they inherit a single recessive allele, while females may be carriers if they have one recessive and one dominant allele.

Crossover and Genetic Linkage

Crossover is a process during meiosis where homologous chromosomes exchange genetic material, leading to genetic variation in gametes. Genes that are closely linked on the same chromosome are less likely to be separated by crossover events. Understanding the likelihood of crossover between linked genes is crucial for predicting offspring phenotypes and determining inheritance patterns.

Phenotype and Genotype

Phenotype refers to the observable characteristics or traits of an organism, which result from the interaction of its genotype (the genetic makeup) with the environment. In this context, the phenotypes of the offspring depend on the combination of alleles inherited from their parents, while the genotype provides the underlying genetic information that determines these traits.