In mice, the dominant allele Gs of the X-linked gene Greasy...

In mice, the dominant allele Gs of the X-linked gene Greasy produces shiny fur, while the recessive wildtype $G$ s $^{+}$allele determines normal fur. The dominant allele Bhd of the X-linked Broadhead gene causes skeletal abnormalities including broad heads and snouts, while the recessive wild-type $B h d^{+}$allele yields normal skeletons. Female mice beterozygous for the two alleles of both genes were mated with wild-type males. Among 100 male progeny of this cross, 49 had shiny fur, 48 had skeletal abnormalities, 2 had shiny fur and skeletal abnormalities, and 1 was wild type. a. Diagram the cross described, and calculate the distance between the two genes. b. What would have been the results if you had counted 100 female progeny of the cross?

In mice, the dominant allele Gs of the X-linked gene Greasy produces shiny fur, while the recessive wildtype $G$ s $^{+}$allele determines normal fur. The dominant allele Bhd of the X-linked Broadhead gene causes skeletal abnormalities including broad heads and snouts, while the recessive wild-type $B h d^{+}$allele yields normal skeletons. Female mice beterozygous for the two alleles of both genes were mated with wild-type males. Among 100 male progeny of this cross, 49 had shiny fur, 48 had skeletal abnormalities, 2 had shiny fur and skeletal abnormalities, and 1 was wild type.
a. Diagram the cross described, and calculate the distance between the two genes.
b. What would have been the results if you had counted 100 female progeny of the cross?

Key Concepts

Coupling and Repulsion Phases

These terms describe the arrangement of alleles on the homologous chromosomes in a heterozygote. In the coupling (or cis) phase, dominant (or mutant) alleles are found together on one chromosome and the recessive (or wild-type) alleles on the other, leading to parental types that are more commonly observed. In the repulsion (or trans) phase, one chromosome carries a dominant allele for one trait and a recessive allele for another, with the opposite arrangement on the homolog, which changes the expected frequencies of recombinant offspring and is crucial for interpreting linkage data.

Genetic Mapping

Genetic mapping involves determining the order and relative distances between genes on a chromosome using recombination frequencies from controlled genetic crosses. By analyzing the distribution of recombinant and parental types in progeny, researchers can construct maps that describe the arrangement of genes, which is essential for understanding genome organization and predicting linkage outcomes.

Recombination Frequency

Recombination frequency is the proportion of recombinant offspring that result from a cross, and it provides a measure of the physical distance between two linked genes. It is calculated by dividing the number of recombinant outcomes by the total number of offspring and then converting this fraction into a percentage or map units. The closer two genes are, the lower the recombination frequency will be.

Gene Linkage

Gene linkage describes the phenomenon of two or more genes that are located close together on the same chromosome, which tend to be inherited together because the likelihood of a crossover event separating them is reduced. This non-independent segregation affects the expected ratios of offspring phenotypes in genetic crosses, particularly in dihybrid or multifactorial crosses.

X-linked Inheritance

This concept refers to genes located on the X chromosome that do not recombine like autosomal genes between sexes. In species with XY sex-determination systems, males (XY) exhibit hemizygosity for X-linked genes, meaning that any allele present on the single X chromosome is expressed. This results in distinct patterns of inheritance, where the phenotype of a male is directly linked to the allele on his single X, while females have two copies and can be heterozygous.

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