Crossover Suppression Help
Many extrinsic and intrinsic factors are known to contribute to the crossover rate. Among these are the effects of sex, age, temperature, proximity to the centromere or heterochromatic regions, chromosomal aberrations such as inversions, and many more. Two specific cases of crossover suppression are presented in this section: (1) complete absence of crossing over in male Drosophila and (2) the maintenance of balanced lethal systems as permanent trans heterozygotes through the prevention of crossing over.
Absence of Crossing Over in Male Drosophila
One of the unusual characteristics of Drosophila is the apparent absence of crossing over in males. This fact is shown clearly by the nonequivalent results of reciprocal crosses.
EXAMPLE 6.27 Testcross of heterozygous females.
Consider two genes on the third chromosome of Drosophila, hairy (h) and scarlet (st), approximately 20 map units apart.
When dihybrid males are crossed to dihybrid females (both in repulsion phase) the progeny will always appear in the ratio 2 : 1 : 1 regardless of the degree of linkage between the genes. The double-recessive class never appears. The student should verify that these expectations are valid.
Drosophila is not unique in this respect. For example, crossing over is completely suppressed in female silkworms. Other examples of complete and partial suppression of crossing over are common in the genetic literature.
Balanced Lethal Systems
A gene that is lethal when homozygous and linked to another lethal with the same mode of action can be maintained in permanent dihybrid condition in repulsion phase when associated with a genetic condition that prevents crossing over (see "inversions" in Chapter 7). Balanced lethals breed true and their behavior simulates that of a homozygous genotype. These systems are commonly used to maintain laboratory cultures of lethal, semilethal, or sterile mutants.
EXAMPLE 6.29 Two dominant genetic conditions, curly wings (Cy) and plum eye color (Pm), are linked on chromosome 2 of Drosophila and associated with a chromosomal inversion that prevents crossing over. Both Cy and Pm are lethal when homozygous. Half the progeny from repulsion heterozygotes die, and the viable half are repulsion heterozygotes just like the parents.
Balanced lethals may be used to determine on which chromosomean unknown genetic unit resides (see Solved Problem 6.7). Sex-linked genes make themselves known through the nonequivalence of progeny from reciprocal matings (Chapter 5). Without the aid of a balanced lethal system, the assignment of an autosomal gene to a particular linkage group may be made through observation of the peculiar genetic ratios obtained from abnormal individuals possessing an extra chromosome (trisomic) bearing the gene under study.
Practice problems for these concepts can be found at:
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