BIOLOGY Volume 3 - A Guide to General Biology - 2004

24. VARIATION AND GENETICS

24.7. Gene Interactions

Up to this point, this chapter has focused on relatively straightforward aspects of genetics: dominance, monohybrid and dihybrid crosses, linkage, Sex Determination, and sex linkage. However, Other types of Gene interactions are known to exist, and these likely account for the majority of an Organism's phenotypic traits. Some of these interactions are examined below.

24.7.1. Codominance

There are instances where two or more alleles do not display complete dominance or recessiveness because neither dominates the other in the heterozygous state. This condition—known as codominance or incomplete dominance—represents an exception to the patterns Mendel observed in his monohybrid cross experiments. Fortunately, Mendel chose traits that did not exhibit codominance; otherwise, it could have significantly complicated his early research.

Codominance is observed in both plants and animals. In most cases, heterozygotes exhibit a phenotype intermediate between those of the dominant and recessive homozygotes. A classic example is the Andalusian fowl, produced by crossing solid black chickens with dirty-white chickens (which have small blue spots on a white Background, often referred to as "splashed white"). The black plumage is produced by an allele responsible for the Synthesis of the black pigment melanin, which is absent in splashed chickens. In heterozygotes, melanin is only partially synthesized, resulting in a bluish sheen to the plumage.

Since there are no universally accepted standard symbols for incompletely dominant alleles, researchers have found it necessary to introduce specific allele designations to explain cross results: black as B, splashed as b, W, B^w, or B^Bw. The results of a cross between homozygous black and splashed chickens are shown in Fig. 24.23.

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Fig. 24.23. Breeding of Andalusian fowl (F₁ hybrids).

When F₁ individuals are crossed with each other, the phenotypic ratio in the F₂ generation deviates from the Mendelian 3:1 ratio typical of Monohybrid Crosses. Instead, a 1:2:1 ratio is observed, where half of the offspring share the same genotype as the F₁ individuals (Fig. 24.24). A 1:2:1 ratio is characteristic of crosses involving codominance. Additional Examples are provided in Table 24.4.

Fig. 24.24. Results of crossing Andalusian fowl (F₂ hybrids).

Table 24.4. Examples of codominance

24.12. In cats, the genes controlling coat color are located on the X Chromosomes and exhibit codominance. When a black female cat is crossed with a red male cat, the F₁ generation yields black males and tortoiseshell females. What is the expected phenotypic ratio in the F₂ generation? Explain your expected results.