10.3 There are many variations of inheritance patterns

Describe how alleles interact in intermediate inheritance. Objectives Describe how alleles interact in intermediate inheritance. Describe inheritance patterns involving multiple alleles. Explain how polygenic inheritance can result in a wide range of phenotypes. Describe how environmental conditions can affect phenotype expression. Key Terms intermediate inheritance codominance polygenic inheritance

Intermediate Inheritance The F1 offspring of Mendel's pea crosses always looked like the dominant homozygous parent. This is because one dominant allele was enough to produce the dominant phenotype, but the recessive phenotype required inheriting two recessive alleles. But for some characters of organisms, neither allele is dominant. The heterozygotes have a phenotype that is intermediate between the phenotypes of the two homozygotes. This pattern of inheritance is called intermediate inheritance

For example, in a particular breed of chicken called Andalusians, black and white parents produce F1 hybrid offspring, called "blues," with grayish-blue feathers. Because neither the black nor white allele is dominant, capital and lowercase letters are not used to represent them. Instead, a C for "color" is paired with a superscript B for "black" or W for "white" to represent the two alleles. A heterozygote chicken has one of each allele, CBCW, and is blue in color.

Multiple Alleles The inheritance patterns you have explored so far have involved only two contrasting alleles for each inherited character. But for many genes several alleles exist in the population. This expands the number of possible genotypes and phenotypes.

For example, multiple alleles control the character of blood type in humans. A person's blood type may be A, B, AB, or O. The letters refer to two carbohydrates, designated A and B, which are found on the surface of red blood cells. A person's red blood cells may be coated with one carbohydrate (type A) or the other (type B), with both (type AB), or with neither (type O).

These four blood types result from various combinations of three alleles, symbolized as IA (for carbohydrate A), IB (for carbohydrate B), and i (for neither A nor B). Each person inherits one of these alleles from each parent. There are six possible ways to pair the alleles—in other words, there are six possible genotypes. The alleles IA and IB exhibit codominance, meaning that a heterozygote expresses both traits. Note that this is different from intermediate inheritance. The individual's phenotype is not intermediate, but rather shows the separate traits of both alleles.

                                                                                                                             

When multiple genes affect a character, the variation in phenotypes can become even greater. When two or more genes affect a single character, it is called polygenic inheritance. In humans, height and skin color have polygenic inheritance. For example, the height of students in a large high school might range from about 125 cm to 200 cm, with students of every possible height in between.

The Importance of Environment An individual's phenotype depends on environment as well as on genes. For example, although a tree's genotype does not change throughout its lifetime, the tree's leaves vary in size, shape, and greenness from year to year depending on exposure to wind and sunlight. In humans, nutrition influences height, exercise affects build, and exposure to sunlight darkens the skin. Even identical twins, who have the same genotype for every character, accumulate some differences in phenotype as a result of their exposure to environmental differences. For example, each twin may have a different diet, activity level, history of illnesses, and exposure to sunlight.