Section 11.3

1. Section 11.3 Other Patterns of Inheritance

2. Beyond Dominant and Recessive Alleles There are important exceptions to most of Mendel’s principles Not all genes show simple patterns of inheritance Majority of genes have more than 2 alleles Many important traits are controlled by more than one gene

3. Incomplete Dominance Some alleles are neither dominant nor recessive Incomplete dominance – cases in which one allele is not completely dominant over another allele The heterozygous phenotype lies somewhere between the 2 homozygous phenotypes Example: cross b/w 2 four o’clock plants cross b/w red flowered (RR) and white flowered (WW) produced an F1 generation with pink flowers (RW)

4. Codominance Codominance – phenotypes produced by both alleles are clearly expressed Example: in certain varieties of chickens, allele for black feathers is codominant with the allele for white feathers. Heterozygous chickens are speckled with both colors: black and white feathers (erminette) This is unlike the blending of red and white to make pink in incomplete dominance Many human genes (cholesterol) show codominance too People with heterozygous form of gene produce 2 different forms of the protein, each with a different effect on cholesterol levels

5. Multiple Alleles So far the examples have described genes for which there are only 2 alleles (a and A) In nature, these genes are the exception; not the rule. Many genes exist in several different forms and are said to have multiple alleles Multiple alleles – gene with more than 2 alleles Individuals only have 2 copies of each gene Many different alleles are found within a population Example: Human Blood type Example: coat color in rabbits (1 genes w/ 4 different alleles) 4 known alleles display simple dominance – can produce 4 coat colors

6. Polygenic Traits Many traits are produced by the interaction of several genes Polygenic (many genes) – traits controlled by 2 or more genes Example: at least 3 genes involved in making reddish-brown pigment in eyes of fruit flies Polygenic traits often show wide range of phenotypes Variety of skin color in humans comes about partly because more than 4 different genes probably control this trait

7. Genes and the Environment Characteristics of any organism are not determined solely by the genes that organism inherits Genes provide a plan for development How the plan unfolds depends on the environment Phenotype is only partly determined by its genotype

8. Mystery Clue – p. 320 Green feathers don’t actually contain green pigments. Rather, they contain a mixture of blue and yellow pigments. Could feather color be controlled by more than one gene?

9. Genes and the Environment Western white butterfly (Pontia accidentalis) – found throughout western North America if hatch in summer = different color patterns on wings than those that hatch in spring Reason: hatch in shorter days of spring = greater level of pigment in their wings = markings appear darker than those hatched in longer days of summer Reasoning: in order to fly effectively, butterfly body temperature must be 84-102oF since spring is cooler, greater pigmentation helps them to reach body temperature needed for flight. In hot summer months  less pigmentation enables them to avoid overheating Conclusion: environment in which the butterflies develop influences the expression of their genes for wing coloration Environmental conditions can affect gene expression and influence genetically determined traits. An individual’s actual phenotype is determined by its environment as well as its genes