Gene Linkage and Patterns of Inheritance

Gene Linkage and Gene Maps  Exception to Mendel’s rule of independent assortment  Thomas Hunt Morgan experimented with Drosophila (the common house fly)  Reddish-orange eyes and miniature wings almost always inherited together  Observed this trend in many genes  Grouped all the fly’s genes into four linkage groups  Drosophila has four linkage groups and four pairs of chromosomes

 Conclusions 1. Each chromosome is actually a group of linked genes 2. Mendel’s law of independent assortment still true  It is the chromosomes that assort independently, not individual genes

Gene Mapping  The relative locations of genes on a chromosome can be determined by using the frequency of crossing-over between genes

Patterns of Inheritance  Exceptions to Mendel’s principles  Most genes have more than two alleles  Many important traits are controlled by more than one gene

Incomplete Dominance  Some alleles are neither dominant nor recessive.  The heterozygous phenotype lies somewhere between the two homozygous phenotypes  four o’ clock plants and flower color

Codominance  The phenotypes produced by both alleles are expressed.  Chicken feathers- heterozygous= “erminette”- speckled with black and white feathers  Blood type- A and B are codominant

Multiple Alleles  A gene with more than tw0 alleles  An individual still only has two copies of each gene  Rabbit coat color  A single gene with at least four different alleles  Blood type  A, B, and O

Polygenic Traits  Traits that are produced by the interaction of several genes  Skin color, height  Show a normal distribution (bell-shaped curve)

Polygenic traits are controlled by many genes and result in gradations where each gene loci has an additive effect. What this means to a biologist is that if 10 gene loci are turned on the plant might be 20 cm tall. If 5 gene loci are turned on the plant might be 10 cm tall. Skin color and height in humans are polygenic and therefore humans come in all colors and heights.

Sex-Linked Inheritance  The genes located on the X and Y chromosome show a pattern of inheritance called sex-linkage  Genes found on the Y chromosome are found only in males and are passed directly from father to son  Genes on the X chromosome are found in both sexes, but the fact that men have just one X chromosome leads to some interesting consequences

Sex-linkage: colorblindness  Humans have 3 genes responsible for color vision, all on the X chromosome  In males, a defective allele for any of these genes results in colorblindness  Red-green colorblindness occurs in 1 in 12 males  1 in 200 in females  Colorblindness must be present in both alleles to be expressed in females

Genes and the Environment  The phenotype of an organism is only partly determined by its genotype  Western white butterfly  Western whites hatching in summer have different color patterns on wings than those hatching in spring  More pigment in butterflies of the shorter days of spring  Spring months are cooler; greater pigmentation helps them reach the body temp needed for flight