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

Conclusions1. Each chromosome is actually a group of

linked genes2. 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