Gene Linkage and Patterns of Inheritance
Post on 24-Feb-2016
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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
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