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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
 Conclusions
1.
2.
Each chromosome is actually a group of
linked genes
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


Rabbit coat color


An individual still only has two copies of each
gene
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