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Genetic Linkage
Chapter 10, Sections 4 and 5
The Chromosome Theory of
Inheritance
It is the chromosomes
that segregate and
assort independently
during gamete formation
 On a pair of homologous
chromosomes, alleles of
a gene reside at the
same location called a
gene locus

Meiosis: Gamete Formation
An organism is either
homozygous or
heterozygous for each
gene
 The alleles carried on
different
chromosomes assort
independently into
gametes

Genetic Linkage and Crossing Over



Genes that are carried on
the same chromosome
are often inherited
together  genetic
linkage
Crossing over can
separate linked genes
In general, alleles with
loci close together will
stay together; Alleles
farther apart are more
likely to be separated by
crossing over
Sex-Linked Genes


Discovered by American
Geneticist Thomas Hunt
Morgan in the early
1900s studying fruit flies
Normally, fruit flies have
red eyes, but Morgan
discovered that some
mutant flies had white
eyes (most of which
where males)
Sex-Linked Inheritance Patterns
About 2,000 genes have been mapped to the X
chromosome, and only about a dozen have been
mapped to the Y chromosome
 Females must inherit two copies of a sex-linked
recessive allele to express it; males only need
ONE COPY of the allele to express it

Ex: XrXr = white-eyed female
XrY = white-eyed male
Morgan’s Experiment
Morgan’s F1 Generation
Sex-Linked Disorders

Colorblindness (effects 1
in 100 males)
– Red-green colorblindness
– Orange-blue colorblindness

Hemophilia (effects 1 in
5,000 male births and 1
in 20,000 female births)
– A complication in which
blood does not clot
normally