Inheritance Patterns for Linked
Genes
Section Summary
• Genes are located at specific chromosome
locations called loci (singular, locus).
• Genes located in close proximity to each other
on a chromosome tend to be genetically
linked. (the genes are inherited together)
• Traits linked to a particular sex chromosome
exhibit unique inheritance patterns.
Gene Loci
• The alleles of a gene reside at the same location,
or locus, on homologous chromosomes.
Gene Loci
• Homologous chromosomes may bear either
the same alleles or different ones at a
particular locus, making an organism either
homozygous or heterozygous for each gene.
Independent Assortment
Genetic Linkage
• If genes are located on separate
chromosomes, they assort independently of
each other during meiosis.
Genetic Linkage
• The alleles for genes located on the same
chromosome are less likely to assort
independently, and instead stay together
during meiosis.
• The only way such alleles can assort
independently is if crossing over during
meiosis separates them.
Genetic Linkage
• The tendency for alleles for different genes on
the same chromosome to be inherited
together is called genetic linkage.
• Genes with loci that are close together are
known as linked genes.
Genetic Linkage
• The closer two genes are on a chromosome,
the greater the degree of genetic linkage
• The farther apart the genes are, the more
likely it is that crossing over will separate
them.
Gene Maps
• Recombination frequency,(the frequency with
which certain genes turn up together) can be
used to map the distance between gene loci
on a chromosome.
• These values allow geneticists to create a gene
map, a diagram that shows the relative
locations and distances of genes on a
chromosome.
Gene Maps
• This is a gene map of one of
a rat’s chromosomes
Inheritance Patterns of Sex-Linked
Traits
• Any gene that is located on a sex chromosome
is called a sex-linked gene.
Scanning electron micrograph of an X and Y chromosome
Image courtesy of Indigo Instruments, Canada
Inheritance Patterns of Sex-Linked
Traits
• In humans, most sex-linked genes are found
on the X chromosome, which is considerably
larger in size than the Y chromosome.
• Genes on the X chromosome are said to be
X-linked.
Inheritance Patterns of Sex-Linked
Traits
• The gene involved in the white-eye
inheritance pattern in fruit flies is located only
on the X chromosome.
• There is no corresponding gene for eye-colour
on the Y chromosome.
Inheritance Patterns of Sex-Linked
Traits
• Females have two X chromosomes (they are
XX), therefore females carry two copies of
every X-linked gene.
• Males have only one X chromosome (they are
XY) and so carry only one copy of the X-linked
gene.
F1 Generation
F2 Generation
Inheritance Patterns of Sex-Linked
Traits
• Since the white-eye trait is recessive, a female
will have white eyes only if both X
chromosomes carry the white-eye allele (XrXr).
• If a male inherits a single copy of the whiteeye allele on his X chromosome, as there is no
other copy of the gene, he will have white
eyes (XrY).
Sex-Linked Disorders in Humans
Sex-Linked Disorders in Humans
Red-Green Colour blindness
Hemophilia
Sex-Linked Inheritance
• A "carrier" is a female who is heterozygous for
the recessive, sex-linked trait.
Sex-Linked Inheritance
• A female "carrier" who is heterozygous for the
recessive, sex-linked trait red color blindness,
marries a normal male. What are the possible
phenotypes of the offspring?
Sex-Linked Inheritance
• Let XR represent the gene for normal colour
vision
• Let Xr represent the gene for red-green colour
blindness
Example
• A female carrier for hemophilia has children
with an unaffected male. What are the
possible phenotypes of their children?
Hemophilia
Homework
• Page 151 # 1 - 11