1. Linked Genes, X-linked
inheritance, and Mutations
Ch. 15

2. Genes are on
Chromosomes
• This is obvious
to us, but it has
only been
known for a few
decades

3. Chromosome Theory
of Inheritance
• The work of scientists in the early
1900s
• Says 2 things:
1. Genes occupy specific loci (positions on
chromosomes)
2. Chromosomes undergo segregation and
independent assortment during meiosis

4. Thomas Hunt Morgan
and his fruit flies
• The first solid evidence associating a specific
gene with a a specific chromosome came from
Thomas Hunt Morgan, an embryologist
• Morgan’s experiments with fruit flies provided
convincing evidence that chromosomes are the
location of Mendel’s heritable factors

5. Fruit flies were a good
choice…
• They breed at a high rate
• A generation can be bred
every two weeks
• They have only four pairs
of chromosomes

6. What Morgan did…
• He spent a year looking
for variant individuals,
those that differed from
the normal, or “wild”
phenotype
• Traits alternative to the
wild type are called
mutant phenotypes

7. He found a
white eyed
male…
• He mated it to a “wild” type red eyed female and got
all red eyed offspring in the F1 generation
• What does that tell us?
• He let the F1s mate, and got the classic 3:1 ratio of
red:white eyes, but only in males
• What does that tell us?
• Morgan figured out that the gene for eye color was on
the X chromosome
• His work supported the chromosome theory of
inheritance

9. Morgan’s other work
• Each chromosome has
hundreds or thousands of
genes
• Genes located on the same
chromosome that tend to be
inherited together are called
linked genes

10. Morgan experimented to see
inheritance of two traits
• Morgan crossed
flies that differed
in traits of body
color and wing
size

11. What he found…
• He found that the
body color and wing
size traits were
usually inherited
together and the
offspring looked like
the parents, mostly…

12. What he found…
• Some of the offspring had phenotypes different from
the parents, but in lower ratios than expected
• Offsrping with phenotypes like the parents called
parental types
• Offspring with new phenotype combos called
recombinant types
• 50% frequency of recombination = genes on 2
different chromosomes
Gametes from yellow-round
heterozygous parent (YyRr)
Gametes from greenwrinkled homozygous
recessive parent (yyrr)
Parental-type
offspring
Recombinant
offspring

13. What he
found…
• He discovered that some genes can be linked, but
there is a process that can break their connection:
crossing over
• This led to the development of genetic maps that
determine the location of each gene on a chromosome
based on the frequency of recombination
• Distance between genes expressed as map units
• 1 map unit = 1% recombination frequency

14. Sex-linked
genes
• An organism’s sex is
an inherited
phenotypic character
determined by the
presence or absence
of certain
chromosomes
• Mammals like humans
have an XX or XY
system of inheritance
• Other organisms have
other systems

15. Genes on the sex
chromosomes are called
sex-linked genes
• Some diseases on the X
chromosome:
• Color blindness
• Rare in females, mild disease
• Duchenne muscular dystrophy
• 1 in 3500 males in US gets it
• Lack the gene for the muscle protein
dystrophin
• Muscles get weaker and lose
coordination
• Usually don’t live past 20s
• Hemophilia
• Lack the protein to cause clotting
• Don’t clot normally

16. Barr bodies
• In mammalian females, 1 of the 2 X chromosomes
is inactivated during embryonic development
• The inactive X condenses into what is called a
Barr body (we can see it under the microscope)
• If she is heterozygous for a sex-linked trait, she
will be a mosaic for that trait

17. • Some cells have the
maternal X
inactivated
• These cells have the
orange color
• Some cells have the
paternal X
inactivated
• These cells have the
black color
• All cells in the
ovaries have active
X chromosomes

18. Chromosomal mutations
• In nondisjunction, pairs
of homologous
chromosomes do not
separate normally
during meiosis
• As a result, one
gamete receives two of
the same type of
chromosome, and
another gamete
receives no copy

19. What results…
• Aneuploidy - a zygote
produced from a normal
gamete and a gamete
produced by
nondisjunction
• Offspring with this condition
have an abnormal number
of a particular chromosome

20. Very rare among
animals
Common in plants,
some fish, some
amphibians
What results…
• Trisomy - having 3 copies of a particular
chromosome
• Monosomy - having just one copy of a particular
chromosome
• Polyploidy is a condition in which an organism has
more than two complete sets of chromosomes
Recent research
has shown that
this Chilean
rodent is a
tetraploid

21. Chromosomal breakage
• Breakage of a chromosome can lead to
four types of changes in chromosome
structure:
• Deletion removes a chromosomal segment

22. Deletion mutation
• Example: retinoblastoma (eye
tumors)

23. Chromosomal breakage
• Duplication repeats a segment

24. Duplication mutation –
fragile X syndrome

25. Chromosomal breakage
• Inversion reverses a segment within a
chromosome

26. Hemophilia A – inversion
mutation patient was given
injection in buttocks

27. Chromosomal breakage
• Translocation moves a segment from one
chromosome to another

28. Translocation mutation
causes Burkitt’s lymphoma
Tumors on hand from
cancer

29. Down syndrome
• Trisomy 21 - 3
number 21
chromosomes
• 1 in 700 children in
US
• Frequency increases
with age of mother

30. Trisomy 18 – Edward’s syndrome
low birth weight, mental
retardation, extra fingers and toes

31. • Klinefelter
syndrome is the
result of an extra
chromosome in a
male, producing
XXY individuals
Trisomy of sex
chromosomes
• Monosomy X, called
Turner syndrome,
produces X0 females, who
are sterile; it is the only
known viable monosomy
in humans