1. Ch. 15 The Chromosomal
Basis of Inheritance
15.3-15.5

2. Main Ideas
1.
2.
3.
4.
5.
Sex-linked inheritance and
issues
Aneuploidy
Chromosomal mutations
Genomic imprinting
Non-nuclear DNA

3. Sex-linked genes exhibit unique
patterns of inheritance
 The
XX, XY
system in humans
 The XX, X system
in grasshoppers
 The ZW, ZZ
system in chickens
 The diploid,
haploid system in
bees

4. Sex Chromosomes in Humans
 Females
- XX
 Males XY
 The
Y chromosome is
homologous with certain
regions on the X chromosome
 Parents pass on one sex
chromosome to their children
through their gametes


Woman always pass on an X
chromosome
Males pass on either an X (to
make a daughter) or a Y (to
make a son)

5. Sex Chromosomes in Humans
 Early
in development,
the embryo has
“generic” sex parts
 Whether these parts
develop into testes or
ovaries is determined by
the presence of
hormones within the
embryo
 The “default” is a female
Weeks of development

6.  On
the Y chromosome is a gene called
the SRY gene (sex-determining region
of Y)
 This gene codes for a protein that
triggers other genes, leading to the
development of a male

7. Quick Think
Why
do you think it is the presence
of a Y chromosome that makes
someone male, and not the presence
of two X chromosomes that makes
someone female? (i.e. - why is
female the default)?

8. Inheritance of Sex-linked Genes
 Sex-linked
genes - genes on the sex
chromosomes (may or may not be related to
gender)
 We see different inheritance patterns in
males and females because females have 2
X chromosomes where males only have 1

9.  Women
have 2 Xchromosomes, men have an
X and a Y.
 For women to express a
recessive phenotype, they
must inherit 2 Xchromosomes, both with the
recessive allele.
 For men to express the
recessive phenotype, they
need only 1 recessive X
X-linked traits

10. Colorblindness
 Colorblindness
is recessive and is a mutation
of a gene on the X chromosome

Men are more likely to be colorblind than
women…WHY

11. Duchenne muscular dystrophy
 1/3500
males in US
 Progressive weakening
of muscles
 Loss of coordination
 Death by early 20s
 Mutation in X
chromosome in the gene
that codes for a muscle
protein (dystrophin)

12. Hemophilia
 Prolonged
bleeding when injured
 Clots slow to form
 Caused by a mutation for a gene on the
X chromosome that codes for a blood
clotting protein
 Can be treated but not cured
Bleeding (bruising) after
an injection

13. X inactivation in
females
 Even
though females get 2 X
chromosomes, 1 becomes inactive
early in the development of the embryo
 The inactive X condenses into a Barr
body
 This means that the cells of males and
females each have 1 active X
chromosome

14.  Which
X becomes inactive in each embryonic
cell is random
 In
some cells, the paternal X will do this
 In other cells, the maternal X will do this
 This
leads to mosaicism in females
 All
cells that developed from the embryonic cells
containing the maternal X will exhibit certain traits
 The cells that develop from the embryonic cells
containing the paternal X will exhibit other traits
Example:
tortoiseshell coat color in cats

16. Quick Think
Can
a male cat be a calico or
tortoiseshell?
Explain your reasoning.

17. Abnormal Chromosome Numbers
 Occasionally,
the chromosomes do not
segregate properly during meiosis
 This
is called nondisjunction
 This results in some gametes with too
many chromosomes and some gametes
with too few

19.  Having
an abnormal
number of chromosomes is
a condition known as
aneuploidy
 Having 3 copies of a
particular chromosome is
called a trisomy
 Having 1 copy of a
particular chromosome is
called a monosomy

20. Down Syndrome
 Caused
by a trisomy of chromosome 21
 1/700 births in US
 The result of nondisjunction during meiosis I
 Risk increases with the age of the mother
 Mental retardation, increased risk of other
diseases, shortened life span, underdeveloped
and likely sterile

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

22. Abnormal Sex Chromosomes
 Extra
or missing sex
chromosomes also cause
abnormal phenotypes.
 If an individual only has one sex
chromosome, they have Turner
syndrome (45, X karyotype).
 These
people are short,
underdeveloped, have a web neck,
and sterile females.

23. Abnormal Sex chromosomes
 XXY
syndrome (47,
XXY karyotype) is
called Klinefelter
syndrome. These
males are very tall
and sexually
underdeveloped, with
diminished
intelligence and some
female phenotypes
due to the extra X
chromosome

24.  Polyploidy
- having more than two complete
chromosome sets


The normal state is having 2 of each chromosome
(homologous pairs)
When organisms have 3 or 4 of each chromosome, that is
called polyploidy
 Triploidy
 Very

(3n), tetraloidy (4n), hexploid (6n), octapolid (8n)
uncommon in animals
Some fish and amphibians, mostly
 Quite
common in plants
Recently discovered tetraploid
mammal, rodent from Chile

25. Quick Think
 Gene
dosage, the number of active
copies of a gene, is important to proper
development.
 Identify and describe two disorders that
are the result of improper gene dosage

26. Alterations of Chromosome Structure
 Chromosomal
include
mutations
 Deletion
 Inversion
 Duplication
 Translocation
 Often
occur during
meiosis
 Chromosomes
break &
rejoin incorrectly

27. Deletion mutations
A
piece of
chromosome is
lost during
meiosis
 Cri du chat is
one condition
that results distinctive cry,
severe
retardation,
shortened life
span

28. Inversion Mutations
A
double break
where the piece
reattaches, but
backwards
 Hemophilia is a
inversion on the X
chromosome

29. Duplication mutation
A
piece breaks off
from one
chromosome and
reattaches to the
sister chromatid
 Fragile
X syndrome
is caused by this,
2nd most common
form of mental
retardation after
Downs syndrome

30. Translocation mutation
A
piece breaks off one
chromosome and reattaches
to a different chromosome
 Burkitt’s lymphoma is caused
by this

31. Quick Think
 At
what point in gamete formation would
these chromosomal mutations occur?
During what particular process might
the chromosomes be especially
vulnerable to these mutations?

32. Exceptions to the chromosome theory
of inheritance
 Sometimes, the phenotype
of the individual is different
depending on which parent
passed along the particular
allele

 If
Example: in mice, there is a
normal lgf2 gene and a
mutant lgf2 gene
the mother gives mutant
& the father gives normal,
the mouse grows to normal
size
 But, if the father gives a
mutant allele, the mouse
will be dwarf
 This
is called genomic
imprinting

33.  In
genomic imprinting –
one copy of a gene is
silenced during gamete
formation by the addition
(usually) of a methyl (-CH3) group to the cytosine
nucleotides of that gene
 Occurs in a small number
of genes critical to
embryonic development in
animals
 Therefore ONLY the
maternal or paternal copy
of that gene can be
expressed in the offspring

34. DNA in organelles
 DNA is also found in
mitochondria and chloroplasts.
 This DNA is not passed to
gametes through meiosis like
nuclear DNA is.
 Mitochondrial DNA is only
passed from Mother to child.
 The genes in mitochondrial DNA
code for proteins of the ETC &
ATP synthase

Mutations in this DNA may
contribute to nervous system
disorders, diabetes, heart disease,
& Alzheimer’s