2. LEARNING OBJECTIVES
▪ Distinguish among sex limited, sex influenced, and sex linked
traits.
▪ Explain the chromosomal basis for genetic linkage and
demonstrate how the chromosomal locations of linked genes can
be mapped.
▪ Identify the specific patterns of inheritance that are characteristic
of sex-linked traits.
▪ Know some human diseases or disorders caused by
chromosomal alteration.
3. TOPICS
▪ Chromosome Theory of Heredity
▪ Gene Dosage and Compensation
▪ Chromosome Mutation
▪ Changes in Chromosome Number
▪ Changes in Chromosome Structure
▪ Chromosomal Alterations in Human
Disease/Disorders
4. CHROMOSOMES
▪ Threadlike structures within
nucleus.
▪ A very long DNA molecule and
associated proteins, that carry
portions of the hereditary
information of an organism.
5. Thomas H. Morgan
▪ Defined the relationships between
chromosomes and heredity.
▪ Morgan's first papers dealt with
the demonstration of sex linkage
of the gene for white eyes in the
fly, the male fly being
heterogametic.
11. Sex-Limited Inheritance
Inheritance in which a trait or phenotype is expressed in one sex
only
Example:
A man may inherit his beard type from his mother rather
than from his father. Both sexes transmit the genes although
only one sex normally expresses the trait.
12. Sex-Influenced Inheritance
Inheritance that is autosomal but has a different intensity of
expression in the two sexes
Example:
In humans, baldness occurs in males with a single copy or
the allele. A woman will lose her hair only if she is
homozygous for the alleles.
13. Sex-Linked Inheritance
Inheritance that may result from a mutant gene located on either
the X- or Y-chromosome.
Example:
Color Blindness – mild disorder
Hemophilia – Any of several hereditary blood-coagulation
disorders, manifested almost exclusively in males, in which
the blood fails to clot normally because of a deficiency or
an abnormality of one of the clotting factors.
15. GENE DOSAGE COMPENSATION
▪ In mammals, only one X chromosome if fully active in most
diploid cells.
▪ In females, one of the two X chromosomes is inactivated
during embrayonic development.
16. GENE DOSAGE COMPENSATION
▪ The inactive X chromosome contracts into a dense object, called
a BARR BODY, which lies along the inside of the nuclear
envelope in cells of females.
▪ Most of the genes of the X chromosomes that forms the BARR
BODY are not expressed, although small regions of that
chromosomes remain active.
17. CHROMOSOMES MUTATIONS
▪ Major changes in the chromosomes of a cell may occur as a result
of errors during meiosis, mitosis or because of MUTAGENS.
▪ CHROMOSOME MUTATIONS – changes in genetic material that
involve entire chromosomes or pieces of them.
18. CHROMOSOMES MUTATIONS
▪ Changes in chromosome structure (chromosomal mutations) are
of great importance.
▪ Chromosomal mutations are used as tools in genetic experiments.
▪ In addition, many human diseases including many types of
cancer are caused by chromosomal rearrangements.
20. NONDISJUNCTION
An accident of meiosis or mitosis in which both members of a
pair of homologous chromosomes or both sister chromatids
fail to separate properly
22. ANEUPLOIDY
A chromosomal aberration in which certain chromosomes are present
in extra copies or deficient in number
a. trisomy- cell has an extra chromosome (2N + 1)
b. monosomy- cell is missing a chromosome (2N-1)
23. POLYPLOIDY
When an organism has more than two complete chromosome sets
a. Triploids – three complete sets
b. Tetraploids – four sets
25. CHANGES IN CHROMOSOME STRUCTURE
DELETION - The simple loss of a chromosomal segment
DUPLICATION -The presence of two copies of a chromosomal
region
26. CHANGES IN CHROMOSOME STRUCTURE
INVERSION -A segment of a chromosome can rotate 180 degrees
and rejoin the chromosome, resulting in a chromosomal mutation
TRANSLOCATION - two nonhomologous chromosomes can
exchange parts to produce a chromosomal mutation
27. MECHANISMS OF CHANGE
▪ breakage of a chromosome can lead to a variety of
rearrangements affecting the gene
▪ fragments without centromeres are usually lost when the cell
divides. the chromosome from which the fragment originated
will then be missing certain genes, an alteration called deletion
▪ in some cases, the fragment may join a homologous
chromosome, producing a duplication
28. MECHANISMS OF CHANGE
▪ it may reattach to the original chromosome, but in reverse
(called inversion)
▪ it may join a non-homologous chromosome (called
translocation)
▪ a homozygous deletion (or a single X in a male) is usually
lethal
▪ position effect- a gene's expression can be influenced by its
location among neighboring genes
30. DOWN SYNDROME
▪ is the most common serious
birth defect in the US (1:700)
▪ is the result of aneuploidy-
extra chromosome 21
▪ therefore, each body cell has a
total of 47 chromosomes
▪ the chances of having a DS
child increase with age (usually
due to age of eggs)
31. PATAU SYNDROME
▪ caused by trisomy for
chromosome 13
▪ characterized by serious eye,
brain, and circulatory defects,
as well as harelip and cleft
palate
▪ occurs one in every 5000 live
births
32. EDWARD SYNDROME
▪ trisomy of chromosome 18
▪ affects almost every organ
system in the body
▪ one in 10000
▪ most victims survive in less than
a year
33. KLINEFELTER SYNDROME
▪ an extra X chromosome in a
male (XXY)
▪ one in every 2000 births
▪ have abnormally small testes
and is sterile
▪ can lead to breast
enlargement and other
feminine body contours.
▪ the affected individual is
usually of normal
intelligence.
34. TURNER SYNDROME
▪ monosomy- female is XO
▪ is the only known viable
human monosomy
▪ individuals with this
syndrome are phenotypically
female but their sex organs do
not mature at adolescence,
and secondary sexual
characteristics fail to develop.
▪ they are sterile and short but
usually have no mental
deficiency. Source: http://www.turner-syndrome-us.org/resource/faq.html
35. METAFEMALES
▪ females with three X
chromosomes
▪ they have limited fertility and
may be mentally retarded.
36. CRI DU CHAT (“CRY OF THE CAT”) SYNDROME
▪ A human disorder caused by
specific deletion in chromosome 5.
▪ An individual with this syndrome
is mentally retarded and has a
small head with unusual facial
features and a cry that sounds like
the mewing of a distressed cat.
▪ Death usually occurs in infancy or
early childhood.
