Power point presentation on Chromosome and Human Genetics.ppt

Errors in the distribution of chromosomes during meiosis causes changes in the chromosome number.
When homologous chromosomes fail to separate, the 2 chromosomes move toward one pole of the cell, none to the other cell.

1. Chromosome and Human
Genetics

2. Human Genetics
•Constructing Pedigrees
•Inheritance Patterns
Because chromosome can be autosomes or sex
chromosomes, and because traits can be dominant or
recessive, we distinguish between 4 patterns:
1. Autosomal Recessive Inheritance
2. Autosomal Dominant Inheritance
3. Sex-Linked (or X-Linked) Recessive Inheritance
4. Sex-Linked (or X-Linked) Dominant Inheritance

3. What is a pedigree?
We study human genetics from past
history of families
A pedigree (family genetic history) a chart
of the genetic connection among
individuals.

4. Constructing a pedigree
A square represents a male
A clear square represents a normal male
A darkened square represents an affected
male
A circle represents a female
A clear circle represents a normal female
A darkened circle represents an affected
female

5. Constructing a pedigree
A single horizontal line connecting
both sexes represents mating

Vertical lines connect
parents to offspring
Generations are labeled
with roman numeral

6. This is a pedigree

7. Autosomal Recessive Inhritance
Because 2 normal parents have affected children the trait is
recessive.
Because both sexes can be affected the gene of the trait is
located on autosomes. The inheritance in this case is called
autosomal recessive

8. Autosomal Recessive Traits
•Examples:
•Phenylketonuria PKU
•Albinism
•Sickle-cell Anemia

9. Phenylketonuria - PKU
•Most people have a dominant gene that
enable them to break down the amino acid
phenylalanine by converting it into the
amino acid tyrosine.
•Those who have the recessive gene are
unable to do so. The phenylalanine they
ingest in food builds up in the body and is
broken down into phenylketones (mousy-
smelling substances are excreted in urine)

10. •Phenylketones cause nerve cells in the
brain to develop abnormally – mental
retardation.
•Genetically:
•A - normal a – affected (PKU)
•The gene is located on autosomes (#12),
therefore both sexes can be affected.
•The genotypes AA or Aa are normal
•The genotype aa is affected - PKU

11. •Can two normal parents have an affected
child?
•Yes

12. PKU test
This disease is known to be caused by an:This disease is known to be caused by an:
Autosomal recessive gene.Autosomal recessive gene.

13. Autosomal Dominant Traits
Because one parent in every generation is affected, the affected trait is
dominant. And because both sexes can be affected the gene of the trait
is located on autosomes. The inheritance in this case is called
autosomal dominant.

14. X-Linked Recessive Inheritance
•Examples:
•Hemophilia

15. X-Linked Recessive Traits
Because 2 normal parents have affected children the trait is
recessive.
Because only males can be affected the gene of the trait is
located on X-chromosome. The inheritance in this case is
called X-linked (or sex-linked) recessive.

16. Pedigree of DMD, an X-linked gene
Who are affected? A. both sexes. B. males only. C. females only
As you see in the pedigree, two normal parents have an affected child,
so the trait is recessive. And because only males are affected, the gene
is located on X chromosome (X-linked inheritance)

17. The Royal Hemophilia Pedigree

18. Genetically
•D – normal d – affected
•The gene is located on X chromosome not
on Y
•X(D)X(D) normal woman
•X(D)X(d) normal woman (carrier)
•X(D)Y normal man
•X(d)Y affected boy

19. Human Chromosomes
•Karyotype Analysis
•Changes in the Chromosome Number
•Changes in the Chromosome Structure

20. Karyotype Analysis
•Preparation of metaphase chromosomes.
•To view human chromosomes, geneticists
remove white blood cells, stain and
photograph their nuclei, then cut the
chromosomes from photo with scissors
and arrange them in pairs in decreasing
size as shown in the next slide.

22. Changes in the Chromosome
Number
•Down Syndrome:
•People normally have 46 chromosomes:
22 pairs of autosomes and 1 pair of sex
chromosomes – XX or XY
•In case of Down Syndrome the cell has 47
chromosomes, 3 copies of chromosome
21 – Trisomy 21
•Syndrome is a set of symptoms that occur
together.

23. •This condition is characterized by lower
than average IQ. Heart malformation,
eyelid folds, and others.

24. Down Syndrome:Trisomy 21 was first
described in 1866 by J. Langdon Down

25. Correlation between maternal age and
the incidence of Down syndrome
•As women age,

the chances they

will bear a child

with Down

syndrome increase

26. Nondisjunction
•Errors in the distribution of chromosomes
during meiosis causes changes in the
chromosome number.
•When homologous chromosomes fail to
separate, the 2 chromosomes move
toward one pole of the cell, none to the
other cell.

27. •Nondisjunction may also affect the sex
chromosomes
–nondisjunction of the X chromosome creates
three possible viable conditions
•XXX female
–usually taller than average but other symptoms vary
•XXY male (Klinefelter syndrome)
–sterile male with many female characteristics and
diminished mental capacity
•XO female (Turner syndrome)
–sterile female with webbed neck and diminished stature

28. Nondisjunction of the X chromosome

29. Turner Syndrome
•A person with one X chromosome and no
Y chromosome (XO) is a sterile female.
•2N=45
•This condition is characterized by by folds
of skin along the neck, a low hairline at the
nape of the neck, and failure to develop
adult sexual characteristics at puberty.

30. Klinefelter Syndrome
•A male with 2 X chromosomes and one Y
chromosome (XXY)
•2N=47
•Affected people develop as sterile males
with small testes, long legs and arms.
Most manage well in society