The nucleus contains chromosomes made of DNA and proteins. Humans normally have 23 pairs of chromosomes, including 22 pairs of autosomes and a pair of sex chromosomes that are XX in females and XY in males. Each chromosome contains a centromere that divides it into short and long arms. DNA is composed of two nucleotide chains that coil around each other to form a double helix. Proteins are synthesized through transcription of DNA into mRNA and translation of mRNA by ribosomes. There are several patterns of inheritance for genetic disorders, including autosomal dominant, autosomal recessive, and X-linked inheritance.

2.  Nucleus contains chromosomes, every chromosome
consist of a single molecule of DNA together with
acidic and basic structures.
 Most human cells contains 46 chromosomes with 22
pairs of autosomes which are a like in male and
female plus a pair of sex chromosomes XX or XY.
 Each chromosome has a narrow waist called
centromere.
 The centromere divides each chromosome into a
long and short arms which are labeled q and p.

4.  At mitosis each chromosome replicates to
form a pair of chromatids.
 DNA :
each molecule of DNA is composed of 2
nucleotide chains coiled a round each other
clockwise to form a double helix.
each nucleotide consist of a nitrogenous
base , a molecule of deoxyribose and a
phosphate molecule.

7.  The nitrogenous bases are of two types,
purines and pyramidines.
 In DNA there are two purine bases adenine (
A) and guanine ( G) and two pyramidine ,
thimine (T) and cytosine( C).
 The 2 nucleotide chains run in opposite
direction and are held together by hydrogen
bonds betweenA andT or G and C.

8.  Proteins ( enzymes, carrier molecules,
hormones or recptors are all composed of a
seris of amino acids. 20 a.a are known and the
order of these determines the form and
function of the resulting protein.
 All proteins are incoded in DNA and the unit
of DNA which codes for a protein is called a
gene.

10.  The first stage in protein synthesis is
transcription.The 2 strands of DNA separate
in the area to be transcribed and one strand
function as a template , then messenger RNA
is formed with a complementary nucleotide
sequence under the influence of the enzyme
RNA polymerase

11.  Each set of 3 DNA base pairs (codons)
encodes an a.a.
 Ther are 4 types of nucleotides(A,G,C andT)
there are 64 possible combinations or
codons.
 Most genes consist of alternating protein
coding segments ( exons) and non-protein
coding segments( introns) whose function is
unknown.

12.  Translation:
occurs in the cytoplasm. Each mRNA
molecule becomes attached to one or more
ribosomes and recognized by a matching
transfer RNA( tRNA) which contributes its a.a
to the end of the growing protein chain.

13.  Normal male normal female
 Affected male affected female

14. 1-Autosomal dominant:
when only one gene of a pair is affected
and can result on expression of the disease.

15.  Autosomal recessive:
when mutation occurs on both members of a
gene pair (homozygotes).Parents are usually
unaffected and heterozygotes with the
chance of 1 in 4 children to be affected.

16.  X-linked recessive disorders:
males have only one x and thus are
affected if any x-linked genes are mutant

17.  X linked dominant:
very rare, males and females are affected .

18.  5 main types:
1. Chromosomal disorders.
2. Single gene disorders
3. Multifactorial.
4. Mitochondrial dis.
5. Somatic cell disorders.

19. 1. Chromosomal disorders.
abnormality in one or more chromosome
which can be visualised microscopically.

20. 2. Single gene disorders:
mutation in one member or both members
of a pair of genes.

21. 3.Multifactorial :
an individual is more prone to
enviromental effect by action of one or
more genes.

22. 4. Mitochondrial dis:
mitochondria have their own
chromosomes which are derived from the
ovum and so show maternal inheritence..

23. 5- Somatic cell disordersr:
arise after conception and are thus not
inherited although they produce their
effects via the DNA.
.

24.  Female carriers are usually clinically normal but may
occasionally be mildly affected because of non-
random inactivation of one of their X chromosome
which called lyon hypothesis or lyonisation. All early
female embryo randomly inactivate either the
paternal or the maternal X chromosome in each cell
at the sixteenth day after fertilization .occasionally
this inactivation is non-random and if the X with the
mutant gene is kept active in many cells of the
target tissue then mild disease can result.

25.  Usually occurs in sex linked diseases.
 Normally all early female embryo randomly
inactivate either the paternal or the maternal
X chromosome in each cell at the sixteenth
day after fertilization .
 Female carriers are usually clinically normal
but in lyonisation of non-random
inactivation of one of their X chromosome
occurs may be mildly affected.