Human Genetic Disorders

I. Genes

a. Definition

• Segments of DNA molecules

• A nucleotide sequence

b. Functions

• Control of hereditary traits

• Sequence coding for a polypeptide which maybe an enzyme, or a part of an enzyme, which in turn is responsible for a certain phenotype or trait.

c. The Human Karyotype

• Ordered arrangement of chromosomes arranged in homologous pairs

• The chromosome complement of a cell or organism characterized by the number, size and morphological features of each chromosome

II. Types of Human Genetic disorders

a. Human Chromosomal Abnormalities

1. Nondisjunction

• Occurs when homologous chromosomes fail to segregate during meiosis

• Produces gametes with three chromosome pairs (trisomy) or only one chromosome of a normal pair (monosomy)

• Involves any chromosomes, including the sex chromosomes

• Causes serious genetic disorders

Ex: Down’s syndrome- trisomy 21

Edward’s syndrome- trisomy 18

Patau syndrome- trisomy 13

2. Klinefelter’s syndrome, XXY

• Results when an abnormal sperm, containing both X and Y-chromosomes, fertilizes a normal egg with one X-chromosome

• Can also result when an XX egg is fertilized by a Y sperm

• Offspring has genotype XXY

• Occurs in 1 in 1000 born males

• Have reduced fertility and maybe mentally retarded

3. Turner’s syndrome, XO

• Results from the absence of an X-chromosome in an egg

• When fertilized by a normal X-carrying sperm, offspring has the genotype XO (“O” indicating the lack of a chromosome)

• XO occur in 1 in 2000 born females

• They do not mature sexually

• Sterile

• Abnormally short an have webbed necks

4. Triplo-X syndrome, XXX

• Occurs at the frequency of 1 in 1000 live female births

• Do not have distinct clinical features except for tallness and menstrual irregularities, secondary amenorrhea and premature menopause

• Mostly have no sexual abnormalities but are generally of subnormal mental abilities

5. Jacob syndrome or Double Y syndrome, XYY

• Occurs in 1 in 1000 male births

• First observed in 1960’s

• Extra Y-chromosome predisposes the male to violent behavior

6. Hermaphrodites

• Have both ovarian and testicular tissues

• Do not have the normal Y; rather they have the minute y

• Mental retardation, congenital anomalies and hypertonia

7. Chromosomal deletions

• Abnormalities in chromosome structure due to some deletions in segments of some autosomal and sex chromosomes

Ex: cri-du-chat syndrome or 5 p sydrome

• Deletion of the short arm of chromosome 5

• Mentally retarded

• With small epiglottis and larynx

• Muscular dystrophy

• Deletion of a small segment of chromosome X

• Involves deletion of genes responsible for synthesis of the protein dystrophin.

b. Human Allelic Disorders (Recessive)

1. Albinism

• not an infectious