INTRODUCTIONTO

GENETIC DISORDERS

Presented By :Velo, John R.DMD2D

What is an Allele?

an alternative form of a gene located at a specific position on a specific chromosome.

These DNA codings determine distinct traits that can be passed on from parents to offspring.

May be dominant or recessive

The process by which alleles are transmitted was discovered by Gregor Mendel and formulated in what is known as Mendel's law of segregation.

Difference between a dominant and recessive allele

If the two alleles from both parents are :

Similar : homozygous offsprings Different : heterozygous

the stronger of the two will show up in the offspring while the weaker one is masked

Difference between a dominant and recessive allele

Dominant Allele A gene (allele) which is expressed clinically in the

heterozygous state In a dominant disorder, only one mutant allele need

be present as it masks the normal allele

Recessive Allele A gene (allele) which is only expressed clinically in the

homozygous state can be suppressed if present with a dominant gene

and will not show it's character in presence of a dominant gene

Recessive alleles or genes will only show up if the offspring inherits recessive copies of the trait from both parents

Difference between a dominant and recessive allele Dominant genes are usually the ones that are oftentimes

observed in an offspring and passed down to subsequent generations while recessive genes will only show for a few generations and eventually disappear.

Dominant genes are represented by capital letters and recessive genes are represented by small letters.

There are three combinations of genotypes or alleles:

AA (receives dominant traits from both parents), Aa (receives a dominant trait from one parent and a recessive

trait from the other), and aa (receives recessive traits from both parents)

What is a Punnett Square?

a diagram that is used to predict an outcome of a particular cross or breeding experiment

is visual representation of Mendelian inheritance

How is a trait transmitted?

Types of Genetic Disorders : Genetic disorder - disease caused by

abnormalities in an individual’s genetic material (genome).

The four different types of genetic disorders are : (1) single-gene, (2) multifactorial, (3) chromosomal, and (4) mitochondrial

(1) Single-gene (Mendelian or monogenic)

caused by changes or mutations that occur in the DNA sequence of one gene.

Genes code for proteins, perform most life functions, and make up the majority of cellular structures. When a gene is mutated, it's protein product can no longer carry out its normal function, a disorder can result.

Some examples are cystic fibrosis, sickle cell anemia, Marfan syndrome, Huntington’s disease, and hereditary hemochromatosis.

Examples of Single-Gene Disorders

A. Marfan’s SyndromeB. Cystic FibrosisC. Huntington’s Disease

A.B.

C.

(2) Multifactorial (complex or polygenic)

caused by a combination of environmental factors and mutations in multiple genes.

Examples include heart disease, high blood pressure, Alzheimer’s disease, arthritis, diabetes, cancer, and obesity

Multifactorial inheritance is also associated with heritable traits such as fingerprint patterns, height, eye color, and skin color.

(3) Chromosomal

Chromosomes - distinct structures made up of DNA and protein, are located in the nucleus of each cell

Because chromosomes are carriers of genetic material, such abnormalities in chromosome structure as missing or extra copies or gross breaks and rejoinings (translocations) can result in disease.

Examples include Down syndrome or trisomy 21, Turner syndrome (45,X), Klinefelter syndrome (47, XXY) and cri du chat syndrome, or the "cry of the cat" syndrome (46, XX or XY, 5p-).

Examples of Chromosomal Disorders

(4) Mitochondrial

This relatively rare type of genetic disorder is caused by mutations in the nonchromosomal DNA of mitochondria.

Examples of mitochondrial disease include:

an eye disease called Leber's hereditary optic atrophy;

a type of epilepsy called MERRF which stands for myoclonus epilepsy with Ragged Red Fibers; and

a form of dementia called MELAS for mitochondrial encephalopathy, lactic acidosis and stroke-like episodes