Human Genetic Diseases (non mutation)

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1 mutation) Pedigrees mutation) 1. Autosomal recessive inheritance: this is the inheritance of a disease through a recessive allele. In order for the person to have the condition they would have to be homozygous (two recessive alleles) Ex. 1 Tay Sachs disease a disease where a recessive allele causes an enzyme to be lacking in the lysosomes of brain cells. Lipids build up inside the cell and destroys it. Patients have brain and spinal chord damage. Most end up with blindness and have developmental delays Ex. 2 phenylketouria (PKU)- a disease where an enzyme that converts phenylalanine to tyrosine is absent. Tyrosine is used to create melanin and some hormones. Phenylalanine is broken down abnormally and in the process neural damage occurs. Patients with the condition are usually babies and end up with nervous system damage. To treat this, they are placed on special diets to reduce phenylalanine intake until the nervous system is developed. 1

2 mutation) cont 2. Co-dominant inheritance: a situation where if a person receives both dominant alleles for the condition, they will have the condition. People who are heterozygous are carriers Ex. Sickle-cell anemia: a disease where the blood cells become sickled or are elongated and blunt. There is a defect in the hemoglobin in these cells causing them to lose their shape. This leads to blood clots and reduced blood flow to organs. Most individuals are often tired and suffer from many other diseases and often die. No known cure or treatments mutation) cont 3. Autosomal dominant inheritance These are conditions caused by dominant alleles and anyone having the dominant allele is affected. Ex. Progeria: a disease causing a person to age rapidly Ex. Huntington s disease: a lethal disease in which the brain deteriorates over about 15 years time. Symptoms include loss of motor coordination, memory and ability to speak. Most people die in their 40 s and 50 s. 2

3 mutation) cont Incomplete dominance inheritance A condition where there is a blending of two traits. The heterozygous individual will have the condition but not to the severity of a homozygous individual. Ex. HF (Familial Hypercholesterolemia): people receiving the allele for this trait will have twice the normal cholesterol levels in the blood. Sex-linked inheritance These are traits usually inherited as a recessive allele on the X-chromosome Ex. Red-green colourblindness, hemophilia, muscular dystrophy Pedigrees Pedigree: a chart/diagram that shows the genetic relationship among a group of related individuals; pedigrees are like family trees used to trace the inheritance of traits (usually diseases) in humans. 3

4 Pedigree Steps in Interpreting a Pedigree chart Determine if the pedigree chart shows an autosomal or X- linked disease. If most of the males in the pedigree are affected, then the disorder is X-linked If it is a 50/50 ratio between men and women the disorder is autosomal Determine whether the disorder is dominant or recessive. If the disorder is dominant, one of the parents must have the disorder. If the disorder is recessive, neither parent has to have the disorder because they can be heterozygous. 4

5 Rules of Inheritance Autosomal Recessive Appears in both sexes with equal frequency Trait tend to skip generations Affected offspring are usually born to unaffected parents When both parents are heterozygous, approx. 1/4 of the progeny will be affected Appears more frequently among the children of consanguine marriages Autosomal Recessive pedigree 5

6 Rules of inheritance Autosomal Dominant Appears in both sexes with equal frequency Both sexes transmit the trait to their offspring Does not skip generations Affected offspring must have an affected parent unless they posses a new mutation When one parent is affected (het.) and the other parent is unaffected, approx. 1/2 of the offspring will be affected Unaffected parents do not transmit the trait Autosomal dominant pedigree 6

7 Rules of Inheritance X-Linked Recessive More males than females are affected Affected sons are usually born to unaffected mothers, thus the trait skips generations Approximately 1/2 of carrier mothers sons are affected It is never passed from father to son All daughters of affected fathers are carriers X-linked recessive pedigree 7