Human Genetic Disorders

A genetic disorder is an abnormal condition that a person inherits

through genes or chromosomes.

Genetic disorders are caused by mutations, or changes in a

person’s DNA.

Human Genes and Mutations

• Mutations happen when an error occurs while copying the DNA sequence

• Not all mutations are harmful, some are beneficial and others have no effect at all

• Certain chemicals, x-rays and radioactive substances can cause mutations

Chromosome Disorders

• Individual, or point, mutations can occur, but an error can occur with the number of chromosomes that should be inherited

• These mistakes usually occur in meiosis– Chromosome number is too many or too few– Results from Non-disjucntion

• An incorrect number of chomosomes is often fatal to an unborn embryo or fetus, or the baby dies shortly after birth– Down Syndrome-3 copies of Chromosome #21

Common Genetic Disorders• Cystic Fibrosis

– Body produces abnormally thick mucus in the lungs and intestines that can lead to bacterial infections

– Causes difficulty breathing and digesting– It is a recessive allele (both parents must carry it) and has no

known cure

• Sickle-cell disease (Sickle-cell Anemia)– Caused by abnormal hemoglobin (protein that carries oxygen)

causing pain and weakness– The allele for it is co-dominant. – People with two sickle cell alleles have it– People with one sickle-cell allele produce both normal and abnormal hemoglobin but don’t usually have symptoms. There are treatments but no cure.

Common Genetic Disorders cont.• Hemophilia

– Disorder that causes the blood to clot slowly or not at all. One of the proteins needed for normal clotting is not produced.

– It is caused by a recessive allele on the X chromosome and thus is sex-linked.

– Occurs more often in males than females.– Can develop after birth but usually genetic.– http://www.sciencecases.org/hemo/hemo.asp

• Down Syndrome– Due to an extra copy of chromosome 21 – Occurs when chromosomes fail to

separate during meiosis– Down Syndrome patients have a distinct

appearance and some degree of mental retardation but many lead full active lives

Modern Approach to Genetic Disorders

• Doctors can detect genetic disorders using amniocentesis and karyotypes

• Amniocentesis: chromosomes from fluid surrounding the baby are examined

• Karyotype: a picture of the chromosomes revealing whether the baby has the right number or whether it is a boy or girl

• Couples with family histories of disorders must weigh the chances of children having a disorder. Karyotypes, pedigrees and Punnett squares can help with determining the potential risks