| Why are humans not good subjects for genetic research? | - Generation time is too long
- Parents produce relatively few offspring
- Breeding experiments are unacceptable
However, basic Mendelian genetics endures as the foundation of human genetics
In human genetics, geneticists analyze the results of human matings that have already occurred |
| Pedigree | - A pedigree is a family tree that describes the inheritance of a trait across generations
- Pedigrees can be used to make predictions about future offspring
- We can use the multiplication and addition rules to predict the probability of specific phenotypes |
| Recessively Inherited Disorders | - Many genetic disorders are inherited in a recessive manner
- These range from relatively mild to life-threatening
- Recessively inherited disorders show up only in individuals homozygous for the allele
- Example: Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair |
| Carriers | - Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal
- Most individuals with recessive disorders are born to carrier parents
- If a recessive allele that causes a disease is rare, it is unlikely that two carriers will meet and mate |
| Consanguineous matings | - Consanguineous matings (that is, between close relatives) increase the chance that both parents of a child carry the same rare allele
- Most societies and cultures have laws or taboos against marriages between close relatives |
| Cystic fibrosis | - Cystic fibrosis is the most common lethal genetic disease in the United States, striking one out of every 2,500 people of European descent
- The cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes, leading to a buildup of chloride ions outside the cell
- Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine
- Untreated, cystic fibrosis can cause death by the age of 5
- Daily doses of antibiotics to stop infection and physical therapies can prolong life
- In the United States, more than half of those with cystic fibrosis now survive into their 40s |
| Sickle-cell disease | - Sickle-cell disease affects one out of 400 African-Americans
- It is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells
- In homozygous individuals, all hemoglobin is abnormal (sickle-cell)
- Symptoms include physical weakness, pain, organ damage, and even paralysis
- Heterozygotes (said to have sickle-cell trait) are usually healthy but may suffer some symptoms
- About one out of ten African-Americans has sickle-cell trait, an unusually high frequency
- Heterozygotes are less susceptible to the malaria parasite, so there is an advantage to being heterozygous in regions where malaria is common |
| Dominantly Inherited Disorders | - Some human disorders are caused by dominant alleles
- Dominant alleles that cause a lethal disease are rare and arise by mutation
- Example: Achondroplasia is a form of dwarfism caused by a rare dominant allele |
| Huntington’s disease | - Huntington’s disease is a degenerative disease of the nervous system
- The disease has no obvious phenotypic effects until the individual is about 35 to 40 years of age
- Once the deterioration of the nervous system begins, the condition is irreversible and fatal
- The timing of onset of a disease significantly affects its inheritance
- There is a test that can detect the presence of the Huntington’s allele in an individual’s genome
- Some individuals with a family history of Huntington’s disease choose to be tested for the allele
- Others decide that it would be too stressful to find out |
| Multifactorial Disorders | - Many diseases, such as heart disease, cancer, alcoholism, and mental illnesses, have both genetic and environmental components
- No matter what our genotype, our lifestyle has a tremendous effect on phenotype |
| Genetic Testing and Counseling | - Genetic counselors can provide information to prospective parents concerned about a family history for a specific disease
- Fetal and newborn testing can also reveal genetic disorders |
| Genetic Counselor Couple Example (For reference) | - Suppose a couple both have a brother who died from the same recessively inherited disease
- A genetic counselor can help determine the risk that this couple will have a child with the disease
- It is important to remember that each child represents an independent event in the sense that its genotype is unaffected by the genotypes of older siblings
- If both members of the couple had a sibling with the recessively inherited illness, both of their parents were carriers
- Thus each has a ⅔ chance of being a carrier themselves
- If both are carriers, there is a ¼ chance of each child having the recessive illness
- The overall probability of them having a child with the illness is ⅔ × ⅔ × ¼ = 1/9 |
| Tests for Identifying Carriers | - For a growing number of diseases, tests are available that identify carriers and help define the odds of having an affected child more accurately
- The tests enable people to make more informed decisions about having children
- However, they raise other issues, such as whether affected individuals fully understand their genetic test results, and how the test results are used |
| Amniocentesis | - Fetal Testing
- In amniocentesis, the liquid that bathes the fetus is removed and tested for certain genetic disorders |
| Chorionic villus sampling (CVS) | - Fetal Testing
- In chorionic villus sampling (CVS), a sample of the placenta is removed and tested |
| Fetal testing techniques | - Amniocentesis
- Chorionic villus sampling
- Other techniques, such as ultrasound, allow fetal health to be assessed visually in utero |
| Newborn Screening | - Some genetic disorders can be detected at birth by simple tests that are now routinely performed in most hospitals in the United States
- One common test is for phenylketonuria (PKU), a recessively inherited disorder that occurs in one of every 10,000–15,000 births in the United States
- The number of conditions that can be tested in newborns is over 100 and ever-increasing |
