Human Genetics

A. The Basics

1. Genetics applies to people just as it does to Mendel’s pea plants

2. The problem with human genetics that the you can’t control matings

3. The advantage to studying human genetics that the good records are kept

4. There are about 1500 human diseases caused by genetic problems

B. Diseases that are Mendelian traits - that is they follow Mendel’s laws

1. Dominant traits

a. Huntington’s disease

1. Woody Guthrie had the disease

2. It is a degenerative disease of the nerves that starts at 35 to 45 years of age

3. Ends in death in 5 to 15 years after onset

4. Behaves as a dominant allele

1. A child of a sufferer has a 50% change of having the disease

2. The disease is not evident during the child-bearing years

5. What are the ethics of detection - Would you want to know if you carried the trait?

b. Other dominant traits - that are more or less debilitating

1. Cleft chin

2. Brown colored teeth

3. Polydactyly - 6 fingers on each hand

2. Recessive traits

a. Phenylketonuria - PKU - detected in newborns

1. The enzyme that breaks down phenylalanine is lacking

2. The child’s urine has a peculiar odor - like mice

3. Other degradation products of phenylalanine build up and prevent nervous system development

4. The treatment is to put the chile on a special diet - control the amount of phenylalanine that the child gets.

a. After the nervous system has developed the person can go off the diet

1. There is a problem with the children of PKU females

2. The child could get exposed to the toxins in utero

a. If a PKU woman gets pregnant she must go back on the special diet

b. This protects the growing fetus

5. To protect against PKU, every child in the US is screened at birth for the trait

b. Sickle-cell anemia

1. Also recessive

2. Sickle-cell patients have a wrong amino acid in their hemoglobin

a. Glutamine is changed to Valine in the 6th position of the b side chain of hemoglobin - to be exact

b. The bad hemoglobin cross-bonds when oxygen levels are low

1. Causes cells to sickle 3. Causes sickling of blood cells - in homozygous individuals

a. Change in shape of red blood cells - can’t go through blood vessels

b. A so-called sickling episode can be fatal to the individual

4. Why should this exist?

a. The bad hemoglobin is less easily eaten by the malaria parasite

1. Thus the person with it is protected against malaria

b. The homozygous normal individual dies of malaria

c. The homozygous recessive dies of sickle cell

d. The heterozygote “carrier” of the trait is protected against malaria and does not have sickling of the blood cells

c. Other recessive diseases

1. Cystic Fibrosis - characteristic of people from Europe

a. 1 in 1,800 births of European Americans

b. 1 in 70,000 births of African Americans

c. Lungs fill with mucous - death around 20 to 30 years of age

d. Only cure is a lung transplant

2. Tay-Sachs disease

a. Characteristic of Ashkenasic Jewish people - European Jewish

b. Baby’s nervous system quickly deteriorates - death at 4 to 5 years

c. The Jewish community is attempting to remove the gene by genetic counseling

3. Sex-linked traits

a. Color-blindness - sex linked - appears mostly in males

b. Hemophilia - problem with clotting

1. Patients bleed excessively

2. Need lots of blood transfusions - problem of AIDS in the blood supply

c. Duchenne Muscular Dystrophy

1. Deterioration of muscles

2. Sex-linked

C. Diseases that are not Mendelian Traits

1. Trisomy 21 - Down’s Syndrome

a. What if the homologous chromosomes do not separate correctly during gamete formation?

1. The result is egg or sperm with extra chromosome

2. The fertilized egg will have an extra chromosome

3. In most cases the fetus is spontaneously aborted

4. Development proceeds if extra chromosome is one of the sex chromosomes or is chromosome 21

b. Trisomy 21 produces stocky individuals with some mental retardation

2. Non-disjunction of sex chromosomes

a. Lots of variants

1. XXY - male

2. XXX - female

b. Variants show various anatomical problems and sterility

D, How to handle genetic diseases

1. PKU - Simply screen every newborn - not a difficult thing

2. Amniocentesis - take sample of uterine fluid (amniotic fluid)

a. Contains cells from fetus

b. Count chromosomes - can pick up Down’s syndrome

c. Would you abort the fetus?

1. This is up to the parents

3. Genetic Screening

a. Cystic fibrosis, sickle cell, Tay-Sachs and Huntington’s are all seen in genetic screening

1. You can even detect heterozygotes

b. Genetic counselor can advise against certain marriages

1. Advise married couples not to have children

2. Couples need to know all of the risks

c. What about Huntington’s - would you want to know if you have it????

d. How far can genetic screening go?

1. Many people have genetic predisposition for particular diseases

2. There might be discrimination by employers

E. Inheritance of Intellectual Ability

1. How much of mental ability is inherited

a. Is there a Mendelian gene for particular abilities?

b. Twin studies suggest there is a predisposition to particular behaviors

1. Raise twins apart - you find that the adults are surprisingly similar

c. It is not known how much is inherited and how much in environmentally controlled

1. Nature vs. Nurture debate

2. There have been repeated attempts over the year to demonstrate the genetic superiority of particular racial or social groups

a. Usually the writer of a particular paper about this topic belongs to the “superior” group

b. What about the inheritance of IQ scores?

1. If you have less access to education, you will score lower

2. How much of intelligence as measured on tests is a result of economic opportunity and has nothing to do with genetic superiority?

3. There is now easy scientific test

4. Make opportunities equal - then you can design a fair test of the hypothesis!