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BSC 2010C: Principles of Biology I: Human Genetics

This library guide supports the FSCJ course BSC 2010C Principles of Biology.

Human Genetics

Human Genetics

Human Blood groups
There are 3 different alleles : IA, IB, and i. Alleles IA and IB are codominant ( both affect the phenotype).
Blood has antibodies that react against factors that are not present in the blood. eg group A has antibodies against B. Blood types
Blood group O can give blood to anyone ( universal donor ). Blood group AB can receive blood from anyone ( universal recipient ).
Blood groups can be used in paternity cases. For example, if a mother is blood group O and her child is blood group B, the father cannot be blood group A or O. So blood groups can prove that someone is not the father. However, blood groups alone cannot prove that someone is definitely the father of a particular child, because millions of people have the same blood group. To prove paternity you must use the more sophisticated genetic fingerprinting (chapter 15).

Blood groups and antibodies
Phenotype Genotype Antigen Antibody Can get blood from
A B AB O
A IA IA or IA i A Anti-B Yes No No Yes
B IB IB or IB i B Anti-A No Yes No Yes
AB IA IB A and B None Yes Yes Yes Yes
O i i None Anti-A
and Anti-B
No No No Yes

Rhesus factor
The Rhesus factor is caused by a different gene from blood groups. It was first found in Rhesus monkeys, but humans have exactly the same gene. Rhesus positive is caused by a dominant allele ( R ).
Women who are Rhesus negative, but have a Rhesus positive baby, will produce antibodies against the babies blood. The drug rhogam prevents these Rhesus problems if it is given during pregnancy.

Pedigree charts
Pedigree charts are family trees that are used to show how traits are inherited.
Dominant alleles : if a child has that trait, at least one parent must have that trait. eg brown hair.
Recessive alleles : children can have that trait even if neither parent shows it (both parents are carriers) eg blond hair.

Genetic Diseases

A) Caused by a recessive gene
Heterozygous individuals are carriers. If two carriers have children, each child has a 25% of being born with the disease. Because the disease is recessive, it can skip several generations.
Examples : Sickle cell anemia : affects red blood cells. Caused by 1 amino acid different in the protein hemoglobin. Sickle cell
Roughly 10% African Americans are carriers. Carriers are resistant to malaria. The gene for sickle cell anemia is on chromosome 11.

Cystic fibrosis : affects the lungs. Roughly 4 % Caucasians are carriers. The gene is on chromosome 7.
There are blood tests to detect carriers of sickle cell anemia or cystic fibrosis. In both diseases carriers live a normal life, but people with the disease often live only into their 30s.

B) Caused by a dominant gene.
Heterozygous individuals suffer from the disease. If a heterozygous person has children with a normal (homozygous recessive) person, each child has a 50% chance of inheriting the disease. The disease is caused by a dominant gene, so the only way someone can have the disease is if one of their parents has it (and one of their grandparents etc). It cannot skip generations.
example : Huntingtons disease : affects the nerves. Symptoms appear usually around age 40. The gene is on chromosome 4. New research. Huntingtons is a disease caused by a "triplet repeat" : the DNA code CAG is repeated 36 to 100 times in people with Huntingtons disease, but only 10 to 30 times in normal people.

Aneuploidy

Aneuploidy means that a cell inherits the wrong number of chromosomes. If the number inherited is very different from normal, the embryo does not survive. Karyotype : a photo of the chromosomes in a person.

Examples of aneuploidy
Down syndrome : extra copy of chromosome 21. Total of 47 chromosomes.
The risk of having a baby with Down syndrome increases as the mother gets older. Down syndrome children are usually short, mild to moderately mentally retarded, and live about 55 years. Men with Down syndrome are sterile, women can have children but there is a 50% chance of the child having Down syndrome.

Turner syndrome : missing an X chromosome ( genotype X ). Total of 45 chromosomes. They are always female, are typically short and almost always sterile.

Metafemale : extra X chromosome ( XXX ). Total of 47 chromosomes. Usually tall females, with normal fertility and two Barr bodies.

Klinefelters syndrome : ( XXY ). Total of 47 chromosomes. They have male sex organs, but high levels of female hormone (because of the two X chromsomes), so often grow breasts. Typically they are sterile.

Jacobs syndrome : ( XYY ). Total of 47 chromosomes. Tall males with very slight mental retardation.

 

Fetal testing

Triple screen test: checks the levels of three proteins in the mothers blood. Triple Screen

Amniocentesis - takes a sample of fluid from the womb. It is done 3 - 4 months into the pregnancy. Amniocentesis is routine for pregnant women who are 35 or older.

Chorionic villus sampling - takes a sample of the placenta. It is done 2 - 3 months into the pregnancy.

Both techniques can cause miscarriage in 1 - 2 % of cases.

New technique : separates fetal blood cells ( that have crossed the placenta ) from a sample of mothers blood. This should reduce the miscarriage rate to zero.

Pre-implantation genetic diagnosis.
This is used for people who are carriers for, or have, a genetic disease (eg sickle cell anemia) but who still want to have children. Eggs are removed from the body, artificially inseminated, and grown to the 8 - 64 cell stage. One or more cells are removed and tested for genetic diseases. Embryos that do not have genetic diseases are implanted back into the uterus. Genetic diagnosis


Last edited July 2014, by David Byres, David.Byres@fscj.edu