Science Question Potpourri

The following are some of the most commonly asked questions I received as a science teacher.

DNA and paternity testing

How accurate can a DNA test for determining paternity be without a sample from the mother? I am a law student and in class we have to present a paper regarding a certain case. Mine is a female with a child has accused a male of being the father, but the results may be considered inconclusive to the courts because her sample was not taken. Is there a possiblity of the paternity test not being accurate due to only one comparison between the child and the male?

No. Since the child obviously belongs to the woman it does not matter that mom was not tested. The question is, how much DNA did the child get from the alleged father. If the child is male, the Y chromosome came from the father. There are genes on the Y chromosome which would definitely prove the child belongs to the man, if the man tested is the father.

If the child is a female, one of her X chromosomes came from the man and can also be matched with his, if he is the father. The gel electrophoresis process basically is a gel with DNA samples inserted into tiny wells at one end of the gel. Electricity is applied at one end. This movement of charge across the gel causes the DNA to move toward the opposite end of the gel. The larger fragments will move more slowly through the gel than the smaller fragments (of DNA). The amount of movement through the gel is compared (the man’s compared to the child’s). If there is a high enough percentage of similarity in this test, more specific tests can be performed – tests such as using enzymes that cleave DNA in specific locations. Only people that share certain genes will have exact matches on some of the portions of DNA that are cleaved.

Follow Up Question:
I don’t know if DNA was used because of the two samples taken, one was blood and the other was a cheek swab.
The testing was not done at a court-approved laboratory.

OK. this puts a little different light on the situation. The test done with blood may not have used DNA. If only red blood cells were used, the DNA couldn’t have been tested because red blood cells lose their DNA because they do not undergo cell division but are made by the bone marrow. Other portions of the blood could have been used to collect DNA, but I suspect a simple blood test was done given the fact that a court-approved laboratory was not used. Without more specific information I can’t say for sure.

Blood tests for parentage typically include basic and extended red cell antigens, red cell and white cell enzymes, and serum proteins. If these results are not conclusive a second tier of tests on white cell antigens (Human Leukocyte Antigens or HLA) is done. The procedures for electrophoresis testing of HLA, serum proteins, and red cell enzymes are sophisticated and may be done only at a laboratory involved in organ transplant donor identification.

The swab test can use DNA. Cheek cells are collected on the swab and each of those cells will have DNA in them. It sounds like the second test with the swab has the potential to be the more accurate test.

By the way, it is better if the mother provides samples for testing, but it is not absolutely necessary. If mom is tested, the genetic markers the child shares with the mother are matched. The remaining genetic markers of the child are then compared to the same markers from the dad’s sample. If mom is not tested a larger number of genetic markers have to be used for comparison. This still works fine and paternity can be established if the child shares half his/her markers with the man.

If the DNA matches above a minumum probability of 99.9%, then the man is the father. I think it only takes 2 differences in the DNA pattern to rule out the father.

The test can even be done with several of the father’s family members if the father is deceased or unavailable for testing. This obviously takes a lot more comparing, but is possible.

It is unfortunate the man didn’t have his test done at a court-approved lab. I understand the need for regulations about labs because some labs may have sloppy procedures and give unreliable results, but it seems the court could have allowed him time to have the swab test done at an approved place.

This is obviously a case meant to give law students a lot of thought.

Is it possible for parents that both have A Positive blood type to have a child that is O Positive?

Yes. I’ll simplify my answer to discuss two genes inherited for blood type (leaving out the Rh factor and other blood proteins). Each person has 2 genes for the ABO blood type, one on each of a pair of chromosomes.
A person with type A blood has either;
1) Gene 1 with type A and gene 2 with type A or
2) Gene 1 with type A and gene 2 with type O.

When 2 people with type A blood have a baby with type O blood, that means that both parents have option #2 above, and that both parents donated the type O gene to the baby. The baby inherited type O from both parents.

The reason the parents can have type A blood but pass on a type O gene is because type A is a dominant gene over type O gene. The O gene in both parents’ chromosomes is not expressed because it is masked by the presence of the type A gene. There is a 25% probability of having a baby with type O blood when both parents have genotypes of AO (one gene with type A and the other gene with type O).

What is the difference between genotypes and phenotypes?

The phenotype refers to the physical characteristics. (Phenotype and physical both begin with the letters ph).
These are traits that are expressed in the organism such as red hair, or color of flower, etc.

The genotype refers to the genes that are present on the chromosomes. Not all genes are expressed physically. Some genes are recessive; that is, they are ‘covered’ by the dominant form of the gene for that trait.
Remember there are 2 chromosomes, one from each parent. Each chromosome of a pair has genes for specific traits. The gene on one chromosome may mask the effect of the same gene on the other chromosome of that pair.

For example:
One of my parents had blood type “O”. This meant that the genes on that parent’s chromosomes were both for type “O” blood. (Type “O” genes are recessive; a person only has type “O” blood when both genes are “O”.)

My blood type is “B”. This means that I inherited an “O” gene from one parent and a “B” gene from the other parent.
MY blood phenotype is “B”.
My blood genotype is “BO”.

The “O” gene is not expressed in me, so the physical trait I inherited is type “B” blood.

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