Qs people asked me on science and my replies to them - part 160 - DNA tests

Q: Dr. Krishna, this Q is based on a TV serial I am watching. The hero, a father in the serial is refusing to have a DNA test to establish his paternity. Are there other ways we can address the paternity dispute?

Krishna: I don't watch TV but watched a few episodes of the serial you mentioned to understand the problem after reading your mail. The serial is misleading because according to it, you cannot do anything if the father refuses to have a DNA test. His wife and daughter are suffering unnecessarily and crying a lot. There is no need to do that in real life. 

My answer to  you and to all the others who face this problem is, "'Yes, you can determine who the father is through paternal grandparents, uncles and aunts also'". In that way it is misleading to say you can't do anything and cry helplessly if the father refuses a DNA test. But once the truth is established, the serial will have no interesting story to continue and that is why, they don't go that way. But in real life, science can help you. You need not worry if the father is unwilling to have a DNA test. You can still establish the facts and go to court to get justice. 

A grandparent DNA (or paternal uncle's or aunt's) test is one in which the genetic material (DNA) of a child is compared to that of one or two other individuals to determine the likelihood that the child is related to the other individual(s) as a biological grandchild, nephew or niece.  In most cases, grandparent tests are performed to determine paternity—whether or not the biological son of a tested individual is the biological father of a child—in situations where the possible father is deceased, incarcerated, unwilling or otherwise unavailable to participate in a paternity test. 

In courts. test results are often presented by grandparents to obtain custody of their grandchild, by mothers to obtain Social Security benefits for their child, and by petitioners to provide evidence of biological relationship for immigration purposes.  In rare cases, a grandparent DNA test may also be performed to determine whether individuals are maternal grandparents of a child, for cases in which maternity is in question but in which the alleged mother is unavailable.

Because a child inherits exactly 50% of its genes from its biological mother and 50% from its biological father, that child shares some 25% of its genes with each biological grandparent due to inheritance.

A grandchild, whether a granddaughter or a grandson, will generally share between 1300-2300 centimorgans with either of their grandparents. Expressed as a percentage, grandparents will between 18-32% of their DNA with the offspring of their children, with the average being about 25%.

 In a grandparent DNA test performed to address paternity, we are asking the question, “Does the DNA of the alleged grandparent(s) contain the genes that the child inherited (or could have inherited) from its biological father?”   If both biological parents of the alleged father are tested and they do not possess the paternal genes between them, then they are excluded (ruled out) as the biological grandparents and the probability of grand-parentage will be 0%.  If both biological parents of the alleged father are tested and they do possess the paternal genes between them, then they are not excluded (not ruled out) as the biological grandparents and the probability of grand-parentage will be more that 99%.  For these tests, we recommend testing samples from both alleged grandparents and the mother’s sample whenever possible to achieve results of greatest accuracy.

Not only this, you can also establish your relatives using DNA tests.  

You can also  find your genetic relatives using genetic database for individuals with whom you share DNA. This is not a trivial task since we all share many small DNA segments with each other. However, on average you share longer stretches of DNA with your relatives than with unrelated individuals.

With the help of science we can confidently detect related individuals if they have at least one continuous region of matching SNPs (Single Nucleotide Polymorphisms) that is longer than our minimum threshold of 7cM (centiMorgans) long and at least 700 SNPs.

Using that criteria, we can estimate the likelihood of detecting a genetic relationship with a 3rd cousin to be approximately 90%, The table below shows the average likelihood of detecting cousins of different degrees:

The more distant the relationship, the less likely it is that a genetic relationship will be detected. Distant relatives are less likely to be detected because fewer of them share DNA segments long enough to meet our threshold. Even though the likelihood of detecting distant cousins is low, you will most likely have many distant cousins listed in the DNA Relatives feature. 

The DNA Relatives feature estimates a predicted relationship and range using the number of segments and percent DNA shared.

In general, you share more segments and a larger percentage of DNA with close relatives than with distant ones.

A notable exception to this general rule is a parent and child relationship. If you are female, you share 23 segments with each parent, and each segment spans the entire chromosome. If you are male, you share 22 segments with father (since the DNA Relatives feature does not use the Y chromosome) and 23 segments with your mother. While you might share more segments with your brother or first cousin than with your father, those segments will be shorter.

The vast majority of genetic relatives found by the DNA Relatives feature share a common ancestor within the last five to ten generations. A few may be more distantly related. A predicted relationship is provided to help you locate your likely recent common ancestor. The predicted relationship uses the percent DNA shared and the number of segments to predict your likely relationship with that relative. In some cases, multiple relationships overlap in the number of segments and percentage DNA shared. Due to this overlap, some relationships can be difficult to pinpoint or can show a predicted relationship that differs from your known relationship with that relative. For distant relationships, tests report uncertainty by also showing a relationship range.

Test results label individuals whose degree of relationship is difficult to estimate due to the small amount of DNA shared as "distant cousins".

Lastly, if the predicted relationship is not what you know the relationship to be, you can set the known relationship instead. You can do this by clicking on the predicted relationship in the DNA Relatives feature.

Percent DNA Shared by Relationship

To help in interpreting your results in the DNA Relatives feature, the following summarizes the average percent (%) DNA shared for different types of relationships. You may notice that several relationships share the same average percent (%) DNA.

You can get more information by visiting this site: https://isogg.org/wiki/Autosomal_DNA_statistics

PS: I want to add something here ... there is a thing called chimerism, where a mother's DNA doesn't match with that of her children. Read here how this happens: Why some people have different natured genomes in their cells