This would make both of his parents Dd genetypes.
Bob's sister could either not have the disease at all, DD, or be a carrier, Dd. If she was a carrier she then poses the chance of passing the disease on through child birth. Her father was unaffected, which means he must have been DD, which would make his mother probably Dd, so the chances are equal of Bob's sister being either DD, or Dd, with a fifty percent chance of being either of the genotypes.
Both DNA and RNA are made up of different tiny particles of information, often referred to as nucleotides. The nucleotides of Adenine, Guanine, Cytosine, and Thymine all make up DNA. In RNA however, Thymine is displaced and Uracil takes its place. It is these four proteins which help make up the coding and information of our bodies. Various proteins can then used these nucleotides, and based on the order they are in, read the information stored there. For example, the order of these nucleotides varies from species to species, and is always different for members of one species to members of another species.
The DNA is copied into RNA, which is almost identical to the original coding of the DNA. through the process of transcription the DNA is copied into RNA, which retains almost all of the original information that was held in the DNA.
These RNA genes are often composed of both exons and introns. However, only the exons are required for the process of protein synthesis. RNA is then divided into only the parts necessary for coding, the extrons. This leaves us with mRNA. It is then this mRNA that helped guide the process or protein synthesis, or also called translation. The information has all been passed down the chain, leaving mRNA holding all the necessary information for the process and coding for protein synthesis to take place. This process helps shape and build the proteins needed for bodily functions. It is these proteins which help code for different traits and features in humans, be it hair color or eye color, or numerous other traits passed on this way.
As with any scientific procedure, deciding if you wanted to be tested has both its positives and its negatives. If his father had the disease, it is possible he to will develop the disease. If he is tested he would find out weather or not he actually would develop it. Some people would rather not know this kind of information, and would not like to know if they had a disease that would kill them. Other people, would like to know so they would be ready, and also could help them make decisions about having kids. If it turns out he has the disease, he may decided to not have children as to not risk passing it on. It is a hard moral decision, and has positives and negatives on both sides of the argument. If he wants to know that he might die earlier than expected, but hoped to help his children, he may want to get tested. If he wishes not to know however, it may be better he doesn't get