How to calculate number of genotypes possible for one locus

DNA is present in nearly every cell of our bodies, and we leave cells behind everywhere we go without even realizing it. Flakes of skin, drops of blood, hair, and saliva all contain DNA that can be used to identify us. In fact, the study of forensics, commonly used by police departments and prosecutors around the world, frequently relies upon these small bits of shed DNA to link criminals to the crimes they commit. This fascinating science is often portrayed on popular television shows as a simple, exact, and infallible method of finding a perpetrator and bringing him or her to justice. In truth, however, teasing out a DNA fingerprint and determining the likelihood of a match between a suspect and a crime scene is a complicated process that relies upon probability to a greater extent than most people realize. Government-administered DNA databases, such as the Combined DNA Index System (CODIS), do help speed the process, but they also bring to light complex ethical issues involving the rights of victims and suspects alike. Thus, understanding the ways in which DNA evidence is obtained and analyzed, what this evidence can tell investigators, and how this evidence is used within the legal system is critical to appreciating the true ethical and legal impact of forensic genetics. How Does DNA Identification Work? Although the overwhelming majority of the human genome is identical across all individuals, there are regions of variation. This variation can occur anywhere in the genome, including areas that are not known to code for proteins. Investigation into these noncoding regions reveals repeated units of DNA that vary in length among individuals. Scientists have found that one particular type of repeat, known as a short tandem repeat (STR), is relatively easily measured and compared between different individuals. In fact, the Federal Bureau of Investigation (FBI) has identified 13 core STR loci that are now routinely used in the identification of individuals in the United States, and Interpol has identified 10 standard loci for the United Kingdom and Europe. Nine STR loci have also been identified for Indian populations. As its name implies, an STR contains repeating units of a short (typically three- to four-nucleotide) DNA sequence. The number of repeats within an STR is referred to as an allele. For instance, the STR known as D7S820, found on chromosome 7, contains between 5 and 16 repeats of GATA. Therefore, there are 12 different alleles possible for the D7S820 STR. An individual with D7S820 alleles 10 and 15, for example, would have inherited a copy of D7S820 with 10 GATA repeats from one parent, and a copy of D7S820 with 15 GATA repeats from his or her other parent. Because there 12 different alleles for this STR, there are therefore 78 different possible genotypes, or pairs of alleles. Specifically, there are 12 homozygotes, in which the same allele is received from each parent, as well as 66 heterozygotes, in which the two alleles are different. Problem: A genetic locus that is analyzed in many forensic and paternity testing laboratories is the human leukocyte antigen locus known as HLA-DQ alpha. There are four major alleles at this locus known as 1, 2, 3, and 4. How many different genotypes are possible for these four alleles? A) 8 B) 10 C) 12 D) 16 E) 24 #geneotype #forensicGenetics #DNA #locus #Allele #STR #GeneticProfile #paternityTesting