Three independently assorting STR markers (A, B, and C) are used to assess the paternity of a colt recently born to a quarter horse mare. Blood samples are drawn from the mare, her colt, and three possible male sires (S₁, S₂, and S₃). DNA at each marker locus is amplified by PCR, and a DNA electrophoresis gel is run for each marker. Amplified DNA bands are visualized in each gel by ethidium bromide staining. Gel results are shown below for each marker. Evaluate the data and determine if any of the potential sires can be excluded. Explain the basis of exclusion, if any, in each case.

Verified step by step guidance

1. Step 1: Understand the principle of paternity testing using STR markers. Each individual inherits one allele from the mother and one allele from the father at each STR locus. Therefore, the colt's alleles at each marker must be a combination of one allele from the mare and one allele from the sire.
2. Step 2: For each STR marker (A, B, and C), compare the alleles present in the colt with those of the mare. Identify which allele in the colt must have come from the sire by excluding the allele inherited from the mare.
3. Step 3: For each potential sire (S₁, S₂, and S₃), check if they possess the allele(s) that the colt must have inherited from the father at each marker. If a potential sire lacks the required allele at any marker, that sire can be excluded as the biological father.
4. Step 4: Repeat this comparison for all three markers independently, since the markers assort independently. A sire must have matching alleles at all markers to be considered a possible father.
5. Step 5: Summarize the results by listing which sires can be excluded based on the absence of required alleles at one or more markers, and explain that exclusion is based on Mendelian inheritance patterns where the colt must inherit one allele from each parent at every locus.

Key Concepts

Independent Assortment of STR Markers

Short Tandem Repeat (STR) markers are specific DNA sequences used in genetic profiling. When markers assort independently, the inheritance of one marker does not affect the others, allowing each locus to be analyzed separately. This principle helps in paternity testing by comparing alleles at multiple loci to increase accuracy.

Paternity Exclusion Principle

Paternity exclusion occurs when an alleged sire lacks one or more alleles present in the offspring that must have been inherited from the father. If the sire’s genotype does not share at least one allele at each marker locus with the offspring, he can be excluded as the biological father.

PCR Amplification and Gel Electrophoresis in DNA Analysis

Polymerase Chain Reaction (PCR) amplifies specific DNA regions, such as STR loci, to detectable levels. Gel electrophoresis separates these amplified fragments by size, visualized by ethidium bromide staining. The resulting band patterns represent alleles, which are compared across individuals for genetic relationship analysis.