A family consisting of a mother (I-1), a father (I-2), and three children (II-1, II-2, and II-3) are genotyped by PCR for a region of an autosome containing repeats of a 10-bp sequence. The mother carries 16 repeats on one chromosome and 21 on the homologous chromosome. The father carries repeat numbers of 18 and 26. Following the layout of Figure 7.26c, which aligns members of a pedigree with their DNA fragments in a gel, draw a DNA gel containing the PCR fragments generated by amplification of DNA from the parents (I-1 and I-2). Label the size of each fragment.

Key Concepts

Polymerase Chain Reaction (PCR)

PCR is a molecular biology technique used to amplify specific DNA sequences, making millions of copies of a particular segment. It involves repeated cycles of denaturation, annealing, and extension, allowing researchers to analyze genetic material in detail. Understanding PCR is crucial for interpreting the results of genotyping, as it directly affects the size and quantity of the DNA fragments produced.

Genotype and Alleles

A genotype refers to the genetic constitution of an individual, specifically the alleles present at a given locus. In this case, the mother has two different alleles for the repeat region (16 and 21 repeats), while the father has alleles of 18 and 26 repeats. Recognizing how these alleles contribute to the offspring's genotype is essential for predicting the sizes of the PCR fragments that will appear on the gel.

Gel Electrophoresis

Gel electrophoresis is a laboratory technique used to separate DNA fragments based on their size. When an electric current is applied, smaller fragments move faster through the gel matrix than larger ones, allowing for the visualization of different DNA sizes. This technique is critical for analyzing the PCR products from the parents, as it helps determine the specific repeat lengths inherited by the children.