Human Pedigrees Answer Key

Working Through Human Pedigrees

I've graded more pedigree problems than I care to count. They show up in every intro biology and genetics course, usually as homework or exam questions, and students consistently struggle with them for the same reasons. You look at a chart full of shaded and unshaded symbols, you're asked to figure out inheritance patterns and genotype predictions, and you just kind of stare at it for twenty minutes. The

Human Pedigrees Answer Key

concept isn't some special tool or software. It's just the worked-out solutions that instructors post so students can check their work. What actually helps is understanding the logic behind how those keys are constructed, because without that foundation you're just memorizing which boxes get shaded in which patterns. Here's how you actually approach these problems instead of guessing.

First, look at the parents and the affected offspring. If both parents are unaffected but they have an affected child, that's autosomal recessive. Period. That's the single most reliable signal in any pedigree. I once had a student who spent forty-five minutes trying to argue it was X-linked because there happened to be more affected males in one generation. The answer was recessive, the sample size was just small and skewed. Don't overthink it when the parent-offspring pattern is clear. Now for the actual process. Map out what you know. Assign letters to the alleles -- I always use A and a for autosomal, X^H and X^h for X-linked stuff. Start with individuals whose genotypes you can determine with certainty. Affected individuals in a recessive cross are homozygous recessive. Unaffected individuals could be heterozygous or homozygous dominant, which is where it gets messy. The thing most people miss is working backward from the children before committing to parental genotypes. Take generation II in a typical three-generation problem. If individual II-4 is affected and you're dealing with a recessive trait, you immediately know both parents must carry at least one copy of the recessive allele. That constrains what generation I could have been. Then you move forward to generation III, checking consistency.

I ran into a genuine edge case recently that wasn't covered in any textbook. The pedigree showed what looked like autosomal dominant inheritance, but every affected individual had an affected parent, and then suddenly there was a skipped generation. At first glance it looked like the answer key would just say dominant, but the skipped generation was the red flag. What was actually happening was incomplete penetrance. The person who "skipped" it carried the dominant allele but didn't express the phenotype. I flagged this to my students with a note about penetrance because standard pedigree interpretation rules don't account for it well. If you see an apparently dominant pattern with a gap, reconsider whether the trait is fully penetrant before concluding anything. For X-linked recessive specifically, there's a shortcut. Affected mothers pass the trait to all their sons. If you see an affected mother with an unaffected son, it's not X-linked recessive. This eliminates half your possibilities in seconds. I use this as my first filter before doing any other analysis. Y-linked traits are rare and straightforward. Only males are affected, and every affected father passes it to every son. If there's a single female affected or an affected male with an unaffected son, it's not Y-linked. These questions almost never appear without being obvious, but they're easy marks when they do.

When you're actually checking against an answer key, don't just verify whether you got the right inheritance pattern. Trace every individual's genotype assignment in the key. If they have you marking someone as Aa when the pedigree doesn't support heterozygosity, ask yourself why. Often the key is making a probability assumption -- like saying "50% chance this individual is a carrier" -- rather than stating a definite genotype. These distinction matters on exams where the question asks for the probability versus the certainty. One honest limitation here: pedigree analysis cannot definitively prove a mode of inheritance with small families. Three generations with four affected individuals total might look convincing, but statistically it's weak evidence. A real genetics lab would want molecular confirmation through sequencing. Pedigrees are inferential tools, not proof. I tell my students this early so they don't walk away thinking they've figured something out with absolute certainty when they've really just made the most reasonable guess given limited data. If you're looking for practice problems with detailed answer keys, most textbook companion websites offer them. Campbell Biology has a solid set. OpenStax Genetics also posts free ones. The key ones are usually in Chapter 14 or 15 of introductory texts, depending on how they organize chapters. Just search for "pedigree analysis practice problems with answers" and you'll find PDFs. I've collected a few over the years but honestly the textbook versions are fine and usually more accurate than random uploads from study sites.

The main categories you'll encounter on these answer keys are autosomal dominant, autosomal recessive, X-linked dominant, X-linked recessive, and the occasional Y-linked or mitochondrial question. Mitochondrial is the one most students blow through wrong because they forget that only mothers pass it on. All children of an affected mother are affected. No children of an affected father are affected. Simple rule, frequently tested, consistently missed. Work through the problems yourself before looking at any key. Even if you get them wrong, the act of assigning genotypes and finding contradictions trains your pattern recognition. Then check the key and trace exactly where your logic diverged. That's where the actual learning happens, not in copying the shaded boxes from a solution document.