Inferring Inheritance Patterns from Pedigrees and Offspring Ratios

Hello! Last lesson began with known parental genotypes and used Punnett squares to predict offspring. Now you’ll work in reverse: given a family history or an observed offspring ratio, what inheritance pattern could have produced it? This is an HSC skill in using evidence to justify an inference, rather than naming a pattern from one visual clue. Allow about 35–40 minutes.

Read a pedigree as evidence

A pedigree tracks a trait through a family. Squares represent males, circles represent females, and shaded symbols represent people who show the trait being tracked. A horizontal line connects parents; lines below it connect them to their children. Read the key on any particular diagram before interpreting its shading.

Watch the opening of Pedigrees by Amoeba Sisters for these conventions, then continue through its autosomal-recessive example. The useful question throughout is: which allele must each parent have been able to pass on?

The strongest pedigree clues come from relationships, not from counting shaded symbols. Under a simple, fully expressed dominant–recessive model:

• If two unaffected parents have an affected child, a dominant explanation does not fit: neither parent displays the proposed dominant trait. A recessive explanation can fit because both parents may be unaffected carriers.
• If a trait appears in successive generations, dominant inheritance may fit—but that observation alone does not establish dominance. A recessive trait can also appear in successive generations.
• An unaffected person is not automatically a carrier. Their family relationships may prove they carry an allele, leave it possible, or rule it out.

“Autosomal” means the gene is on a chromosome other than a sex chromosome. “Recessive” tells you which allele combination produces the trait. These are separate questions: first consider whether the trait behaves as dominant or recessive, then whether its transmission fits an autosome or a sex chromosome.

Work backwards from a family

The pedigree below tracks alkaptonuria, an autosomal-recessive condition. Blue symbols are affected. Let $a$ be the recessive allele associated with the condition and $A$ the dominant allele. In this model, affected people are $aa$; unaffected people may be $AA$ or $Aa$.

Start with people whose genotypes are certain. The affected father in the first generation is $aa$, so he can pass only $a$. His affected son in the second generation is also $aa$, which means the unaffected mother must have supplied $a$. She is therefore $Aa$, not $AA$. Her appearance alone would not have revealed that.

Now look at the left-hand branch of the third generation. The affected woman is $aa$, and her unaffected partner is marked 3. They have an affected daughter. Since that daughter needed an $a$ from each parent, individual 3 must be $Aa$. The cross $aa\times Aa$ predicts a $\frac{1}{2}$ chance of $aa$ and a $\frac{1}{2}$ chance of $Aa$ for each child.

On the right-hand branch, both parents are $Aa$. Their affected daughter is $aa$. Their unaffected daughter is labelled $A?$ for good reason: she could be $AA$ or $Aa$. From $Aa\times Aa$, the possible genotypes have probabilities $\frac{1}{4}AA$, $\frac{1}{2}Aa$, and $\frac{1}{4}aa$. Given that this daughter is unaffected, the $aa$ possibility is excluded. Of the three equally likely unaffected outcomes in the Punnett square, two are $Aa$, so her probability of being a carrier is $\frac{2}{3}$—not $\frac{1}{2}$.

The pedigree also tests alternatives. An X-linked recessive explanation would require an affected daughter to receive an affected X chromosome from her father. Yet the affected daughter on the left has an unaffected father. That relationship rules out X-linked recessive inheritance under the stated simple model. A Y-linked explanation cannot account for affected females. Combined with the unaffected-parent evidence, the family supports autosomal recessive inheritance much more strongly than an impression such as “the condition skips a generation.”

Check sex-linked possibilities by tracing chromosomes

For an X-linked trait, a father supplies his X chromosome to each daughter and his Y chromosome to each son. A son receives his X chromosome from his mother. Those transmission rules give you tests that are more precise than “more males are affected.”

In an X-linked recessive model, write an affected male as $X^{a}Y$. An unaffected mother can be a carrier, $X^A{X}^a$, and have an affected son: he receives $X^a$ from her. But an affected daughter, $X^a{X}^a$, must receive an $X^a$ from both parents. An unaffected father, $X^{A}Y$, cannot supply it.

In an X-linked dominant model, an affected father passes his affected X chromosome to all his daughters and none of his sons. Thus an unaffected daughter of an affected father rules out that model, assuming the trait is fully expressed. A Y-linked allele passes from a father to all his sons and to no daughters.

Watch the sex-linked portion of the same video to see why chromosome notation helps keep the parental contributions straight.

These are tests of what is possible, not a rule that every small pedigree identifies one pattern with certainty. More affected males can suggest X-linked recessive inheritance, but chance can produce a sex imbalance for an autosomal trait. An affected father and son can also coexist in an X-linked family if the son inherited the relevant allele from his mother; the father did not pass his X to that son. When several models still fit, state what additional family relationship or genetic evidence would distinguish them.

Use offspring ratios without overclaiming

An offspring ratio supplies another kind of evidence. If two unaffected parents produce some affected children, test the recessive-carrier cross $Aa\times Aa$. It predicts $1AA:2Aa:1aa$, or 3 unaffected:1 affected. A different cross, $Aa\times aa$, predicts 1 unaffected:1 affected for that same recessive trait.

But a 1:1 phenotype ratio alone does not prove recessive inheritance. If $A$ instead represents a dominant affected trait, an affected heterozygote crossed with an unaffected $aa$ parent also predicts 1 affected:1 unaffected. You need the parents’ phenotypes, family relationships, or other evidence to distinguish the explanations. Always state what your allele letters mean before interpreting a ratio.

Observed numbers need not equal predicted fractions. For example, 29 unaffected and 11 affected offspring are reasonably close to a predicted 3:1 ratio; they are not evidence that every set of four offspring contains exactly one affected child. With few offspring, chance variation is greater. A ratio is therefore consistent with a model, not proof of it.

For an HSC response, make your reasoning visible in a compact sequence: identify a decisive parent–child relationship, propose an inheritance model, show the allele contributions or expected cross, and explain why an alternative does not fit. For the illustrated pedigree, that could read:

Alkaptonuria is consistent with autosomal recessive inheritance. The unaffected male labelled 3 must carry $a$, because his affected daughter is $aa$ and must receive one $a$ from him. His being unaffected makes him $Aa$. An X-linked recessive model cannot account for an affected daughter with this unaffected father.

Key takeaways

A pedigree is most informative when you trace what each parent must be able to contribute. Unaffected parents with an affected child can reveal recessive carriers; sex-chromosome transmission can rule out particular X- or Y-linked explanations; and offspring ratios can check whether a proposed cross is plausible. Keep certain genotypes, possible genotypes, and probabilistic predictions distinct.

Next, you’ll extend this evidence-based approach to experimental data about reproduction or inheritance, where the task is to draw a conclusion from results rather than from a family diagram alone.