Each puzzle is a family made by the lab’s genetics model and checked so that its answer is clear. 1. Mode: decide how the trait is inherited. 2. Genotypes: tap people and choose their genotypes. 3. Probability: work out the odds for the next child. Tap any person to read their label.
Swipe the chart sideways to see everyone.
Which mode of inheritance fits this family best?
A_ means “one A, and the other allele can’t be told from the chart”. A genotype counts as right only if the chart proves it.
Learn the symbols geneticists use to draw a family. Pick Basic (6 labels) or Advanced (10), then try the blank diagram, the label quiz and match the jobs. On a phone, swipe the diagram sideways.
In this family, IV-1 has a recessive trait. Her parents, III-2 and III-3, are first cousins, and both are carriers.
The 2022 update of the NSGC nomenclature uses the shapes for gender identity (square, circle, or a diamond for other or unknown) and adds a note of sex assigned at birth below the symbol, such as AFAB or AMAB, when it matters. Genetics classes commonly use the simpler square = male, circle = female, as this page does.
Tap a word, then tap the numbered box it belongs to (on the diagram or in the list below). Tap a filled box to take the word back.
Each line describes the job of one part. Tap a part name, then tap the box next to the job it does. Tap a filled box to take the word back.
These are well-known human traits. The small charts for Huntington’s disease, cystic fibrosis, color blindness and X-linked hypophosphatemia are example families drawn to show the typical pattern, not real families. The Queen Victoria chart is a simplified version of a real family. Under each chart, the lab’s checker says which modes fit.
A brain disorder caused by an expanded repeat (CAG) in the HTT gene on chromosome 4. One altered copy is enough, so each child of an affected parent has a 1 in 2 chance of inheriting it, sons and daughters alike, and it usually shows up in every generation. Symptoms usually begin in a person’s thirties or forties, often after they have had children.
Caused by two altered copies of the CFTR gene, which codes for a channel for chloride ions; thick, sticky mucus builds up in the lungs and digestive system. Carriers (one copy) are healthy, so most children with cystic fibrosis are born to two unaffected carrier parents, who have a 1 in 4 chance with each pregnancy. The trait often appears with no family history.
Simplified: only some of Victoria’s children and grandchildren are shown, and the slash for “deceased” is left off. Dots mark carriers that the chart itself proves.
Queen Victoria (1819–1901) was a carrier of hemophilia, a disorder in which blood does not clot normally. Her son Leopold had it. Her daughters Alice and Beatrice were carriers and passed the allele on: Alice’s daughter Alix became Empress Alexandra of Russia, and her son, Tsarevich Alexei, had hemophilia; Beatrice’s daughter Victoria Eugenie (“Ena”) became Queen of Spain, and two of her sons, Alfonso and Gonzalo, had it. No earlier case is known in Victoria’s family, so the mutation probably arose new in Victoria or in one of her parents.
In 2009, scientists studied DNA from the remains of the Russian imperial family (Rogaev et al., Science 326:817). They found a mutation in the F9 gene, which codes for clotting factor IX, so the “royal disease” was hemophilia B. Alexei had only the mutant allele and his mother Alexandra was a carrier, exactly as the pedigree predicts. Victoria’s son Edward VII, an ancestor of today’s British royal family, was unaffected, so he could not pass it on: a man’s only X is the one he gives to his daughters.
The OPN1LW and OPN1MW genes for red- and green-sensitive light receptors are on the X chromosome. Among people with Northern European ancestry, about 1 in 12 males but only 1 in 200 females have red-green color vision deficiency (MedlinePlus). A typical path: an affected grandfather → his carrier daughter → an affected grandson.
The textbook example of an X-linked dominant trait (also called hypophosphatemic or vitamin D–resistant rickets). Changes in the PHEX gene lower the phosphate level in the blood, so bones are soft: children can have rickets, bowed legs and short stature. An affected father passes it to all of his daughters and none of his sons; an affected mother passes it to about half of her children.
Each small family is made by the model: two parents and their children. Here nothing is assumed about either parent (either may be a carrier). Tap every mode that this family rules out, then press Check. 8 families per round.
Which modes are ruled out? (Tap to mark ✘; tap again to unmark.)
A worksheet with an answer key: label the symbols, read a four-generation pedigree (mode, evidence, genotypes, probabilities), rule out modes for four small families, and solve classic problems. Every answer in the key is computed by the same model as the lab.
AP Biology Unit 5 (Heredity) uses pedigrees in Topic 5.3 (Mendelian Genetics: rules of probability for single-gene traits) and Topic 5.4 (Non-Mendelian Genetics: sex-linked traits). Here is the reasoning behind the rules in the lab.
A female has two X chromosomes; a male has one X and one Y. Most genes on the X have no partner on the Y, so a male has only one copy of each X-linked gene: he is hemizygous. One recessive allele is then enough to show the trait (XaY), while a female needs two (XaXa). That is why X-linked recessive traits are much more common in males.
The rules in the lab follow from how the chromosomes are passed on: a father gives his X to every daughter and his Y to every son; a mother gives one of her two X chromosomes to each child. So an X-linked trait is never passed from father to son.
Two rarer patterns are not used in the puzzles. Y-linked: passed from a father to all of his sons and never to daughters; very few such traits are known. Mitochondrial: passed on by the mother, never by the father, because the egg, not the sperm, provides the mitochondria. It can be passed to all of her children; how many of them are affected, and how severely, varies (heteroplasmy: cells carry a mix of normal and altered mitochondrial DNA).
Penetrance is the share of people with a disease genotype who actually show the trait. The lab assumes complete penetrance (100%). Many real traits have incomplete penetrance: for example, people with 36 to 39 CAG repeats in HTT may or may not develop Huntington’s disease, while 40 or more repeats almost always cause it. A dominant trait with incomplete penetrance can seem to “skip” a person, which breaks the rule “two unaffected parents → recessive”.
Variable expressivity means that people who show the trait show it to different degrees. In neurofibromatosis type 1 (autosomal dominant), members of the same family can have anything from a few light-brown skin spots to many tumors. Genetic counselors keep both effects in mind when they read a real pedigree.
Two unaffected parents, two affected sons. The trait is clearly recessive, but is it autosomal or X-linked? Both fit: a carrier mother (XAXa) could give Xa to two sons, or two carrier parents (Aa × Aa) could have two aa sons. With only four children, the lab cannot tell them apart; more relatives or a genetic test are needed.
The answer also depends on the assumptions. Below is puzzle 7 from the lab. If people who marry in may be carriers, a common recessive allele could explain it too. Tick the box to drop the assumption and see which modes still fit.
When two modes fit, ask which one makes the chart more likely. The X-linked hypophosphatemia chart above fits both autosomal and X-linked dominant, but only X-linked dominant makes “every daughter of an affected father affected, every son unaffected” certain; under autosomal dominant the chance of that pattern in 4 children is (1/2)4 = 1/16.
For an autosomal recessive trait: pick what is known about each partner. The child is affected (aa) only if both partners pass an a, so multiply the two chances. This uses only Mendel’s rules and conditional probability, with no extra information from the partners’ own children.
Why 2/3? Two carriers have children AA, Aa, Aa and aa in equal numbers. A brother or sister who is known to be unaffected is not aa, which leaves three outcomes, and two of them are Aa.
Every couple in a pedigree is a cross. Once you know the parents’ genotypes, a Punnett square gives the chances for each child, and Mendel’s laws explain why: the two alleles separate in meiosis, so each gamete carries one. New alleles first arise by mutation.
A single family is usually too small for a chi-square test to say much: with four or five children, almost any ratio can happen by chance. Geneticists instead combine many families, or (to find where a gene lies) use statistical scores such as LOD scores that add up evidence across families. Use chi-square for large crosses, such as fruit-fly or corn data, not for one pedigree.
A pedigree is a family tree drawn with a simple code. Squares are males, circles are females, and a filled-in symbol means the person has the trait. A line between a man and a woman joins a couple; a line hanging down from it leads to their children, oldest on the left. Each row is a generation, numbered I, II, III from the top, so “III-2” is the second person in the third row.
Look for two clues. If two unaffected parents have an affected child, the trait must be recessive: both parents are carriers who each passed on a hidden allele. If two affected parents have an unaffected child, the trait must be dominant: for an autosomal trait both parents are heterozygous (Aa) and the child got a from each; for an X-linked dominant trait the mother is XAXa and the unaffected child is a son who got her Xa. A recessive trait often seems to skip generations; a dominant trait usually appears in every generation. These words describe how alleles act, not how common or how serious a trait is.
Use what fathers and sons do. A father gives his X to every daughter and his Y to every son. So for an X-linked recessive trait, an affected woman has only affected sons, and an affected daughter has an affected father; the trait is far more common in males. For an X-linked dominant trait, an affected father has only affected daughters and no affected sons. If an affected father has an affected son (and the mother cannot explain it), the trait is not X-linked. Then write genotypes, starting with the people whose phenotype tells you the most, and use Punnett squares for the odds.
Each rule rules a mode out; it never proves one. A careful answer checks every mode against every family in the chart, which is exactly what the lab’s exact search does, and it states its assumptions (complete penetrance, no new mutations, a rare allele). When two modes still fit, compare how likely the chart is under each one. Probabilities must use everything known: an unaffected sibling of an affected child is a carrier with probability 2/3, not 1/2.
In the Pedigree Lab, choose a wrong mode on purpose and read which people rule it out (circled in orange). Solve the mode, then in step 2 find one person you can’t be sure about and answer A_. In step 3, check that an unaffected child of two carriers gives 2/3. Then press Next puzzle until you have seen all four modes.
Pedigrees are one of the oldest tools in genetics, and they are still used every day.
| Area | How pedigrees are used |
|---|---|
| Genetic counseling | A genetic counselor usually starts by drawing a family pedigree of at least three generations, using standard symbols. It helps to spot an inheritance pattern, estimate the chance that a person or a future child is affected or a carrier, and decide whether a genetic test is useful. |
| Finding disease genes | Large families with many affected members help scientists find where a gene lies. Studies of a very large family in Venezuela with Huntington’s disease helped locate the gene on chromosome 4 (1983) and then identify it (1993). |
| Medicine | Doctors ask about family health history because many conditions run in families, from inherited disorders to raised risks of some cancers and heart disease. A pedigree turns that history into a picture. |
| Animal and plant breeding | Breeders keep pedigree records (for example studbooks for horses, dogs and zoo animals) to plan matings, avoid close inbreeding, and keep harmful recessive alleles from becoming common. |
| History | Royal family trees are well recorded, so traits such as hemophilia in Queen Victoria’s descendants could be followed for generations, and later confirmed with DNA. |
Related pages: Punnett squares, Mendelian genetics, meiosis, DNA mutations.
Squares are males and circles are females. A filled-in symbol means the person has the trait. A horizontal line between a man and a woman joins a couple, and a vertical line from it leads down to a horizontal sibship line with their children, oldest on the left. Each row is a generation, numbered with Roman numerals from the top, and people in a row are numbered from left to right, so II-3 is the third person in generation II.
Key takeaway: shapes give the sex, shading the trait, rows the generations.If two unaffected parents have an affected child, the trait is recessive: both parents must be carriers. If two affected parents have an unaffected child, the trait is dominant: for an autosomal trait both parents are Aa; for an X-linked dominant trait the mother is XᴬXᵃ and the unaffected child is a son. Without either clue, a trait that appears in every generation suggests dominant, and one that skips generations suggests recessive. These rules assume complete penetrance and no new mutations.
Key takeaway: unaffected parents with an affected child means recessive.Use the fact that a father gives his X to every daughter and his Y to every son. An X-linked recessive trait is much more common in males; an affected woman has only affected sons, and an affected daughter has an affected father. An X-linked dominant trait passes from an affected father to all of his daughters and none of his sons. If an affected father has an affected son whose mother cannot explain it, the trait is not X-linked.
Key takeaway: look at fathers: they never pass an X-linked allele to sons.In many textbooks a half-shaded symbol marks a carrier: a heterozygous person who has one copy of a recessive allele but does not show the trait. The standard nomenclature used by genetic counselors marks a carrier with a dot in the middle of the symbol instead. Either way, carriers are often not marked at all, because many carriers do not know their status.
Key takeaway: a carrier mark, but carriers are often not drawn.A carrier has one copy of a recessive allele and one dominant allele (Aa, or XᴬXᵃ for an X-linked trait), so they do not show the trait but can pass the allele to their children. Two carriers of the same autosomal recessive allele have a 1 in 4 chance with each child of having an affected child. For an X-linked recessive trait, carriers are female: a male with the allele shows the trait.
Key takeaway: has the recessive allele, does not show it, can pass it on.Yes, if the trait is dominant. For an autosomal dominant trait with two heterozygous parents (Aa × Aa), each child has a 1 in 4 chance of being aa and unaffected; for an X-linked dominant trait, with a heterozygous mother (XᴬXᵃ) and an affected father (XᴬY), each son has a 1 in 2 chance of being unaffected (XᵃY), while every daughter is affected. For a recessive trait, the answer is no (with complete penetrance): affected parents are both aa, so every child is aa. That is why an unaffected child of two affected parents proves the trait is dominant.
Key takeaway: yes for a dominant trait; no for a recessive one.The Roman numerals on the left mark the generations: I is the oldest generation at the top, then II, III and so on. Within each generation, people are numbered with ordinary numbers from left to right. Together they give each person a label, such as I-2 or III-4, so you can talk about them clearly.
Key takeaway: Roman numerals are generations; numbers are people.Males have one X chromosome and one Y, and most genes on the X have no partner on the Y, so a male is hemizygous: a single recessive allele on his X is enough to show the trait. A female needs the recessive allele on both of her X chromosomes. For red-green color blindness, about 1 in 12 males but only 1 in 200 females of Northern European ancestry are affected.
Key takeaway: males have only one X, so one recessive allele is enough.For an autosomal recessive trait with unaffected parents, both parents must be carriers (Aa × Aa). Their children are AA, Aa, Aa and aa in equal proportions. An unaffected sibling is not aa, which leaves three equally likely outcomes, and two of them are Aa. So the chance is 2/3, not 1/2. If that sibling has a child with a known carrier, the chance the child is affected is 2/3 × 1/4 = 1/6.
Key takeaway: 2/3, because aa is ruled out.When a trait disappears in one generation and returns in the next, it is usually recessive: the allele was passed on by carriers who did not show it. This happens with both autosomal recessive and X-linked recessive traits, so skipping alone does not tell you the trait is X-linked. A dominant trait with incomplete penetrance can also seem to skip a person.
Key takeaway: usually recessive, autosomal or X-linked.