20 Punnett Square Problems to Practice On

A Punnett square is just a grid of every possible way two parents' alleles can combine. Once you can build that grid without thinking, every genetics problem turns into the same three moves: figure out the gametes, fill the grid, read off the ratio.

Twenty problems below, starting with a single gene and complete dominance, then working up through incomplete dominance, codominance, two genes at once, and sex-linked traits. Try each one yourself before opening the solution. That's where the pattern actually sticks.

Section 1: Monohybrid Crosses with Complete Dominance

Start with one gene, two alleles, and a clear dominant-recessive relationship. This is the grid mechanics you'll reuse in every section after this one.

Problem 1

In pea plants, tall ($T$) is dominant over short ($t$). Two heterozygous plants ($Tt \times Tt$) are crossed. What are the genotype and phenotype ratios of the offspring?

Problem 2

A homozygous tall pea plant ($TT$) is crossed with a homozygous short plant ($tt$). What fraction of the offspring will be tall?

Problem 3

A heterozygous tall plant ($Tt$) is crossed with a short plant ($tt$). What's the phenotype ratio of the offspring?

Problem 4

In pea plants, purple flowers ($P$) are dominant over white ($p$). Two heterozygous plants ($Pp \times Pp$) produce 160 offspring. How many are expected to have white flowers?

Section 2: Working Backward from Offspring Ratios

Now flip the question around. Instead of predicting offspring from known parents, you're given what showed up and have to figure out a genotype or a probability. Same grid, read in reverse.

Problem 5

In guinea pigs, black fur ($B$) is dominant over white ($b$). A black guinea pig is crossed with a white guinea pig, and the offspring come out $1:1$ black to white. What is the genotype of the black parent?

Problem 6

Both parents are $Aa$ for a gene with complete dominance. What is the probability that a given offspring is homozygous, either $AA$ or $aa$?

Problem 7

In fruit flies, red eyes ($R$) are dominant over sepia eyes ($r$). Two red-eyed flies are crossed and produce 96 offspring, of which 24 have sepia eyes. What are the genotypes of the parents?

Problem 8

Both parents are carriers ($Ff$) for a recessive genetic disorder. What is the probability that a child is unaffected but still a carrier?

Section 3: Incomplete Dominance and Codominance

Not every trait follows a clean dominant-recessive rule. Sometimes the heterozygote blends both phenotypes, and sometimes it shows both at once. The grid mechanics don't change, but the phenotype you read off the genotype does.

Problem 9

In snapdragons, flower color shows incomplete dominance. Red ($C^RC^R$) and white ($C^WC^W$) plants are crossed. What phenotype do the offspring have?

Problem 10

Two pink snapdragons ($C^RC^W \times C^RC^W$) are crossed. What is the phenotype ratio of the offspring?

Problem 11

In cattle, coat color is codominant: red ($C^R$) and white ($C^W$) alleles are both expressed in the heterozygote, producing roan. A roan bull ($C^RC^W$) is crossed with a red cow ($C^RC^R$). What fraction of the offspring are expected to be roan?

Problem 12

In humans, ABO blood type alleles $I^A$ and $I^B$ are codominant, and both are dominant over $i$. A parent with blood type AB ($I^AI^B$) and a parent with blood type O ($ii$) have a child. What blood types are possible, and in what ratio?

Section 4: Dihybrid Crosses

Now track two genes at once. The grid grows to $4 \times 4$, but each gene still segregates on its own, independent assortment just means you can multiply the two single-gene ratios together instead of tracking every combination by hand.

Problem 13

In pea plants, round seeds ($R$) are dominant over wrinkled ($r$), and yellow seed color ($Y$) is dominant over green ($y$). The genes assort independently. A double heterozygote is self-crossed ($RrYy \times RrYy$). What is the phenotype ratio of the offspring?

Problem 14

From the same cross in Problem 13 ($RrYy \times RrYy$), 320 offspring are produced. How many are expected to have wrinkled, green seeds?

Problem 15

A double heterozygote is test-crossed with a double recessive ($RrYy \times rryy$). What is the phenotype ratio of the offspring?

Problem 16

Cross $RRYy \times rrYy$. What is the phenotype ratio of the offspring?

Section 5: Sex-Linked Traits and Combined Problems

X-linked genes behave differently because males only carry one X. These problems also stack a second skill on top, multiple alleles or a second gene, so you're combining two things you've already practiced.

Problem 17

In humans, red-green color blindness is X-linked recessive. A color-blind man ($X^cY$) has children with a homozygous normal woman ($X^CX^C$). What are the possible genotypes of the daughters and sons?

Problem 18

A carrier woman ($X^CX^c$) has children with a color-blind man ($X^cY$). What fraction of the daughters are expected to be color-blind, and what fraction of the sons?

Problem 19

For human ABO blood type, a heterozygous type A parent ($I^Ai$) and a heterozygous type B parent ($I^Bi$) have a child. What blood types are possible, and what fraction of each?

Problem 20

In fruit flies, red eye color is X-linked dominant ($R$) over white ($r$), and long wings are autosomal dominant ($L$) over vestigial ($l$). A female heterozygous for both traits ($X^RX^rLl$) is crossed with a white-eyed, vestigial-winged male ($X^rY\,ll$). What fraction of the offspring are expected to be white-eyed sons with vestigial wings?

A note on notation: superscript letters on the X, like $X^R$ or $X^c$, mark which allele of an X-linked gene a chromosome carries. A $Y$ chromosome never carries that gene, which is exactly why males only need one copy of an X-linked allele to show its phenotype.