AP Biology Quiz
Practice Mendelian Genetics in AP Biology with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.
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A genetic counselor is analyzing a pedigree for a rare genetic condition. Two unaffected parents have a child who is affected by the condition.
The inheritance pattern of this condition is most likely which of the following?
This quiz focuses on Mendelian Genetics, giving you a quick way to practice the rules, question types, and explanations that matter most for AP Biology.
Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.
A genetic counselor is analyzing a pedigree for a rare genetic condition. Two unaffected parents have a child who is affected by the condition.
The inheritance pattern of this condition is most likely which of the following?
Explanation: When two unaffected individuals have an affected offspring, it is a classic sign of a recessive inheritance pattern. The parents must both be heterozygous carriers of the recessive allele. If the trait were dominant, at least one parent would have to be affected to pass the allele to the child.
In a species of plant, the allele for tall stems (T) is dominant to short stems (t), and the allele for red flowers (R) is dominant to white flowers (r). The genes for stem height and flower color are on different chromosomes.
If two plants that are heterozygous for both traits are crossed, what fraction of the offspring is expected to be tall with white flowers?
Explanation: In a cross of TtRr×TtRr, the probability of a tall offspring (T−) is 3/4, and the probability of a white-flowered offspring (rr) is 1/4. Using the product rule for independent events, the probability of an offspring being tall and white is (3/4)×(1/4)=3/16.
In tomato plants, red fruit (R) is dominant to yellow fruit (r), and tall vines (T) are dominant to dwarf vines (t). The genes are unlinked.
A plant that is heterozygous for both traits is crossed with a plant that has yellow fruit and dwarf vines. What is the expected phenotypic ratio of the offspring?
Explanation: This is a dihybrid test cross (RrTt×rrtt). The heterozygous parent produces four types of gametes (RT,Rt,rT,rt) in equal proportion. The homozygous recessive parent produces only one type of gamete (rt). The resulting offspring will have four different genotypes (RrTt,Rrtt,rrTt,rrtt) and four corresponding phenotypes, each in equal proportion, leading to a 1:1:1:1 ratio.
A pedigree chart traces the inheritance of a particular genetic disorder in a family. Affected individuals are represented by shaded symbols. The disorder appears in every generation and is passed from an affected father to his son.
Based on this information, what is the most likely mode of inheritance for this disorder?
Explanation: The disorder appearing in every generation suggests a dominant pattern. The transmission from father to son rules out X-linked inheritance, as a father passes his Y chromosome, not his X, to his son. Therefore, the pattern is most consistent with autosomal dominant inheritance.
In fruit flies, the allele for long wings (L) is dominant to the allele for vestigial wings (l). A researcher has a male fruit fly with long wings but an unknown genotype.
To determine the genotype of the long-winged male fly, the researcher should perform a test cross. This involves crossing the male fly with a female of which of the following genotypes?
Explanation: A test cross is used to determine an unknown genotype of an organism showing the dominant phenotype. This is done by crossing the organism with a homozygous recessive individual. The phenotypes of the offspring will reveal the unknown genotype.
In snapdragons, the allele for red flowers is completely dominant over the allele for white flowers. A botanist has a large population of red-flowered snapdragons.
Which of the following statements about this population is most accurate?
Explanation: Because red is dominant, the red-flowered phenotype can be produced by both the homozygous dominant (RR) and heterozygous (Rr) genotypes. Therefore, the population of red-flowered plants can contain individuals with two different genotypes while exhibiting only one phenotype.
In a plant species, three genes are unlinked. Tall (T) is dominant to short (t), purple flowers (P) are dominant to white (p), and round seeds (R) are dominant to wrinkled (r).
In a cross between two plants with genotype TtPpRr, what is the probability of an offspring being short, having purple flowers, and having wrinkled seeds?
Explanation: For each gene, the cross is between two heterozygotes. The probability of short (tt) is 1/4. The probability of purple flowers (P−) is 3/4. The probability of wrinkled seeds (rr) is 1/4. Since the genes are unlinked, the probabilities are multiplied: (1/4)×(3/4)×(1/4)=3/64.
In corn, smooth kernels (K) are completely dominant to wrinkled kernels (k). A plant with smooth kernels is self-fertilized, and the offspring show a 3 smooth : 1 wrinkled phenotype ratio. Assume one autosomal gene with complete dominance and no linkage to other genes. Which genotype must the parent plant have had?
Which parental genotype is required to produce a 3:1 smooth-to-wrinkled ratio by self-crossing?
Explanation: This question tests Mendelian inheritance analysis, requiring inference of parental genotype from self-cross offspring ratios. The 3:1 smooth-to-wrinkled ratio indicates the parent is heterozygous Kk, as self-fertilization (Kk x Kk) produces KK, Kk, and kk genotypes. Alleles segregate equally into gametes, yielding 1/4 KK, 1/2 Kk (smooth), and 1/4 kk (wrinkled), for a 3:1 phenotypic ratio. This fits because only a heterozygote can produce both dominant and recessive phenotypes in that proportion. A tempting distractor is KK or Kk, which might stem from thinking homozygotes could yield recessives, ignoring that KK self-cross gives all smooth. For inheritance questions, work backward from ratios to parental genotypes using standard Mendelian patterns like 3:1 for heterozygote self-crosses.
In snapdragons, tall stems (T) are completely dominant to short stems (t). A tall plant of unknown genotype is crossed with a short plant (tt). In the offspring, 48 plants are tall and 52 plants are short. Assume random fertilization and a single gene with complete dominance. Which genotype is most likely for the tall parent?
Explanation: This question tests the skill of analyzing Mendelian inheritance patterns. The cross between a tall snapdragon of unknown genotype and a short one (tt) yields approximately equal numbers of tall and short offspring, indicating a 1:1 phenotypic ratio. This ratio suggests the tall parent is heterozygous (Tt), as it produces gametes with 1/2 T and 1/2 t, combining with the t gametes from the short parent to give Tt and tt equally. If the tall parent were homozygous (TT), all offspring would be tall, but the observed data matches the segregation expected from a heterozygous parent. A tempting distractor is TT, which might arise from the misconception that dominant phenotypes always indicate homozygous genotypes without considering test cross results. To solve similar inheritance questions, use test crosses with recessive individuals to reveal unknown genotypes through offspring ratios.
In fruit flies, red eyes (R) are completely dominant to sepia eyes (r). Two red-eyed flies are crossed, and both are known to be heterozygous (Rr × Rr). Each fly produces gametes carrying one allele, and fertilization is random. Assume a single gene controls eye color with complete dominance. Which genotype ratio is expected among the offspring?
Explanation: This question tests the skill of analyzing Mendelian inheritance patterns. In the cross between two heterozygous red-eyed fruit flies (Rr × Rr), each parent produces gametes with 1/2 R and 1/2 r alleles due to independent segregation. The Punnett square reveals offspring genotypes of RR, Rr, and rr in a 1:2:1 ratio, directly reflecting the combination of alleles from random fertilization. This classic monohybrid ratio confirms the expected genotypic distribution. A tempting distractor is 3 RR : 1 rr, which might stem from the misconception of equating genotypic ratios with phenotypic ones by ignoring the heterozygous category. To solve similar inheritance questions, calculate genotypic ratios using Punnett squares and distinguish them from phenotypic ratios based on dominance.
In rabbits, long ears (L) are completely dominant to short ears (l). A breeder crosses a homozygous dominant rabbit (LL) with a heterozygous rabbit (Ll). Each offspring inherits one allele from each parent, and the gene shows complete dominance. Which proportion of offspring is expected to be heterozygous (Ll)?
Explanation: This question tests the skill of analyzing Mendelian inheritance patterns. In the cross between a homozygous dominant long-eared rabbit (LL) and a heterozygous one (Ll), the LL parent produces only L gametes, while the Ll parent produces 1/2 L and 1/2 l. Offspring genotypes are LL from L + L combinations and Ll from L + l, each occurring with 1/2 probability due to segregation. Therefore, half of the offspring are expected to be heterozygous (Ll), as the recessive allele appears in half the gametes from the heterozygous parent. A tempting distractor is 3/4, which could arise from the misconception of applying a monohybrid ratio without accounting for one parent's homozygosity. To solve similar inheritance questions, identify gamete probabilities for each parent and use them to compute specific genotypic proportions.
In dogs, curly fur (C) is completely dominant to straight fur (c), and black coat color (B) is completely dominant to brown coat color (b). Two dogs with genotypes CcBb are crossed (CcBb × CcBb), and the genes assort independently. Assume complete dominance for both traits. Which proportion of offspring is expected to show straight fur and brown coat?
Explanation: This question tests the skill of analyzing Mendelian inheritance patterns. In the dihybrid cross between two CcBb dogs (CcBb × CcBb), each trait follows monohybrid segregation: 1/4 cc for straight fur and 1/4 bb for brown coat. Since the genes assort independently, the combined probability for both recessive phenotypes (cc and bb) is 1/4 × 1/4 = 1/16. This reflects the random combination of alleles during fertilization for unlinked genes. A tempting distractor is 9/16, which could stem from the misconception of calculating the dominant phenotype ratio instead of the double recessive one. To solve similar inheritance questions, break down dihybrid crosses into monohybrid components and multiply probabilities for independent events.
In corn, smooth kernels (S) are completely dominant to wrinkled kernels (s). A plant with genotype SS is crossed with a plant with genotype ss. Each offspring receives one allele from each parent. Assume a single gene controls kernel texture and inheritance is Mendelian with complete dominance. Which outcome is expected among the offspring?
Explanation: This question tests the skill of analyzing Mendelian inheritance patterns. In the cross between a homozygous smooth-kernel corn plant (SS) and a homozygous wrinkled one (ss), the SS parent produces only S gametes, while the ss parent produces only s gametes. All offspring receive one S and one s allele, resulting in the heterozygous Ss genotype, which expresses the dominant smooth phenotype due to complete dominance. Thus, all offspring are expected to have smooth kernels, as no homozygous recessive combinations occur. A tempting distractor is 3/4 smooth and 1/4 wrinkled, which could come from the misconception of assuming both parents are heterozygous like in a monohybrid cross. To solve similar inheritance questions, determine gamete types from each parent's genotype and predict outcomes using principles of dominance and segregation.
In a plant, red petals (R) are completely dominant to white petals (r), and round seeds (S) are completely dominant to wrinkled seeds (s). A plant heterozygous for both traits (RrSs) is testcrossed with a plant homozygous recessive for both traits (rrss). Assume independent assortment. Which proportion of offspring is expected to have white petals and wrinkled seeds?
Explanation: This problem tests understanding of dihybrid testcrosses using Mendelian genetics with independent assortment. In RrSs × rrss, the heterozygous parent produces four gamete types (RS, Rs, rS, rs) in equal proportions (1/4 each), while the homozygous recessive parent only produces rs gametes. The offspring genotypes are RrSs, Rrss, rrSs, and rrss, each occurring at 1/4 frequency. The rrss offspring (white petals, wrinkled seeds) represent 1/4 of the total. A common mistake is multiplying 1/2 × 1/2 to get 1/4, which works here but for the wrong reason—in testcrosses, offspring ratios directly reflect the heterozygous parent's gamete ratios. For dihybrid testcrosses, each phenotype class appears at 1/4 frequency when genes assort independently.
In pea plants, purple flowers (P) are completely dominant to white (p). A plant with purple flowers that is heterozygous (Pp) is crossed with a white-flowered plant (pp). Each offspring receives one allele for flower color from each parent. Assuming Mendelian segregation and no linkage to other traits, the offspring phenotypes depend only on whether they inherit at least one P allele. Which proportion of offspring is expected to have white flowers?
Explanation: This question tests your ability to predict offspring phenotypes using Mendelian inheritance principles. In this cross between Pp (purple) × pp (white), the heterozygous parent produces two types of gametes: P and p in equal proportions, while the homozygous recessive parent only produces p gametes. The possible offspring genotypes are Pp (purple) and pp (white), each occurring with 50% probability since half the gametes from the Pp parent carry p. Therefore, one-half of offspring will have white flowers (pp genotype). A common error is thinking all offspring will be white because one parent is white, but this ignores that the Pp parent contributes a dominant P allele half the time. When solving inheritance problems, always construct a Punnett square to systematically track all possible gamete combinations.
In pea plants, purple flowers (P) are completely dominant to white flowers (p). A gardener crosses two purple-flowered plants and records that some offspring have white flowers. The gardener concludes both parents must carry the recessive allele. Assume the trait is controlled by a single gene and that fertilization is random among gametes. Which proportion of offspring from this cross is expected to have white flowers?
Explanation: This question tests your ability to analyze Mendelian inheritance patterns and predict offspring ratios from a monohybrid cross. Since both purple-flowered parents produced some white-flowered offspring (pp), each parent must carry at least one recessive p allele, making both parents heterozygous (Pp). When crossing Pp × Pp, each parent produces gametes with P or p in equal proportions, resulting in offspring genotypes of 1 PP : 2 Pp : 1 pp. Since only pp individuals have white flowers, the expected proportion is 1/4 or 25% white-flowered offspring. Students often mistakenly think that if both parents show the dominant phenotype, all offspring must also show it, forgetting that heterozygous parents can produce homozygous recessive offspring. To solve inheritance problems, always determine parental genotypes from the given information, then use a Punnett square to predict offspring ratios.
In a flowering plant, axial flowers (A) are completely dominant to terminal flowers (a). A heterozygous axial plant (Aa) is self-crossed. Assume the trait is controlled by a single gene with complete dominance. Which proportion of offspring is expected to be terminal-flowered?
Explanation: This question involves analyzing Mendelian inheritance in a self-cross of a heterozygous individual. When a heterozygous axial plant (Aa) self-crosses, it's equivalent to Aa × Aa, where each parent produces A and a gametes in equal proportions. The Punnett square yields offspring genotypes of 1 AA : 2 Aa : 1 aa, creating the classic 1:2:1 genotypic ratio. Since terminal flowers only appear in homozygous recessive (aa) individuals, and aa represents 1/4 of the offspring, the expected proportion of terminal-flowered plants is 1/4 or 25%. Students sometimes confuse self-crossing with producing offspring identical to the parent, but heterozygous self-crosses segregate into multiple genotypes following Mendel's laws. To solve self-cross problems, treat them as standard crosses between two individuals with identical genotypes.
In a laboratory strain of yeast, the ability to grow on medium X (A) is completely dominant to inability to grow (a). A yeast cell that can grow is crossed with one that cannot grow (aa). Among many offspring, approximately half can grow and half cannot. Assume one gene with complete dominance and equal segregation of alleles. Which genotype is most likely for the parent that can grow?
Which genotype of the growing parent best matches a 1:1 grow-to-no-grow offspring ratio in an A?×aa cross?
Explanation: This question tests Mendelian inheritance analysis, inferring genotype from testcross ratios in yeast. The 1:1 grow-to-no-grow ratio suggests the growing parent is heterozygous Aa, producing A and a gametes equally. Crossed with aa (only a gametes), this yields Aa (grow) and aa (no-grow) in equal proportions via allele segregation. This matches the approximate half-and-half, as expected in a testcross with a heterozygote. A tempting distractor is AA or Aa, because dominance masks genotype, but this ignores that AA would produce all growers, contradicting the data. For inheritance questions, utilize testcrosses to reveal heterozygosity by checking for recessive phenotypes in offspring.
In mice, black fur (B) is completely dominant to brown fur (b). A black mouse of unknown genotype is crossed with a brown mouse known to be homozygous recessive (bb). The litter contains 8 pups: 4 black and 4 brown. Assume a single autosomal gene with complete dominance and that the sample reflects the expected ratio. Which genotype is most likely for the black parent?
Which genotype best explains the observed 1:1 black-to-brown offspring ratio?
Explanation: This question tests Mendelian inheritance analysis, emphasizing genotype inference from testcross results. The black parent crossed with bb yields a 1:1 black-to-brown ratio, indicating the black parent is heterozygous Bb, as it contributes B or b alleles equally. During meiosis, the Bb parent segregates alleles into gametes with 1/2 B and 1/2 b, and when fertilized by b from the recessive parent, this produces Bb (black) and bb (brown) in equal proportions. This matches the observed 4:4 ratio, confirming the logic of allele segregation in a testcross. A tempting distractor is BB or Bb equally likely, stemming from the misconception that dominance prevents distinguishing homozygotes from heterozygotes without ratios, ignoring the testcross purpose. For inheritance questions, use testcross outcomes to deduce unknown genotypes by comparing observed ratios to expected Mendelian proportions.
In a species of beetle, long antennae (L) are completely dominant to short antennae (l). Two beetles with long antennae are crossed. One parent is known to be homozygous dominant (LL), while the other is heterozygous (Ll). Assume a single autosomal gene with complete dominance. Which genotype ratio is expected among the offspring?
Which offspring genotype ratio is expected from an LL×Ll cross?
Explanation: This question tests Mendelian inheritance analysis, involving genotype ratio prediction from a monohybrid cross. In an LL x Ll cross, the homozygous LL parent produces only L gametes, while the Ll parent produces L and l equally due to allele segregation. The offspring genotypes are LL (from L x L) and Ll (from L x l), each in 1/2 proportion, leading to a 1 LL : 1 Ll ratio. This occurs because the heterozygous parent's alleles separate independently, with no ll possible since one parent lacks l. A tempting distractor is 1 LL : 2 Ll : 1 ll, which confuses this with a heterozygous cross, mistakenly assuming both parents are Ll. For inheritance questions, clearly note parental genotypes and track allele contributions to avoid ratio misapplication.