Mendelian Genetics - AP Biology

1. Based on this information, what is the most likely mode of inheritance for this disorder?
   Answer: Autosomal dominant 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.
2. Which of the following outcomes is most consistent with the principles of Mendelian genetics and probability?
   Answer: The observed outcome can deviate from the expected 3:1 ratio because inheritance involves events of chance that do not always match theoretical predictions in small samples. Mendelian ratios are statistical probabilities, not fixed outcomes. Each fertilization event is independent. With a small sample size, like four offspring, random fluctuations can easily lead to observed ratios that differ from the expected 3:1 ratio. This does not invalidate Mendelian principles.
3. 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?
   Answer: One-half of offspring will have white flowers 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.
4. In dogs, the allele for curly fur (C) is completely dominant to straight fur (c). A breeder crosses two curly-furred dogs and obtains some straight-furred puppies. Assume a single autosomal gene and complete dominance. Based on the appearance of straight-furred offspring, which parental genotype combination is most likely?
   Answer: Cc and Cc This question tests Mendelian inheritance analysis, deducing parental genotypes from the presence of recessive offspring. Producing straight-furred (cc) puppies requires both curly parents to carry and contribute the c allele, so they must both be heterozygous Cc. In a Cc x Cc cross, alleles segregate to allow 1/4 cc offspring, as each parent provides c with 1/2 probability. This is consistent because homozygous CC parents cannot produce cc, confirming the need for heterozygosity. A tempting distractor is CC and Cc, stemming from the misconception that one heterozygote suffices, but that would require the other to be cc, contradicting both being curly. For inheritance questions, remember that recessive traits in offspring indicate both parents carry the recessive allele.
5. In rabbits, a dominant allele (R) produces round ears and a recessive allele (r) produces pointed ears. Two rabbits with round ears are crossed. Each parent could be RR or Rr. If one offspring is born with pointed ears (rr), the parents' genotypes can be inferred using Mendelian inheritance. Which parental genotypes are consistent with producing a pointed-eared offspring?
   Answer: Rr × Rr This question requires reverse genetics—using offspring phenotypes to deduce parental genotypes through Mendelian principles. For a pointed-eared offspring (rr) to appear, it must receive one r allele from each parent, meaning both parents must carry at least one r allele. Since both parents have round ears (dominant phenotype), they cannot be rr, so both must be Rr (heterozygous). The cross Rr × Rr produces RR (1/4), Rr (1/2), and rr (1/4), explaining how round-eared parents can have pointed-eared offspring. Students often think dominant phenotype parents cannot have recessive phenotype offspring, forgetting that heterozygotes carry hidden recessive alleles. When offspring show a recessive phenotype that neither parent displays, both parents must be heterozygous carriers.
6. The results of this cross demonstrate which of the following Mendelian principles?
   Answer: The principle of dominance. The cross is between homozygous dominant ($BB$) and homozygous recessive ($bb$) parents. The F1 generation are all heterozygous ($Bb$) and display the blue phenotype. This directly illustrates the principle of dominance, where the dominant allele ($B$) masks the expression of the recessive allele ($b$).
7. Mendel's law of segregation describes how alleles for a single gene are distributed into gametes. This law is physically explained by the behavior of chromosomes during which phase of meiosis?
   Answer: Anaphase I, when homologous chromosomes are pulled to opposite poles of the cell. A diploid organism has two alleles for each gene, located on homologous chromosomes. The law of segregation is a direct result of the separation of these homologous chromosomes during Anaphase I of meiosis, ensuring that each gamete receives only one of the two alleles.
8. 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?
   Answer: 1 RR : 2 Rr : 1 rr 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.
9. Mendel's law of segregation is a direct consequence of which process in meiosis?
   Answer: The separation of homologous chromosomes during anaphase I. The law of segregation states that the two alleles for a heritable character separate from each other during gamete formation. This occurs when homologous chromosomes, each carrying one allele for a gene, are separated into different daughter cells during anaphase I of meiosis.
10. In dogs, a dominant allele (S) produces short hair and the recessive allele (s) produces long hair. A short-haired dog of unknown genotype is crossed with a long-haired dog (ss). Among 8 puppies, 4 are short-haired and 4 are long-haired. Assume a single gene with complete dominance and that this sample reflects expected ratios. Which genotype is most consistent with the short-haired parent?
    Answer: Ss This problem uses offspring ratios to deduce parental genotypes through Mendelian analysis. The long-haired parent must be ss (homozygous recessive), and the 1:1 ratio of short-haired to long-haired puppies is the key clue. If the short-haired parent were SS, all offspring would be Ss (short-haired), but we observe 50% long-haired puppies. The only way to get a 1:1 phenotypic ratio is if the short-haired parent is Ss, producing S and s gametes equally, which combine with only s gametes from the ss parent to yield 50% Ss (short) and 50% ss (long). Students often assume a dominant phenotype means homozygous dominant genotype, but heterozygotes also express the dominant phenotype. When you see a 1:1 ratio in a cross with a homozygous recessive parent, the other parent must be heterozygous.
11. The inheritance pattern of this condition is most likely which of the following?
    Answer: Autosomal recessive 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.
12. 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 $A? \times aa$ cross?
    Answer: Aa 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.
13. Which statement best describes the relationship between a gene and its alleles in a diploid organism?
    Answer: Alleles are different versions of a specific gene that arise from mutations and are located at the same locus on homologous chromosomes. Alleles are alternative forms of a single gene. They occupy the same position (locus) on a pair of homologous chromosomes and control the same trait, though they may result in different expressions of that trait.
14. In corn, tall plants (T) are completely dominant to short plants (t). A tall plant of unknown genotype is crossed with a short plant (tt). The offspring include both tall and short individuals in approximately equal numbers. Assume a single gene controls height. Which genotype most likely describes the tall parent?
    Answer: Tt This question involves analyzing Mendelian inheritance through a testcross to determine an unknown genotype. The tall plant crossed with a short plant (tt) produces both tall and short offspring in approximately equal numbers, indicating a 1:1 phenotypic ratio. Since the short parent (tt) can only contribute t alleles, the presence of short offspring (tt) means the tall parent must have contributed t alleles to some offspring. The equal ratio suggests the tall parent produces T and t gametes in equal proportions, which only occurs with a heterozygous (Tt) genotype. If the tall parent were homozygous (TT), all offspring would be Tt and tall, contradicting the observed results. Students often assume a dominant phenotype indicates homozygous dominant genotype, but testcrosses reveal heterozygotes by producing recessive offspring. When determining unknown genotypes, use offspring ratios as evidence of parental gamete frequencies.
15. 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?
    Answer: 1 red/tall : 1 red/dwarf : 1 yellow/tall : 1 yellow/dwarf This is a dihybrid test cross ($RrTt \times rrtt$). 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.
16. 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?
    Answer: Bb 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.
17. 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?
    Answer: $3/64$ 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: $(1/4) \times (3/4) \times (1/4) = 3/64$.
18. In a beetle species, spotted wings (W) are completely dominant to unspotted wings (w). A cross between two beetles produces offspring with a 1:1 ratio of spotted to unspotted wings. Assume a single gene controls the trait and inheritance is Mendelian. Which parental genotypes most likely produced this ratio?
    Answer: Ww × ww This question tests ability to determine parental genotypes from offspring phenotypic ratios in Mendelian inheritance. The 1:1 ratio of spotted to unspotted wings indicates that half the offspring show the dominant phenotype and half show the recessive phenotype. This ratio only occurs when one parent is heterozygous (Ww) and the other is homozygous recessive (ww). The Ww parent produces W and w gametes equally, while the ww parent produces only w gametes, resulting in offspring genotypes of 1 Ww : 1 ww, which translates to the observed 1:1 phenotypic ratio. Students might think Ww × Ww could produce a 1:1 ratio, but that cross yields a 3:1 ratio instead. When identifying parental genotypes from offspring ratios, remember that a 1:1 phenotypic ratio is diagnostic of a testcross between a heterozygote and a homozygous recessive.
19. In rabbits, the allele for spotted coat (S) is completely dominant to solid coat (s). A spotted rabbit is crossed with a solid rabbit (ss). The spotted parent's genotype is unknown. The first 12 offspring include 6 spotted and 6 solid. Assume this reflects the expected Mendelian ratio for a single gene. Which is the most likely genotype of the spotted parent?
    Answer: Ss This question tests Mendelian inheritance analysis, focusing on deducing parental genotype from offspring ratios in a testcross. The 1:1 spotted-to-solid ratio suggests the spotted parent is heterozygous Ss, producing S and s gametes equally. When crossed with ss, which provides only s, the offspring are Ss (spotted) and ss (solid) in equal proportions due to random fertilization. This matches the observed 6:6, confirming segregation of alleles in the heterozygote. A tempting distractor is SS or ss, arising from the misconception that phenotypes alone determine genotypes without considering ratios, overlooking testcross utility. For inheritance questions, compare observed data to expected Mendelian ratios to infer hidden genotypes accurately.
20. 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?
    Answer: Tt 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.

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