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Biology Help: Explain Inheritance Patterns With Evidence

Review real example questions for Explain Inheritance Patterns With Evidence in Biology.

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A pedigree shows that an affected father and an unaffected mother have four children: all daughters are affected and all sons are unaffected.

Which inheritance pattern best matches this evidence?

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Question 1

A pedigree shows that an affected father and an unaffected mother have four children: all daughters are affected and all sons are unaffected.

Which inheritance pattern best matches this evidence?

Autosomal recessive, because affected fathers pass recessive alleles only to daughters
X-linked dominant, because fathers pass their X chromosome to all daughters and their Y to all sons (correct answer)
Y-linked, because only daughters inherit the Y chromosome from their father
Autosomal dominant, because autosomal traits affect only females when inherited from the father

Explanation: This question tests your ability to explain inheritance patterns using evidence from offspring ratios, Punnett squares, and pedigrees to determine whether traits follow dominant/recessive, incomplete dominance, or other inheritance patterns. Mendelian inheritance patterns can be identified from characteristic offspring ratios: DOMINANT/RECESSIVE pattern shows 3:1 phenotype ratio when two heterozygous parents cross (Aa × Aa → 1 AA : 2 Aa : 1 aa genotypes, which gives 3 dominant phenotype : 1 recessive phenotype because both AA and Aa show dominant trait while only aa shows recessive). This 3:1 ratio is evidence that one allele is dominant and one is recessive. In pedigrees, RECESSIVE traits often skip generations (two unaffected heterozygous parents Aa can have affected child aa—the recessive allele was hidden in parents but appears in child), while DOMINANT traits typically appear in every generation (can't hide—even one copy shows). INCOMPLETE DOMINANCE shows 1:2:1 phenotype ratio (matching genotype ratio) because heterozygote shows intermediate phenotype: red (RR) × white (WW) → all pink (RW), then pink × pink (RW × RW) → 1 red : 2 pink : 1 white, and the 1:2:1 ratio with intermediate phenotype is evidence for incomplete dominance rather than dominance. Recognizing which ratio or pattern appears in data allows you to determine the inheritance type! The pedigree where an affected father and unaffected mother produce all affected daughters and unaffected sons matches X-linked dominant inheritance, as the father passes his X chromosome (with the dominant allele) to daughters (who express it) and his Y to sons (who get the mother's normal X and remain unaffected), consistent with sex-linked patterns without male-to-male transmission. Choice B correctly explains the inheritance pattern by properly interpreting the pedigree evidence to identify X-linked dominant, highlighting how fathers transmit the X to daughters and Y to sons, resulting in the observed sex-specific affection. Choice C fails because it suggests Y-linked inheritance, but Y-linked traits are passed only to sons (not daughters), and daughters do not inherit the Y chromosome, misidentifying the pattern as Y-linked when the evidence shows affected females inheriting from the father. Using evidence to determine inheritance patterns: (1) RATIO EVIDENCE: Observe offspring phenotype ratios from known or unknown parent crosses. 3:1 ratio (like 75 purple : 25 white) → dominant/recessive, heterozygous parents (Pp × Pp). 1:1 ratio (like 50 purple : 50 white) → heterozygous × homozygous recessive (Pp × pp). 1:2:1 ratio (like 25 red : 50 pink : 25 white) → incomplete dominance, heterozygous parents (RW × RW). All same phenotype → depends on parents (could be homozygous × homozygous). Ratios reveal parent genotypes and dominance patterns! (2) PUNNETT SQUARE: Use to PREDICT ratios from parent genotypes, then compare to actual data. Example: Parents both Aa → Punnett square predicts 3:1 ratio. Observed: 74 dominant, 26 recessive (close to 3:1). Match confirms model! Punnett square also determines parent genotypes from offspring: if offspring show recessive trait (aa), BOTH parents must have at least one a allele (either Aa or aa). (3) PEDIGREE PATTERNS: Trait skips generation (grandparents → grandchildren but not parents) → recessive (parents are carriers Aa, don't show trait). Trait in every generation → dominant usually. Affected parent, affected children → dominant often (one affected parent sufficient if dominant). Two unaffected parents, affected child → recessive (both parents Aa carriers). The generational pattern reveals dominance! Example full analysis: Data shows 300 purple flowers, 100 white flowers from cross (3:1 ratio). INTERPRET: 3:1 suggests dominant/recessive with heterozygous parents. INFER parent genotypes: both Pp (if P dominant for purple, p recessive for white). PREDICT: Pp × Pp should give 1 PP : 2 Pp : 1 pp = 3 purple : 1 white. CHECK: predicted 3:1 matches observed 3:1 ✓. CONCLUDE: evidence supports P dominant, p recessive, parents both heterozygous Pp. This systematic analysis uses evidence to determine inheritance pattern!

Question 2

A single-gene trait is X-linked recessive. A mother who does not show the trait has an affected son. The father does not show the trait. Which statement best explains how the son could be affected?

The son must have inherited the recessive allele on the X chromosome from his mother, who is a carrier (correct answer)
The son must have inherited the recessive allele on the X chromosome from his father
The son must have two recessive X alleles, one from each parent
X-linked recessive traits cannot appear unless the mother is affected

Question 3

In rabbits, black fur (B) is dominant to white fur (b). A black rabbit is crossed with another black rabbit. Their offspring include both black and white rabbits. Which Punnett-square-based claim is supported?

Both parents are BB, and white offspring (bb) appear due to blending
At least one parent must be Bb, and the appearance of white offspring indicates both parents carry b (correct answer)
Both parents are bb, but black fur appears because dominant alleles form after fertilization
One parent must be BB and the other must be bb, producing a 3:1 ratio

Question 4

A dominant/recessive trait is studied in mice: black fur (B) is dominant to brown fur (b). A black mouse crossed with a brown mouse produces 12 offspring: 6 black and 6 brown. Which conclusion is best supported?

The black parent is $BB$ and the brown parent is $bb$
The black parent is $Bb$ and the brown parent is $bb$ (correct answer)
Both parents are $Bb$
The brown trait is dominant, so the brown parent must be $Bb$

Explanation: This question tests your ability to explain inheritance patterns using evidence from offspring ratios, Punnett squares, and pedigrees to determine whether traits follow dominant/recessive, incomplete dominance, or other inheritance patterns. Mendelian inheritance patterns can be identified from characteristic offspring ratios: DOMINANT/RECESSIVE pattern shows 3:1 phenotype ratio when two heterozygous parents cross (Aa × Aa → 1 AA : 2 Aa : 1 aa genotypes, which gives 3 dominant phenotype : 1 recessive phenotype because both AA and Aa show dominant trait while only aa shows recessive). This 3:1 ratio is evidence that one allele is dominant and one is recessive. In pedigrees, RECESSIVE traits often skip generations (two unaffected heterozygous parents Aa can have affected child aa—the recessive allele was hidden in parents but appears in child), while DOMINANT traits typically appear in every generation (can't hide—even one copy shows). INCOMPLETE DOMINANCE shows 1:2:1 phenotype ratio (matching genotype ratio) because heterozygote shows intermediate phenotype: red (RR) × white (WW) → all pink (RW), then pink × pink (RW × RW) → 1 red : 2 pink : 1 white, and the 1:2:1 ratio with intermediate phenotype is evidence for incomplete dominance rather than dominance. Recognizing which ratio or pattern appears in data allows you to determine the inheritance type! The black × brown cross produces 6 black and 6 brown offspring (1:1 ratio), which matches the Punnett square for Bb × bb predicting 50% Bb (black) and 50% bb (brown), supporting dominant/recessive with the black parent heterozygous. Choice B correctly explains the inheritance pattern by properly interpreting the 1:1 offspring ratio to identify Bb × bb genotypes. For example, choice A fails by suggesting BB × bb, but that would predict all black offspring, not the observed 1:1 ratio. Using evidence to determine inheritance patterns: (1) RATIO EVIDENCE: Observe offspring phenotype ratios from known or unknown parent crosses. 3:1 ratio (like 75 purple : 25 white) → dominant/recessive, heterozygous parents (Pp × Pp). 1:1 ratio (like 50 purple : 50 white) → heterozygous × homozygous recessive (Pp × pp). 1:2:1 ratio (like 25 red : 50 pink : 25 white) → incomplete dominance, heterozygous parents (RW × RW). All same phenotype → depends on parents (could be homozygous × homozygous). Ratios reveal parent genotypes and dominance patterns! (2) PUNNETT SQUARE: Use to PREDICT ratios from parent genotypes, then compare to actual data. Example: Parents both Aa → Punnett square predicts 3:1 ratio. Observed: 74 dominant, 26 recessive (close to 3:1). Match confirms model! Punnett square also determines parent genotypes from offspring: if offspring show recessive trait (aa), BOTH parents must have at least one a allele (either Aa or aa). (3) PEDIGREE PATTERNS: Trait skips generation (grandparents → grandchildren but not parents) → recessive (parents are carriers Aa, don't show trait). Trait in every generation → dominant usually. Affected parent, affected children → dominant often (one affected parent sufficient if dominant). Two unaffected parents, affected child → recessive (both parents Aa carriers). The generational pattern reveals dominance! Example full analysis: Data shows 300 purple flowers, 100 white flowers from cross (3:1 ratio). INTERPRET: 3:1 suggests dominant/recessive with heterozygous parents. INFER parent genotypes: both Pp (if P dominant for purple, p recessive for white). PREDICT: Pp × Pp should give 1 PP : 2 Pp : 1 pp = 3 purple : 1 white. CHECK: predicted 3:1 matches observed 3:1 ✓. CONCLUDE: evidence supports P dominant, p recessive, parents both heterozygous Pp. This systematic analysis uses evidence to determine inheritance pattern!

Question 5

In pea plants, tall stems (T) are dominant to short stems (t). Two tall plants are crossed and produce 80 offspring: 61 tall and 19 short (about a 3:1 ratio). Which inheritance pattern and parent genotypes best explain the data?

Incomplete dominance; parents are $TT \times tt$ producing all intermediate-height offspring
Dominant/recessive; parents are $Tt \times Tt$ , producing about 3 tall : 1 short (correct answer)
Recessive trait is tall; parents are $tt \times tt$ producing mostly tall offspring
Codominance; parents are $TT \times tt$ producing 1 tall : 1 short

Explanation: This question tests your ability to explain inheritance patterns using evidence from offspring ratios, Punnett squares, and pedigrees to determine whether traits follow dominant/recessive, incomplete dominance, or other inheritance patterns. Mendelian inheritance patterns can be identified from characteristic offspring ratios: DOMINANT/RECESSIVE pattern shows 3:1 phenotype ratio when two heterozygous parents cross (Aa × Aa → 1 AA : 2 Aa : 1 aa genotypes, which gives 3 dominant phenotype : 1 recessive phenotype because both AA and Aa show dominant trait while only aa shows recessive). This 3:1 ratio is evidence that one allele is dominant and one is recessive. In pedigrees, RECESSIVE traits often skip generations (two unaffected heterozygous parents Aa can have affected child aa—the recessive allele was hidden in parents but appears in child), while DOMINANT traits typically appear in every generation (can't hide—even one copy shows). INCOMPLETE DOMINANCE shows 1:2:1 phenotype ratio (matching genotype ratio) because heterozygote shows intermediate phenotype: red (RR) × white (WW) → all pink (RW), then pink × pink (RW × RW) → 1 red : 2 pink : 1 white, and the 1:2:1 ratio with intermediate phenotype is evidence for incomplete dominance rather than dominance. Recognizing which ratio or pattern appears in data allows you to determine the inheritance type! The observed 61 tall and 19 short offspring approximate a 3:1 ratio, which matches the prediction for a dominant/recessive trait where tall (T) is dominant and both parents are heterozygous tall (Tt), as a Punnett square for Tt × Tt would predict 3 tall : 1 short phenotypes. Choice B correctly explains the inheritance pattern by properly interpreting offspring ratios as about 3:1 to identify dominant/recessive inheritance with Tt × Tt parents. For instance, choice A fails because it misidentifies the pattern as incomplete dominance, which would show a 1:2:1 ratio with intermediate heights, not the observed 3:1 without intermediates. Using evidence to determine inheritance patterns: (1) RATIO EVIDENCE: Observe offspring phenotype ratios from known or unknown parent crosses. 3:1 ratio (like 75 purple : 25 white) → dominant/recessive, heterozygous parents (Pp × Pp). 1:1 ratio (like 50 purple : 50 white) → heterozygous × homozygous recessive (Pp × pp). 1:2:1 ratio (like 25 red : 50 pink : 25 white) → incomplete dominance, heterozygous parents (RW × RW). All same phenotype → depends on parents (could be homozygous × homozygous). Ratios reveal parent genotypes and dominance patterns! (2) PUNNETT SQUARE: Use to PREDICT ratios from parent genotypes, then compare to actual data. Example: Parents both Aa → Punnett square predicts 3:1 ratio. Observed: 74 dominant, 26 recessive (close to 3:1). Match confirms model! Punnett square also determines parent genotypes from offspring: if offspring show recessive trait (aa), BOTH parents must have at least one a allele (either Aa or aa). (3) PEDIGREE PATTERNS: Trait skips generation (grandparents → grandchildren but not parents) → recessive (parents are carriers Aa, don't show trait). Trait in every generation → dominant usually. Affected parent, affected children → dominant often (one affected parent sufficient if dominant). Two unaffected parents, affected child → recessive (both parents Aa carriers). The generational pattern reveals dominance! Example full analysis: Data shows 300 purple flowers, 100 white flowers from cross (3:1 ratio). INTERPRET: 3:1 suggests dominant/recessive with heterozygous parents. INFER parent genotypes: both Pp (if P dominant for purple, p recessive for white). PREDICT: Pp × Pp should give 1 PP : 2 Pp : 1 pp = 3 purple : 1 white. CHECK: predicted 3:1 matches observed 3:1 ✓. CONCLUDE: evidence supports P dominant, p recessive, parents both heterozygous Pp. This systematic analysis uses evidence to determine inheritance pattern!

Question 6

A single gene controls earlobe type in a class example: free earlobes (E) are dominant to attached earlobes (e). Two students with free earlobes have a child with attached earlobes. Which statement is best supported by this evidence?

At least one parent must be $EE$ because dominant traits require two dominant alleles
Both parents must be carriers ( $Ee$ ), and the child is $ee$ (correct answer)
The attached-earlobe trait must be dominant because it appeared in the child
The child must be $Ee$ because recessive traits cannot appear if parents show the dominant trait

Explanation: This question tests your ability to explain inheritance patterns using evidence from offspring ratios, Punnett squares, and pedigrees to determine whether traits follow dominant/recessive, incomplete dominance, or other inheritance patterns. Mendelian inheritance patterns can be identified from characteristic offspring ratios: DOMINANT/RECESSIVE pattern shows 3:1 phenotype ratio when two heterozygous parents cross (Aa × Aa → 1 AA : 2 Aa : 1 aa genotypes, which gives 3 dominant phenotype : 1 recessive phenotype because both AA and Aa show dominant trait while only aa shows recessive). This 3:1 ratio is evidence that one allele is dominant and one is recessive. In pedigrees, RECESSIVE traits often skip generations (two unaffected heterozygous parents Aa can have affected child aa—the recessive allele was hidden in parents but appears in child), while DOMINANT traits typically appear in every generation (can't hide—even one copy shows). INCOMPLETE DOMINANCE shows 1:2:1 phenotype ratio (matching genotype ratio) because heterozygote shows intermediate phenotype: red (RR) × white (WW) → all pink (RW), then pink × pink (RW × RW) → 1 red : 2 pink : 1 white, and the 1:2:1 ratio with intermediate phenotype is evidence for incomplete dominance rather than dominance. Recognizing which ratio or pattern appears in data allows you to determine the inheritance type! In this case, two parents with the dominant free earlobe phenotype (E) produce a child with the recessive attached phenotype (e), which indicates the trait is recessive and both parents must be heterozygous carriers (Ee) to pass on two e alleles to the child (ee), as shown in a Punnett square for Ee × Ee predicting 25% ee. Choice B correctly explains the inheritance pattern by properly interpreting the pedigree evidence of unaffected parents producing an affected child to identify recessive inheritance with carrier parents. For example, choice C fails by misinterpreting the attached trait as dominant, but a recessive trait can appear in the child if hidden in carrier parents, not requiring dominance. Using evidence to determine inheritance patterns: (1) RATIO EVIDENCE: Observe offspring phenotype ratios from known or unknown parent crosses. 3:1 ratio (like 75 purple : 25 white) → dominant/recessive, heterozygous parents (Pp × Pp). 1:1 ratio (like 50 purple : 50 white) → heterozygous × homozygous recessive (Pp × pp). 1:2:1 ratio (like 25 red : 50 pink : 25 white) → incomplete dominance, heterozygous parents (RW × RW). All same phenotype → depends on parents (could be homozygous × homozygous). Ratios reveal parent genotypes and dominance patterns! (2) PUNNETT SQUARE: Use to PREDICT ratios from parent genotypes, then compare to actual data. Example: Parents both Aa → Punnett square predicts 3:1 ratio. Observed: 74 dominant, 26 recessive (close to 3:1). Match confirms model! Punnett square also determines parent genotypes from offspring: if offspring show recessive trait (aa), BOTH parents must have at least one a allele (either Aa or aa). (3) PEDIGREE PATTERNS: Trait skips generation (grandparents → grandchildren but not parents) → recessive (parents are carriers Aa, don't show trait). Trait in every generation → dominant usually. Affected parent, affected children → dominant often (one affected parent sufficient if dominant). Two unaffected parents, affected child → recessive (both parents Aa carriers). The generational pattern reveals dominance! Example full analysis: Data shows 300 purple flowers, 100 white flowers from cross (3:1 ratio). INTERPRET: 3:1 suggests dominant/recessive with heterozygous parents. INFER parent genotypes: both Pp (if P dominant for purple, p recessive for white). PREDICT: Pp × Pp should give 1 PP : 2 Pp : 1 pp = 3 purple : 1 white. CHECK: predicted 3:1 matches observed 3:1 ✓. CONCLUDE: evidence supports P dominant, p recessive, parents both heterozygous Pp. This systematic analysis uses evidence to determine inheritance pattern!

Question 7

In snapdragons, flower color shows incomplete dominance: red is $RR$ , white is $WW$ , and pink is $RW$ . Two pink plants are crossed and produce 96 offspring: 23 red, 49 pink, and 24 white. Which conclusion is best supported?

This supports incomplete dominance because the phenotype ratio is about $1$ red : $2$ pink : $1$ white (correct answer)
This supports simple dominant/recessive inheritance because the phenotype ratio is about $3:1$
This supports codominance because pink flowers show both red and white patches at the same time
This supports sex-linked inheritance because the offspring are split into three groups

Explanation: This question tests your ability to explain inheritance patterns using evidence from offspring ratios, Punnett squares, and pedigrees to determine whether traits follow dominant/recessive, incomplete dominance, or other inheritance patterns. Mendelian inheritance patterns can be identified from characteristic offspring ratios: DOMINANT/RECESSIVE pattern shows 3:1 phenotype ratio when two heterozygous parents cross (Aa × Aa → 1 AA : 2 Aa : 1 aa genotypes, which gives 3 dominant phenotype : 1 recessive phenotype because both AA and Aa show dominant trait while only aa shows recessive). This 3:1 ratio is evidence that one allele is dominant and one is recessive. In pedigrees, RECESSIVE traits often skip generations (two unaffected heterozygous parents Aa can have affected child aa—the recessive allele was hidden in parents but appears in child), while DOMINANT traits typically appear in every generation (can't hide—even one copy shows). INCOMPLETE DOMINANCE shows 1:2:1 phenotype ratio (matching genotype ratio) because heterozygote shows intermediate phenotype: red (RR) × white (WW) → all pink (RW), then pink × pink (RW × RW) → 1 red : 2 pink : 1 white, and the 1:2:1 ratio with intermediate phenotype is evidence for incomplete dominance rather than dominance. Recognizing which ratio or pattern appears in data allows you to determine the inheritance type! The observed 23 red, 49 pink, and 24 white offspring from two pink (RW) parents approximate a 1:2:1 phenotype ratio, which matches the Punnett square prediction for RW × RW in incomplete dominance, where pink is the intermediate heterozygote. Choice A correctly explains the inheritance pattern by properly interpreting the offspring ratios as about 1:2:1 to identify incomplete dominance. For example, choice B fails by misidentifying the ratio as 3:1 for dominant/recessive, but the three distinct phenotypes and 1:2:1 ratio do not fit that pattern. Using evidence to determine inheritance patterns: (1) RATIO EVIDENCE: Observe offspring phenotype ratios from known or unknown parent crosses. 3:1 ratio (like 75 purple : 25 white) → dominant/recessive, heterozygous parents (Pp × Pp). 1:1 ratio (like 50 purple : 50 white) → heterozygous × homozygous recessive (Pp × pp). 1:2:1 ratio (like 25 red : 50 pink : 25 white) → incomplete dominance, heterozygous parents (RW × RW). All same phenotype → depends on parents (could be homozygous × homozygous). Ratios reveal parent genotypes and dominance patterns! (2) PUNNETT SQUARE: Use to PREDICT ratios from parent genotypes, then compare to actual data. Example: Parents both Aa → Punnett square predicts 3:1 ratio. Observed: 74 dominant, 26 recessive (close to 3:1). Match confirms model! Punnett square also determines parent genotypes from offspring: if offspring show recessive trait (aa), BOTH parents must have at least one a allele (either Aa or aa). (3) PEDIGREE PATTERNS: Trait skips generation (grandparents → grandchildren but not parents) → recessive (parents are carriers Aa, don't show trait). Trait in every generation → dominant usually. Affected parent, affected children → dominant often (one affected parent sufficient if dominant). Two unaffected parents, affected child → recessive (both parents Aa carriers). The generational pattern reveals dominance! Example full analysis: Data shows 300 purple flowers, 100 white flowers from cross (3:1 ratio). INTERPRET: 3:1 suggests dominant/recessive with heterozygous parents. INFER parent genotypes: both Pp (if P dominant for purple, p recessive for white). PREDICT: Pp × Pp should give 1 PP : 2 Pp : 1 pp = 3 purple : 1 white. CHECK: predicted 3:1 matches observed 3:1 ✓. CONCLUDE: evidence supports P dominant, p recessive, parents both heterozygous Pp. This systematic analysis uses evidence to determine inheritance pattern!

Question 8

A plant with purple flowers is crossed with a plant with white flowers. All 30 offspring have purple flowers.

Assuming a single gene with complete dominance, what is the most likely genotype of the purple-flowered parent?

$Pp$ , because crossing with $pp$ always gives all purple offspring
$PP$ , because crossing with $pp$ gives all $Pp$ (all purple) offspring (correct answer)
$pp$ , because recessive parents can sometimes produce dominant offspring
$PP$ or $Pp$ are equally likely because the offspring ratio must be $3:1$

Explanation: This question tests your ability to explain inheritance patterns using evidence from offspring ratios, Punnett squares, and pedigrees to determine whether traits follow dominant/recessive, incomplete dominance, or other inheritance patterns. Mendelian inheritance patterns can be identified from characteristic offspring ratios: DOMINANT/RECESSIVE pattern shows 3:1 phenotype ratio when two heterozygous parents cross (Aa × Aa → 1 AA : 2 Aa : 1 aa genotypes, which gives 3 dominant phenotype : 1 recessive phenotype because both AA and Aa show dominant trait while only aa shows recessive). This 3:1 ratio is evidence that one allele is dominant and one is recessive. In pedigrees, RECESSIVE traits often skip generations (two unaffected heterozygous parents Aa can have affected child aa—the recessive allele was hidden in parents but appears in child), while DOMINANT traits typically appear in every generation (can't hide—even one copy shows). INCOMPLETE DOMINANCE shows 1:2:1 phenotype ratio (matching genotype ratio) because heterozygote shows intermediate phenotype: red (RR) × white (WW) → all pink (RW), then pink × pink (RW × RW) → 1 red : 2 pink : 1 white, and the 1:2:1 ratio with intermediate phenotype is evidence for incomplete dominance rather than dominance. Recognizing which ratio or pattern appears in data allows you to determine the inheritance type! The cross of a purple-flowered plant with a white-flowered one producing all 30 purple offspring indicates the purple parent is likely homozygous dominant (PP), as a Punnett square for PP × pp predicts all Pp (purple) offspring, whereas Pp × pp would yield a 1:1 purple:white ratio, which doesn't match the uniform results. Choice B correctly explains the inheritance pattern by properly interpreting the offspring ratio evidence to identify the purple parent as PP, since the all-purple outcome from crossing with pp confirms homozygosity for the dominant allele. Choice A fails because it suggests the purple parent is Pp, but that would produce half white offspring in a test cross with pp, not the observed 100% purple, misinterpreting the ratio as supportive of heterozygosity rather than homozygosity. Using evidence to determine inheritance patterns: (1) RATIO EVIDENCE: Observe offspring phenotype ratios from known or unknown parent crosses. 3:1 ratio (like 75 purple : 25 white) → dominant/recessive, heterozygous parents (Pp × Pp). 1:1 ratio (like 50 purple : 50 white) → heterozygous × homozygous recessive (Pp × pp). 1:2:1 ratio (like 25 red : 50 pink : 25 white) → incomplete dominance, heterozygous parents (RW × RW). All same phenotype → depends on parents (could be homozygous × homozygous). Ratios reveal parent genotypes and dominance patterns! (2) PUNNETT SQUARE: Use to PREDICT ratios from parent genotypes, then compare to actual data. Example: Parents both Aa → Punnett square predicts 3:1 ratio. Observed: 74 dominant, 26 recessive (close to 3:1). Match confirms model! Punnett square also determines parent genotypes from offspring: if offspring show recessive trait (aa), BOTH parents must have at least one a allele (either Aa or aa). (3) PEDIGREE PATTERNS: Trait skips generation (grandparents → grandchildren but not parents) → recessive (parents are carriers Aa, don't show trait). Trait in every generation → dominant usually. Affected parent, affected children → dominant often (one affected parent sufficient if dominant). Two unaffected parents, affected child → recessive (both parents Aa carriers). The generational pattern reveals dominance! Example full analysis: Data shows 300 purple flowers, 100 white flowers from cross (3:1 ratio). INTERPRET: 3:1 suggests dominant/recessive with heterozygous parents. INFER parent genotypes: both Pp (if P dominant for purple, p recessive for white). PREDICT: Pp × Pp should give 1 PP : 2 Pp : 1 pp = 3 purple : 1 white. CHECK: predicted 3:1 matches observed 3:1 ✓. CONCLUDE: evidence supports P dominant, p recessive, parents both heterozygous Pp. This systematic analysis uses evidence to determine inheritance pattern!

Question 9

In a family, a recessive trait (a) causes a certain phenotype only in genotype $aa$ . Two parents do not show the trait, but they have two children: one shows the trait and one does not. Which set of genotypes is most consistent with this information?

Parents: $AA$ and $AA$ ; children: $AA$ and $AA$
Parents: $Aa$ and $Aa$ ; children: $aa$ and $Aa$ (correct answer)
Parents: $aa$ and $aa$ ; children: $aa$ and $aa$
Parents: $AA$ and $aa$ ; children: $Aa$ and $Aa$

Explanation: This question tests your ability to explain inheritance patterns using evidence from offspring ratios, Punnett squares, and pedigrees to determine whether traits follow dominant/recessive, incomplete dominance, or other inheritance patterns. Mendelian inheritance patterns can be identified from characteristic offspring ratios: DOMINANT/RECESSIVE pattern shows 3:1 phenotype ratio when two heterozygous parents cross (Aa × Aa → 1 AA : 2 Aa : 1 aa genotypes, which gives 3 dominant phenotype : 1 recessive phenotype because both AA and Aa show dominant trait while only aa shows recessive). This 3:1 ratio is evidence that one allele is dominant and one is recessive. In pedigrees, RECESSIVE traits often skip generations (two unaffected heterozygous parents Aa can have affected child aa—the recessive allele was hidden in parents but appears in child), while DOMINANT traits typically appear in every generation (can't hide—even one copy shows). INCOMPLETE DOMINANCE shows 1:2:1 phenotype ratio (matching genotype ratio) because heterozygote shows intermediate phenotype: red (RR) × white (WW) → all pink (RW), then pink × pink (RW × RW) → 1 red : 2 pink : 1 white, and the 1:2:1 ratio with intermediate phenotype is evidence for incomplete dominance rather than dominance. Recognizing which ratio or pattern appears in data allows you to determine the inheritance type! In this family, unaffected parents produce one affected child (aa) and one unaffected, which fits a Punnett square for Aa × Aa predicting 25% aa and 75% A_ (unaffected), consistent with recessive inheritance. Choice B correctly explains the inheritance pattern by properly interpreting the pedigree evidence to identify Aa × Aa parents with aa and Aa children. For example, choice A fails by suggesting AA × AA, but that couldn't produce an aa child showing the recessive trait. Using evidence to determine inheritance patterns: (1) RATIO EVIDENCE: Observe offspring phenotype ratios from known or unknown parent crosses. 3:1 ratio (like 75 purple : 25 white) → dominant/recessive, heterozygous parents (Pp × Pp). 1:1 ratio (like 50 purple : 50 white) → heterozygous × homozygous recessive (Pp × pp). 1:2:1 ratio (like 25 red : 50 pink : 25 white) → incomplete dominance, heterozygous parents (RW × RW). All same phenotype → depends on parents (could be homozygous × homozygous). Ratios reveal parent genotypes and dominance patterns! (2) PUNNETT SQUARE: Use to PREDICT ratios from parent genotypes, then compare to actual data. Example: Parents both Aa → Punnett square predicts 3:1 ratio. Observed: 74 dominant, 26 recessive (close to 3:1). Match confirms model! Punnett square also determines parent genotypes from offspring: if offspring show recessive trait (aa), BOTH parents must have at least one a allele (either Aa or aa). (3) PEDIGREE PATTERNS: Trait skips generation (grandparents → grandchildren but not parents) → recessive (parents are carriers Aa, don't show trait). Trait in every generation → dominant usually. Affected parent, affected children → dominant often (one affected parent sufficient if dominant). Two unaffected parents, affected child → recessive (both parents Aa carriers). The generational pattern reveals dominance! Example full analysis: Data shows 300 purple flowers, 100 white flowers from cross (3:1 ratio). INTERPRET: 3:1 suggests dominant/recessive with heterozygous parents. INFER parent genotypes: both Pp (if P dominant for purple, p recessive for white). PREDICT: Pp × Pp should give 1 PP : 2 Pp : 1 pp = 3 purple : 1 white. CHECK: predicted 3:1 matches observed 3:1 ✓. CONCLUDE: evidence supports P dominant, p recessive, parents both heterozygous Pp. This systematic analysis uses evidence to determine inheritance pattern!

Question 10

A class investigates a trait where allele $A$ is dominant to allele $a$ . Two heterozygous parents have 100 offspring. Which observed outcome would best support the prediction from a Punnett square for $Aa \times Aa$ ?

About 50 show the dominant phenotype and 50 show the recessive phenotype
About 75 show the dominant phenotype and 25 show the recessive phenotype (correct answer)
About 25 show the dominant phenotype and 75 show the recessive phenotype
All 100 show an intermediate phenotype between the two parents

Explanation: This question tests your ability to explain inheritance patterns using evidence from offspring ratios, Punnett squares, and pedigrees to determine whether traits follow dominant/recessive, incomplete dominance, or other inheritance patterns. Mendelian inheritance patterns can be identified from characteristic offspring ratios: DOMINANT/RECESSIVE pattern shows 3:1 phenotype ratio when two heterozygous parents cross (Aa × Aa → 1 AA : 2 Aa : 1 aa genotypes, which gives 3 dominant phenotype : 1 recessive phenotype because both AA and Aa show dominant trait while only aa shows recessive). This 3:1 ratio is evidence that one allele is dominant and one is recessive. In pedigrees, RECESSIVE traits often skip generations (two unaffected heterozygous parents Aa can have affected child aa—the recessive allele was hidden in parents but appears in child), while DOMINANT traits typically appear in every generation (can't hide—even one copy shows). INCOMPLETE DOMINANCE shows 1:2:1 phenotype ratio (matching genotype ratio) because heterozygote shows intermediate phenotype: red (RR) × white (WW) → all pink (RW), then pink × pink (RW × RW) → 1 red : 2 pink : 1 white, and the 1:2:1 ratio with intermediate phenotype is evidence for incomplete dominance rather than dominance. Recognizing which ratio or pattern appears in data allows you to determine the inheritance type! For two heterozygous parents (Aa × Aa), the Punnett square predicts a 3:1 phenotype ratio, so observing about 75 dominant and 25 recessive in 100 offspring would best match this prediction for dominant/recessive inheritance. Choice B correctly explains the inheritance pattern by properly interpreting the Punnett square predictions to identify the 3:1 ratio as supporting evidence. For example, choice A fails by suggesting a 1:1 ratio, which would better fit Aa × aa, not Aa × Aa. Using evidence to determine inheritance patterns: (1) RATIO EVIDENCE: Observe offspring phenotype ratios from known or unknown parent crosses. 3:1 ratio (like 75 purple : 25 white) → dominant/recessive, heterozygous parents (Pp × Pp). 1:1 ratio (like 50 purple : 50 white) → heterozygous × homozygous recessive (Pp × pp). 1:2:1 ratio (like 25 red : 50 pink : 25 white) → incomplete dominance, heterozygous parents (RW × RW). All same phenotype → depends on parents (could be homozygous × homozygous). Ratios reveal parent genotypes and dominance patterns! (2) PUNNETT SQUARE: Use to PREDICT ratios from parent genotypes, then compare to actual data. Example: Parents both Aa → Punnett square predicts 3:1 ratio. Observed: 74 dominant, 26 recessive (close to 3:1). Match confirms model! Punnett square also determines parent genotypes from offspring: if offspring show recessive trait (aa), BOTH parents must have at least one a allele (either Aa or aa). (3) PEDIGREE PATTERNS: Trait skips generation (grandparents → grandchildren but not parents) → recessive (parents are carriers Aa, don't show trait). Trait in every generation → dominant usually. Affected parent, affected children → dominant often (one affected parent sufficient if dominant). Two unaffected parents, affected child → recessive (both parents Aa carriers). The generational pattern reveals dominance! Example full analysis: Data shows 300 purple flowers, 100 white flowers from cross (3:1 ratio). INTERPRET: 3:1 suggests dominant/recessive with heterozygous parents. INFER parent genotypes: both Pp (if P dominant for purple, p recessive for white). PREDICT: Pp × Pp should give 1 PP : 2 Pp : 1 pp = 3 purple : 1 white. CHECK: predicted 3:1 matches observed 3:1 ✓. CONCLUDE: evidence supports P dominant, p recessive, parents both heterozygous Pp. This systematic analysis uses evidence to determine inheritance pattern!