Scientists use observable evidence to direct their questions about phenomena. For which question would the karyotype provide the most evidence?

Transcription

1 1. A karyotype shows the visual appearance of an individual s chromosomes. The karyotype below shows the chromosomes of a person with a genetic disorder. Scientists use observable evidence to direct their questions about phenomena. For which question would the karyotype provide the most evidence? (A) Since the person is missing one sex chromosome, did the two homologous Y chromosomes fail to separate as the male gamete was undergoing meiosis before fertilization? This answer suggests the student may understand that nondisjunction of chromosomes can produce a monosomy (2n-1), as evidenced by the karyotype, but does not understand that this could not occur as described because the Y chromosome does not form a homologous pair with another Y chromosome (as it pairs with an X chromosome in males). (B) Since the person is missing one sex chromosome, did nondisjunction occur during meiosis when forming gametes before fertilization? Rationale: This answer suggests the student understands that nondisjunction of chromosomes during meiosis can produce a monosomy (2n-1) because a gamete with a missing chromosome (n- 1) can be produced, and when this gamete combines with a normal gamete (n), it produces a monosomy (2n-1), as evidenced by the karyotype. Page 1 of 8

2 (C) Since the person is missing one sex chromosome, possibly the Y chromosome provided by a sperm cell, did an unfertilized egg undergo DNA replication to change from a haploid cell to a diploid cell? This answer suggests the student may understand that if a gamete replicates its DNA, it will become a diploid cell, but does not understand that if this did occur in an egg cell, the offspring would have two X chromosomes and the resulting ploidy would produce a 2n+1 karyotype after fertilization because the affected gamete would pair with a haploid gamete (2n+n=3n), instead of the 2n-1 karyotype shown. (D) Since the person is missing one sex chromosome, did the X and Y chromosomes cross over during meiosis I to produce a nonfunctional chromosome? This answer suggests the student may understand that crossing over causes genetic reshuffling and occurs in meiosis I, but does not understand that crossing over only occurs between homologous chromosomes (not sex chromosomes) and does not create mutations like translocations, duplications, or deletions because it does not change the ultimate number of chromosomes, which is needed to result in a genetic disorder involving ploidy. Aligned to: LO 3.14 CA 3.14: Apply Math to Genetics 2. A karyotype shows the visual appearance of an individual s chromosomes. The karyotype below shows a set of chromosomes found in a fertilized human egg. Page 2 of 8

3 Which scientific question would be the most logical to ask if a scientist wanted to investigate the cause of a genetic condition shown in the karyotype? (A) Is this condition caused by a sex cell that completed meiosis I but not meiosis II before fertilization? Rationale: This answer suggests the student understands that without meiosis II (or meiosis I) sex cells will be diploid, not haploid, and that after fertilization, a triploid individual will be produced because the error in the conservation of chromosome number that occurred during meiosis II will lead to the genetic disorder (triploidy) shown in the karyotype (2n+n=3n). (B) Is this condition caused by nondisjunction in a pair of homologous chromosomes in one of the sex cells before fertilization? This answer suggests the student may understand that nondisjunction in homologous chromosomes can cause extra chromosomes in sex cells and offspring, but does not understand that this would not produce a triploidy condition because there would only be one additional chromosome (trisomy), not an entire extra set of chromosomes (triploidy), as shown in the karyotype. (C) Is this condition caused by a fertilized egg that underwent DNA replication, but not mitosis or cell division, after fertilization? This answer suggests the student may understand that DNA replication without mitosis or cell division in a cell would produce a polyploidy condition, but does not understand that this would produce a tetraploidy condition, not the triploidy condition shown in the karyotype, because when a diploid cell divides there are four sets of chromosomes (2n 2=4n). (D) Is this condition caused by the fertilization of a sex cell by another sex cell that had not undergone DNA replication before meiosis? Page 3 of 8

4 This answer suggests the student may understand that if DNA replication does not occur before meiosis, sex cells will be produced with an incorrect number of chromosomes, but does not understand that this would not result in a triploidy offspring because the sex cell would be ½n instead of n, which would result in an offspring that is 1½n, not the 3n offspring shown in the karyotype. Aligned to: LO 3.14 CA 3.14: Apply Math to Genetics 3. Huntington s disease is an autosomal dominant disorder. Which pedigree could represent a family with a history of Huntington s disease? (A) This answer suggests the student may understand that dominant traits are expressed if an individual has at least one dominant allele (right side of pedigree), but does not understand that dominant traits cannot skip a generation, because a dominant allele will always be expressed, unlike a recessive allele, which can be masked by a dominant allele and can reappear in the next generation if two recessive alleles are passed down (left side of pedigree). The student may not understand how to apply a Punnett square to determine the mode of inheritance observed in the pedigree. (B) Page 4 of 8

5 This answer suggests the student may understand that dominant traits appear about 50% of the time in successive generations, but does not understand that dominant traits cannot skip a generation, because a dominant allele will always be expressed, unlike a recessive allele, which can be masked by a dominant allele and can reappear in the next generation if two recessive alleles are passed down (left side of pedigree). The student may not understand how to apply a Punnett square to determine the mode of inheritance observed in the pedigree. (C) This answer suggests the student may understand that dominant traits appear in every successive generation and do not have to be passed to future generations (right side of pedigree), but does not understand that dominant traits cannot skip a generation, because a dominant allele will always be expressed, unlike a recessive allele, which can be masked by a dominant allele and can reappear in the next generation if two recessive alleles are passed down (left side of pedigree). The student may not understand how to apply a Punnett square to determine the mode of inheritance observed in the pedigree. (D) Rationale: This answer suggests the student understands that a dominant trait can be passed to an offspring if at least one parent expresses the dominant phenotype, but that the dominant trait cannot appear in an offspring if both parents do not express the dominant phenotype (and therefore do not have the dominant allele), as seen in the last generation in the pedigree. Aligned to: LO 3.14 CA 3.14: Apply Math to Genetics Page 5 of 8

6 4. A scientist is studying the pedigrees of four families in order to identify a family that may have a sex-linked, recessive genetic disorder. Which statement provides the best justification for identifying a family as having a sex-linked recessive genetic disorder based on its pedigree? (A) Pedigree 1 shows a recessive, sex-linked disorder, because a mother passes the trait down to her sons in generation II, and they pass the trait on to their sons in generation III. This answer suggests the student may understand that sex-linked, recessive traits are generally passed from mother to son and are mostly observed in males, but does not understand that these traits cannot be passed from father to son because the son does not inherit an X chromosome from his father; thus the student may mistakenly believe that the son could inherit an X or Y chromosome from either parent. The student may not understand how to apply a Punnett square to determine the mode of inheritance of a trait that shows the pattern of an affected father (X Y) not passing the trait to his son (and an affected daughter not being produced unless a father is also affected), which is only seen in pedigree 3. Page 6 of 8

7 The prime symbol ( ) denotes a chromosome carrying the mutant allele. (B) Pedigree 2 shows a recessive, sex-linked disorder, because the trait affects mostly males, skips generations, and is carried, but not expressed, by females. This answer suggests the student may understand that sex-linked, recessive traits are mostly expressed in males, but does not understand that these traits cannot be observed in a daughter with a father who does not express the condition, as seen in the affected daughter in generation III, because the daughter always inherits the X chromosome from her father and the father must have the condition in order to pass it on to his daughter. The student may not understand how to apply a Punnett square to determine the mode of inheritance of a trait that shows the pattern of an affected father (X Y) not passing the trait to his son and an affected daughter (X X ) not being produced unless a father is also affected, which is only seen in pedigree 3. The prime symbol ( ) denotes a chromosome carrying the mutant allele. (C) Pedigree 3 shows a recessive, sex-linked disorder, because the trait affects mostly males and, since it is on the X chromosome, it is never passed from father to son. Rationale: This answer suggests the student understands that sex-linked, recessive traits are passed on the X chromosome, so females often carry, but do not normally express, the phenotype (unless both X chromosomes are affected), and males inherit the condition from their mother, not their father. (D) Pedigree 4 shows a recessive, sex-linked disorder, because the trait affects mostly males, skips generations, and is passed from mother to son. Page 7 of 8

8 This answer suggests the student may understand that sex-linked, recessive traits are mostly expressed in males, but does not understand that these traits cannot be observed in a daughter with a father who does not express the condition because the daughter always inherits the X chromosome from her father and the father must have the condition in order to pass it on to his daughter. The student may not understand how to apply a Punnett square to determine the mode of inheritance of a trait that shows the pattern of an affected father (X Y) not passing the trait to his son and an affected daughter (X X ) not being produced unless a father is also affected, which is only seen in pedigree 3. The prime symbol ( ) denotes a chromosome carrying the mutant allele. Aligned to: LO 3.14 CA 3.14: Apply Math to Genetics Page 8 of 8