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IB Biology HL D2.1.10 Down Syndrome Questions

Meiotic non-disjunction occurs when homologous chromosomes or sister chromatids fail to separate. If chromosome 21 is affected, fertilization can result in trisomy 21 and Down syndrome.

Syllabus
First assessment 2025
Course
Biology HL
Level
HL

Exam points

  • Identify meiosis I non-disjunction as failure of homologous chromosomes to separate, or meiosis II non-disjunction as failure of sister chromatids to separate.
  • Explain how a gamete with an extra chromosome 21 can lead to trisomy 21 and Down syndrome after fertilization.

IB Biology HL D2.1.10 Down Syndrome Questions question 1

[Maximum number: 1]

In a strain of soybeans, high oil content (H) in seeds is dominant to low oil content (h) and four seeds in a pod (F) is dominant to two seeds in a pod (f). A farmer crosses two soybean plants, both with high oil content and four seeds in a pod. The offspring have a phenotypic ratio of 9: 3: 3: 1.

The image shows the karyotype of a person who developed as a female.

Karyotype photograph with chromosomes arranged in numbered groups 1–22 and a separate unnumbered chromosome at the right.

Determine, with a reason, whether this karyotype shows that non-disjunction has occurred.

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