5.6 Chromosomal Inheritance

01Nondisjunction

If homologs (meiosis I) or sister chromatids (meiosis II) fail to separate, gametes get an extra or missing chromosome. Fertilization then produces aneuploidy: trisomy (2n + 1) or monosomy (2n − 1).

Down syndrome = trisomy 21. Turner syndrome = XO (monosomy X). Klinefelter syndrome = XXY.

02Karyotypes and pedigrees

A karyotype shows chromosomes arranged by size and can reveal aneuploidy and large structural changes (deletions, duplications, inversions, translocations).

Polyploidy (extra full sets) is common and often beneficial in plants — wheat is hexaploid.

03Notebook box

Nondisjunction in meiosis I → all 4 gametes abnormal. In meiosis II → 2 abnormal, 2 normal.

Worked example: A karyotype shows 47 chromosomes with three copies of chromosome 21. Cause? → Nondisjunction during gamete formation.

Key takeaways

✦Nondisjunction → trisomy or monosomy.
✦Karyotypes reveal chromosome number problems.
✦Polyploidy is common in plants.

Watch outA karyotype can't detect single-gene mutations like cystic fibrosis.

Quick check

Did it stick?

1.Down syndrome results from…

2.Nondisjunction in meiosis I produces how many abnormal gametes?

3.A person with XXY has…