Can a triploid XY offspring with genotype EEF result from non-disjunction in meiosis II of an XY parent with genotype EE and an XX parent with genotype EF?

Answer: False

The statement is false because non-disjunction in meiosis II of the XY parent (genotype EE) cannot produce the observed triploid XY offspring with genotype EEF.

Genetic Analysis of the Scenario

Parent Genotypes and Expected Gametes

• XY Parent (EE): Can only produce gametes containing the E allele
• XX Parent (EF): Can produce gametes containing either E or F alleles 1

Why Meiosis II Non-disjunction in the XY Parent Cannot Explain EEF

Non-disjunction in meiosis II occurs after sister chromatids fail to separate, resulting in gametes that contain two identical copies of the same chromosome 1.

• If non-disjunction occurs in meiosis II of the XY parent (genotype EE), the abnormal gamete would contain two E alleles (both identical sister chromatids) 1
• When this abnormal gamete (EE) fertilizes a normal gamete from the XX parent containing F, the resulting offspring would be EEF 1
• However, this offspring would also receive two Y chromosomes (YY) from the paternal non-disjunction event, resulting in genotype EEFY with sex chromosomes XYY, not XY as stated 2

The Correct Mechanism

To produce a triploid XY offspring with genotype EEF, non-disjunction must occur in meiosis II of the XX parent (genotype EF):

• The XX parent undergoes meiosis II non-disjunction, producing a diploid gamete containing both E and F alleles (EF) plus two X chromosomes 1
• This abnormal gamete (EF, XX) is fertilized by a normal sperm from the XY parent containing E and Y 3
• The resulting offspring has genotype EEF with sex chromosomes XXY, which would then need to lose one X chromosome through anaphase lag or other mechanism to become XY 3

Alternatively, meiosis I non-disjunction in the XX parent could produce a diploid egg with EF that, when fertilized by a normal sperm (E, Y), directly produces the triploid EEF genotype 3.

Key Mechanistic Principles

Meiosis II non-disjunction produces gametes with identical sister chromatids (isodisomy), while meiosis I non-disjunction produces gametes with non-identical homologous chromosomes (heterodisomy) 1. Since the XY parent is homozygous (EE), any non-disjunction event in this parent can only contribute E alleles, never F 3.