Because of the relatively high frequency of meiotic errors that lead to developmental abnormalities in humans, many research efforts have focused on identifying correlations between error frequency and chromosome morphology and behavior. Tease et al. (2002) studied human fetal oocytes of chromosomes 21, 18, and 13 using an immunocytological approach that allowed a direct estimate of the frequency and position of meiotic recombination. Below is a summary of information [modified from Tease et al. (2002)] that compares recombination frequency with the frequency of trisomy for chromosomes 21, 18, and 13. (Note: You may want to read appropriate portions of Chapter 8 for descriptions of these trisomic conditions.) Trisomic Mean Recombination Live-born Frequency Frequency Chromosome 21 1.23 1/700 Chromosome 18 2.36 1/3000–1/8000 Chromosome 13 2.50 1/5000–1/19,000 What conclusions can be drawn from these data in terms of recombination and nondisjunction frequencies? How might recombination frequencies influence trisomic frequencies?

Key Concepts

Meiotic Recombination

Meiotic recombination is a process during meiosis where homologous chromosomes exchange genetic material, leading to genetic diversity in gametes. The frequency of recombination can vary between different chromosomes and is influenced by factors such as chromosome structure and the presence of recombination hotspots. Understanding this process is crucial for analyzing how variations in recombination rates may correlate with the occurrence of chromosomal abnormalities, such as trisomy.

Nondisjunction

Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division, which can occur in meiosis I or II. This error can lead to aneuploidy, where cells have an abnormal number of chromosomes, such as in trisomy conditions. The relationship between nondisjunction and recombination frequency is significant, as lower recombination rates may increase the likelihood of nondisjunction events, resulting in higher trisomy frequencies.

Trisomy

Trisomy is a genetic condition where an individual has three copies of a particular chromosome instead of the usual two. This can lead to developmental abnormalities and various health issues, depending on which chromosome is affected. The data presented in the question highlights the correlation between recombination frequency and the incidence of trisomy for chromosomes 21, 18, and 13, suggesting that higher recombination rates may be associated with lower frequencies of these chromosomal abnormalities.