Genetics - A. V. Sivolob 2008

Formal Genetics: Patterns of Trait Inheritance
Crossing-over
Mitotic crossing-over

All the Examples discussed so far refer to crossing-over that occurs during Meiosis in gametogenesis. However, crossing-over can also take place in somatic Cells—specifically during mitosis at the four-chromatid stage. Such instances have been documented in Drosophila. For example, a female heterozygous for alleles at the w locus (woo — coral eyes, and v — white eyes) has pink eyes. If developing larvae are exposed to X-rays, mitotic crossing-over may occur via an exchange of segments between non-sister homologous chromatids. As a result, one daughter Cell will receive two Chromosomes with the v allele, while the other will receive two with the woo allele. During subsequent Cell Division, the first cell will give rise to a clone of identical cells forming a white spot against the pink Background of the eye, whereas the second will produce a dark red spot.

Mitotic crossing-over is a consequence of the precise Repair of Double-strand breaks during Replication via Homologous Recombination (see Fig. 1.25). Following the completion of repair and the resolution of the Holliday junction formed between two daughter DNA molecules, these molecules may exchange genetic segments. In Unicellular Eukaryotes, mitotic recombination likely serves as a crucial mechanism for generating genetic variation during Vegetative Reproduction.

METABOLISM/35.html">Selection/41.html">Review Questions and Exercises

1. Formulate Mendel's three laws and explain them in terms of the molecular and cytological mechanisms of heredity.

2. What are genotype, phenotype, phenotypic radical, homozygote, and heterozygote?

3. How do recessive alleles typically arise?

4. What is the difference between complete dominance, incomplete dominance, and codominance?

5. Calculate the phenotypic ratio resulting from the cross of a dihybrid (AaBb), assuming incomplete dominance at the Aa Gene locus.

6. Calculate the χ2 value for a coin toss resulting in 25 heads and 32 tails. Is the null hypothesis accepted?

7. Define complementation, epistasis, and polygenic inheritance in the context of non-allelic gene interactions.

8. How do the methodological approaches for studying Quantitative Traits differ from those for qualitative traits?

9. Describe the statistical parameters used to characterize the expression of quantitative traits. Explain the meaning of the heritability coefficient.

10. How many linkage groups exist in Drosophila?

11. Outline the scheme and expected results of a cross between a female Drosophila with yellow body (a sex-linked recessive mutation y) and a wild-type male (black body, y+). What segregation ratio will be observed in the F2 progeny from crossing the F1 offspring? Also, analyze the corresponding reciprocal cross.

12. How is recombination frequency determined? What units are used to measure map distance between genes based on this frequency?

13. Under what conditions is the detection of double crossovers possible?

14. What is Interference, and how does it affect crossover frequency? How is the coefficient of coincidence calculated?

15. What is Gene Conversion, and what mechanisms drive it?

16. What causes unequal crossing-over, and what are its consequences for The Structure of homologous chromosomes?



Last update: 11/08/2026

Editorial and Educational Adaptation: This material has been compiled based on the primary/original source text. The project team performed an editorial review, corrected technical inaccuracies, structured sections, and adapted the content for an educational format.

What was processed:

  • elimination of formatting defects (OCR errors, structural breaks, corrupted characters);
  • editorial organization of content;
  • standardization of terminology in accordance with academic sources;
  • verification of factual statements against the original source text.

All mentions of the author, publication year, and origin of the primary text have been preserved in accordance with the source.