Genetic recombination is the reshuffling of DNA that helps make sexually produced offspring genetically unique. During meiosis, homologous chromosomes pair up and exchange matching segments, creating new combinations of alleles on the same chromosome. This process is a major reason siblings can look different even though they have the same parents.
Understanding crossing over also helps explain inheritance patterns that do not follow simple independent assortment.
Key Facts
- Crossing over occurs during prophase I of meiosis, after homologous chromosomes pair to form tetrads.
- A chiasma is the visible X-shaped contact point where nonsister chromatids exchange DNA.
- Recombination creates new allele combinations, such as AB and ab producing Ab and aB chromatids.
- Genes that are close together on the same chromosome are linked and are less likely to be separated by crossing over.
- Recombination frequency = recombinant offspring / total offspring x 100 percent.
- 1 map unit = 1 centimorgan = 1 percent recombination frequency.
Vocabulary
- Homologous chromosomes
- A pair of chromosomes, one from each parent, that carry the same genes in the same order but may have different alleles.
- Tetrad
- A group of four chromatids formed when two replicated homologous chromosomes pair during prophase I of meiosis.
- Nonsister chromatids
- Chromatids from different homologous chromosomes that can exchange DNA during crossing over.
- Chiasma
- The physical crossover point where nonsister chromatids touch and exchange genetic material.
- Linkage map
- A chromosome map that uses recombination frequencies to estimate the relative distances between genes.
Common Mistakes to Avoid
- Confusing sister chromatids with nonsister chromatids is wrong because crossing over normally occurs between chromatids from homologous chromosomes, not between identical sister chromatids.
- Saying crossing over happens in mitosis is wrong because the recombination that creates new allele combinations is a normal feature of prophase I in meiosis.
- Assuming linked genes always stay together is wrong because crossing over can separate linked alleles, especially when the genes are far apart on the chromosome.
- Treating recombination frequency as an exact physical distance is wrong because it is an estimate based on crossover events and becomes less accurate for genes that are very far apart.
Practice Questions
- 1 In a testcross, 84 offspring have parental phenotypes and 16 offspring have recombinant phenotypes. What is the recombination frequency between the two genes, and how many map units apart are they?
- 2 Two linked genes have a recombination frequency of 12 percent. In 500 offspring, how many recombinant offspring would you expect?
- 3 Explain why crossing over between nonsister chromatids increases genetic variation, but crossing over between sister chromatids usually does not create new allele combinations.