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.
Understanding Biology: Genetic Recombination and Crossing Over
Before DNA is exchanged, each chromosome has already copied itself. It therefore consists of two sister chromatids, which are nearly identical copies. Each chromosome then finds its matching homolog from the other parent.
The homologs carry the same kinds of genes in the same order, though they may carry different alleles. A protein structure called the synaptonemal complex holds the homologs closely aligned. Enzymes make carefully controlled breaks in DNA.
A broken strand can use the matching DNA sequence on a nonsister chromatid as a guide for repair. This repair process can swap corresponding DNA sections between the two chromatids.
The exchange must occur at matching positions for the chromosomes to keep a complete set of genes. It is not a random cutting and joining process. Cells use many proteins to check alignment, cut DNA, reconnect strands, and repair errors.
Crossing over has a second job beyond producing new gene combinations. The physical links between homologs help them orient correctly when meiosis separates them. If exchanges fail or occur in the wrong places, chromosomes may separate unevenly.
Gametes can then receive too many or too few chromosomes. Such errors are one cause of chromosome conditions in humans.
A crossover does not guarantee that every gene near it will appear recombined in offspring. The result depends on where the break occurs relative to the genes being studied. Genes far apart have more DNA between them, so there are more opportunities for an exchange to occur between them.
Scientists can use offspring counts to estimate this distance. They divide the number of recombinant offspring by the total number of offspring, then multiply by one hundred percent. This estimate has an important limit.
A recombination frequency cannot rise above fifty percent because widely separated genes can behave like genes on different chromosomes. Multiple exchanges can even restore the original outer gene arrangement, hiding some crossover events from an experiment.
Genetic maps are built from these patterns in carefully planned crosses. A map unit represents one percent observed recombination, but it is an estimate rather than a fixed physical length of DNA. Different chromosome regions have different crossover rates.
Some regions rarely recombine because of their DNA structure or position near a centromere. In school genetics problems, pay close attention to whether allele pairs are written on the same homolog or on opposite homologs.
Keep parental combinations separate from recombinant combinations. It also helps to distinguish sister chromatids from homologous chromosomes, since only nonsister chromatids exchange segments during the usual crossing over process.
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.