Unit 3
Genetics
Chapter 3.3
Linkage and Crossing OverClass 12 Biology – Linkage and Crossing Over Notes PDF
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Introduction
Linkage explains why some genes do not assort independently, while crossing over creates new combinations of alleles called recombinants.
1. Linkage
Genes situated on the same chromosome form a linkage group. The closer two genes are, the stronger their tendency to remain together because a crossover is less likely to occur between them.
- Linked genes deviate from simple independent assortment.
- Genes on different chromosomes usually assort independently.
- Genes far apart on the same chromosome may recombine frequently enough to appear nearly independent.
Fig. 1 – Linked Genes on Homologous Chromosomes
2. Complete and Incomplete Linkage
Complete Linkage
Linked genes are transmitted together without detectable crossing over between them, producing only parental combinations.
Incomplete Linkage
Crossing over occurs between linked genes, so both parental and recombinant combinations are produced.
Fig. 2 – Parental and Recombinant Gametes
3. Crossing Over
Crossing over occurs during prophase I of meiosis, especially after homologous chromosomes pair. Non-sister chromatids exchange corresponding segments at sites that become visible as chiasmata.
Important Terms
- Synapsis: pairing of homologous chromosomes.
- Bivalent/tetrad: paired homologues containing four chromatids.
- Chiasma: visible point of contact associated with crossover.
- Recombinant chromatids: chromatids containing new allele combinations after exchange.
Fig. 3 – Crossing Over Between Non-Sister Chromatids
4. Sequence of Crossing Over
- Homologous chromosomes pair by synapsis.
- A bivalent/tetrad is formed.
- Non-sister chromatids break at corresponding positions.
- Equivalent segments are exchanged and rejoined.
- Chiasmata become evident as homologues begin to separate.
- Meiosis produces parental and recombinant chromatids.
Fig. 4 – From Homologues to Recombinants
5. Recombination Frequency and Gene Mapping
The proportion of recombinant offspring can be used to estimate the distance between linked genes.
Genes that are farther apart generally show a higher probability of crossover between them than genes that are close together. Recombination data therefore help construct linkage maps.
Fig. 5 – Simple Linkage Map
6. Significance of Linkage and Crossing Over
- Linkage keeps certain gene combinations together.
- Crossing over generates genetic variation.
- Recombination creates new allele combinations useful in evolution and breeding.
- Recombination frequency helps estimate gene order and map distance.
- Crossing over contributes to diversity among gametes produced by meiosis.
7. Quick Revision & Exam Points
- Define linkage and linkage group.
- Differentiate complete and incomplete linkage.
- Define crossing over and chiasma.
- At what stage of meiosis does crossing over occur?
- Explain the mechanism of crossing over.
- What are parental and recombinant types?
- Define recombination frequency.
- State the significance of crossing over.
- Linked genes lie on the same chromosome.
- Closer genes show stronger linkage.
- Crossing over occurs between non-sister chromatids.
- Crossing over occurs in prophase I.
- Chiasma marks crossover association.
- Recombinants have new allele combinations.
- Recombination frequency helps gene mapping.
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