BIOLOGY Lecture Notes - Golden Pages 2003

6. BASICS OF HEREDITY

Linked Inheritance

The independent assortment of traits (Mendel's third law) occurs under the condition that the genes determining these traits are located in different pairs of homologous Chromosomes. Consequently, in any Organism, the number of genes capable of independent assortment during Meiosis is limited by the number of chromosome pairs. Typically, the number of genes in an organism significantly exceeds the number of chromosomes. Genes localized in the same chromosome form a linkage group and are inherited together. Morgan proposed calling this joint inheritance of genes linked inheritance. The number of linkage groups corresponds to the haploid chromosome set.

The mode of inheritance of linked genes differs from that of genes localized in different pairs of homologous chromosomes. The generation of new Gametes is driven by the exchange of segments between homologous chromosomes, a process known as Crossing-over.

Crossing-over occurs during prophase I of meiosis, at the time of homologous chromosome pairing (synapsis). At this stage, parts of two chromosomes may cross over and exchange segments. As a result, qualitatively new chromosomes are formed, containing segments (genes) from both maternal and paternal chromosomes. Individuals developing from such gametes with novel allele combinations are referred to as crossover or recombinant individuals. The frequency (percentage) of crossing-over between two genes located in the same chromosome is proportional to the distance between them. The closer two genes are to each other, the lower the frequency of crossing-over between them.

The distance between genes characterizes the strength of linkage and is expressed in centimorgans (map units) or as a percentage of recombination (crossing-over). One centimorgan corresponds to one percent of crossing-over. There are genes with a high degree of linkage, as well as those where linkage is hardly detectable.

The Biological Significance of crossing-over is immense, as genetic recombination generates novel, previously non-existent Gene combinations, thereby enhancing organismal survival in the course of evolution.



Last update: 06/08/2026

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