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Inheritance Pattern – Crossing Over

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Inheritance from parents to the offsprings has certain patterns. Several Inheritance patters that will be studied are non-Mendelian inheritance, linkage, crossing over, non-disjunction, Lethal gene, blood type, and sex determination.

Crossing over

cell division in prophase I, each homologous chromosome pairs up with a very close distance. It causes the chromosome's arms cross over with another chromosome. The point where crossing over occurs induces chemical reaction and breaks the chromosome's arm. The broken arms then reconnect but there occurs an exchange of homologous chromosome's

arm and gene between the two chromosome. The exchange of homologous chromosome's arm and gene is called crossing over. The point of cross over is called chiasma. Chiasma in crossing over can occur at more than one Point.

Crossing over can result new offspring formation, either in parental type or recombinant type. So, crossing over also has a role in increasing diversity of living organisms. Moreover, crossing over may also induce mutation. Crossing over that only occurs between homologous chromosomes can cause duplication and deletion of chromosome. The crossing over that occurs in non-homologous chromosome can cause chromosome translocation.

arm and gene between the two chromosome. The exchange of homologous chromosome's arm and gene is called crossing over. The point of cross over is called chiasma. Chiasma in crossing over can occur at more than one Point.

Crossing over can result new offspring formation, either in parental type or recombinant type. So, crossing over also has a role in increasing diversity of living organisms. Moreover, crossing over may also induce mutation. Crossing over that only occurs between homologous chromosomes can cause duplication and deletion of chromosome. The crossing over that occurs in non-homologous chromosome can cause chromosome translocation.

Crossing Over can produce new offsprings, either parental or combinant type

There are several factors causing crossing over, which are as follows.

  •  Distance between genes. The farther the distance between genes, the bigger the chance for crossing over to occur. 
  • Age .The older the age of a person, the smaller the chance for crossing over to occur (because ceII division more rarely happens) 
  • Temperature. The lower or higher the temperature, the bigger the chance for crossing over to occur. 
  • X ray. The x ray can increase the possibility for crossing over to occur. 
  • Sex type. Generally, crossing over occurs to both male and female. However, for some organisms, crossing over only occurs in particular sex type. For example, in silk moth caterpillar (Bombyx mori), the crossing over only occurs in male. Meanwhile, in fruit fly, it only occurs to female. 


We can count the crossing overvalue (cov) in an organism by using The following formulation.



Read carefully the following case. In fruit fly, red eye is determined By allele M, while allele m determines purple color (mutant). Long wing is determined by allele P, while shrivel wing (mutant) is determined by allele p. If the homozygous red eye Long wing fruit fly is crossed with purple eye shrivel wing, then all of the F, Produced have red eye and long wings. In test cross of F, offsprings, the F, produced are 382 red eye with normal wings, 16 red eye with shrivel wings, 22 purple eye with long wings, and 353 purple eye with shrivel wings. According to the above formula, the value of COV is:


The crossing over value shows the percentage value of formed recombinant. The value also determines the linkage strength between genes in chromosome. The bigger the crossing over value, the smaller the linkage strength between genes because the distance is farther. The measurement used for distance between gene locus is unit or centiMorgan as an honor to T. H. Morgan who discovered chromosome map. One unit or one centiMorgarr = 1% crossing over. Therefore, the distance between genes in the example above is 4.9 units.

Download Patterns of Inheritance


Source: Life: The science of Biology, 2003

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