Chromosome Deficiencies

Content
Structural Changes in Chromosomes
Deficiencies
Duplications
Translocations 
Inversions 
Deficiency is due to loss of a part of a chromosome. Smaller deficiencies, present in heterozygous condition (only on. one of the two homologous chromosomes), can be tolerated by an organism. Such individuals at meiosis will form a loop in a bivalent that can be observed at pachytene stage (Fig. 19.3). Loops can also be observed in salivary gland chromosomes of Drosophila which are found in a permanent state of pairing, so that even small deficiencies could be detected in these chromosomes (Fig. 19.4).
Deficiencies have an effect on inheritance also. In presence of a deficiency, a recessive allele will behave like a dominant allele (pseudodominance). This principle of pseudodominance exhibited by deficiency heterozygotes has been utilized for location of genes on specific chromosomes in Drosophila, maize and other organisms. L.J. Stadler, who was a pioneer in radiation work in plants devised a method where a homozygous recessive stock was pollinated by irradiated pollen from dominant stock, so that if irradiation induced a deletion, recessive allele will express due to pseudodominance. As shown in Figure 19.5, if homozygote abc is pollinated by ABC, heterozygous F1 (ABC/abc)will be produced expressing only dominant characters. If pollen with dominant alleles ABC is irradiated, a deletion may be induced leading to expression of pseudodominance by one or more recessive alleles. If meiosis at pachytene is examined in such a deficiency heterozygote, presence of loop will indicate location of gene. Several genes were located on different chromosomes of maize and tomato, utilizing deficiencies. In Drosophila also deficiencies were recorded particularly on X-chromosomes in regions of genes w (white eye), fa (facet eye)and v (vermilion coloured eye).
 
Chromosome pairing in a deficiency heterozygote.
Fig. 19.3. Chromosome pairing in a deficiency heterozygote.

Diagrammatic representation of pairing ot salivary gland X-chromosomes in a deficiency heterozygote in a ♀ Drosophila, showing a loop. Note that the bands 3C2 to 3C11 are missing in one chromosome.
Fig. 19.4. Diagrammatic representation of pairing ot salivary gland X-chromosomes in a deficiency heterozygote in a ♀ Drosophila, showing a loop. Note that the bands 3C2 to 3C11 are missing in one chromosome.
 
Detection of deletion due to pseudodominance.
Fig. 19.5. Detection of deletion due to pseudodominance.


Deficiencies have also been recorded in waltzingmice in region of gene v inducing nervous abnormality. In human beings, a deficiency was discovered, which was associated with cat like-cry so that the child carrying this deficiency had a cat like cry and also had microcephally (small head and low mental faculty). This deficiency was found in a segment of chromosome 5.