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Aneuploidy of chr

Answer :

Aneuploidy refers to the failure of segregation of chromatids during cell division cycle results in the gain or loss of chromosomes.

Common disorders of this kind in human beings;

Down’s syndrome

• Caused due to the presence of an additional copy of chromosome number 21 9trisomy of 21)

• The affected individual is short statured with a small round head, furrowed tongue and partially opened mouth. Palm is broad with characteristic palm crease. Physical, psychomotor and mental development is retarded.

• Karyotype with 23 pairs of autosomes and 1 pair of sex chromosomes. (total of 24 pairs i.e. 47 chromosomes).

Turner’s syndrome

• Caused due to the absence of one X chromosome, i.e., 45 with X0,

• Affected female individuals are sterile as ovaries are rudimentary and may lack secondary sexual characters.

• Karyotype of 45 chromosomes.


The genes determining the above characters are located on the X chromosome. When two genes in the dihybrid cross are situated on the same chromosome, the proportion of parental combination becomes much higher than the non-parental type of combination. The linkage is the physical association of genes on a chromosome. Recombination explains the non-parental gene combination. Genes on the same chromosomes are tightly linked (show low recombination), or it may be loosely linked (showed high recombination).

1.3 percent recombinants indicate that genes are loosely held, so there are less parental type characters and. 98.7 per cent progeny with parental type combination indicates genes were tightly held and less recombination.

In Mendelian dihybrid cross, the genes segregate independent of each other, and the F2 ratio was 9:3:3:1 (when two genes are independent).

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