Non disjunction 2nd year biology Lecture no. 10 F.Sc. I Cell cycle chapter 21 скачать в хорошем качестве

Non disjunction 2nd year biology Lecture no. 10 F.Sc. I Cell cycle chapter 21

#biologyfsc #chapter21 #cellcycle #2ndyear MEIOTIC ERRORS (NON-DISJUNCTION) Meiosis is an orderly occurring phenomenon, which ensures every phase with appropriate finish, but sometimes, at any point the result may be unexpected, causing abnormalities Non Disjunction. One of such abnormalities is chromosome non-disjunction, in which chromosomes fail to segregate during anaphase and telophase and do not finish with equal distribution of chromosome among all the daughter nuclei. This results either increase or decrease in the number of chromosomes, causing serious physical, social and mental disorders. This non-disjunction may be in autosome or in sex chromosome. Some examples of each type may be discussed below in some detail. Down’s Syndrome (Mongolism). Definition. It is one of the consequences of autosomal non-disjunction in man, during which 21st pair of chromosome fails to segregate, resulting in gamete with 24 chromosomes. Cause. When this gamete, fertilizes normal gamete the new individual will have 47 (2n + 1) chromosomes. Chance. Non-disjunction appears to occur in the ova and is related to the age of mother. The chances of teenage mother having Down’s syndrome child is one in many thousands, forty years old mother, one in hundred chances and by forty-five the risk-is three times greater. Effects. The affected individuals have flat, broad face, squint eyes with the skin fold in the inner corner, and protruding tongue, mental retardation, and defective development of central nervous system. Autosomal non-disjunction may occur in other than 21st chromosome which usually results in abortion, or death in very early age. Klinefelter’s Syndrome. These individuals have additional sex chromosome e.g., 47 chromosomes (44, autosome + XXY). Effects. They are phenotypically male but have frequently enlarged breasts, ‘tendency to tallness, obesity, small testes with no sperms at ejaculation and under developed secondary sex characters. Other Possibilities. Males with 48 chromosomes (44 autosomes + XXXY), with 49 chromosomes (44 autosomes + XXXXY) and with 47 chromosomes (44 autosomes + XYY) are also observed. Turner’s Syndrome. These affected individuals have one missing X chromosome with only 45 •chromosomes (44 autosomes + X). Effects. Individuals with this condition often do not survive pregnancy and are aborted. Those who survive have female appearance with short stature, webbed neck, without ovaries and complete absence of germ cells.