How Many Pairs Of Chromosomes Do Humans Have

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In humans, each diploid cell contains 2n : 46 chromosomes. These 46 chromosomes are arranged in 23 pairs.
Number of chromosomes 46
Number of pairs of chromosomes 23
Autosomes 22 pairs
Allosomes (Sex Chromosomes) 1
Sex chromosomes also called asheterosomes, allosomes
X-chromosomecontributed by mother or father
Y-Chromosomecontributed only by father
Allosomes in MalesXY (Heteromorphic)
Allosomes in femalesXX (Homomorphic)
Picture of chromosomes lined up in pairs Karyotype.
When human karyotype arranged in decreasing lengthIdiogram
Literal MeaningColored bodies
Number of chromosomes in sperm or egg23 (haploid)
Chromosomes in Zygote46
Chromosomes duplicated bymitosis
Chromosomes halved bymeiosis
Short armp-arm
long armq-arm



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Size and Composition

Length0.1- 33 microns
Thickness0.2 - 2 microns
Contains DNA40%
Histone50%
Proteins8.5%
RNA1.5%

Sex Chromosome Table

-XY
LengthLongShort
ShapeLetter XLetter Y
SizeX is 5 times larger than Y
GenesX has 10 times more genes than Y
one X chromosome 'silenced'YesNo
Sex determinerNoYes
Holandric GenesNoYes
Discovered byHenking (1891)Stevens (1902)
Amount of EuchromatinLargeSmall
Amount of HeterochromatinSmallLarge
DNA contentMoreLess
Genetically ActiveMoreLess
Homologous PartSameSame
Non-homologous partLargeSmall

Types of Sex Determination Methods

MethodMaleFemaleFound in Animals
XY MethodXYXXMammals, some insects, Humans
ZW MethodZZZWBirds, reptiles, fishes
XO MethodXXXORoundworms, insects
ZO MethodZOZZMoths, butterfly

Barr Bodies

Number of Barr Bodies in a human is one less than the number of X-chromosome(s)
Barr bodies count
Male0
Female1
Super male0
Super Female2
Klinefelter's Syndrome1

Classification of Chromosomes on the basis of position of Centromere

Depending on the position of centromere, four categories of chromosomes are identified.
TypePosition of CentromereShape during Anaphasic Movement
MetacentricMiddleV-shaped
Submetacentricsome distance from the middleL or J-shaped
Acrocentricnear the endJ-shaped
TelocentricTip of chromosomeI-shaped

Classification of Chromosomes on the basis of Centromere

TypeDescription
DicentricChromosomes with two centromeres [maize]
PolycentricChromosomes with many centromeres. Centromeres lies in diffused condition along the length of chromosome. [Ascaris]
HolocentricThe chromosomes whose centromeres are diffused rather than discretely located.

Variation in Number of Chromosomes

Euploidy is the presence of exact multiple set of chromosomes
Haploidn (Normal sex cells)
Diploid2n (Normal body cells)
Triploid3n
Tetraploid4n
Pentaploid5n
Hexaploid6n
Polyploidmn (m is an integer)

Aneuploidy is change in chromosomes with just one or two additions or substraction of chromosomes, and does not involve entire set.

Hypoploidy is aneuploidy with loss of chromosomes
Monosomy2n - 1
Double monosomy2n - 1 - 1
Nullisomy2n - 2

Hyperploidy is aneuploidy with gain of chromosomes
Trisomy2n + 1
Tetrasomy2n + 2
Double tetrasomyn + 2 + 2
Mixed Aneuploidy2n + 1A - 1B



Syndromes in Humans

SyndromeDescription
Cri-du-chat or Lejeune's syndromeThis is caused by deletion on p-arm of chromosome 5. 
Turner's syndromeMonosomy in humans (44 + XO)
Down's syndromeTrisomy of 21st chromosome.
Edward's syndrome Trisomy of 18th chromosome.
Patau's syndrome Trisomy of 13th chromosome.
Jacob's or criminal syndrome
(Super males)
Extra Y chromosome in male. (44 + XYY)
Klinefelter's syndrome Extra X chromosome in male. (44 + XXY)
Super female Extra X chromosome in female. (44 + XXX)
Reasons for variationNondisjunction during meiosis


Darryl Leja and Ernesto del Aguila, NHGRI

Mutations are of following types:


  • Single chromosome mutations are
    • Deletion - Cri du Chat syndrome (5p-)
    • Duplication
    • Inversion
  • Mutations involving two chromosomes
    • Insertion
    • Translocation - Reciprocal translocation between chromosome-9 and 22 causes 'Philadelphia' Chromosome

  • Common Genetic Disorders involving mutations

    DisorderMutationChromosomes
    Angelman SyndromeDCP15
    Canavan disease 17p
    Color BlindnessPX
    Cri du chat syndromeD5
    Cystic fibrosisP7q
    Down syndromeC21
    Duchenne muscular dystrophy DXp
    HaemochromatosisP6
    HaemophiliaPX
    Klinefelter syndromeCX
    Prader-Willi syndromeDC15
    Sickle-cell diseaseP11p
    Tay-Sachs diseaseP15
    Turner syndromeCX
    D - Deletion of a gene or genes.
    C - Whole chromosome extra, missing or both.
    P - Point mutation

    Number of Chromosomes (2n) in Different Plants and Animals

    Maize20
    Wheat14
    Adder's tongue fern,
    Ophioglossum reticulum
    1262
    (Maximum known)
    Common fruit fly8
    Earthworm36
    Cat38
    Mouse40
    Rabbit44
    Gorilla, Chimpanzee48
    Sheep54
    Elephant56
    Cow60
    Donkey62
    Horse64
    Dog78
    Kingfisher132


    Multiple Choice Questions


    Percentage of Histone in a chromosome is
    (1) 100
    (2) 75
    (3) 50
    (4) 25
    
    Ans
    

    Which of the following statement is true?
    (1) Length of X is more than that of Y
    (2) X chromosomes is the sex determiner
    (3) Length of X is equal to that of Y
    (4) X chromosomes has Holandric genes
    
    Ans
    

    Which of the following statement is true?
    (1) Amount of Euchromatin is more in X than Y
    (2) Amount of Euchromatin is less in X than Y
    (3) Amount of Heterochromatin is more in X than Y
    (4) X chromosomes has less DNA content
    
    Ans
    

    Which of the following statement is true?
    (1) X chromosome is more genetically active
    (2) Amount of Euchromatin is more in Y than X
    (3) Amount of Heterochromatin is more in X than Y
    (4) X chromosomes has less DNA content
    
    Ans
    

    Down`s Syndrome occurs due to trisomy of which chromosome?
    (1) 18th 
    (2) 19th
    (3) 13th
    (4) 21st
    
    Ans
    

    Edwards`s Syndrome occurs due to trisomy of which chromosome?
    (1) 18th
    (2) 19th
    (3) 13th
    (4) 21st
    
    Ans
    

    Patau`s Syndrome occurs due to trisomy of which chromosome?
    (1) 18th
    (2) 19th
    (3) 13th
    (4) 21st
    
    Ans
    

    The chromosome whose centromere is situated at the tip of chromosome. It is truly terminal. Chromatids are not divided into arms. Such chromosome is called ______ chromosome.
    (1) Acrocentric 
    (2) Telocentric
    (3) Metacentric
    (4) Holocentric
    
    Ans
    

    The shape of Submetacentric chromosome during Anaphasic movement is:
    (1) V-shape 
    (2) K-shape
    (3) J-shape
    (4) I-shape
    
    Ans
    

    Reciprocal translocation between chromosome-9 and chromosome-22 results in 
    (1) Patau`s syndrome
    (2) Klinefelter`s syndrome
    (3) Edward`s syndrome
    (4) Philadelphia chromosome
    
    Ans
    


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