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GENETICS
Multiple Allelism And Co Dominance:
¾ In this phenomenon, the offspring shows resemblance to both the parents
¬ Landsteiner discovered ABO system of blood groups.
¬ The fourth group AB was discovered by de Castello and Steini.
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¾ A well-known example of these phenomena is the inheritance of ABO blood groups in man
¾ Bernstein showed that these groups are controlled by 3 alleles — IA, IB and I°/i. These alleles are
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autosomal and follow Mendelian pattern of inheritance.
¾ Any individual carries two of these three alleles
¾ The allele IA produces a glycoprotein A, found on the surface /membrane of red blood cells.
¾ The allele IB produces glycoprotein B, found on the surface /membrane of red blood cells.
¾ The allele i does not produce any glycoprotein
¾ The allele IA is dominant over I and the allele IB is also dominant over i.
¾ When the alleles IA and IB are together , they are equally dominant and both the glycoproteins,
A and B are produced
¾ The blood group is determined by the presence or absence of one or both the glycoproteins. I.e
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group A has glycoprotein A, group B has glycoprotein B, group AB has both the glycoproteins
while group O has neither of them.
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¾ The inheritance of blood group character follows mendedelian pattern of inheritance
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¾ Since there are three alleles, six different genotypes are possible, which produce four different
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phenotypes, i.e, blood groups A,B,AB and O.
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Pleiotropy
¾ In this phenomenon ,a single gene product may produce more than one phenotypic effect
¾ Even in garden peas, such a phenomenon has been observed in the following characters :
¾ Starch synthesis/size of starch grains and the shape of seeds are controlled by one gene
¾ Flower colour and seed coat colour are found to be controlled by the same gene
¾ Starch is synthesized efficient in starch synthesis, BB and hence the starch grains are large and
the seed at maturity are round
¾ The homozygotes, bb are less efficient in starch synthesis; hence they have small starch grains
and the seeds are wrinkled
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¾ The heterozygous, Bb produce round seeds, indicating that B is the dominant allele; but the
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starch grains are intermediate in size and hence for the starch grain size, the alleles show
incomplete dominance
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¾ It is example of pleiotropy as the same gene controls two traits, i.e, seed shape and size of strch
grains
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¾ The genotypes and the phenotypes for the two traits are as follows:
¾ BB – round seeds, large starch grains.
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¾ Bb –round seeds, intermediate sized starch grains
¾ Bb –wrinkled seeds , small starch grains
¾ Here ,It can also be mentioned that dominance is not autonomous feature of the gene or its
product, but it depends on the production of a particular phenotype from the gene product
Polygenic Inheritance
¾ The traits studied by Mendel in garden pea have two distinic alternate forms ; but there are
several characters which are not so distinct in their occurrence
¾ Polygenic inheritance is the phenomenon in which a character is controlled by three or more
genes (multiple genes) and the graded phenotypes are due to the additive or cumulative effect of
all the different genes of the trait
¾ Such trait are called polygenic traits, since they are controlled by a number of genes
¾ Human skin colour, height and intelligence are some examples of polygenic traits
¾ Polygenic inheritance also takes into account the influence of environment; it is also called
quantitative inheritance, as the character /phenotype can be quantified like amount of pigment,
intelligence, etc.
darkness is dominant over light skin colour.
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¾ Human skin colour is believed to be controlled by three different genes, A, B and c, where
¾ An individual with the genotype, AABBCC, all dominat alleles, will have the darkest skin
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colour and the recessive individual with all recessive alleles (aabbcc), will have the lightest skin
colour; an individual with three dominant alleles (like AABbcc , AAbbCc , AaBbCc ,..Etc) have
an intermediate skin colour.
¾ In this manner , the number of each type of alleles in the genotype is responsible for the darkness
or lightness of the skin
¾ The alleles of the three genes show Mendelian pattern of inheritance
¾ In this following there is mating between individuals with the darkest skin and the lightest skin
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produce offspring with intermediate skin colour
Rediscovery of Mendel’s Laws
¾ Though Mendel published his work and the laws of inheritance in 1865, they remained
unrecognized till 1900, for the following reasons:
i. His work could not be widely publicized as communication was not easy
ii. His concept of ‘ factors’ as stable and discrete units that controlled the expression of
traits and that of pair of alleles which did not blend with each other were not accepted by
his contemporaries as the explanation for variation
iii. Mendel’s approach of using mathematics to explain the biological phenomena was new
and unacceptable to many biologists
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iv. Though Mendel’s work suggested that factors were discrete units, he could not provide
any physical proof for the existence of factors or prove what they are made of.
¾ In 1900 , de vries, Correns and Tchermark independently rediscovered mendel’s results on the
inheritance of characters
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¾ By then , there had been advancements in microscopy and scientists were able to observe cell
division , nucleus, chromosomes, etc
¾ By 1902, chromosome movements during cell division had been worked out.
Chromosomal Theory Of Inheritance
¾ Walter Sutton and theodor Boveri independently postulated this theory in 1902
¾ They found that the behavior of chromosomes was parallel to the behavior of mendelian factors
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(genes) and used the chromosome movements to explain Mendel’s laws
¾ The similarities are as follows:
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i) Both genes and chromosomes occur in pairs in normal diploid cells
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ii) Both of them segregate during gamete formation and only one member of each pair enters a gamete
iii) Members of each pair segregate independently of the members of the other pair(s)
¾ Sutton and Boveri argued that the pairing and separation of the homologous pair of
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chromosomes would lead to the segregation of a pair of factors they carried.
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Thomas Hunt Morgan and Genetics
¾ T.H. Morgan worked on fruit flies (Drosophila melanogaster)
¾ They are found suitable for studies in genetics for the following reasons:
i) They could be grown on simple synthetic medium in the laboratory
ii) The flies complete their life cycle in about two weeks
iii) They could produce a large number flies in the progeny of a single mating
iv) The male and female flies are distinct and show many types of hereditary variations that
could be easily observed
¾ Morgan carried out many dihybrid crosses in Drosophila, with the genes that were sex-linked i.e.
the genes are present on the X-chromosome
¾ He observed that the two genes under consideration in his experiments did not segregate
independently as in the case of characters studied by Mendel
Linkage and Recombination
¾ Morgan et al observed that when the two genes in a dihybrid cross are located on the same
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chromosome , the proportion of parental gene combinations in the progeny was much higher
than the non- parental or new combinations (also called recombination) of genes
¾ They also found that the proportion of recombinants varies even if the two genes are present on
the same chromosome
low, and vice versa.
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¾ If the linkage is stronger between two genes (tightly linked), the frequency of recombination is
¾ He hybridized yellow- bodied and white – eyed females with brown –bodied and red-eyed males
(wild type ) (cross 1) and intercrossed their F1 progeny
¾ The F2 generation contained the following : the parental combinations were 98.7% and the
recombinants were 1.3%
¾ In another cross (cross2) between white-bbodied female fly with miniature wing and a male fly
with yellow-body and normal –wing , the progeny contained the following : the parental
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combinations were 62.8% and the recombinants were 37.2%
¾ It is evident that the linkage between genes for yellow- body and white –eyes is stronger than
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that between genes for white-body and miniature wings
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¾ Sturtevant used the frequency of recombination between the gene pairs on the same chromosome
as a measure of the distance of the genes and mapped their position on the chromosome
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¾ Today genetic/chromosome maps are used in the sequencing of organisms
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Sex-Determination
¾ The concept of genetic/ chromosomal basis of sex-determination came from the cytological
observations made in a number of insects
¾ H. Henking (1891) could trace a specific nuclear structure all through spermatogenesis in a few
insects
¾ He observed that 50% of the sperms received this structure , while the remaining 50% did not
receive it
¾ Henking named the structure as X body , but could not explain its significance
¾ Later it was found to be a chromosome and it was named as X-chromosome
a) XO-type of sex determination
¾ A large no of insects like grasshopper show XO type of sex determination
¾ It is a case of male heterogamety , where 50% of the sperms bear an X-chromosome and the
other 50% of the sperms do not have the X-chromosome, but only the autosomes ; all the ova
bear an X-chromosome
¾ When an ovum is fertilized by a sperm having X –chromosome , the zygote develops into a female
¾ When an ovum is fertilized by a sperm having X–chromosome, the zygote develops into a male
b) XY-type of sex determination
¾ In insects like Drosophila melanogaster and in human beings, this type of sex–determination is seen
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¾ The males have an X-chromosome and anther small ,but characteristically –shaped Ychromosome , i.e., males have XY chromosomes along with other autosomes
¾ The females have two X-chromosomes along with the other autosomes
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¾ It is a case of male heterogamety, where the males produce two types of sperms ,50% of sperms
having one X-chromosome and the other 50% with one Y-chromosome
¾ The females are homogametic and produce all ova with one X-chromosome
¾ Sex of an individual is decided at the time of fertilisation by the type of sperm fertilizing the ovum
c) ZW-type of sex –determination
¾ This type of sex determination is seen in certain birds
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chromosomes
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¾ The females have ZW chromosomes along with the autosomes and the males have ZZ
¾ It is a case of female heterogametry, where the females produce two types of ova, 50% of ova
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with one Z-chromosome and the other 50% with one W-chromosome
¾ The males are homogametic and produce sperms all, with one Z-chromosome
¾ In this case, the sex of the individual is determined by the type of ovum that is fertilsised to
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produce the offspring
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d) Haplodiploid sex-determination
¾ This type of sex –determination is seen in honeybees; it is based on the no of sets of
chromosomes, an individual receives.
¾ When the ovum is fertilized by a male gamete, the diploid the zygote (2n=32) develops into a
female, i.e., a queen or worker.
¾ When the ovum develops by parthenogenesis, i.e., with out fertilization, a male individual,
called drone, is formed.
¾ The male honeybee (drone) is haploid (with n=16 chromosomes ) and forms sperms by mitosis
¾ The special characteristic feature of this system of sex-determination are :
1. The male honeybees do not have a father , but have a grandfather
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2. They do not have sons, but can have grandsons
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Mutation
¾ Gene mutation result in alteration in the sequences of bases of bases of DNA or a change in the
base and thereby change in the genotype and phenotype of an organism
¾ A mutation that involves a single base change , is called point mutation; a classical example of
point mutation is sickle –cell anaemia
¾ Delection or insertion /deplication /addition of one or two bases in the DNA results in a change
in the reading frame , there by resulting in a polypeptide with a different set of amino acids
¾ Structural alterations of the chromosomes result due to less or gain of large segment of DNA , as
DNA/genes are located on the chromosomes
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¾ When the members of a homologous pair of chromosomes fail to segregate during anaphase I of
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meiosis, aneuploidy results; there is loss or gain of one or more chromosomes
¾ Failure of separation of the duplicated chromosomes in to daughter nuclei results in polyploidy,
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a phenomenon in which the cell has three ,four or more sets of chromosomes
¾ The physical and chemical agents/factors that bring about mutations, are called mutagens
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¾ Mutations are also responsible for variation
Pedigree Analysis
¾ Inheritance pattern in human beings cannot by studied by crosses as in other organisms, for the
following reasons:
i) Controlled crosses cannot be performed
ii) The progeny produced is very small (usually one) and takes a long time
¾ Pedigree analysis is the important tool to trace the inheritance of a specificit, abnormality or
disease to study human genetics
¾ In a pedigree chart, conventionally the following symbols are user:
ii. Squares denote males
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i. Circles denote females
iii. Mating is shown by a horizontal line connecting a male symbol with a female symbol.
iv. Offspring symbols are arranged from left to right in the under of birth and connected by
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a horizontal line below the parents, and this line is connected to parent/marriage line by a
vertical line
v. A solid/blackened symbol represents the absence of the trait or abnormality under study
vi. The offspring are connected to a horizontal line below the parents and the line is
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connected to the parental line by a vertical line
Genetic Disorders
¾ Genetic disorders can be grouped into two categories:
a) Mendelian disorders and b) Chromosomal disorders
Mendelian Disorders
¾ These are mainly due to alteration or mutation in a single gene
¾ These disorders are transmitted from one generation to the next following Mendel’s principles of
heredity ; these disorders may be:
i. Autosomal as in cystic fibrosis , sickle- cell anaemia and phenylketonuria or
ii. Sex-linked as in haemophilia, colour blindness and myotonic dystrophy
pedigrees are given as examples for
(a) An autosomal dominant
(b) An autosomal recessive disorder and
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(c) A sex – linked
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¾ Inheritance of mendelian disorders can be represented in pedigree charts; the following
¾ Some mendelian disorders are:
Haemophilia
i. It is a sex-linked , recessive disorder, whose gene is present on the X-chromosome
ii. The defective allele produces a defective protein, which is part of the cascade of proteins
involved in the clotting of blood
iii. Clotting of blood is abnormally delayed that even a simple/ small cut will result in non
stop bleeding in the affected individual
iv. More males than females suffer from the disorder as they have only one X-chromosome,
and the recessive allele on it is expressed
v. The possibility of female becoming hemophilic is very rare , as she has to receive the
must atlest be a carrier and father hemophilic
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defective alleles from both the parents and be homozygous recessive ,i.e., her mother
vi. Heterozygous female is a carrier and passes on the disease to some of her sons; Queen
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Victoria was a carrier of this disease and produced hemophilic descendents.
Colour Blindness
i.
It is a sex –linked , recessive disorder with the mutant gene on the X-chromosome
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The defect is in the red or green cones of the eye and the victim is unable to discriminate
between red and green colours
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It occurs in 8 percent of males and only 0.4 percent females
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Inheritance pattern is very similar to that of hemophilia in that more males than females
suffer from the disorder
A colour –blind female is rare , as her mother has to be at least a carrier and father colour
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blind and she must be homozygous for the recessive defective allele
A heterozygous female has normal vision , but is a carrier and passes on the disorder to
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vi.
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some of her sons
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Cystic Fibrosis
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It is caused by a recessive mutant allele on an autosome (chromosome 7)
The gene produces a unique glycoprotein that leads to the formation of mucus of
abnormally high viscosity.
This type of mucus interferes with the functioning of many exocrine glands like sweat
glands, liver, pancreas and lungs
Sickle-Cell Anaemia
i.
It is caused by a mutant recessive allele on chromosome 11.
ii.
The mutant gene causes the substitution of glutamic acid (glu) by valine (val) at the sixth
position of the beta globin chain of hemoglobin ; this substitution of the amino acid is the
result of a single base substitution in the sixth codon of the beta globin gene, from GAG
to GUG
The normal (Hb-A) and the defective (Hb-S) peptides are
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The defective haemoglobin undergoes polymerization under low oxygen tension and
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iii.
changes the shape of RBC from biconcave cell to sickle – shaped elongated cells
v.
The disease is controlled by a single pair of allele ,HbA and HbS
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Of the three possible genotypes only individuals homozygous for HbS (Hbs Hbs) show
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the disease, though hertozygous individuals (HbA Hbs) are carriers.
When both parents are heterozygous/ carrier for the gene , the disease appears in some of
their children
Phenylketonuria
i. It is caused by a recessive mutant allele on chromosome 12
ii. The affected individuals lack an enzyme that catalyses the conversion of the amino acid
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phenylalanine into tyrosine
iii. Consequently phenyl alanine is metabolized into phenyl pyruvate and other derivatives
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iv. Accumulation of these chemicals in the brain results in mental retardation
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v. These are also excreted in the urine as they are not absorbed by the kidney
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Thalassemia
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i. It is an autosomal recessively- inherited disorder, transmitted to the offspring, when both
the parents are heterozygous, i.e.., carrier of the disease
ii. The defect arises due to either mutation or deletion which results in the reduced rate of
synthesis of one of the globin chains of haemoglobin
iii. The characteristic symptom of the disease, anaemia results due to the abnormal
haemoglobin
iv. Depending on the globin chain affected thalassemia is of two types :
a. Alpha Thalassemia and
b. Beta Thalassemia
(a) Alpha Thalassemia:
i. Alpha Thalassemia is controlled by two closely –linked genes, HBA-1 and
HBA-2 ,on chromosome16
ii. Due to mutation or delection of one or more of the four allele, there is reduced
rate of synthesis of alpha globin chain of haemoglobin; more the number of
alleles affected, less will be the globin synthesis
(b) Beta thalassemia :
i. Beta Thalassemia is controlled by a single gene HBB on chromosome 11
ii. The disease is caused due to mutation of one or both the alleles of the gene
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iii. There is reduced synthesis of beta globin of haemoglobin
iv. Thalassemia is a quantitative problem, were synthesis of few globin molecules
leads to anaemia, whereas sickle-cell anaemia is qualitative problem, where
Chromosomal disorders
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synthesis of a defective globin that is non– functional causes the disease.
¾ These are caused due to absence or excess or abnormal arrangement (Structure) of one or more
chromosomes.
¾ Such a situation leads to serious consequences in the individual.
¾ The chromosomal abnormalities/disorders in an individual can be found out by karyotyping.
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Down’s Syndrome
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¾ Some of the chromosomal disorders are discussed below.
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¾ It is caused by the presence of an extra copy of the chromosome 21, i.e., trisomy of 21st
chromosome
¾ This disorder was first reported by Langdon Down (1866).
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¾ The afflicted individual shows the following symptoms:
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i. Shot stature and small round head with a flat back
ii. Partially open mouth with furrowed tongue
iii. Broad, flat face with slanting eyes
iv. Broad plam with characteristic palm crease
v. Many loops on finger tips
vi. Small and arched palate
vii. Epicanthic eye fold
viii. Congenital heart diseases
ix. Physical, psychomotor and mental retardation
Klinefelters syndrome
¾ It is caused by the presence of an extra X-chromosome in the male , i.e., XXY; the
individual has 47 chromosomes
¾ Though the individuals a male , he shows a number of feminized characters
¾ Following are the characteristic symptoms of the disorder
(a) Tall stature with feminised physique
(c) Female type of public hair pattern
(d) Poor beard growth
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(e) Sterility
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(b) Breast development (gynaecomastia)
Turner’s syndrome
¾ This disorder is caused due to the absence of the X- chromosomes in a female ; the
karyo type has only 45 chromosomes and they are called as OX females
¾ The following are the symptoms of an affected individual
(a) Shot stature
(b) Rudimentary overies
(c) Poor development of breasts
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(d) Lack of secondary sexual characters
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(e) Sterility