SEX DETERMINATION

FEW TERMS:-
1. Genetics: Study about heredity & variations.
2. Gene: Functional segments of DNA.
3. Genome: The exact sets of chromosomes carried by one gamete.
4. cValue (paradox): The amount of DNA carried by one gamete.
5. Allele: Alternate form of one gene (T t).
6. Trait: possible expression of a character.
7. Phenotype: External appearance of an individual or a character.
8. Genotype: Genetic makeup of a phenotype.
9. Homozygous: similar allele in both loci (TT, tt)
10. Heterozygous: Dissimilar allele in both loci (Tt).
11. Emasculation: Removal of male parts of a bisexual flower before maturation.
12. Hemizygous: presence of a single allele.

SEX:
 Sex is the aggregation of morphological, physiological & behavioral traits that differentiate egg producing
animals (Females) from sperm producing animals (males).
 Sexually reproducing animals are mostly unisexual & show sexual dimorphism.
 Sex determination is a 3 step process
(i) Chromosomal sex determination by sex chromosomes
(ii) Gonadal sex determination by separate gonads.
(iii) Phenotypic sex determination by hormones secreted from gonads.

CHROMOSOME:
 Chromosomes are made up of DNA.
 Eukaryotic chromosomes are found coiled around histone proteins.
 Eukaryotes chromosomes are present in sets. Sets of chromosomes are called ploidy.
 Ploidy level may be;
1 set monoploid (n), 2 sets Diploid (2n), 3 sets Triploid (3n), 4 sets Tetraploid (4n), Half set Haploid (1/2 n)

TYPES OF CHROMOSOMES: Two types of chromosomes are found,

AUTOSOMES ALLOSOMES
 These are somatic chromosomes, which determine  Allosomes are sex chromosomes which determine the
somatic characters. sex and sex linked characters.
 Each pair appears in similar form (1-1’).  The pair may be same or different. (XX, XY).
 Not involved in sex determination.  They play pivotal role in sex determination.
 Humans have 22 pairs of autosomes.  Humans have only one Pair of allosome.

SEX CHROMOSOMES/ALLOSOMES:-
 Allosomes determine the sex of the baby.
 Sex chromosomes are designated as X, Y, Z &W, Autosomes are represented as AA.
 In human sex chromosomes are called X&Y and they differ from each other.

Y-CHROMOSOME:-
 Y-chromosome was discovered by N. Steven & was established as sex chromosome by Painter.
 Y-chromosome carries genes for expression of male sex and male sexual characters.
 The genes present in Y-chromosome are called hollandric gene.
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  Y-Chromosome bears a gene called "sry gene", which codes for testis determining factor (TDF).
 TDF is essential for differentiation of testes.
 Absence of sry gene in males resulted in formation of ovaries.
 Y-chromosome is dominant in nature.
X-CHROMOSOME:
 X-chromosome was discovered & established as sex chromosome by Henking (1891).
 X-chromosome carries genes for expression of female sex and female sexual characters.
 X-chromosomes is recessive in nature.
 In females one X-chromosome is reduced called barr body.

SEX DETERMINATION

 The chief biological function of sex is reproduction.

THEORIES OF SEX DETERMINATION:-

1) Chromosomal theory of sex determination.

2) Genic balance theory
3) Hormonal theory
4) Environmental factors for sex determination,

(1) CHROMOSOMAL THEORY OF SEX DETERMINATION:-

 This theory was given by Wilson & Steven.

 Allosomes or sex chromosomes participate in sex determination & expression of sex linked characters.
 Sex chromosomes carry genes for expression of sex and are the responsible for development of sexual
characters.
 Sex determination on chromosomal basis is of following types;

(i) XX-XY Type [Human]:-

 It is called Lygaeus type.
 In this case females are homozygotic with two X-chromosomes & produce only one type of ova (A+X).
 Males are heterozygotic and produce two types of sperms; Androsperm (A+Y) & gynosperm (A+X).
 Fusion of androsperm (A+Y) makes male Sex & fusion of gynosperem (A+X) produces female sex.
 Thus, males or nature of sperm determines the sex of the baby.

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(ii) XX-XO Type: (Protener type) [Grasshoppers]
 In this type females are homozygotic with two x-chromosomes & produce only one type of ova (A+X).
 Males are hemizygotic & produce two types of sperms, (A+X) & (A+O). The nature of sperm determines the
sex of the baby.
 Males have one chromosome less than females.
 Fusion of (A+X) sperm ensures female sex and fusion (A+O) sperm confirms male sex
e.g. Grasshoppers cockroach etc.

(iii) ZW-ZZ type: [Birds]

 The letter Z &W are used to distinguish the system from XY-chromosome system.
 In this type females are heterozygous and produce two types of ova; (A+Z) & (A+W).
 Males are homogametic & produce only one type of sperm, (A+Z).
 Fusion of (A+Z) ovum forms male sex & fusion of (A+W) ovum produces female sex.
 Thus the nature of egg determines the sex of the baby.
e.g. Birds

(IV) ZO-ZZ Type: [Butterfly]

 In this type females are hemizygotic & produce two types of ova (A+Z) & (A+O)
 Males are homozygotic & produce only one type of sperm; (A+Z).
 Fusion of (A+Z) ovum forms male sex & fusion of (A+O) ovum forms female sex.
 Females have one chromosome less than males.
 Thus the nature of egg determines the sex of the baby in ZO-ZZ type.
eg. Butterfly

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(V) Haplo-diploidy (n-2n)/ Honey bee method:

 It is found in honey bees, wasps & ants.

 In this case females are diploid (2n) & males are haploid (n).
 Males produce sperms by mitosis & females produce ova by meiosis.
 Unfertilized eggs are developed into males by parthenogenesis & fertilized eggs developed into females.
 Females are two types; Fertile queen & sterile worker.
 Females fed with royal jelly become queen.

Parthenogenesis: It is the process of formation of new individuals without fertilization.

 Parthenogenesis is of two types; Arrhenotoky and Thelytoky.
 In arrhenotoky males are produced and in thelytoky females are produced.
 When the offspring are fertile it is called complete and when offspring become sterile it is
called incomplete both in Arrhenotoky and Thelytoky.

 Flight of queen honey bee for mating is called Nuptial flight.

 Flight for making new home is called Swarming.
 Replacement of new queen is called Supersedure
2. GRENIC BALANCE THEORY:-
 This theory was given by C.B. Bridges.
 This mechanism of sex determination is found in Drosophila melanogaster.
 According to this theory sex is determined by genic balance ore sex ratio between no. of X-chromosomes. &
sets of autosomes.
 As per this theory Y-chromosome has no role in sex determination.
 X-chromosomes carry genes for femaleness & Autosomes carry genes for maleness.
 Sex ratio is calculated by taking ratio of X-chromosomes & sets of autosomes.

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 No. of X −chromosomes
Sex index ratio=
Sets of Autosomes
X
=1.0 [ Normal female ]
A
= 0.5 Normal male
= between 0.5 to 1.0 = Intersex
= Above 1.0 - super female
= Below 0.5 = super male

(3) HORMONAL THEORY:-

 This theory was given by crew.

 According to this theory hormones play an important role in sex determination.
 In few species like fowl have both testes & ovaries. Male sex develops due to secretion of testosterone & female
sex develops due to progesterone.

SEX REVERSAL:-
 Artificial removal of gonads of either sex resulted in development of secondary sexual characters of opposite
sex in few species.
e.g. Fish, amphibian birds etc.
FREE MARTIN:-
 It is found in cattle where twins of opposite sex are born, the male is normal and the female become sterile with
few male traits. This sterile female is called free martin.
 During embryogenesis twins have a common blood circulation.
 The female hormones are produced a little later than the male hormones. The male hormones influence the
female foetus to become sterile, which become free martin.

(4) ENVIRONMENTAL FACTORS FOR SEX DETERMINATION:-

 In certain species environmental factors play vital role in sex differentiation.

Example-1: Bonelia:
 In Bonelia sex determination is Chemotactic (chemical dependent).
 In Bonelia Larvae are predominantly bisexual.
 The larvae which settle down in substratum become female.
 The larvae which come in contact of female proboscis become male.
 It is established that the proboscis of mature female Bonelia secrete a chemo-tactic substance that transform
Larvae into males.

Example: 2: Temperature dependent:

 In turtles incubation of eggs in lower temperature makes male sex and in higher temperature female sex.
 In crocodile & lizard incubation of eggs in lower temperature makes female sex & in higher temperature male
sex.

Example: 3 Metabolism:
 Increased metabolism makes male sex. e-g. Pigeon.

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GYNANDROMORPHISM:-
 The term Gynandromorphism was introduced by Goldschmidt in 1915.
 Gynandromorph is an individual which shows traits of both sexes.
 It is generally found in insects. Gynandromorphs are also reported in humans.

Types:-
(a) Bilateral gynandromorphism:
 In this case an individual shows male characters in one lateral & female character in other lateral.
(b) Antero- posterior gynandromophism:
 In this case an individual shows male characters on ventral side & female characters on dorsal side. These cases
may also be vice versa.
(c) Sex piebald:-
 The individual is predominantly male or female with patches of tissues of opposite sex.
 E.g. Gynacomastia: A man with functional and enlarged breasts.

SEX LINKED INHERITANCE:-

 Inheritance of certain somatic characters along with sex chromosome is called sex linked inheritance.

Characters of sex linked inheritance:-

(a) Criss-cross inheritance:-
 It is a type of inheritance in which genes of one parent are inherited to grandchildren through the children of
opposite sex.
 It is of two types;
(i) Digynic: It is the transfer of characters from male to female to male.
(ii) Diandric: It is the transfer of characters from female to male to female.
(b) Hologynie: Transfer of characters from mother to daughter.
(c) X-chromosome linked characters are recessive. These characters are more expressive in males than females.
(d) Females are found as found as carrier but males never found as carrier.
(e) Female are found to be affected when father is affected and mother is either affected or carrier.

SEX LINKED INHERITANCE IN HUMAN:-

 X-chromosome carries diseased genes in humans.

Example: 1 COLOUR BLINDNESS:

 It is an X-linked disease in which a person suffering fails to distinguish different colours.
 This disease was discovered by John Dalton in 1794.
 Colour blindness is of following types;
(a) Red blindness-Protanopia
(b) Green blindness - Deuteronopia
(c) Blue blindness - Tritanopia.

 Red & green blindness are X-linked but blue blindness is autosomal disorder.
 Complete inability to distinguish any colour is called achromatopsia.
 Colour blindness is tested by Ishihara test.
 On the basis of presence of colorblind gene chromosomal complements (Karyotype) of male & females in
colour blindness are;

C C C
Female: XX- Normal X X −Carrier X X −Coloublind / Affected /Diseased
C
Male: XY- Normal X Y −Colourblind

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Inheritance pattern of Colour blindness:

Case-1: Normal father and Colour blindness Mother

C C
F ather ( XX ) x Mother (X X )

♂ X Y
♀
C C C
X X X Carrier daughter X Y Colour blind son
C C C
X X X Carrier daughter X Y Colour blind son

Result: All the daughters are carrier and all the sons are colour blind.

Case-2: Normal father and carrier Mother

c
Father ( XX ) x Mother (X X )

♂ X Y
♀
C C C
X X X Carrier daughter X Y Colour blind son

X XX Normal daughter XY Normal son

Result: 1 carrier daughter: 1 Normal daughter. 1 Colour blind son: 1 Normal son

Case-3: Colour blind father and Normal Mother

Father( X ¿¿ C Y )x Mother (XX )¿

♂ C Y
X
♀
X C XY Normal blind son
X X Carrier daughter

X C XY Normal blind son

X X Carrier daughter

Result: All daughters are carrier and all sons are normal.
Case-4: Colour blind father and carrier mother
Father ( X C Y ) x Mother ( X C X)

♂ C Y
X
♀
C C C C
X X X Colour blind daughter X Y Colour blind son

X C
X X Carrier daughter XY Normal son

Result: 1 Colour blind daughter : 1 Carrier daughter and 1 Colour blind son : 1 normal son

Case-5: Colour blind father and colour blind mother

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 Father ( X C Y ) x Mother ( X C X C )

♂ C Y
X
♀
C C C C
X X X Colour blind daughter X Y Colour blind son
C C C C
X X X Colour blind daughter X Y Colour blind son

Result: All the children are colour blind.

EXAMPLE-2: HAEMOPHILIA:
 Haemophilia is also called bleeders disease or Royal disease.
 It is an x-linked disorder in which persons suffering fail to clot blood when injured.
 This disease was first observed by John Otto in 1803.
 In queen Victoria of UK this disease was diagnosed by Haldane.

Types:
(i) Haemophilia-A:-
 This disease occurs due to deficiency of blood clotting factor VIII.
(ii) Haemophilla-B:
 It occurs due to deficiency of blood clotting factors IX. Haemophilia-B is called Christmas disease.
(iii) Haemophilia-C:
 It occurs due to deficiency of clotting factor XI.

Chromosomal complements in haemohpilia:

h h h
Female: XX- Normal X X −Carrier X X −Haemophilic/ Affected / Diseased
h
Male: XY- Normal X Y −Haemophilic
Inheritance pattern of haemohpilia:
Case-1: Normal father and Haemophilic Mother
h h
Father ( XX ) x Mother (X X )

♂ X Y
♀
h h h
X X X Carrier daughter X Y Hemophilic son
h h h
X X X Carrier daughter X Y Hemophilic son

Result: All the daughters are carrier and all the sons are haemophlic.
Case-2: Normal father and carrier Mother
h
Father ( XX ) x Mother (X X)

♂ X Y
♀
C h h
X X X Carrier daughter X Y Haemophilic son

X XX Normal daughter XY Normal son

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Result: 1 carrier daughter: 1 Normal daughter. 1 Colour blind son: 1 Normal son
Case-3: Haemophilic father and Normal Mother
Father( X ¿¿ h Y )x Mother ( XX)¿

♂ h Y
X
♀
X h XY Normal son
X X Carrier daughter

X h XY Normal son
X X Carrier daughter

Result: All daughters are carrier and all sons are normal.

Case-4: Haemophilic father and carrier mother

Father ( X h Y ) x Mother ( X h X )

♂ h Y
X
♀
h h h h
X X X Haemophilic daughter X Y Haemophilic son

X h
X X Carrier daughter XY Normal son

Result: 1 Colour blind daughter : 1 Carrier daughter and 1 haemophilic son : 1 normal son

Case-5: Haemophilic father and haemophilic mother

Father ( X h Y ) x Mother ( X h X h )

♂ h Y
X
♀
h h h h
X X X Haemophilic daughter X Y Haemophilic son
h h h h
X X X Haemophilic daughter X Y Haemophilic son

Result: All the children are haemophilic.

GENETIC DISORDERS:-
 Genetic disorders arise due to defect or mutation in chromosomes.
 Genetic disorders are grouped into two categories.
(A) Chromosomal disorders
(B) Mendelian disorders.
(A) CHROMOSOMAL DISORDERS:-

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STRUCTURAL DISORDERS:-

1. Deletion: Loss of one or more genes from a Chromosome.

2. Duplication: Repetition of one or more genes on a chromosome.

3. Inversion: Here genes exchange their places. Inversion is of two types.

(i) Paracentric inversion: -

 It does not involve centromere.
(ii) Pericentric inversion:-
 It involves centromere.
 Pericentric inversion can change a sub-meta centric chromosome into a meta-centric chromosome.
4. Translocation: In this process two homologous chromosomes exchange genes with each other.

Types of Chromosomes on the basis of Centromere:

1. Acentric:- No centromere
2. Telocentric:- centromerce at end.
3. Acrocentric:- centromere near end. 13, 14, 15, 21, 22 & Y
4. sub-meta centric:- centromere near centre. 2, 4-12, 17, 18, & X
5. metacentric: centromere at centre. 1, 3, 16, 19 & 20
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POINT MUTATION ON GENE MUTATION:-
 A change which affects only one gene is called gene mutation or point mutation.
 Gene mutation is of following types;

ATG CAT ⎯⎯→ GTG TAT

1. Transition: When a purine base is replaced by another purine base or a pyrimidine base by another pyrimidine

ATG ATG ⎯⎯→ TTG ACG

2. Transversion : When a purine base is replaced by pyrimidine base and similarly a pyrimidine base by a purine

3. Frameshift: Sometimes due to loss or gain of one nucleotide the reading frame of the genetic code for an entire

CAT CAT CAT CAT ⎯⎯→ CAT ATC ATC ATC when C gets lost after CAT
protien changes.

4. Missense: A change in the genetic code due to replacement of a nucleotide (base) may give rise to a different protein
e.g. sickle cell haemoglobin.

GAAGAAGAA ⎯⎯→ GAAUAAAA synthesis stops as UAA in stop condon

5. Nonsense: If a genetic code changes such that it becomes a stop codon mid way, no protein is formed e.g.

6. Silent: When the changed nucleotide does not bring about any phenotypic change because it also codes for same
amino acid.
7. Null mutation: - A mutation that completely eliminates a gene is called null mutation.

CHROMOSOMAL DISORDERS:-
 Chromosomal disorders are two types;
(A) Allosomal
(B) Autosomal

(A) Allosomal: These occur due to defect in sex chromosomes.

Example-1: Klinefelter syndrome [44+XXY]

 This disorder is caused by trisomy of X-chromosome in males.
 The chromosomal complements or karyotype of these persons is 44+XXY=47.
 It is an allosomal disorder in which males possess an X-chromosome as barr body.
 This disorder was first reported by Harry klinefelter.
 Somatic cells of klinefelter males possess one barr body.
Characteristics of Klinefelter syndrome:-
(i) They are sterile males.
(ii) Tall body stature with long limbs.
(iii) Scanty hair on the body.
(iv) They have enlarged breasts like females.
(v) They possess penis.
(vi) Sperms produced are defective & non-motile.
(vii) Under developed testes, prostate & genitalia.
Example-2: Turner syndrome [44+X or 44+X0=45]
 This genetic disorder is caused due to monosomy of X-chromosome in females.
 chromosomal complement of these persons is 44+X or 44+XO=45
 It is allosomal disorder in which females possess one less X-chromosome.
 Turner’s females do not possess barr body.
Characteristics of turner syndrome: -
(i) They are sterile females.
(ii) Body stature is short.
(iii) Shield shaped broad chest.

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 (iv) They have webbed neck.
(v) Ovaries are rudimentary.
(vi) Menstrual cycle is rarely happen.
(vii) Breasts are under development.
(B) Autosomal disorders:-
 These occur due to defect in autosomes.
Example-1: Down syndrome
 It is a genetical disorder that causes delay in physical & mental growth of children.
 Genetical cause of Down syndrome is trisomy of chromosome-21.
 Chromosomal complement of Down syndrome persons is 45+ XX or 45+XY (Trisomy of 21st chromosome).
 Late reproductive age of mother is a factor for Down syndrome.
Characteristics of Down syndrome persons:-
(i) Short body stature.
(ii) Small rounded & enlarged forehead.
(iii) Furrowed tonque
(iv) Partial open mouth.
(v) Small & scanty eyeball
(vi) Swollen lips & flattened nose.
(vii) Sagging salivary mouth.
(viii) Reduced limbs with short fingers & toes.

Other autosomal disorders:

Disorders Genetical cause Chromosome Symptoms
Edward’s syndrome Trisomy of 18th 45+XX Or 45+XY Cardiac and kidney malfunction,
chromosome mental retardation
Patau’s syndrome Trisomy of chromosome- 45+XX Or 45+XY Mental retardation, deafness,
13 cryptorchidism in males, small chin
Cri-du-chat Deletion of half part in 46 Cat like cry in newborns
syndrome short arm of chromosome-
5
Huntington’s Defect in chromosome-4 46 Gradual degradation of brain tissue,
disease shrinkage of brain.
Alzheimer’s disease Autosomal recessive (old 46 Mental deteriotion, memory loss.
age)

(B) Mendelian disorders:-

 These disorders inherits in Mendelian pattern of inheritance.
 These are of two types; sex linked & autosomal. Sex linked mendelian disorders are Haemophilia and
colourblindness.
Autosomal disorders:-
Example-1: Thalassemia
 Thalassemia is an autosomal recessive blood disorder.
 This disorder is caused due to excessive destruction of RBCs due to defective formation of globin chain because
of mutation.
 Generally haemoglobin molecule consists of 4 haeme parts & 4 polypeptides (two alpha & two beta globin
chains).
 Defect in these polypeptide chains causes thalassemia.
 Based on the defective globin chain, thalassemia is following types;
(i) Alpha-Thalassemia:
 It occurs due to the presence of defective alpha-globin genes on chromosome-16.

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  There are two genes responsible for alpha-globin synthesis; HBA1 & HBA 2.
 Persons with one defective allele are carrier while two defective alleles produce a-thalassemia minor.
 Three defective alleles produce Haemoglobin bart's in infants and haemoglobin-H in adults.
 When all the alleles are defective, they kill foetus, called hydrops fetalis.
(i) Beta-thalassemia:
 In this case decreased synthesis of Beta-globin occurs.
 Defect is due to alleles of HBB-genes present one chromosome - 11.
 Persons with one defective allele suffer from thalassemia minor.
 Persons with both defective alleles suffer from cooley's anaenia. (Thalassemia major)
(iii) Delta thalassemia :-
 It occurs due to defective alleles of HBD genes of chromosome-11.
 It is very rare. About 3% adults have delta globin chain.
LINKAGE:-
 Physical association of genes on a chromosome is called linkage.
 Phenomenon of linkage was demonstrated in Drosophila by T.H. Morgan in 1910.

Types:
Complete linkage Incomplete linkage
Two genes inherit together without being crossovered. crossover may occurs betn two linked genes.

Linkage group:-
 All genes located on the similar chromosome are called linked genes & the phenomenon as linkage.
 Each such chromosome form one Linkage group. Thus number of linkage groups would be equal to haploid
number of chromosomes in an organism.

Crossing over:-
 Exchange of chromosomal segments between two non- sister chromatids of homologous chromosomes is called
crossing over.
 Crossing over occurs during pachytene sub-stage of prophase-I of meiosis-I.
 The point of crossing over is called chiasma.
Timeline of crossing over:-
(i) Synapsis: - The process of pairing of homologous chromosomes. It occurs during zygotene.
(ii) Cross over: Chromatids overlap. It occurs during Pachytene.
(iii) Chiasma formation: Chiasma is visible in diplotene
(iv) Terminalization: it is the process of shrinkage of chiasma to the terminal of chromatids. It occurs during
Diakinesis sub stage

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