Klinefelter's syndrome - 44AA+XXY
Turner's syndrome - 44AA+XO

(a) A = Phenylalanine hydroxylase
(b) B = Tyrosinase

Red-green colour blindness or daltonism, haemophilia and Duchenne's muscular dystrophy

SRY - Sex Determining region Y
TDF - Testes Determining Factor

Wiener proposed the existence of eight alleles (R¹, R², R⁰,R^Z, r, r¹, r¹¹, r^Y) at a single Rh locus. All genotypes carrying a dominant 'R allele' (R^l, R² ,R⁰,R^Z)will produce 'Rh-positive' phenotype and double recessive genotypes (rr, rr¹, rr¹¹, rr^y) will give rise to Rh-negative phenotype.

Antigens A and Antigen B was discovered by Karl Landsteiner. Antigen AB was discovered by Von De Castelle and Sturli.

A person with a recessive gene for haemophilia lacks a normal clotting substance (thromboplastin) in blood, hence minor injuries cause continuous bleeding, leading to death

Symptoms:
1. Severe mental retardation
2. Defective development of the CNS.
3. Increased separation between the eyes.
4. Flattened nose, ears are malformed
5. Mouth is constantly open and the tongue protrudes.

The genes present in the differential region of Y chromosome are called Y- linked or holandric genes. The Y linked genes have no corresponding allele on X chromosome. The Y linked genes inherit along with Y chromosome and they are phenotypically express only in the male sex.

It is the transmission of a gene from mother to son or father to daughter.
These patterns of inheritance are called criss-cross inheritance (or) skip generation inheritance in which a character is inherited to the second generation through the carrier of first-generation.
E.g., Wing color in the magpie moth (Abraxus) feather pattern in chickens

Haplodiploidy is a sex determination system in which males develop from unfertilized eggs are haploid, and female develops from fertilized egg and is diploid.
It is sometimes called arrhenotoky.
e.g., Honeybees, ants, and wasps.

(a) Thalassemia
Thalassemia is an autosomal recessive disorder. It is caused by gene mutation resulting in excessive destruction of RBC's due to the formation of abnormal hemoglobin molecules. Normally haemoglobin is composed of four polypeptide chains, two alpha, and two beta globin chains. Thalassemia patients have defects in either the alpha or beta globin chain causing the production of abnormal haemoglobin molecules resulting in anaemia.
Thalassemia is classified into alpha and beta based on which chain of haemoglobin molecule is affected. It is controlled by two closely linked genes HBA1 and HBA2 on chromosome 16. Mutation or deletion of one or more of the four alpha gene alleles causes Alpha Thalassemia. In Beta Thalassemia, production of beta globin chain is affected. It is controlled by a single gene (RBB) on chromosome 11. It is the most common type of Thalassemia and is also known as Cooley's anaemia. In this disorder, the alpha chain production is increased and damages the membranes of RBC.
(b) Albinism
Albinism is an inborn error of metabolism, caused due to an autosomal recessive gene. Melanin pigment is responsible for skin colour. Absence of melanin results in a condition called albinism. A person with the recessive allele lacks the tyrosinase enzyme system, which is required for the conversion of dihydroxyphenyl alanine (DOPA) into melanin pigment inside the me1anocytes. In an albino, melanocytes are present in normal numbers in their skin, hair, iris, etc., but lack melanin pigment.
 3, 4 - dihydroxy phenylalanine (DOPA) \(\underrightarrow { Tyrosinase } \)  Melanin

Rh incompatability has great significance in childbirth. If a woman is Rh negative and the man is Rh positive, the foetus may be Rh positive having inherited the factor from its father. The Rh negative mother becomes sensitized by carrying Rh-positive foetus within her body. Due to damage of blood vessels, during childbirth, the mother's immune system recognizes the Rh antigens and gets sensitized. The sensitized mother produces Rh antibodies. The antibodies are IgG type which are small and can cross placenta and enter the foetal circulation. By the time the mother gets sensitized and produce anti 'D' antibodies, the child is delivered.
Usually, no effects are associated with exposure of the mother to Rh positive antigen during· the first childbirth, subsequent Rh positive children carried by the same mother may be exposed to antibodies produced by the mother against Rh antigen, which are carried across the placenta into the foetal blood circulation. This causes haemolysis of foetal RBCs resulting in haemolytic jaundice and anaemia. This condition is known as Erythroblastosis foetalis or Haemolytic disease of the newborn (HDN).

In human beings the diploid (2n) body cell has 46 chromosomes (23 pairs).
i) chromosomal abnormalities are caused by errors is the number or structure of chromosome.
ii) Failure of chromatids to segregate during cell division resulting in the gain or loss of of one or more choromosomes called aneuploidy. It is caused by the nondisjunction of chromosomes.
iii) There abnormalities causes various syndromes in human beings. They are

1. Down's Syndrome/ Trisomy-21:
Trisomic condition of chromosome - 21 
Symptoms :
(i) severe mental retardation,
(ii) Defective development of the central nervous system,
(iii) increased separation between the eyes,
(iv) flattened nose, ears are malformed,
(v) mouth is constantly open and the tongue protrudes.
2. Patau's Syndrome/ Trisomy-13
i) Trisomic condition of chromosome l3 results in Patau's syndrome.
ii) Meiotic non disjunction is thought to be the cause for this chromosomal abnormality.
ii) It is characterized by
symptoms
(i) multiple and severe body malformations as well as profound mental deficiency.
(ii) Small head with small eyes,
(iii) cleft palate,
(iv) malformation of the brain
(v) internal organs 
b. Allosomal abnormalities is human beings:
Mitotic or meiotic non-disjunction of sex chromosomes causes allosomal abnormalities. Several sex chromosomal abnormalities have been detected. Eg. Klinefelter's syndrome and Turner's syndrome.
1. Klineletter's Syndrome(XXY Males):
(i) It is due to the presence of an additional copy of the X chromosome resulting in a karyotype of 47, XXY.
(ii) Persons with this syndrome have 47 chromosomes (44AA+XXY).
symptoms
sterile males, tall, obese, with long limbs, high pitched voice, under developed genitalia and have feeble breast (gynaecomastia) development.
2. Turner's Syndrome (XO Females):
(i) It is due to the loss of a X chromosome resulting in a karyotype of 45, X.
(ii) Persons with this syndrome have 45 chromosomes (44 autosomes and one X chromosome) (44AA + XO) and are sterile females.
symptoms
Low stature, webbed neck, under developed breast, rudimentary gonads lack of menstrual cycle during puberty, are the main symptoms of this syndrome.

In human beings a dominant X - linked gene is necessary for the formation of colour sensitive cells, the cones. The recessive form of this gene is incapable of producing colour sensitive cone cells. Homozygous recessive females (X^cX^c) and hemizygous recessive males (X^cY) are unable to distinguish red and green colour. The inheritance of colour blindness can be studied in the following two types of marriages.
(i) Marriage between colour blind man and normal visioned woman:
A marriage between a colour blind man and a normal visioned woman will produce normal visioned male and female individuals in Fl generation but the females are carriers. The marriage between a Fl normal visioned carrier woman and a normal visioned male will produce one normal visioned female, one carrier female, one normal visioned male and one colour blind male. Colour blind trait is inherited from the male parent to his grandson through carrier daughter, which is an example of criss-cross pattern of inheritance.

(ii) Marriage between normal visioned man and colour blind woman:
a) If a colour blind woman (X^CX^C) marries a normal visioned male (X^Cy), all F₁ sons will be colourblind and daughters will be normal visioned but are carriers.
b) Marriage between F₁ carrier female with a colour blind male will produce normal visioned carrier daughter, colourblind daughter, normal visioned son and a colourblind son in the F₂ generation.

(i) The inheritance of a trait that is determined by a gene located on one of the sex chromosomes is called sex linked inheritance.
(ii) Genes present on the differential region of X or Y chromosomes are called sex linked genes. The genes present in the differential region of "X" chromosome are called X linked genes. The X-linked genes have no corresponding alleles in the Y chromosome.
(iii) The genes present in the differential region of Y chromosome are called Y- linked or holandric genes. The Y linked genes have no corresponding allele in X chromosome.
(iv) The Y linked genes inherit along with Y chromosome and they phenotypically express only in the male sex.
(v) Sex linked inherited traits are more common in males than females because, males are hemizygous and therefore express the trait when they inherit one mutant allele.
(vi) The X - linked and Y - linked genes in the differential region (non-homologus region) do not undergo pairing or crossing over during meiosis. The inheritance of X or Y linked genes is called sex-linked inheritance.
X linked Inheritance - Eg. Colour blindness
Y linked Inheritance - Eg. hypertrichosis 

Heterogametic Males:
In this method of sex determination the males are heterogametic producing dissimilar gametes while females are homogametic producing similar gametes.
It is of kinds XX-XO type and XX-XY type.
(i) XX-XO Type
(a) This method of sex determination is seen in bugs and some insects such as cockroaches and grasshoppers.
(b) The female with two X chromosomes are homogametic (XX) while the males with only one X chromosome are heterogametic (XO).
(c) The presence of an unpaired X chromosomes determines the male sex. The males with unpaired 'X' chromosome produce two types of sperms, one half with X chromosome and other half without X chromosome.
(d) The sex of the offspring depends upon the sperm that fertilizes the egg.
 
(ii) XX-XV type (Lygaeu Type):
This method of sex determination is seen in human beings and in Drosophila.
(a) The females are homogametic with XX chromosome, while the males are heterogametic with X and Y chromosome.
(b) Homogametic females produce only one kind of egg, each with one X chromosome, while the heterogametic males produce two kinds of sperms some with X chromosome and some with Y chromosome.
(c) The sex of the embryo depends on the fertilizing sperm. An egg fertilized by an 'X' bearing sperm produces a female, if fertilized by a 'Y' bearing sperm, a male is produced.

Genetic basis of blood grouping in human population
1. ABO blood grouping in human shows the phenomenon of codominance.
2. Alleles are able to express themselves independently when present together.
3. More than two alleles govern the same character.
4. Karl Landsteiner discovered two kinds of antigens called antigen 'A' and antigen 'B' on the surface of RBC's of human blood.
5. In 1902 two of Landsteiner's students Von De Castelle and Sturli discovered fourth rarest blood group 'AB' (universal recipient).
6. Bernstein in 1925 discovered that the inheritance of different blood groups in human beings is determined by a number of multiple allelle series.
7. The three autosomal alleles located on chromosome are concerned with the determination of blood group in any Person.
8. The gene controlling blood types has been labeled as 'L' (after the name of the discoverer, Landsteiner) or I (from iso -agglutination).
9. The I gene exists in three allelic forms, I^A, I^B and I^o specifies A antigen.
10. I^B allele determines B antigen and I^o allele specifies no antigen.
11. Individuals who possess these antigens in their fluids such as the saliva are called secretors
Genetic basis of the human ABO blood groups
Genotype ABO blood group phenotype Antigens present on red blood cell Antibodies present in blood plasma I^AI^A Type A A Anti -B I^AI^O Type A A Anti -B I^BI^B Type B B  Anti -A I^BI^O Type B B Anti -B I^AI^B Type AB A and B Neither Anti - A nor Anti -B I^OI^O Type O Neither A nor B Anti - A and anti -B

Trisomic condition of chromosome 13 results in Patau's syndrome. Meiotic non-disjunction is thought to be the cause for this chromosomal abnormality. It is characterized by multiple and severe body malformations as well as profound mental deficiency. Small head with small eyes, cleft palate, malformation of the brain and internal organs are some of the symptoms of this syndrome.

(a) Gypsy moth -ZW - ZZ type (ZW-females, ZZ - males)
(b) Human beings - XX - XY type (XX-females, XY - males)
(c) Butterflies - ZO - ZZ type (ZO-females, ZZ - males)

(i) A gene, sex-lethal (SxL) located on the X chromosome, in Drosophila directs female development.
(ii) This gene has two states of activity. When it is 'on' it directs female development and when it is 'off maleness ensures.
(iii) Other genes located on the X chromosome and autosomes regulates sex switch gene.
(Iv) However, the Ychromosome of Drosophila is required for male fertility.

Affected individuals lack the tyrosinase enzyme which converts DOPA to melanin pigment responsible for skin colour. It is caused due to autosomal recessive genes

Karyotyping is a technique through which a complete set of chromosomes is separated from a cell and the chromosomes are arranged in pairs. An idiogram refers to a diagrammatic representation of chromosomes.

(i) A dominant X - linked gene is necessary for the formation of colour sensitive cells, the cones. which helps to distinguish colours.
(ii) A recessive form of the gene is incapable of producing the cones. Hence the affected person is unable to distinguish red and green colour.

(i) In Drosophila the sex is determined by the balance between the genes'for femaleness located on the 'X' chromosome and those for maleness located on the 'auto somes' (A).
(ii) Hence the sex of an individual is determined by the ratio of its X chromosome to that of the autosome A sets. This ratio is termed as sex index and is expressed as
Sex Index \(=\frac{Number\ of\ X\ chromosomes(X)}{Number\ of\ X\ automosome}\)
change in this ratio leads to a changed sex phenotype Eg: Drosophila

(i) It helps in gender identification.
(ii) It is used to detect the chromosomal aberrations like deletion, duplication, translocation, invasion and nondisjunction of chromosomes.  
(iii) It helps to identify the abnormalities of chromosomes like aneuploidy.
(iv) It is also used in predicting the evolutionary relationships between species.
(v) Genetic diseases in human beings can be detected by this technique.

Sex determination in honeybees
sex determination is as follows
1. Haploid sex determination is seen in honeybees, it is based on the number of sets of chromosome, an individual receives.

2. When the ovum is fertilized by a male gamete the zygote (2n = 32) develops into a female i.e., a queen or worker.
3. When the haploid ovum develops by parthenogenesis. i.e., without fertilization, a male individual, called drone is formed as adult male.
4. The male honeybees (drone) is haploid (with n = 16) and forms sperms by mitosis.