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

Trisomic condition of chromosome - 21 results in Down's syndrome. It is characterized by severe mental retardation, defective development of the central nervous system, increased separation between the eyes, flattened nose, ears are malformed, mouth is constantly open and the tongue protrudes.

Genes in the non-homologous region of the Y-chromosome are inherited directly from male to male. In humans, the Y-linked or holandric genes for hypertrichosis (excessive development of hairs on pinna of the ear) are transmitted directly from father to son, because males inherit the Y chromosome from the father. Female inherits only X chromosome from the father and are not affected.

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

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.

The possible genotypes of a B-blood group person are I^BI^Bor I^Bl^O.

When three or more alleles of a gene that control a particular trait occupy the same locus on the homologous chromosome of an organism, they are called multiple alleles and their inheritance is called multiple allelism.

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

Individuals have parts of their body expressing male characters and other parts of the body expressing female characters. The organism is made up of tissues of male and female genotypes and represents a mosaic pattern.

Mary Lyon suggested that Barr bodies represented an inactive chromosome, which in females becomes tightly coiled into a heterochromatin, a condensed and visible form of chromatin. The number of Barr bodies observed in cell was one less than the number of X-Chromosome. XO females have no Barr body, whereas XXY males have one Barr body.

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

The inactivation of an X chromosome one of the two X chromosomes in every cell in some female mammal is randomly inactivated early in embryonic development and named after geneticist Mary Lyon.
The inactive X chromosome is silenced by it being packaged in such a way that it has a transcriptionally inactive structure called heterochromatin found in nucleoplasm of nucleus.

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.

1. The X chromosome which is common to male and female carries a number of genes. Such genes are called sex linked genes and the characters controlled by them are termed as sex-linked characters. The inheritance of sex linked genes or traits is known as sex linked inheritance.
2. Sex linked inherited traits are more common in males than females, because males are hemizygous (XY) and therefore express that traits when they inherit one mutant allele.
3. Any male receiving X linked recessive allele from his mother will express the trait as his y  chromosome has no corresponding allele.
4. If a recessive X linked gene causes disorder then more males than females have the disorder because the disorder is caused by a single allele in males. Their y chromosome does not carry any corresponding allele. So that sex linked recessive characters are mole in moles.