To understand the concepts of Mendelian genetics, we need to understand (and define) a few terms - so as to have the basic vocabulary for a genetics discussion.

Gregor Johann Mendel

The article has to begin with a brief account of G. J. Mendel.
Birth 22-Jul-1822, Austria
Education Graduation
Interest Hybridization
Famous Work (1856) "Experiments in Plant Hybridization"
Major Contributions
  • Mendelian Laws of Inheritance.
  • A new branch of biology - Genetics

Glossary of Terms used in Heredity and Variation

What is a genetic character?

It is something which is observed / studied / of interest. More formally, it be stated as "a well defined morphological or physiological feature of an organism". Examples are Height, ginning percentage for cotton.

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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Modes of Genetic Inheritance

There are six modes in which an offspring can inherit a trait from his/her parents.
Mode of InheritanceExamples
Autosomal DominantPolycystic kidney disease, Huntington's disease, hereditary spherocytosis
Autosomal RecessiveSickle Cell anemia, cystic fibrosis, Tay-Sachs disease
X-Linked DominantRett Syndrome, Vitamin-D resistant rickets, Alport syndrome
X-Linked Recessivehemophilia, color blindness
Y-LinkedHypertrichosis pinnae, Azoospermia, Retinitis Pigmentosa
Maternal (Cytoplasmic)All the children are affected if mother is affected.


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When fertilization takes place, the sperm fertilizes the egg.
Now, sperms are of two types: Haploid-Y or Haploid-X,
whereas the egg is of just one type: Haploid-X.
So the resulting child may be Diploid-XY (Boy) or Diploid-XX (Girl).

However, in girls, the double dose of X chromosomes isn't really necessary, and one of the X chromosome is rendered inactive, by a process called Lyonization.