Principles of Inheritance and Variation

Down's syndrome is due to trisomy of 21^st chromosome. Patau's syndrome is trisomy of 13^th pair of chromosome and Edward's syndrome is due to trisomy of 18^th chromosome. Therefore, these all syndromes are caused due to change in autosomes.

Frameshift mutation occurs when one or more nucleotides are either added r deleted from DNA. Eg.

Original (wild type) message or reading frame

CAT GAT CAT GAT CAT GAT CAT

After Deletion

CAT GAT ATG ATC ATG ATC AT

Message out of frame.

Turner's Syndrome is a chromosomal condition that mainly affects development in females. It is a monosomic condition with 44 + XO genotype. It occurs by the union of an abnormal egg and normal sperm or vice versa. These are sterile females with rudimentary ovaries, underdeveloped breasts, small uterus, short stature and mental retardation.

The most common form of red-green colour blindness is an 'X chromosome linked recessive' disorder. The 'red' and 'green' genes are known to reside at the tip of the long arm of the X chromosome. Women have two copies of the X-chromosome, and so they may have normal colour vision, even if they carry one copy of the defective gene. Men have only one X-chromosome, and so will be colour blind.

In humans, sex of offspring is determined at the time of fertilization. Man is heterogametic i.e. having two types of gametes 22 + X and 22 + Y and is responsible for sex determination whereas female is homogametic i.e. having single type of gametes 22 + XX.

2n - 1 condition represents diploid set of chromosomes having loss of one chromosome, the presence of one unpaired chromosomes along with diploid set is called monosomy.

Trisomy is a chromosomal disorder characterized by an additional chromosome. Eg. Down syndrome.

 Nullisomy is a genetic condition involving the lack of both the normal chromosomal pairs for a species. This condition is represented by 2n - 2.

Tetrasomy is a form of aneuploidy with the presence of four copies, instead of normal two, of particular chromosomes.

Test cross is a cross to know the genotype of the individual.

For eg, Tall white and dwarf black can be assumed as TtWw and ttww respectively.

Gametes of TtWw = TW, Tw, tW, tw

Gametes of ttww == tW

Thus the ratio of TtWw : Ttww : ttWw : ttww is 1 : 1 : 1 :1.

When a homozygous red flowered plant is crossed with homozygous white flowered plant in F₁ generation, then all red flowered are because red colour is dominant on white colour of flower.

Crossing over is the mutual exchange of segments of homologous chromatids during first meiotic prophase.

Synapsis is the fusion of chromosome pairs at the start of meiosis.

DNA replication precedes the start of Meiosis I. During Prophase I, homologous chromosomes pair and form synapses. The paired chromosomes are called bivalents and the formation of chiasmata caused by genetic recombination becomes apparent.

Dyad formation is a double chromosome resulting from the splitting of a tetrad during meiosis (germ cell formation).

Pure line is true breeding genotype. A line in which homozygous individuals produce only homozygous offspring. Thus pure line breed refers to homozygosity only.