INHERITANCE AND VARIATION

INHERITANCE AND VARIATION

 

Heredity or Inheritance: The transmission of genetic information from one generation to the next generation is known as heredity or inheritance.

Factor: The particle that are responsible for the expression of a particular trait or character is called factor.

Genes: A segment of DNA responsible for inheritance and variation are called genes.

Character: The specific feature of an organism e.g. height of the stem.

Trait: An inherited character and its detectable variant e.g. Tall or dwarf.

Gene: It is a particular segment of DNA which is responsible for the inheritance and expression of that character.

Alleles or Allelomorphs : The two or more alternative forms of  a  gene  (factor)  are called alleles of each other.

 Dominant allele: The allele which expresses its trait even in the presence of an alternative allele   is   called dominant allele

Recessive  allele:  The allele which can not express its trait in presence of an alternative allele   is   called recessive allele

Phenotype: The external appearance of an individual for any trait is called a phenotype.

Genotype: Genetic constitution or genetic makeup of an organism with respect to a particular trait is called a genotype.

Homozygous (pure): An individual possessing identical alleles for a particular trait is called homozygous or pure for that trait.

Heterozygous: An individual possessing different alleles for a particular trait, is called heterozygous. The heterozygous individual is also called hybrid.

Pure line: An individual or a group of individuals (population) which is homozygous or true breeding for one or more traits is called pure line.

Monohybrid: It is heterozygous for one trait and is produced from a cross between two pure parents differing in a single pair of contrasting character.

F1 generation: It refers to the first filial generation obtain by the cross between any two pure parents.

F2 generation: The second-generation (progeny) produced by selfing (inbreeding) of the F1 generation offsprings are called second filial generation.

Punnett square/checkerboard: It is a probability table representing different permutations and combination of fertilization between gametes of the opposite mating types or it can also be defined as a diagrammatic representation of a particular cross to predict the progeny of a cross.

Homologous Chromosomes: The morphologically, genetically and structurally identical chromosomes are called homologous chromosomes.

Back cross: The cross of F1 progeny with either of  its parent is called back cross.

Test cross: The cross of F1 progeny with its recessive parent only is called a test cross.

Phenotypic ratio: The ratio of offsprings produced in F2 and subsequent generation with respect to their physical appearance is called phenotypic ratio.

Genotypic ratio:  The ratio of offsprings produced in F2 and subsequent generation with respect to their genetic makeup or genotype is called genotypic ratio.

Monohybrid cross: A cross between two pure parents differing from each other in only one pair of contrasting character is called a monohybrid cross.

Dihybrid cross: A cross between two pure parents differing from each other in two pair of contrasting character is called a monohybrid cross.

Law of Dominance:  The law states that “When two homozygous individuals with one or more sets of  contrasting  characters  are  crossed,  the alleles (characters) that appear in F1 are dominant and those which do not appear in F1 are recessive”.

Law of Segregation : The law states that “When hybrid (F1) forms gametes, the alleles segregate from each other and enter in different gametes”. The gametes formed are pure in that they carry only one allele each (either dominant allele or recessive allele). Hence, this law is also described as the “Law of purity of gametes”.

Law of Independent Assortment: The law states that “When hybrid possessing two (or more) pairs of contrasting factors (alleles) forms gametes, the factors in each pair segregate independently of the other pair”.

Intragenic interactions : Interaction occurs between the alleles of the same gene is called intragenic interaction e.g. incomplete dominance and co-dominance.

Intergenic (non-allelic) interactions: Interaction occurs between the alleles of different genes present on the same or different chromosomes is called intergenic interactions. e.g. pleiotropy, polygenes, epistasis, supplementary and complementary genes, etc.

Incomplete dominance: In the incomplete dominance, both the alleles (genes) of an allelomorphic pair express themselves partially. One allele (gene) cannot suppress the expression of the other allele  (gene)  completely.

Co-dominance: In co-dominance, both the alleles (genes) of an allelomorphic pair express themselves equally in F1 hybrids. Such alleles which are able to express themselves equally independently in F1 generation is called co-dominance allele and the phenomenon is called co-dominance.

Multiple alleles: More than two alternative forms (alleles) of a gene occupying the same locus on a chromosome is known as multiple alleles.

Pleiotropy: When a single gene controls two (or more) different traits, it is called pleiotropic  gene  and  the  phenomenon  is  called  pleiotropy  or  pleiotropism.

Chromosome: Chromosomes are filamentous bodies present in the eukaryotic nucleus.

Ploidy: The number of homologous sets of chromosomes in a cell is called ploidy.

Euploidy: When the chromosome number in a cell is the exact multiple of the haploid number of the chromosome, then it is called euploidy.

Aneuploidy: When the chromosome number is not the exact multiple of the haploid set, it is described as Aneuploidy. or the abnormal sets of the chromosome is called aneuploidy.

Haploid: One set of chromosomes (n).

Diploid: Two sets of chromosomes (2n).

Triploid: Three sets of chromosomes (3n).

Tetraploid: Four sets of chromosomes (4n).

Centromere: The point at which two chromatids of the chromosome are attached are called centromere or primary constriction.

Kinetochore: Primary constriction consists of a disk shape plate called the kinetochore.

Secondary constriction: In addition to the primary constriction, the chromosome also shows the presence of secondary constriction which is also called nucleolar organizer as it helps in the organisation of nucleolus.

Chromatid: One half of a replicated chromosome is called a chromatid.

Chromonema: A long, unbranched, slender, highly coiled DNA thread  like structure is called Chromonema.

Acrocentric chromosome: In this, the centromere is present more towards the one end   (j shaped).

Telocentric chromosome: In this, the centromere is present at the tip of the chromosome (i shaped).

  

Sub-metacentric: In this, the centromere is present slightly away from the centre of the chromosome  (L shaped ).

Metacentric: In this, the centromere is present in the centre of the chromosome (V-shaped).

Telomeres: The ends of chromosome (i.e. chromatids) are known as telomeres.

Sex chromosome: The chromosomes which are responsible for the determination of sex are known as sex chromosomes (Allosomes).

Euchromatin: The chromosome material which represents the major genes that are involved in transcription, loosely packed and stained strongly are called euchromatin.

Heterochromatin: The chromosome material which represents the major genes that are transcriptionally inactive, densely packed and stained lightly are called heterochromatin.

Linkage: Tendency of two or more genes present on the same chromosomes to inherit together is known as linkage.

Linked genes: the genes having a tendency to be inherited together are called linked genes.

Complete linkage: The linked genes which are closely located on the chromosome do not separate by crossing over and inherit together are called completely linked genes and the phenomenon of their inheritance is called complete linkage.

Incomplete linkage: The linked genes which are located away from each other on the same chromosome and have chances of separation by crossing over are known as incompletely linked genes.

Sex-linkage: The transmission of  X - linked and Y-linked genes from parents to offspring is called sex-linked inheritance.

Complete sex linkage: It is exhibited by genes located on non-homologous regions of  X and Y chromosomes. They inherit together because crossing over does not occur in this region.

Incomplete sex linkage: The linkage exhibited by genes located on homologous regions of X and Y chromosomes are called incomplete sex linkage.

Synopsis: The pairing of the homologous chromosome is called synapsis.

Autosomal inheritance: Transmission of body characters other than the sex-linked traits from parents to their offsprings through autosomes are called autosomal inheritance.

Widow’s peak: A prominent “V” shaped hairline on the forehead is called a widow’s peak.

Phenylketonuria (PKU): It is an inborn metabolic disorder that causes an amino acid called phenylalanine to build up in the body due to recessive autosomal genes

Hypertrichosis: Excessive development of hair on ear pinna is called hypertrichosis.

Ishihara’s Test: It is a colour perception test for red-green colour deficiency.

Colour blindness: It is X-linked recessive disorder in which a person fails to identify the colour.

Haemophilia (Bleeder’s disease): Haemophilia is an X-linked recessive disorder in which blood fails to clot or coagulates very slowly. 

Mendelian disorders: The genetic disorder which is mainly caused by the changes or alteration in a single gene or due to the abnormalities in the genome.

Chromosomal disorders: The genetic disorder is mainly caused due to the anomaly or abnormalities in the chromosome.

Thalassemia: The haemolytic disease caused by faulty synthesis of haemoglobin.

Down’s syndrome: The trisomy of the 21st chromosome is called Down’s syndrome.

Turner’s syndrome: A sex chromosomal disorder in which female is born with only one X-chromosome. (44+X0).

Klinefelter’s syndrome: A sex chromosomal disorder in which male is born with one extra X-chromosome. (XXY males) :