1. Heredity and Variation:

(i) Genetics is a branch of biology that deals with the collective study of heredity and variations.

(ii) Heredity is the transmission of genetic characters from parent to offspring.

(iii) Individuals of the same species have some differences; these are called variation.

(iv) Term genetics was given by W. Bateson (Father of modern genetics).

2. Mendelian Genetics:

Characters	Contrasting Traits
Stem height	Tall/dwarf
Flower colour	Violet/white
Flower position	Axial/terminal
Pod shape	Inflated/constricted
Pod colour	Green/yellow
Seed shape	Round/ wrinkled
Seed colour	Yellow/green

(i) In the mid- $19^{th}$ century, Gregor Johann Mendel, conducted hybridisation experiments on garden peas (Pisum sativum) for seven years ( $1856 - 1863$ ) and proposed the laws of inheritance.

(ii) Mendel for the first time applied statistical analysis and mathematical logic in biology with a large sampling size, giving greater credibility to his work.

(iii) Mendel selected $14$ true-breeding pea plant varieties, as pairs that were similar except for one character with contrasting traits.

(iv) Mendel’s experiment involved $4$ steps as selection, hybridization, selfing and calculations. His results led to the formation of laws of genetics.

(v) Mendel performed monohybrid and dihybrid crosses and gave three principles of inheritance.

(a) Law of dominance

(b) Law of segregation or law of purity of gametes.

(vi) He proposed that the ‘factors’ (later named as genes) regulates the characters that are found in pairs known as alleles.

(a) The dominant characters are expressed when factors are in heterozygous condition (Law of Dominance).

(vii) The recessive characters are only expressed in homozygous conditions. The characters never blend in heterozygous condition.

(a) Law/principle of segregation states that when a pair of contrasting factor or gene are brought together in a hybrid, these factors do not blend or mix up but simply associate themselves and remain together and separate at the time of gamete formation.

(b) The above law is also known as the law of purity of gametes because each gamete is pure i.e., having either $T$ (i.e., the gene for tallness) or $t$ (i.e., the gene for dwarfness). Mendel formulated this law with the help of a monohybrid cross.

(c) Principle of independent assortment states that genes of different characters located in different pairs of chromosomes are independent of one another in this segregation during gamete formation.

3. Principles of segregation and independent assortment can be explained based on the chromosomal theory of inheritance.

(i) To explain this a cross made to study the inheritance of two pairs of contrasting traits and this is explained by a dihybrid cross

4. Test cross:

(i) This is a cross that is performed for those plants whose genotype is to be determined and it is done when an organism showing a dominant phenotype is crossed with the recessive parent instead of self-crossing.

(ii) The results of a typical test cross where the violet colour flower $(W)$ is dominant over the white colour flower $(w)$ .

5. Deviation From Mendelian Genetics:

(i) Incomplete dominance: Deviation from Mendel's law of dominance

(a) Incomplete dominance was first discovered by Correns in Mirabilis jalapa, called ' $4$ o'clock plant’ and is also seen in Antirrhinum majus plant (Snapdragon or Dog flower).

(b) $F_{1}$ generation is different from both parents.

(d) Dominant factors are unable to express completely.

(e) Three different types of flower colour, such as red, white, and pink is observed in Mirabilis jalapa.

(f) When a cross between red and white flower plants was done, the pink flower was obtained in $F_{1}$ generation. The reason for this is that the genes of red colour are incompletely dominant over the genes of white colour.

(g) After selfing the pink flower of $F_{1}$ generation of pink flower the F₂ generation showed red, pink, and white in the ratio $1 : 2 : 1$ instead of a normal monohybrid cross $3 : 1$ .

(h) The ratio of phenotype and genotype of $F_{2}$ generation in incomplete dominance is always the same.

(ii) Multiple Allelism: Deviation from Mendel's law of unit factors

(a) The presence of more than two alleles for a gene is known as multiple allelism.

(b) A good example is $A B O$ blood grouping in humans an example of multiple alleles.

(c) The $R B C$ ’s has glycoproteins (sugar)on its surface and is controlled by the gene $I$ , which has three alleles I^A, I^B and i.

(d) The alleles I^A and I^B produce a slightly different form of sugar while allele ‘i’ does not produce any sugar.

(e) Humans are diploid organisms; each person possesses any two of the three I gene alleles.

(f) $I^{A}$ and $I^{B}$ are completely dominant over $i$ , in other words, when $I^{A}$ and $i$ are present only $I^{A}$ expresses (because 'i' does not produce any sugar), and when $I^{B}$ and i are present $I^{B}$ expresses.

(g) But when $I^{A}$ and $I^{B}$ are present together they both express their own types of sugars: this is because of co-dominance. Hence $R B C$ have both $A$ and $B$ types of sugars. Since there are three different alleles, there are six different combinations of these three alleles that are possible, and therefore, a total of six different genotypes of the human $A B O$ blood type is present.

Blood group	Genotype	An antigen on the surface of RBC	The antibody in blood plasma	Sugar moiety on the surface of RBC
$A$	$I^{A} I^{A}; I^{A} i$	Antigen $A$	Antibody b	N-acetylgalactosamine
$B$	$I^{B} I^{B}; I^{B} i$	Antigen $B$	Antibody a	$β - galactose$
$AB$	$I^{A} I^{B}$	Antigen $A$ , Antigen $B$	Nil	N-acetylgalactosamine; $β - galactose$
$O$	$ii$	Nil	Antibody a, Antibody b	Nil

(iii) Co-dominance: Deviation from Mendel's law of dominance

$AB$ blood group is an example of this:

Table showing the genetic basis of blood grouping in the human population
Allele from Parent $1$	Allele from Parent $2$	Genotype of Offspring	Blood-types Offspring
$I^{A}$	$I^{A}$	$I^{A} I^{A}$	$A$
$I^{A}$	$I^{B}$	$I^{A} I^{B}$	$AB$
$I^{A}$	$i$	$I^{A} i$	$A$
$I^{B}$	$I^{A}$	$I^{A} I^{B}$	$AB$
$I^{B}$	$I^{B}$	$I^{B} I^{B}$	$B$
$I^{B}$	$i$	$I^{B} i$	$B$
$i$	$i$	$ii$	$O$

The $F_{1}$ generation resembles both parents.

(iv) Polygenic Inheritance:

(a) The traits that are generally controlled by three or more genes (multiple genes).

(b) It accounts for the influence of the environment.

(d) In a polygenic trait the phenotype reflects the contribution of each allele, i.e., the effect of each allele is additive.

(e) The dominant alleles $(AABBCC)$ will have the darkest skin colour, the recessive alleles $(aabbcc)$ will have the lightest skin colour and the genotype with three dominant alleles and three recessive alleles $(AaBbCc)$ will have an intermediate skin colour.

(v) Pleiotropy:

(a) Gene which controls more than one character showing multiple phenotypic effects.

(b) In a pea plant a single gene influences the seed coat colour, red spot-on leaf, flower colour.

(c) In Drosophila, the recessive gene of vestigial wings also influence some other characters like the structure of reproductive organs, longevity, bristles on wings, and reduction in egg production.

(d) Examples of the pleiotropic gene in human is phenylketonuria, sickle cell anaemia, cystic fibrosis etc.

6. Chromosomal Theory of Inheritance:

(i) It is proposed by W.S. Sutton and Boveri in $1902$ .

(ii) This theory states that chromosomes are the vehicle of hereditary information and expression as genes are present over them.

(iii) Gametes serve as the bridge between two successive generations.

(iv) Male and female gametes play an equal role in contributing hereditary components of the future generation.

(v) Only the nucleus of sperm combines with the ovum. Thus, hereditary information is contained in the nucleus.

(vi) Chromatin in the nucleus is associated with cell division in the form of chromosomes.

(vii) Any type of deletion or addition in the chromosomes can cause structural and functional changes in living beings.

(viii) A sort of parallelism is observed between Mendelian factors and chromosomes.

(a) A few genes or Mendelian factors are found in each chromosome.

(ix) Determination of sex in most animals and plants is affected by specific chromosomes. These chromosomes are called sex chromosomes.

(a) Chromosomes are also transferred from one generation to the next as in the case of genes (Mendelian factors).

(x) The number of chromosomes is fixed in each living species. These are found as homologous pairs in diploid cells. One chromosome from the father and the other contributed by the mother constitute a homologous pair.

(xi) Before cell division, each chromosome as a whole and the alleles of genes get replicated and are separated during mitotic division.

(a) Meiosis takes place during gamete formation. Homologous chromosomes form synapsis during the Prophase-I stage which in a later course get separated and transferred to daughter cells. Each gamete or a haploid cell has only one allele of each gene present in the chromosome.

(b) A characteristic diploid number is again established by the union of the two haploid gametes.

(xii) In the homologous chromosomes of a pure tall plant, allele $(T)$ is found for tallness in each chromosome. Likewise, in a pure dwarf plant $(t t)$ , allele $(t)$ is present in each chromosome. These homologous chromosomes get separated during meiotic division. Hence, each gamete possesses only one chromosome of each pair.

(xiii) Two kinds of gametes will be formed during gametogenesis, $50 %$ with the allele $(T)$ for tallness and $50 %$ with the allele for dwarfness $(t)$ . Random combination of these gametes will produce offspring in $F_{2}$ generation in the ratio of $25 %$ pure tall $(T T)$ , $50 %$ hybrid tall $(T t)$ and $25 %$ dwarf $(t t)$ .

(xiv) Verification of the chromosomal theory of inheritance was done by Thomas Hunt Morgan and his colleagues and proved the variation is brought about by sexual reproduction.

(xv) Morgan worked with fruit flies, Drosophila melanogaster.

(xvi) The fruit flies are the best choice because,

(a) Grown on a simple synthetic medium in the laboratory.

(b) They complete their life cycle in about two weeks.

(d) Clear differentiation of the sexes.

7. Linkage and Recombination:

(i) Morgan carried out several dihybrid crosses in Drosophila to study genes that were sex-linked.

(a) Morgan hybridised yellow-bodied, white-eyed females to brown-bodied, red-eyed males and inter-crossed their $F_{1}$ progeny.

(b) He observed that the two genes did not segregate independently of each other and the $F_{2}$ ratio deviated vary significantly from the $9 : 3 : 3 : 1$ ratio.

(c) Morgan explained that the two genes in a dihybrid cross were situated on the same chromosome, the proportion of parental gene combinations were much higher than the non-parental type.

(ii) Morgan attributed to the physical association or linkage of the two genes and coined the term linkage and recombination to describe the generation of non-parental gene combinations.

(iii) If genes were very tightly linked in the same chromosome, some genes showed very low recombination, while loosely linked showed higher recombination.

(iv) Alfred Sturtevant explained the genetic maps and used them to analyse the frequency of recombination between gene pairs by measuring the distance between genes.

$Linkage \propto \frac{1}{Distance between genes} \propto \frac{1}{% of recombination} \propto \frac{1}{Independent assortment}$

8. Sex Determination:

(i) It is the establishment of sex through differential development in an individual at an early stage of life.

(ii) It is the study of factors that are responsible for making individual male, female or hermaphrodite.

(iii) Wilson and Stevens proposed chromosomal theory for sex determination.

(a) Chromosomes are of two types – autosome and allosomes

Autosomes or somatic chromosomes regulate somatic characters.
Allosomes or heterosomes or sex chromosome are associated with sex determination.

(iv) Sex chromosomes first discovered by McClung in grasshopper.

(a) $X$ chromosome (called $X$ body) was discovered by Henking when he found that in the testes of male bug one chromosome has no homologue.

(v) There are $5$ types of mechanisms of sex determinations-

(a) In $X X - X Y$ type of sex determination, the female is homogametic i.e., produces only one type of gamete. Males are heterogametic producing two types of gametes. Example: Man, Drosophila, etc.

(b) In $Z W - Z Z$ type of sex determination, the female is heterogametic i.e., produces two types of gamete $(Z W)$ and the male individual is homogametic i.e., produces one type of gamete $(Z Z)$ .

(d) In $X X - X O$ type of sex determination, there is a deficiency of one chromosome in a male. In this type, the female is homogametic, and the male is heterogametic. It is found in grasshopper, cockroach, Ascaris, etc.

(e) In $Z O - Z Z$ type of sex determination, the female had an odd sex chromosome while males have two homomorphic sex chromosomes $(A A + Z Z)$ . The females are heterogametic and produce two types of eggs, male forming with one sex chromosome $(A + Z)$ and female forming without the sex chromosome $(A + O)$ . The males are heterogametic, forming similar types of sperms. It is found in some butterflies and moths.

(vi) Haplodiploidy is the sex determination method in which one sex is haploid (male) while the other is diploid (female), for example, honeybees. It also occurs in wasps and ants.

(a) The sex determination in honeybee is based on the number of sets of chromosomes an individual receives.

(b) An offspring formed from the union of a sperm and an egg develops as a female (queen or worker), and an unfertilised egg develops as a male (drone) by means of parthenogenesis.

(d) The males produce sperms by mitosis, they do not have a father and thus cannot have sons but have a grandfather and can have grandsons.

9. Mutation:

(i) Phenomenon which results in alteration of $D N A$ sequences that results in the changes in the genotype and the phenotype.

(ii) Loss (deletions) or gain (insertion/duplication) of a segment of $D N A$ , result in alteration in chromosomes resulting in abnormalities or aberrations.

(iii) Mutation also arises due to change in a single base pair of $D N A$ -point mutation example, sickle cell anaemia.

(iv) Deletions and insertions of base pairs of $D N A$ , causes frame-shift mutations.

(v) Chemical and physical factors that induce mutations, referred to as mutagens. Example, $U V$ radiations, ethidium bromide, etc.

10. Pedigree Analysis:

(i) Study of ancestral history of man of transmission of genetic characters from one generation to the next.

(ii) Dwarfism, albinism, colour blindness, haemophilia etc. are genetically transmitted characters.

(iii) To study and analyse them, a pedigree of genetic facts/data and the following symbols are used. This provides valuable information on the genetic makeup of human beings.

(iv) Used to identify the genetic disease that occurs in a family.

(v) Symbol used in pedigree.

(vi) The usual practice is to place the males first, on the left if one parent is not shown in a pedigree, it indicates that this individual was phenotypically normal. Solid circle indicate affected female and solid box indicates affected males.

The siblings are indicated in chronological order of birth

Affected male and female individual

Heterozygous for autosomal recessive

Carrier female of sex-linked recessive character or disease

Death of individual

Abortion or stillbirth (sex unspecified)

Consanguineous marriage

Parent with a male child affected with the disease

Number of the progeny of a couple

Monozygotic twins

Dizygotic twins

11. Mendelian Disorder in Humans:

(i) A disease caused by an abnormality in an individual’s $D N A$ .

(ii) Grouped into two categories: Mendelian disorders and chromosomal disorders.

(iii) Mendelian disorders are chiefly determined by alteration or mutation in a single gene. E.g., Haemophilia, cystic fibrosis, sickle cell anaemia, thalassemia, colour blindness, phenylketonuria, etc.

(a) Haemophilia is an X-linked inherited disorder of the blood in which an essential clotting factor is either partly or completely missing.

(b) In sickle-cell anaemia, glutamic acid (glutamine) is replaced by valine at the sixth position in $β$ chain of haemoglobin. The RBCs become sickle-shaped as compared to the normal ones.

It is an autosomal recessive disorder, whose genes are present on chromosome number 11.

(c) Thalassemia is due to an autosomal mutant gene. It is a group of genetic disorder which results from the defective synthesis of subunits of haemoglobin (α and β chains of haemoglobin).

It is an autosomal recessive disorder, whose genes are present on chromosome number 11 and chromosome number 16.

(d) Gaucher's disease is a genetic disorder associated with abnormal fat metabolism.

(e) Huntington's chorea is a disease caused by a dominant gene.

(f) Phenylketonuria is a metabolic genetic autosomal recessive disorder caused due to defective genes present on chromosome number 12.

The affected individual lacks a liver enzyme called phenylalanine hydroxylase that converts the amino acid phenylalanine into tyrosine.

Symptoms: Since phenylalanine is not converted, phenylalanine is accumulated and converted into phenylpyruvic acid (keto derivative) and other derivatives. Accumulation of these in brain results in mental retardation. These are also excreted through urine because of its poor absorption by kidney.

(g) Colour blindness is a recessive sex-linked trait in which the eye fails to distinguish red-green colours. The gene for normal vision is dominant. The normal gene and its recessive alleles are carried by X-chromosome.

Most colourblind people cannot tell the difference between red or green.

(iv) Chromosomal disorders are caused due to the absence or excess or abnormal arrangement of one or more chromosomes. Failure of segregation of chromatids during cell division results in the gain or loss of a chromosome(s) called aneuploidy.

(v) Types of chromosomal aberration:

Monosomy : $(2 n - 1)$

Nullisomy : $(2 n - 2)$

Trisomy : $(2 n + 1)$

Tetrasomy : $(2 n + 2)$

(vi) Types of chromosomal disorders are - Down’s syndrome, Klinefelter’s syndrome and Turner’s syndrome.

(a) Down’s syndrome is caused by the presence of an additional copy of the chromosome number $21$ (trisomy of $21$ ).

It is also called Mangolian idiocy.

Discovered by Langdon Down.

It is a case of autosomal trisomy.

Symptoms: The affected individual is short-stature with a small round head, furrowed tongue and partially open mouth. Palm is broad with characteristic palm crease. Physical, psychomotor and mental development is retarded.

Karyotype : $(45 + XX / XY, + 21)$

Karyotype of Down's syndrome female:

Karyotype of Down's syndrome male:

(b) Klinefelter’s syndrome is caused due by the presence of an additional copy of $X$ -chromosome resulting in a karyotype of $47$ , $X X Y$ .

It is a case of allosomal trisomy.

Karyotype: $(44 + XXY)$

Symptoms: Such an individual has overall masculine development, however, the feminine development (development of breast, i.e., gynaecomastia) is also expressed. Such individuals are sterile.

It is a case of allosomal monosomy.

Karyotype: $(44 + XO)$

Symptoms: Such females are sterile as ovaries are rudimentary besides other features including lack of other secondary sexual characters.

Inheritance of blood group is a condition of (A) Co-dominance (B) Incomplete dominance (C) Multiple allelism (D) Multiple gene (E) Dominance

Important Questions on Principles of Inheritance and Variation

Learn and Prepare for any exam you want !

Inheritance of blood group is a condition of
(A) Co-dominance
(B) Incomplete dominance
(C) Multiple allelism
(D) Multiple gene
(E) Dominance