Explain the Law of Dominance using a monohybrid cross.

(i) Genetics is a branch of biology that deals with principles of inheritance and its practices.

(ii) The first scientific study leading to formulations of laws of inheritance was carried out by Gregor Johann Mendel, known as the 'father of genetics.'

(iii) Mendel chose a garden pea (also called edible pea), Pisum sativum, as plant material for his experiments.

(iv) Based on his experiments, Mendel proposed the principles of inheritance, which are today referred to as 'Mendel's Laws of Inheritance.'

(v) In 1865, Mendel presented the results of his work before the local Natural History Society. In $1900$, Mendel's work finally came to light, and the science of genetics was born. Mendel's laws were rediscovered simultaneously by three great botanists: Hugo de Vries, Eric von Tschermak-Seysenegg, and Carl Correns.

(vi) The number of characters studied by Mendel in the pea plant was seven.

2. Inheritance of One Gene:

(i) Monohybrid cross is the simplest cross performed by Mendel, which is the cross made to study the inheritance of a single character (trait).

(ii) Two crosses between the same pair of genotypes or phenotypes in which the sources of the gametes are reversed in one cross are known as the reciprocal cross.

(iii) The genotypes and phenotypes resulting from various combinations of gametes can be determined by checkerboard or Punnett square method.

(iv) Back cross is a cross of $F_1$ progeny back to one of their parents, and back cross to the recessive parent is known as a test cross.

(v) Mendel proposed the existence of a particulate unit factor for each trait, and these factors serve as basic units of heredity.

(vi) When two unlike unit factors are present in one individual, one unit factor is dominant to the other, which is said to be recessive.

(vii) Mendel's first law is known as the law of segregation or the law of purity of gametes.

3. Inheritance of Two Genes:

(i) A cross between two individuals for studying the inheritance of two characters is known as a dihybrid cross.

(ii) Mendel's second law, the principle of independent assortment, states that the members of one pair of factors (alleles) segregate independently of the member of other gene pairs at the time of gamete formation.

(iii) Genes that are present on the same chromosomes are called syntenic.

(iv) Two alleles are incompletely dominant if the heterozygotes have a phenotype intermediate between the two homozygotes.

(v) Co-dominance is similar to incomplete dominance, but here the heterozygote displays both alleles.

(vi) When any of three or more genes occupy the same locus in a given pair of homologous chromosomes, they are said to constitute a series of multiple alleles. A classic example of multiple alleles is found in humans' ABO blood group system.

(vii) A gene that masks another gene's expression is said to be epistatic, and a gene whose expression is masked by a non-allelic gene is said to be hypostatic.

4. Pleiotropy and Polygenic Inheritance:

(i) Genes that affect multiple, apparently unrelated phenotypes are thus called pleiotropic genes. One of the most widely cited examples of pleiotropy in humans is phenylketonuria (PKU) which is caused by a deficiency of the enzyme phenylalanine hydroxylase, necessary to convert the essential amino acid phenylalanine to tyrosine.

(ii) A character whose expression is brought out by a number of genes is called polygenic inheritance or quantitative inheritance, and human skin colour is controlled by the polygenic effect at least by three separate genes.

(iii) A gene whose phenotypic effect kills the bearer is known as a lethal gene; death from different lethal genes may occur at any time from fertilization of the egg to an advanced stage.

5. Linkage and Recombination:

(i) At present, Drosophila is widely used in classical as well as molecular genetics. It is a popular experimental animal.

(ii) The genes located on the same chromosome are linked, and nonhomologous chromosome genes are unlinked.

(iii) Genes borne on homologous chromosomes belong to the same linkage groups, and the number of linkage groups is equal to the haploid number of chromosomes.

(iv) The linkage is said to be complete when linked genes tend to remain together through many generations and are not separated by crossing over.

(v) In the incomplete linkage, the two genes linked genes separate by crossing over.

(vi) A gene is a unit of specific biological function located at a fixed position on a chromosome, and it is the hereditary unit and is made of polynucleotides(DNA).

(vii) Barbara McClintock ($1902-92$) discovered "controlling elements or jumping genes" that could move from one location to another on the chromosome in maize (Nobel Prize in $1983$).

6. Sex Determination:

(i) Nettie Stevens and Edmund Wilson both independently developed the idea of sex determination by chromosomes.

(ii) Different system of sex determination is $\mathrm{XY}-\mathrm{XX}$ in Drosophila and human beings, $\mathrm{XX}-\mathrm{XO}$ in grasshopper and cockroaches, $\mathrm{ZW}-\mathrm{ZZ}$ in snakes and birds and $\mathrm{ZO}-\mathrm{ZZ}$ some butterflies and moths.

(iii) In honeybees, sex is determined by the fertilization or non-fertilization of eggs rather than the presence or absence of sex chromosomes (Haplo-diploidy).

(iv) Temperature-dependent sex determination (TSD) is reported in turtles and crocodiles.

(v) Sex determination in man is related to the $\mathrm{X}-\mathrm{Y}$ chromosome makeup; the Y chromosome determines the maleness, with even a single one outweighing any number, and the $\mathrm{X}$ chromosome determines femaleness in the absence of any $\mathrm{Ys}$.

(vi) The key to sex determination in humans is the SRY (for Sex Region on the $\mathrm{Y}$) gene located on the $\mathrm{Y}$ chromosome.

(vii) $\mathrm{X}$-linked genes are represented twice in females and once in males and the recessive $\mathrm{X}$-linked genes have a characteristic criss-cross inheritance, i.e., the male transmits his $\mathrm{X}$-linked genes to his grandson through his daughter.

(viii) First X-linked gene was discovered by T. H. Morgan ($1910$) for white eye mutation.

(ix) The most important and common $\mathrm{X}$-linked diseases in human beings are red-green colorblindness, haemophilia, Duchenne muscular dystrophy and fragile $\mathrm{X}$-syndrome.

(x) Holandric genes are those that occur on the Y chromosome only; they are not expressed in females; 'Hairy ears' (hypertrichosis) in man is inherited through genes on $\mathrm{Y}$ chromosomes.

(xi) The genes which occur in homologous sections of $\mathrm{X}$ and $\mathrm{Y}$ chromosomes are called $\mathrm{XY}$-linked genes, and they have inheritance like the autosomal genes.

(xii) Inheritance of pattern baldness in men is an example of a sex-influenced trait which is dominant in males and recessive in females.

(xiii) In sex-limited traits, the phenotypic expression is determined by the presence or absence of one of the sex hormones, e.g., a beard in a man.

7. Mutation:

(i) Mutation refers to any sudden, heritable change in the genotype of an organism.

(ii) A mutation that visibly changes chromosome structure is known as a chromosomal aberration; four types of chromosomal aberrations are deletion, duplication, inversion, and translocation.

(iii) Variations or numerical changes in chromosomes (heteroploidy) can be mainly of two types: euploidy and aneuploidy.

(iv) The term euploidy is applied by organisms with chromosomes that are multiples of some basic number.

(v) Aneuploidy is the term applied for the chromosomal mutation involving only a part of a set, i.e., loss (hypoploidy) or addition (hyperploidy) of one or more chromosomes.

(vi) Any substance or agent-inducing mutation is called a mutagen which may be broadly grouped into two classes: physical mutagen and chemical mutagen.

(vii) Radiation has been used to induce mutations for the first time by H.J. Muller ($1927$) on animals and LJ. Stadler ($1928$) on plants.

(viii) Radiation that can produce mutation is known as effective radiation; $\mathrm{X}$-rays and $\mathrm{y}$ rays cause DNA damage and mutations.

8. Twins:

(i) Monozygotic twins are also called identical twins. The two blastomeres resulting from the first cleavage of the zygote completely separate from each other and develop into independent embryos.

(ii) Dizygotic twins are also called fraternal or non-identical twins, formed by simultaneous fertilization of two different ova by two different sperms.

(iii) In human beings, pedigree analysis is used to know the transmission of specific traits.

9. Genetic Disorders:

(i) Sickle-cell anaemia, a genetic disease, is caused due to change of one base in the gene coding for beta-chain of haemoglobin.

(ii) Down's syndrome is due to trisomy of chromosome $21$, where there is an extra copy of chromosome $21$, and consequently, the total number of the chromosome becomes $47$.

(iii) In Turner's syndrome, one $\mathrm{X}$ chromosome is missing, and the sex chromosome is as $\mathbf{XO}$, and in Klinefelter's syndrome, the condition is $\mathbf{XXY}$.

(iv) The correct human chromosome number $2n - 46$ was reported by Tjio and Levan in $1956$.

(v) Karyotype is the chromosomal complement of an organism, and the arrangement makes it of metaphase chromosome pairs in a standard sequence.

(vi) Cri-du-chat syndrome (from the French word for "cry of the cat") in human beings is caused by a conspicuous deletion in the short arm of one of the $5th$ autosomes.

(vii) Chronic myelogenous leukaemia (CML) in human beings is fatal cancer involving uncontrolled replication of myeloblasts (stem cells of white blood cells) and this condition is associated with an aberration of chromosome $22$ called the 'Philadelphia chromosome'.

(viii) Sex chromatin body was discovered by M. L. Barr and E.G. Bertram ($1949$) in the interphase nerve cell of female cats and is named as "Barr body" after the first discoverer.