Notes-Class-12-Biology-Chapter-3-Inheritance and Variations-Maharashtra Board

Reasons for Mendel’s success :

• Carefully planned experiments.
• Large sample of study. Meticulous recordings which gave the ratios.
• Well-chosen contrasting characters which were recognizable.
• Each character controlled by single factor.
• Dominance and recessiveness among the pair of characters.

Seven pairs of contrasting characters of pea plant studied by Mendel were :

Dihybrid : When two traits are considered during inheritance, it is called dihybrid. A cross between parents differing in two heritable traits, is called dihybrid cross.

• A phenotypic ratio of 9 : 3 : 3 : 1 obtained in the F₂ generation of a dihybrid cross is called a dihybrid ratio.
• Thus for example, when we cross a true breeding pea plant bearing round and yellow seeds with a true breeding pea plant bearing wrinkled and green seeds we get pea plants bearing round and yellow seeds in the F₁ generation.
• When F₁ plants are selfed, we get a ratio of 9 : 3 : 3 : 1 in the F₂ generation, where 9 plants bear yellow round seeds, 3 plants bear yellow wrinkled seeds, 3 plants bear green round seeds and 1 plant bears green wrinkled seeds.

Parents (P₁) : RRYY x rryy

Gametes of P1 : RY and ry

F₁ generation : RrYy(Yellow round)

On selfing F₁ : RrYy x RrYy

Gametes of F₁ : RY, Ry, rY, ry

 F2 generation :

Round Yellow : 9 Round green : 3 Wrinkled yellow : 3 Wrinkled green : 1

Phenotypic ratio : 9 : 3 : 3 : 1

Genotypic ratio : 1:2:1:2:4:2:1:2:1

Mendel’s Laws of Inheritance : Mendel proposed three basic postulates on the basis of which three laws were formulated. These are described below:

Law of Dominance : 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 or Law of purity of gametes : When F₁ hybrid forms gametes, the alleles segregate from each other and enter in different gametes. The gametes formed are pure because they carry only one allele each (either dominant one or recessive one.).

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

Deviations from Mendel’s findings :

• Mendel’s experiments gave the following generalizations :
• Single trait due to single gene which has two alleles.
• Two alleles interact with each other in which one allele is completely dominant.
• Factors or genes for different traits are present on different chromosomes and they show independent assortment.

Later, different deviations were noted by the scientists who worked in post-Mendelian era. These findings form Neo—Mendelism.

The deviations from Mendel’s findings are of following types :

• Phenomena of co-dominance and incomplete dominance : Shown by single trait due to single gene but its two alleles show interactions.
• Multiple allelism : Single trait shown by single gene which has more than two alleles.
• Polygenic inheritance : Single trait due to more than one gene which show interactions which are either epistastic or additive effect.
• Pleiotropy : Many traits which are influenced by a single gene.

Gene interactions : When phenotypic expression of a gene is modified or influenced by the other gene, then it is called gene interactions.

• Intragenic interactions : Interactions between the alleles of same gene. E.g. incomplete dominance and co-dominance. Also seen in the multiple allele series of a gene.
• Intergenic (non-allelic) interactions : Interactions between the alleles of different genes present on the same or different chromosomes E.g., Pleiotropy, polygenes, supplementary and complementary genes.

Incomplete dominance : When both the alleles express themselves partially then it is called the incomplete dominance. Unlike as in complete dominance, here one allele cannot completely suppress the expression of the other allele. Thus there is formation of an intermediate expression in the F₁ hybrid.

E.g. the flower colour of Mirabilis jalapa.

• Red-flowered (RR) plant x White-flowered (rr) plant,
• F₁ offspring : Pink (Rr) flowers.

• Genotypic ratio — 1 RR: 2 Rr: 1 rr
• Phenotypic ratio — 1 Red : 2 Pink : 1 White.

Co-dominance : When both the alleles of an allelomorphic pair express themselves equally in F₁ hybrids then it is called co-dominance.

Alleles that express themselves equally independently in hybrids, are co-dominant alleles.

E.g. Coat colour in cattle.

• Red coat (RR) x white coat (W W).
• F₁ hybrids (RW) :Roan. (mixture of red and white). Both traits expressed equally.
• In F₂ generation red (RR) : roams (RW) : white (WW) = 1 :2: 1.
• Co-dominance shows the genotypic and phenotypic ratios identical.

Multiple alleles :

• When more than two alleles of a gene in a population occupy the same locus on achromosome or its homologue, then such alleles are called multiple alleles.
• Wild and original gene is mutated to form multiple alleles. There is a series of alleles which show dominance, co-dominance or incomplete dominance with the other recessive alleles among them. The most dominant is the wild type.

• g. (1) Size of wings in Drosophila. From normal Wings to vestigial wings (vg) in homozygous condition. (2) A, B, O blood groups in human beings.

Pleiotropy :

• Pleiotropic gene is the single gene that controls two (or more) different traits.
• This phenomenon is called pleiotropy or pleiotropism.
• Phenotypic ratio : 1 : 2 instead of 3 : 1 because of the death of recessive homozygote.
• E.g. Sickle-cell anaemia, is caused by a gene Hbs.
• Normal or healthy gene is dominant : HbA
• Heterozygotes are carriers (HbA/Hbs). They show signs of mild anaemia.
• Homozygotes with recessive gene Hbs die due to severe anaemia.
• Thus, the gene for sickle-cell anaemia is lethal in homozygous condition and produces sickle-cell trait in heterozygous carrier.
• Two different expressions are produced by a single gene. A marriage between two carriers will produce normal carriers and sickle-cell anaemic children in 1 : 2 : 1 ratio.

Ploidy : The degree of repetition of the primary basic number of chromosomes

(i.e. ‘x’) in a cell.

• Euploidy : Euploidy is the condition in which the chromosome number in a cell is the exact multiple of the primary basic number.
• Euploids are further divided into monoploid / haploid (n), diploids (2n), triploids (3n), tetraploid (4n), etc.
• Aneuploidy : Aneuploidy is the condition in which the chromosome number shows either addition or deletion by one or more.

Structure of chromosome :

Parts of a typical chromosome :

• Two chromatids arms joined together at centromere. Chromatid arm bears long, unbranched, slender, highly coiled DNA thread called chromonema.

• Centromere or primary constriction: Has kinetochore. (Disk shaped structure at which spindle fibres are attached during cell division.)
• Secondary constriction : At secondary constriction I, nucleolus becomes organized during interphase.
• A satellite body (SAT body) attached at secondary constriction II.
• Telomeres : The ends of chromosomes.

Types of chromosomes :

The centromere has a specific position in each chromosome. Depending upon the position of centromere and the length of the arms of the chromosome, there are four types of chromosomes.

1. Metacentric :

• Structure : The centromere is exactly at the mid-point in this chromosome.
• Pattern : This chromosome looks like the English letter ‘V’.
• Arm : The arms of this chromosome are equal in length.

2. Sub-metacentric :

• Structure : The centromere is somewhere near the mid-point in this chromosome
• Pattern : It looks like English letter ‘L’.
• Arm : One arm is slightly shorter than the other.

3. Acrocentric :

• Structure : The centromere is near one end of this chromosome
• Pattern : It looks like the English letter ‘j’.
• Arm : One arm is much smaller than other.

4. Telocentric :

• Structure : The centromere is right at  the end of this chromosome.
• Pattern : This chromosome look like the English letter ‘i’.
• Arm : This chromosome consists of only one arm.

Types of chromosomes according to their function :

• Homologous chromosomes : If the pair consists of similar chromosomes by shape and organization, they are called homologous chromosomes.
• Heterologous chromosomes : If the chromosomal pairs are not similar they are called heterologous chromosomes.
• Sex chromosomes or allosomes : The chromosomal pair that decides the sex of the sexually reproducing organisms.
• Somatic chromosomes or autosomes : Chromosomes that decide the body characters other than sex.

Crossing Over :

• Process of formation of re-combinations of genes by interchanging and exchanging the segments of non-sister chromatids of homologous chromosomes is called crossing over. Crossing over takes place in pachytene of prophase-I of meiosis.
• Morgan coined the term crossing over.
• The mechanism of crossing over : There are four sequential steps such as synapsis, tetrad formation, crossing over and terminalisation.
• Due to crossing over, variations are produced which act as raw material for natural selection and thus helps in evolution.

Morgan’s Experiments showing linkage and crossing over :

Drosophila melanogaster used for genetic experiments because,

• Easily cultured in laboratory.
• Life span is short of about two weeks.
• High rate of reproduction.

Morgan explained the principle of linkage, sex linkage and crossing over.

According to his experiments :

• Genes grouped on the same chromosome are strongly linked. Re-combinations among them are only 1.3%.
• Genes present far away from each other on chromosome are loosely linked and hence show more 37.2% re-combinations.

PKU : PKU means phenylketonuria which is an autosomal recessive inborn error.

• In this disorder the metabolism of phenylalanine does not occur due to deficiency of phenylalanine hydroxylase (PAH) enzyme.
• This enzyme is necessary to metabolize the amino acid phenylalanine to the amino acid tyrosine.
• When PAH activity is reduced, phenylalanine accumulates in blood and cerebrospinal fluid and is converted into phenylpyruvate or phenyl-ketone which is a toxic compound.
• This may cause mental retardation. Excess phenylalanine is excreted in urine, hence this disease is called phenylketonuria.
• PKU is caused by mutations in the PAH gene on chromosome no. 12.
• Untreated PKU causes abnormal phenotype which includes growth failure, poor skin pigmentation, microcephaly, seizures, global developmental delay and severe intellectual impairment. However, at birth if an infant is checked for PKU, the further abnormalities can be avoided.

Widow’s peak :  Widow’s peak is a prominent ‘V’ shaped hairline on forehead.

• It is due to autosomal dominant gene.
• Widow’s peak occurs in homozygous dominant (WW) and also heterozygous (Ww) individuals.
• Individuals with homozygous recessive (ww) genotype do not have widow’s peak but have a straight hair line.
• Both males and females have equal chance of inheritance.

Red green Colour blindness : X-linked recessive disorder. Inability to distinguish red and green colours.

Genotypes of male and female individuals for colour blindness are as follows :

Sex linked inheritance (colour blindness) :

X-body :

• German biologist, Henking in 1891, was studying spermatogenesis of the squash bug (Anasa tristis).
• He noted that 50% of sperms receive the unpaired chromosomes while other 50% sperms do not receive it.
• Henking gave a name to this structure as the X-body. He was unable to explain its role in sex determination.
• Further investigations by other scientists led to conclusion that the ‘X-body’ of Henking was a chromosome and gave the name as X-Chromosome to X-body.

Sex determination in human beings :

• In human beings, the sex is determined with the help of X and Y chromosomes. This chromosomal mechanism of sex determination is of XX-XY type.
• In male, the nucleus of each cell contains 46 chromosomes or 23 pairs of chromosomes. Of these 22 pairs are autosomes and one pair of sex chromosomes. Males are thus heteromorphic as they have two different types of sex chromosomes.
• Autosomes or somatic chromosomes are responsible for determination of other characters of the body, but not the sex.
• In female cells, there are 22 pairs of autosomes and one pair of X chromosomes. Females are thus homomorphic as they have similar sex chromosomes.
• Thus the genotypes of female and male are
• as follows :

• Female : 46 chromosomes = 44 autosomes + XX sex chromosomes
• Male : 46 chromosomes: 44 autosomes + XY sex chromosomes

• During gamete formation, the diploid germ cells in the testes and ovaries undergo meiosis to produce haploid gametes (sperms and eggs). The homologous chromosomes separate and enter into different gametes during this process.
• The human male produces two different types of sperms, one containing 22 autosomes and one X chromosome and the other containing 22 autosomes and one Y chromosome. Human males are therefore called heteroganietic, i.e. they produce different types of gametes.
• The human female produces only one type of eggs containing 22 autosomes and one X chromosome and therefore she is homogametic.
• During fertilization, if X containing sperm fertilizes the egg having X chromosome, then a female child with XX chromosomes is conceived.
• If Y containing sperm fertilizes the egg having X chromosome then a male child with XY chromosomes is conceived.
• The sex of the child thus depends upon the type of sperm fertilizing the egg. The heterogametic parent determines the sex of the child and thus the father is responsible for the determination of the sex of the child and not the mother.

On crossing :

Sex determination in Birds :

• Sex determination in birds is by ZW-ZZ mechanism.
• In birds, males are homogametic while females are heterogametic.
• Males are homogametic and produce one type of sperms. Each sperm carries a Z chromosome.
• Females produce two types of eggs; 50% eggs carry Z- chromosome, while 50% eggs carry W- chromosome.
• Thus sex of individual depends  on the kind of egg (ova) fertilized by the sperm.
• When Z bearing egg is fertilized by a sperm a male offspring is produced. If W bearing egg is fertilized then female offspring is produced.

[Note : The diploid autosomal number may be different for different birds. It should be remembered as AA and not 2n (as given in Textbook page no. 65, Fig. 3.15). Because 2n will be diploid chromosome number including autosomes and sex chromosomes.]

Sex determination in honey bee :

• In honey bee, Sex determination is by haplodiploid system.
• Sex is determined by the number of sets of chromosomes received by an individual.
• The egg which is fertilized by sperm, becomes diploid and develops into female.
• The egg which is not fertilized develops by parthenogenesis and develops into a male.
• The queen and worker bee therefore contain 32 chromosomes. The drone, i.e. male bears 16 chromosomes.
• The sperms are produced by mitosis while eggs are produced by meiosis.

Thalassemia :

Thalassemia is an autosomal recessive disorder. The synthesis of alpha chains are controlled by two genes, (HBA1 and HBA2) on chromosome 16.

Beta chain synthesis is controlled by gene HBB located on chromosome 11. Two alpha chains and two beta chains together form four polypeptide chains that make heterotetrameric haemoglobin molecule. But when there is defective gene on either of chromosome 16 or 11, there is quantitative abnormality of polypeptide globin chain synthesis. This results into thalassemia.

Depending upon which chain is affected, thalassemia is classified as, alpha (a) thalassemia and beta ( b) thalassemia.

The clinical symptoms of thalassemia are as follows :

• Pale yellow skin.
• Anaemia due to inability to synthesize haemoglobin.
• Slow growth and development.
• Variation in the shape and size of RBCs.

Patients need regular blood transfusions to cope with the disorder.

Down’s Syndrome : Caused due to aneuploidy.

• Aneuploidy is due to non-disjunction of chromosome at the time of gamete formation during meiosis. Due to non-disjunction, chromosomes fail to separate.
• In addition to a homologous pair of 21^st chromosome there is an extra 21st, therefore it is called trisomy (2n+1) of 21^st chromosome.

Symptoms of Downs syndrome :

• Typical facial features.
• An epicanthal skin fold, over the inner corner of eyes causing downward slanting eyes.
• Typical flat face, rounded flat nose, mouth always open with protruding tongue.
• Mental retardation.
• Poor skeletal development.
• Short stature, relatively small skull and arched palate.
• Flat hand with simian crease that runs across the palm.

Turner’s syndrome : Turner’s syndrome is a genetic disorder caused due to monosomy of X chromosome.

• It was first described by H. H. Turner.
• Turner’s syndrome is caused due to non-disjunction of sex chromosomes which takes place during gamete formation.
• Chromosomal complement of Turner’s syndrome is 44+ XO, having a total of
• 45 chromosomes.

Symptoms of Turner's syndrome :

• Female phenotype.
• Short stature with Webbing of neck.
• Low posterior hair line.
• Secondary sexual characters fail to develop.
• Mental retardation.

Klinefelter‘s syndrome: Klinefelter‘s syndrome is a genetic disorder caused due to trisomy of X chromosome.

• It was first described by Harry Klinefelter.
• Klinefelter’s syndrome is caused due to non-disjunction of sex chromosomes which takes place during gamete formation.
• Chromosomal complement of Klinefelter’s syndrome is 44+XXY, having a total of 47 chromosomes.

Symptoms of Klinefelter's syndrome :

• The Klinefelter’s syndrome individuals are tall, thin and eunuchoid.
• They are sterile with poorly developed sexual characteristics.
• Testes are underdeveloped and small. Spermatogenesis does not take place.
• They have subnormal intelligence and show partial mental retardation.