Principles of Inheritance and Variation {Class 12th Biology}

Unit VII: Genetics

Chapter 5 : Principles of Inheritance and Variation

Class: 12th Biology

Presented By: Dr. Priyanka Patade

Terminology

Genetics: Genetics is the study of principles and mechanism of heredity and variation.

Gregor Johann Mendel is known as ‘Father of Genetics’.

Inheritance: It is the process by which characters are passed on from parent to progeny. It is the basis of heredity.

Variation: It is the degree by which progeny differ from their parents. Variation may be in terms of morphology, physiology, cytology and behaviouristic traits of individual belonging to same species.

Variation arise due to:

⤷Reshuffling of gene/chromosomes.

⤷Crossing over or recombination.

⤷Mutation and effect of environment.

Terminology

✓Genetics is the branch of life science that deals with the study of heredity and variation.

✓Heredity is the transmission of characters from parents to their offsprings.

✓Variation is the difference among the offsprings and with their parents.

✓Hereditary variations: These are genetical and inheritable.

✓Environmental variation: These are acquired and non inheritable.

Terminologies

✓Phenotype: The external appearance of an organism due to the influence of genes and environmental factors.

✓Genotype: The genetic constitution of an individual responsible for the phenotype.

✓Phenotypic ratio: The correct proportion of phenotype in population.

✓Genotypic ratio: The correct proportion of genotype in population.

✓Homozygous: The individual heaving identical genes in an allelic pair for a character. Ex: TT, tt.

✓Heterozygous: The individual heaving un-identical genes in an allelic pair for a character. Ex: Tt.

✓Dominant gene: The gene that expresses its character in heterozygous condition.

✓Recessive: The gene that fails to express its character in heterozygous condition.

✓Hybrid: The progeny obtained by crossing two parents that differ in characters.

✓Back cross: The cross between F1 hybrid and one of its parents.

✓Test cross: The cross between hybrid and its homozygous recessive parent. It is used to identify the genotype of the hybrid.

Mendel’s Law of Inheritance

Mendel conducted hybridization experiments on garden pea (Pisum sativum) for seven years and proposed the law of inheritance in living organisms.

Selection of pea plant

The main reasons for adopting garden pea (Pisum sativum) for experiments by Mendel were –

⤷Pea has many distinct contrasting characters.

⤷Life span of pea plant is short.

⤷Flowers show self-pollination, reproductive whorls being enclosed by corolla.

⤷It is easy to artificially cross-pollinate the pea flowers. The hybrids thus produced were fertile.

Mendel’s Working Method

Mendel’s success was also due to his meticulous planning and method of work –

⤷He studied only one character at a time.

⤷He used all available techniques to avoid cross pollination by undesirable pollen grains.

⤷He applied mathematics and statistics to analyse the results obtained by him.

⤷Mendel selected 7 contrasting characters of garden pea for his hybridization experiments.

Contrasting Characters Studied by Mendel in Pea

Character - Contrasting character (Dominant/Recessive)

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

Inheritance of one gene

Inheritance of one gene can be explained by monohybrid cross.

The cross between two parents differing in one pair of contrasting character is called monohybrid cross.

Crossed tall & dwarf pea plants- Collected seeds & grew to generate first hybrid generation/ Filial generation/F1.

F1 plants- Tall & none were dwarf.

For other traits also- F1 generation resembled only one parent & trait of other parent were not shown.

Self pollinated F1 – Filial 2 generation/ F2.

F2 generation- 1/4th were dwarf & 3/4th tall- identical to parents.

F1 generation one parent trait shown & F2 both parent trait shown in the ratio- 3:1 & no blending were seen.

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Mendel’s interpretation

Mendel called the ‘factors’ that passes through gametes from one generation to next generation. Now a day it is called as genes (unit of inheritance).

Genes that code for a pair of contrasting traits are known as alleles.

Alphabetical symbols are used to represent each gene, capital letter (TT) for gene expressed in F1 generation and small letter (tt) for other gene.

Mendel also proposed that in true breeding tall and dwarf variety allelic pair of genes for height is homozygous (TT or tt). TT, Tt or tt are called genotype and tall and dwarf are called phenotype.

The hybrids which contain alleles which express contrasting traits are called heterozygous (Tt).

The monohybrid ratio of F2 hybrid is 3:1(phenotypic) and 1:2:1(genotypic).

Principle or Law of Inheritance

Based on observations of monohybrid cross, Mendel proposed two law of inheritance-

1. Law of Dominance– states that –

a. Characters are controlled by discrete units called factors.

b. Factors always occur in pair.

c. In a dissimilar pair of factors one member of pair dominate the other.

2. Law of Segregation-

Alleles do not blends and both the characters are recovered during gametes formation as in F2 generation.

During gametes formation traits segregate (separate) from each other and passes to different gametes.

Homozygous produce similar kinds of gametes but heterozygous produce to different kinds of gametes with different traits.

Incomplete Dominance

•It is a post Mendelian discovery. Incomplete dominance is the phenomenon of neither of the two alleles being dominant so that expression in the hybrid is a fine mixture or intermediate between the expressions of two alleles.

•In snapdragon (Mirabilis jalapa), there are two types of pure breeding plants, red flowered and white flowered. On crossing the two, F1 plants possess pink flowers. On selfing them, F2 generation has 1red: 2 pink: 1white. The pink flower is due to incomplete dominance.

•Correns discovered Incomplete dominance in Merabilis jalapa.

•It is also called partial dominance, semi dominance.

•The inheritance in which allele for a specific character is not completely dominant over other allele is called Incomplete dominance.

•Snapdragon or Antirrhinum sp.- Cross between true breed red flower (RR) & white flower (rr), F1 generation- Pink (Rr) & after self pollination in F2 generation- 1 (RR) Red: 2 (Rr) Pink: 1 (rr) white.

•Genotype ratio same as Mendelian cross & Phenotype ratio different than Mendelian cross

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Co-dominance

•It is the phenomenon of two alleles lacking dominance-recessive relationship and both expressing themselves in the organism.

•Human beings, ABO blood grouping are controlled by gene I. The gene has three alleles IA, IB and i. Any person contains any two of three allele IA, IB are dominant over i.

•The plasma membrane of the red blood cells has sugar polymers that protrude from its surface and the kind of sugar is controlled by the gene.

•When IA and IB are present together, both express their own types of sugars because of co-dominance.

•Both the alleles for a character are dominant and express its full character is called co-dominance.

•Ex AB blood group of human being.

•Blood group in humans are controlled by 3 alleles of a gene I.

•They are IA IB and i.

•The ABO locus is located on chromosome 9.

•IA is responsible for production of antigen –A.

•IB is responsible for production of antigen –B.

•i does not produces any antigen.

Multiple alleles

When a gene exists in more than two allelic forms, the phenomenon is known as multiple allelism.

“For example: multiple alleles is the inheritance of A, B and O blood groups in human being”. The gene for blood group occurs in three allelic forms IA, IB and i.

An individual can possess any two of these alleles.

The gene IA codes for glycoprotein A which is responsible for A blood group and gene IB codes for glycoprotein B which is responsible for blood group B.

The gene ‘i’ do not produce any glycoprotein and so the person who is homozygous for it, will have O group blood.

The genes IA and IB are dominant over ‘i’.

Pleiotropy

A single gene product may produce more than one effect.

E.g. starch synthesis/size of starch grain and shape of seeds are controlled by a gene having two allele B & b.

Starch synthesis effective if homozygote BB & produce large starch grains.

⤷Homozygote bb – lesser efficiency in starch synthesis & seeds are wrinkled.

⤷Heterozygote Bb – round seeds, intermediate size.

Flower colour and seed colour are controlled by one gene.

Polygenic inheritance

Polygenic inheritance is defined as quantitative inheritance, where multiple independent genes have an additive or similar effect on a single quantitative trait. Polygenic inheritance is also known as multiple gene inheritance or multiple factor inheritance.

Example: Skin colour in humans: If we take an example of a pair of alleles of three different and unlinked loci as A and a, B and b, C and c. The capital letters represent the allele for dark skin colour. The more capital letters show skin colour towards the darker range and small letters towards the lighter colour of the skin.

Parents having genotype AABBCC and aabbcc will produce offsprings of intermediate colour in the F1 generation, i.e. AaBbCc genotype.

In the F2 generation of two triple heterozygotes (AaBbCc x AaBbCc) mate, they will give rise to varying phenotypes ranging from very dark to very light in the ratio 1:6:15:20:15:6:1.

Inheritance of Two genes (Dihybrid Cross)

A cross made to study simultaneous inheritance of two pairs of Mendelian factors of genes.

Law of independent Assortment – The law states that ‘when two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters’.

In Dihybrid cross two new combinations, round green & wrinkled yellow are formed due to independent assortment of traits for seed shape i.e. round, wrinkled and seed colour i.e., yellow and green.

The ratio of 9:3:3:1 can be derived as a combination series of 3 yellow: 1 green, with 3 round : 1 wrinkled.

This derivation can be written as follows: (3 Round : 1 Wrinkled) (3 Yellow : 1 Green) = 9 Round, Yellow : 3 Wrinkled, Yellow: 3 Round, Green : 1 Wrinkled, Green

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• Mendel work published on 1865 but remain unrecognized till 1900.

•Reasons for that:

1. Lack of communication.

2. Concept of genes / factors- clear.

3. Mathematical approach for biology was not accepted.

4. No proof for existence of factors.

Chromosomal Theory of Inheritance

•It was proposed by Walter Sutton and Theodore Boveri.

• They work out the chromosome movement during meiosis.

•The movement behavior of chromosomes was parallel to the behavior of genes. The chromosome movement is used to explain Mendel’s laws.

•The knowledge of chromosomal segregation with Mendelian principles is called chromosomal theory of inheritance.

•According to this, Chromosome and genes are present in pairs in diploid cells.

•Homologous chromosomes separate during gamete formation (meiosis).

•Fertilization restores the chromosome number to diploid condition.

•Chromosome as well as gene both occurs in pair. The two alleles of a gene pair are located on the same locus on homologous chromosomes.

•Sutton and Boveri argued that the pairing and separation of a pair of chromosomes would lead to segregation of a pair of factors (gene) they carried.

•Sutton united the knowledge of chromosomal segregation with mendelian principles and called it the chromosomal theory of inheritance.

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•He selected Drosophila because,

•It is suitable for genetic studies.

•Grown on simple synthetic medium in the laboratory.

•They complete their life cycle in about two weeks.

•A single mating could produce a large number of progeny flies.

•Clear differentiation of male and female flies.

•Many types of hereditary variations can be seen with low power microscopes.

Linkage and Recombination

When two genes in a Dihybrid cross were situated on same chromosome, the proportion of parental gene combination was much higher than the non-parental type. Morgan attributed this due to the physical association or the linkage of the two genes and coined the linkage to describe the physical association of genes on same chromosome.

The generation of non-parental gene combination during Dihybrid cross is called recombination. When genes are located on same chromosome, they are tightly linked and show very low recombination.

•Morgan carried dihybrid cross in Drosophila to study genes that are sex linked.

•Crossed- yellow bodied, white eyed females with brown bodied, red eyed males & intercourse F1 progeny.

•Two genes did not segregate independently of each other & F2 ratio deviated from 9:3:3:1.

•The genes present on X –chromosome & two genes in a dihybrid cross- situated on same chromosome- parental gene combination is much higher than non parental- this is due to physical association/ linkage of two genes on chromosome- Linkage.

•Generation of non parental combination- Recombination.

•He found genes are grouped in same chromosome, some genes are tightly linked- less recombination.

•When genes are present in different chromosome- higher recombination.

•Eg.- Genes for white & yellow- tightly linked- 1.3% recombination while genes for white & miniature wings- 37.2% recombination.

•Student Alferd Sturtevant used frequency of recombination between gene pairs on chromosome as a measure of the distance between genes & mapped genes position on chromosome.

•Linkage: physical association of genes on a chromosome is called linage.

•Recombination: The generation of non-parental gene combinations is called recombination.

•It occurs in crossing over of chromosomes during meiosis.

Sex determination

 

XX-XO Type

Plants – Vallisneria & insects of order Hemiptera & Orthptera female have 2 X-Chromosomes(XX) and male have only one X- Chromosomes(XO).

Male produce two types of sperms half with X & half without X.

The presence of one unpaired X-Chromosomes determine the masculine sex.

XX-XY Type

Found in mammals including humans, insects like Drosophila & in certain angiospermic plants – Melandrium, Humulus, Coccinia Indica etc.

Female have 2 homomorphic X-Chromosomes in their body & produce same kind of egg.

Male possess one X & one Y chromosome (XY). So two kinds of sperms are produced.

The sex of the embryo depends upon the kind of sperm.

ZO-ZZ System

Found in certain moth & butterflies.

Female possess single Z-Chromosomes in its body hence called ZO (Heterogametic). Produce 2 kinds of egg.

Male possess two Z-Chromosomes ( referred as ZZ) & produce 1 type of sperm.

The sex of offspring depends upon the kind of egg.

ZW-ZZ System

Occur in certain insects & vertebrates like – amphibians,reptiles, birds & plants –Fragaris elatior.

Female have one Z and one W chromosome. So it produce two types of egg.

Male have two homomorphic Z-Chromosomes.

The sex of the offsprings depends upon the kind of egg.

Z bearing egg produce male & W bearing egg produce female.

MALE HAPLOIDY OR HAPLODIPLOIDY MECHANISM

Particularly common in insects as bees, ants, saw flies & wasps.

In these insects fertilized egg develop into diploid female & unfertilized ones into haploid male.

Meiosis is normal in females but crossing over and reductional division fail to occur during spermatogenesis in male due to haploidy.

Example

 Honey bee Queen (2n=32) can produce two types of egg.

Fertilized egg (Diploid zygote) having 32 chromosome.

Unfertilized egg (haploid zygote) having 16 chromosome develops into a male.

The diploid zygote can differentiate into either workers (sterile) & queens (fertile) depending on the diet they consumed during development.

Mutation and types of mutation

Mutation

Mutation is the change in sequence of nucleotide of DNA.

Change in sequence of nucleotide brings sudden change in morphological characteristics of an organism. If such change are heritable, then it is called as mutation.

So, mutation is defined as any heritable change in the sequence of nucleotide of DNA.

Organism with mutation is called mutant while the organism without mutation is wild type.

Genetic mutations

A mutation is a change in the amount or structure of DNA. There are two types of mutation:

A gene or point mutation – a change in the base sequence of a gene, which can cause a change in the polypeptide chain. It is caused by errors that occur during DNA replication.

A chromosome mutation – a change in the number or structure of the chromosomes. It is caused by errors that occur during cell division.

Pedigree Analysis

The analysis of traits in several of generation of a family is called the pedigree analysis.

The inheritance of a particular trait is represented in family tree over several generations. It is used to trace the inheritance of particular trait, abnormality and disease.

A very important tool for studying human inherited diseases  These diagrams make it easier to visualize relationships with in families, particularly large extended families.  Pedigrees are often used to determine the mode of inheritance (dominant, recessive, etc.) of genetic diseases.

Genetic Disorders

Mendelian Disorders	Chromosomal disorders
These are due to alteration in a single gene.	These are caused due to absence or excess of one or more chromosomes or abnormal arrangement of one/more chromosomes.
They are transmitted into generations through Mendelian principles of inheritance.	They may be recessive or dominant in nature.
Examples: Colour blindness Phenylketunuria	Examples: Downs syndrome, Turner’s syndrome

Mendelian Disorders

These disorders occur mainly due to the alteration or mutation in a single gene. These disorders may be dominant or recessive. These are transmitted from one generation to the next following Mendel’s principles of heredity.

Examples of Mendelian disorders are haemophilia, colour blindness, sickle-cell anaemia, phenylketonuria, thalassemia, etc.

Haemophilia

This is a type of sex-linked recessive disorders. According to the genetic inheritance pattern, the unaffected carrier mother passes on the haemophilic genes to sons.

It is a very rare type of disease among females because for a female to get the disease, the mother should either be haemophilic or a carrier but the father should be haemophilic.

This is a disorder in which blood doesn’t clot normally as the protein which helps in clotting of blood is affected. Therefore, a person suffering from this disease usually have symptoms of unexplained and excessive bleeding from cuts or injuries.

This type of genetic disorder is caused when the affected gene is located on the X chromosomes. Therefore males are more frequently affected.

Colour Blindness

It is a condition, in which certain colours cannot be distinguished, due to the lack of one or more colour absorbing pigments in the cone cells of retina.

In humans, the most common colour blindness is red-green colour blindness, which is a sex linked (i.e. X-linked recessive) defect caused by a recessive gene and is thus more common in males than females.

Sickle-cell Anaemia

This is a type of autosomal recessive genetic disorder.

According to Mendelian genetics, its inheritance pattern follows inheritance from two carrying parents.

It is caused when the glutamic acid in the sixth position of the beta-globin chain of haemoglobin molecule is replaced by valine. The mutant haemoglobin molecule undergoes a physical change which changes the biconcave shape into the sickle shape.

This reduces the oxygen-binding capacity of the haemoglobin molecule.

Phenylketonuria

This genetic disorder is autosomal recessive in nature.

It is an inborn error caused due to the decreased metabolism level of the amino acid phenylalanine.

In this disorder, the affected person does not have the enzyme that converts phenylalanine to tyrosine. As a result, phenylalanine accumulation takes place in the body and converted into many derivatives which result in mental retardation.

Thalassemia

This is a type of disorder in which the body makes an abnormal amount of haemoglobin. As a result, a large number of red blood cells are destroyed that leads to anaemia.

It is an X-linked recessive disease.

Facial bone deformities, abdominal swelling, dark urine are some of the symptoms of thalassemia.

Cystic Fibrosis

This is an autosomal recessive disorder.

This disease affects the lungs and the digestive system and the body produces thick and sticky mucus that blocks the lungs and pancreas.

People suffering from this disorder have a very short life-span.

Chromosomal Abnormalities

Chromosomal abnormalities are the type of genetic disorders caused due to the change in many chromosomes or the abnormal arrangement of the chromosomes. There are different types of chromosomal abnormalities as follows:

Aneuploidy – It is a condition in which there is a loss or gain of chromosomes due to abnormal segregation of genes during cell division.

Polyploidy – It is a condition in which the count of the entire set of chromosomes increases due to the failure of cytokinesis in cell division. It is mostly observed in plants.

Down syndrome

This syndrome is a type of trisomy as there is an extra copy of chromosome 21.

It is named so after the person who discovered this chromosomal disorder – Langdon Down.

The symptoms in a person include the following:

⤷The person is short and has a small and round head

⤷Physical and mental development is retarded

⤷Furrowed tongue and partially open mouth

⤷Broad palm

Klinefelter syndrome

This genetic disorder arises due to the presence of an additional X chromosome in males.

Thus, resulting in a chromosome count of 47 (44 + XXY) instead of 46.

Such a person has a masculine physique but has feminine development like the development of breasts.

Such individuals are sterile, i.e.; they cannot reproduce.

Turner syndrome

Unlike Klinefelter syndrome, in this chromosomal disorder there is the absence of one X chromosome in females.

Hence, decreasing the chromosomes count to 45 (44 + X0).

The symptoms include the following:

⤷Such females are sterile

⤷Have rudimentary ovaries and there is the absence of secondary sexual characters.

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