Principles of Inheritance and Variation

PRINCIPLES OF INHERITANCE

AND VARIATION

Genetics

• Organisms reproduce- formation of offspring of the same kind.

• The resulting offspring most often do not totally resemble the parent.

• Branch of biology that deals with the inheritance and variation- Genetics.

• Inheritance- the process by which characters are passed on from parent to progeny.

• Variation-it is the degree by which progeny differ from their parents.

history

• Human knew before 8000- 1000 B. C variation is due to sexual reproduction

• Exploited variations present in wild plants & animals to selectively breed & select organism with desirable characters

• Artificial selection & domestication of wild cow- Sahiwal cows in Punjab

• 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.

Terminology

Gregor Johann Mendel: Father of Genetics

• Known as the father of modern genetics

• Gregor Mendel developed the principles of heredity while studying seven pairs of inherited characteristics in pea plants.

• Although the significance of his work was not recognized during his lifetime, it has become the basis for the present-day field of genetics.

• Conducted hybridization (artificial pollination/ cross pollination) experiment for 7 years 1856-1863 & proposed law of inheritance

• Applied statistical analysis & mathematical logic for biology problems

• Large sampling size- greater credibility to data• Experiments- true breeding pea lines (continuous self

pollination)• Confirmation of inference from experiments on successive

generations of test plants, proved general rules of inheritance• Mendel investigated two opposing traits- tall & dwarf, yellow &

green seed

Mendel’s Approach

Seven pair of contrasting characters selected by Mendel for his experiment.

• 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.

Terminologies

• 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.

Why Mendel selected pea plant?

• Pure variety are available.• Pea plants are easy to cultivate.• Life cycle of plants are only few months. So that result

can be got early.• Contrasting trait are observed.• Flowers are bisexual and normally self pollinated.• Flowers can be cross pollinated only manually.• Hybrids are fertile.

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

• Mendel proposed- Something is stably being passed to the next generation through gametes ‘factors’ – genes

• Genes/factors- unit of inheritance, contain the information required to express particular trait

• Genes which code for pair of contrasting trait- alleles• Alphabetical symbols were used; T-Tall, t- dwarf• Plants pair of alleles for height- TT, Tt & tt• Mendel proposed- true breeding tall or dwarf plant- identical

or homozygous allele pair of TT or tt (genotype)• Descriptive term tall or dwarf- phenotype• Mendel found phenotype of heterozygote Tt of F1 was same

as parent with TT & proposed, in a pair of dissimilar factors one dominates the other & hence called dominant (T) & recessive (t)

TallP Dwarfx

F1All Tall

Phenotype

Monohybrid cross

F2

Tall is dominant to Dwarf

TT tt

Tt

GenotypeHomozygous Dominant Homozygous Recessive

HeterozygousSelf pollinatedGamets T t

T TTtall

Tttall

t TtTall

ttdwarf

Phenotypic ratio 3:1 Genotypic ratio: 1:2:1

• Production of gametes & formation of zygotes- Punnett Square

• Developed by- British scientist Reginald C. Punnett• Graphical representation- calculate probability of possible

genotypes in genetic cross• Gametes- on two sides, top row & left columns

• Self- pollination- 50%• F2- 3/4th tall & 1/4th Dwarf-

phenotypically• 1/4 : ½ : ¼ ratio of TT: Tt: tt-

genotype

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

Mendelian laws of heredity.

• Rules were proposed- Principles or Laws of Inheritance: First Law or Law of Dominance & Second law or Law of Segregation

• Law of dominance1. Characters are controlled by discrete units called Factors2. Factors occurs in pair3. In a dissimilar pair of factors one member of the pair

dominates (dominant) the other (recessive) Used to explain the expression of only one of the parental characters in monohybrid cross (F1) & expression of both in F2.Also explains proportion 3:1 in F2

Law of segregation• It states that, ‘when a pair of factors for a character brought

together in a hybrid, they segregate (separate) during the formation of gametes.

• Alleles do not blend & both characters recovered in F2 & one in F1

• Factors which is present in parent segregate & gametes receives only one of two factors

• Homozygous parent- one kind gamete• Heterozygous parent- two kind gamete each one have one

allele with equal proportion

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

Incomplete dominance: Ex snapdragon.( Dog flower plant)

Parent: Red X White Genotype. RR WWGametes R WF1 generation Pink (Hybrid)

RW Self pollinationF2 generation

Gametes R WR RR

RedRWPink

W RWPink

WWwhite

The phenotypic ratio is 1:2:1.The genotypic ratio is 1:2:1

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.

• IA and IB are co-dominant and dominant over i.

Blood Group Genotype

A- Group IAIA or IA i

B-Group IBIB or IBi

AB-Group IAIB

O-Group ii

• ABO blood grouping- multiple allele• Three alleles govern same character• Multiple allele is found when population studies are made• Single gene product may produce more than one effect• Eg.- Starch Synthesis in Pea seeds- 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

Inheritance of two gene:

Mendel’s 2nd law or Law of independent assortment:

• It states that, ‘factors for different pairs of contrasting

characters in a hybrid assorted (distributed) independently

during gamete formation.

Mendel’s 2nd law can be explained by dihybrid cross.

• Dihybrid cross: The cross between two parents, which differs

in two pairs of contrasting characters.

Dihybrid cross:

Parents

Round Yellow Wrinkled Green

Genotype

Phenotype

RRYY rryy

Gametes RY ry

F1 generation

Round Yellow

RrYy

Phenotypic ratio : 9 : 3 : 3 : 1

Dihybrid test cross.

• F1 hybrid is crossed with recessive green wrinkled pea plant.

• Recessive green wrinkled – rryy, Gamete ry

• F1 hybrid : round yellow- RrYy, Gametes:

RY, Ry, rY, ry.

Gametes RY Ry rY ryry RrYy Rryy rryY rryy

Phenotypic ratio – 1 : 1 : 1 :1

• Mendel work published 0n 1865 but remain unrecognized till 1900

• Reasons for that:1. Lack of communication2. Concept of genes / factors- clear3. Mathematical approach for biology was not acceted4. 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.

Chromosomal Theory of inheritance

• Thomas Hunt Morgan and his colleagues conducted experimental verification of chromosomal theory of inheritance

• Morgan worked with tiny fruit flies, Drosophila melanogaster.

• 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.

SEX DETERMINATION

• Henking (1891) traced specific nuclear structure during spermatogenesis of some insects.

• 50 % of the sperm received these specific structures, whereas 50% sperm did not receive it.

• He gave a name to this structure as the X-body. • This was later on named as X-chromosome.

XX-XO type

• Sex-determination of grass hopper:• The grasshopper contains 12 pairs or 24 chromosomes. The

male has only 23 chromosome.• All egg bears one ‘X’ chromosome along with autosomes.• Some sperms (50%) bear’s one ‘X’ chromosome and 50% do

not.• Egg fertilized with sperm having ‘X’ chromosome became

female (22+XX).• Egg fertilized with sperm without ‘X’ chromosome became

male (22 + X0)

XX-XY type

Sex determination in insects and mammals• In this type both male and female has same number of

chromosomes.• Female has autosomes and a pair of X chromosomes. (AA+

XX)• Male has autosomes and one large ‘X’ chromosome and one

very small ‘Y-chromosomes. (AA+XY)• In this type male is heterogamety and female homogamety.

ZZ – ZW type

Sex determination in birds:• In this type female birds has two different sex chromosomes

named as Z and W.• Male birds have two similar sex chromosomes and called ZZ.• In this type of sex determination female is heterogamety and

male is homogamety.

Linkage & 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.

MUTATION

• Phenotypic variation occurs due to change in gene or DNA sequence is called mutation. The organism that undergoes mutation is mutant.

• Phenomenon which result in alternation of DNA sequence & result in change in genotype & phenotype

1. Loss (deletion) or gain (insertion/duplication) of a segment of DNA results in alteration in chromosomes- abnormalities/ aberrations- Chromosomal aberrations

2. Gene Mutations: The mutation takes place due to change in a single base pair of DNA is called gene mutation or point mutation. E.g. sickle cell anemia.

3. Frame shift mutations: Deletion or insertions of base pairs of DNA is called frame shift mutations.

Pedigree Analysis:

• The study of inheritance of genetic traits in several generations of a family is called the pedigree analysis.

• Pedigree study- strong tool of human genetics to trace inheritance of specific trait/ abnormality/ diseases

• Pedigree analysis of inheritance of a traits is represented in family tree

• It helps in genetic counseling to avoid genetic disorders.

Genetic disorders • Genetic disorders grouped into two categories – 1. Mendelian disorder2. Chromosomal disorderMendelian Disorders• Mendelian disorders are mainly determined by alteration or

mutation in the single gene.• It obey the principle of Mendelian inheritance (principles of

inheritance) during transmission from one generation to other.• Mendelian disorder- traced in family by pedigree analysis• E.g. Haemophilia, Colorblindness, Cystic fibrosis, Sickle cell

anemia, Phenylketonuria, Thalesemia etc.• Dominant or recessive- pedigree analysis• Trait may linked to sex chromosome, Eg. Haemophilia• X- linked recessive trait- transmitted from carrier female to male

progeny

Hemophilia:

• It is a sex linked recessive disease.• The defective individual continuously bleed to a simple cut. • The gene for hemophilia is located on X chromosome.• In this disease a single protein that is a part of cascade of

proteins that involved in the clotting of blood is affected.• The diseases transmitted from unaffected carrier female to some

of the male progeny.• Heterozygous female (carrier)- transmit to sons• Female being hemophilic is rare- Mother should be carrier &

father Haemophilic

H

Sickle cell anemia

• Autosome linked recessive trait• Transmitted from parents- both partners are carrier/

heterozygous• Controlled by single pair of allele HbA & Hbs

• Homozygous individuals of Hbs (HbSHbS)- diseased• Heterozygous individuals HbAHbS- unaffected but carrier• Defect is due to substitution of Glutamic acid(Glu) by Valine

(Val)- at the 6th position beta globin chain of Hb• Due to substitution of single base at 6th codon of beta globin

gene from GAG to GUG• Mutant haemoglobin- polymerization under low oxygen

tension causing change in shape of RBC from biconcave to sickle like structure

phenylketonuria

• Inborn error of metabolism- inherited as autosomal recessive trait

• Affected individual lack enzyme that convert amino acid phenylalanine to tyrosine

• Phenylalanine accumulates & convert to phenylpyruvic acid & other derivatives

• Accumulation in brain result- mental retardation• Excreted in urine- poor absorption by kidney

Chromosomal disorder

• Caused due to absence or excess or abnormal arrangement of one or more chromosome.

Causes:1. Failure of segregation of chromatids- cell division cycle- gain or

loss chromosome- Aneuploidy, Eg.- Down’s syndrome (Extra copy of 21 chromosome)- Trisomy, Turner’s syndrome (loss of an X chromosome in female)- Monosomy

2. Failure of cytokinesis after telophase- increase in whole set chromosomes- Polyploidy, seen in plants

down's syndromes

• Presence of an additional copy of chromosome no. 21- Trisomy of 21

• Described- Langdon Down (1866)• Short statured, small round head, furrowed tongue, partially

open mouth, broad palm with palm crease; physical, psychomotor & mental- retardation

Klinefelter’s syndrome

• Presence of an additional copy of X- chromosome• Karyotype- 47, XXY• Overall masculine development along with feminine

development- development of breast (Gyanaecomastia), Sterile

Turner’s syndrome

• Absence of one of X- chromosome, Monosomy• Karyotype- 45, X0• Females-sterile, ovaries are rudimentary, lack of secondary

sexual character