PRINCIPLES OF INHERITANCE AND VARIATION
1. GENETICS is study of Inheritance & Variations in organism.
2. HERIDITY Transfer of information from one generation to next
3. Father of genetics G.J. MENDEL
4. Father of Cytogenetics is Morgan
5. Physical basis of heredity is GAMETES (It’s Mendel’s finding)
6. Chemical basis of HERIDITY GENES
7. Vehicle of heredity Chromosomes
8. Biochemical nature of genes DNA (Proved by Avery et all in 1944)
9. Mendel published his findings on garden pea plant in “Annual proceedings of the natural history society” of Brunn in 1866.
10. Gregor Johann Mendel, conducted hybridisation experiments on Garden peas for seven years (1856-1863) and proposed the laws of inheritance in living organisms.
11. Rediscovery of Mendel work
➤By CORRENS,TSCHERMARK & DE VRIES in1900.
12. IMPORTANT TERMINOLOGY :
o Character term is used for a heritable feature
o Trait term is used for variants of character
o GENE ➤JOHANSSEN used this term for Mendel’s FACTOR.
o Gene is a specific segment of DNA. It is unit of inheritance.
o ALLELOMORPH:
➤Contrasting pair of genes
Control the two alternative form of same character & present on same locus
o Homologous chromosomes Carry genes of same characters.
o Gene locus –
●A specific region of a chromosome which represents single gene.
● Genes or alleles of same character occupies same locus on a homologous chromosomes.
o Wild Allele
An original allele, dominant in expression & wildly distributed in population.
o Mutant allele
It originate from wild type through mutation during course of evolution.
it is recessive in expression, less distributed in population.
All the variants of a character are mutant of original gene.
o HOMOZYGOUS
If both the gene of a character are same e.g. TT /tt
o HETEROZYGOUS
If both the genes of a character are different alleles e.g. Tt
o DOMINANT GENE
Express in homozygous as well as heterozygous state.
o RECESSIVE GENE
The gene which express in only homozygous state.
o PURE LINE:
Associated with homozygous state (either dominant or recessive).
Obtained by continuous selfing (inbreeding).
Pure line for genes of all characters in an organism in population is never possible. @ an organism may be pureline for few characters
Pure line can be maintain through vegetative propagation & through asexual reproduction.
o GENOTYPE:
Genetic organization of an organism. Example – TT or Tt or tt
o PHENOTYPE
Expression of genes which is in form of observable, structural or functional. Example – Tall or Dwarf
o GENOME:
A complete set of gene in haploid cell.
o GENE POOL:
All the genotypes of all the individuals in an inbreeding population constitute the genepool.
o MONOHYBRID CROSS:
A breeding experiment dealing with only 1 character at a time
o DIHYBRID CROSS:
A breeding experiment with 2 characters at the same time
o Punnet square is used to predict the outcome of a cross if genotype of parents are known.
o Test cross : Cross between F1 plant and Recessive plant
Why Called test cross ? because It is done to know the genotype of unknown dominant parent (either Homozygous or Heterozygous)
o BACK CROSS
The cross between F1 progeny with either of parent.
o RECIPROCAL CROSS
A reciprocal cross involves the same traits but carried by sexes opposite to those in the first cross.
13. MENDEL’S WORK –
o On Pisum sativum (Garden Pea)
o Studied 7 Pair of characters
o Mendel selected 14 true-breeding pea plant varieties, as pairs
o Each one character with contrasting traits.
o 7 Contrasting Characters = 14 Traits @ conventionally called 7 pair of characters
o Some of the contrasting traits selected were
●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
14. MENDALIAN LAW OF INHERITANCE:
o LAW OF DOMINANCE:
When two homozygous plant having contrasting traits of a character are crossed Only one character expressed in progeny (Dominant)
In heterozygous state ,only one gene express and it is called dominant gene.
Dominant gene express both in homozygous & heterozygous state.
Recessive gene express only in homozygous state
This law has many Exceptions
o LAW OF SEGREGATION / LAW OF SPILLITING OF HYBRID / LAW OF PURITY OF GAMETE:
● It states that genes of a character which occurs in pair segregate during gametes formation (Anaphase I of meiosis) and gamete receive only one gene of a pair.
● For example, a gamete receive either T or t of tallness
o LAW OF INDEPENDENT ASSORTMENT:
● It states that two characters of dihybrid cross can move apart independently with each other if they are not linked.
●Because of this we get 4 types of phenotypes in F2 generation.
●It is related with study of two character so Law of independent assortment is not applicable for monohybrid cross.
15. MONOHYBRID CROSS Findings
●F1 Generation -100% dominant trait.
● F2 generation:
●Phenotypic Ratio – 3:1
●Genotypic Ratio -1:2:1
●Test cross ratio – 1:1
● Types of genotype -3
●Types of gametes in F2 Generation -2
16. DIHYBRID CROSS Findings
o F1 ratio:100% dominant.
o Types of gametes in F2 generation – 4.
o F2 phenotype ratio 9:3:3:1.
o F2 genotype ➤9 types.
o Test cross ratio – 1:1:1:1
17. INCOMPLETE DOMINANCE
o Also called Blended inheritance/ Partial dominance.
o The expression of traits of two pure parents as an intermediate condition in the F1 hybrid .
o Reason for this that so called dominant gene is not in position to supress the recessive gene.
o Therefore the F1 ratio is 100% intermediate type & F2 ratio is 1:2:1.
● Examples of incomplete dominance are
• Mirabilis jalapa (four ‘o clock plant)
• Antirrhinum majus (snapdragon flower).
18. CODOMINANCE
o It is the expression of both allele simultaneously in heterozygous state.
o ABO blood group in human is best example of codominance
o There are more than two alleles are present in a population .
o But an individual carries only two allele of a character.
19. Codominance & Incomplete dominance are exception of law of dominance.
20. Pleiotrophy
o A single gene product may produce more than one effect.
o Example in Pea seed – Starch grain size & Seed shape (Gene B & b)
●For one character it shows complete dominance (Seed shape) while for other it shows incomplete dominance (Starch grain size)
21. PHENYLKETONURIA – Example of Pleiotropy.
22. POLYGENIC INHERITANCE
o Continuous variations are due to polygene
o Best example continuous gradation in height and skin colour.
o It is controlled by three or more genes( multiple genes).
o Here, more dominant gene result into strong expression of characters.
o Polygenic inheritance also takes into account the influence of environment.
23. CHROMOSOMAL THEORY OF INHERITANCE
o Proposed by Sutton & Boveri (1902)
o Sutton & Boveri noted that behaviour of chromosome was parallel to the behaviour of gene (factor ) during cell division (behaviour in meiosis)
o They used these findings to explain Mendel’s law.
24. Separation of homologous chromosome during anaphase I is the basis of law of segregation.
25. Chromosomes & Genes both occur in pair. so it became clear that genes are present on chromosomes.
26. Chromosomal theory is all about the relation of genes with chromosomes
27. LINKAGE AND CROSSING OVER
o T.H. Morgan studied the Linkage & Recombination.
o Linkage means physical association of genes . All genes present on a chromosome
o Morgan worked on Drosophila melanogaster (Fruit fly)
o He found that if genes for different characters are located on same chromosomes they will not follow the law of independent assortment.
28. Mendel was lucky that the two character he selected for dihybrid cross were located on different chromosome otherwise he could not become able to formulate the law of independent assortment.
29. Why Morgan selected Drosophila as an experimental material ?
o They could be grown on simple synthetic medium in the laboratory
o Their life cycle is completed in two weeks.
o A single mating could produce a large number of progeny.
o They have clear differentiation of male & female flies.
o They have many types of heredity variations which are easily visible under low power microscope.
30. Morgan used recombination term for non-parental combination (separation of linked gene due to crossing over).
31. The distance between genes on same chromosome is determining factor for production of recombination .
●If the distance is less between there will be less recombinant.
●First time indicated by Morgan
●Sturtewant (morgan ‘s student ) used this concept in Gene Mapping.
●Frequency of recombination between genes pairs on same chromosome as a measure of distance between two genes.
32. SEX DETERMINATION
o The chromosomal mechanism /genetic mechanism of sex determination was studied in insect by Henking
o He found that 50% sperms receive a ‘structure’ which is present on nucleus after spermatogenesis. ▪︎He called this structure X body.
33. SEX DETERMINATION IN INSECT
o It is XO type (XX – Female; XO -Male) @Male have one chromosome less
o Egg -Autosomes + one X chromosomes
o Sperms two types,
▪︎ one with autosome + one X chromosome
▪︎one with only autosomes (No X chromosome)
34. SEX DETERMINATION IN HUMAN
o Sperm is responsible for sex determination.
o There are two types of sperm Biomentors online classes, Mumbai
– (22 + X ) & (22 + Y )
o Male heterogametic condition (Two type of Sperms)
o Egg is only one type (22 + X ).
o If post fertilization —
●44 + XY -Male
● 44 + XX – Female
o Y chromosome in male is smaller than X chromosome
o In drosophila same mechanism is found for sex determination.
35. SEX DETERMINATION IN BIRDS
o ZW – ZZ type Male – ZZ; Female – ZW
o Female heterogametic condition seen (Two type of eggs)
o One type of sperm
●Autosomes + Z.
o Two types of egg formed.
● Autosome + Z
● Autosomes + W
36. MUTATION
o Mutation is change in DNA sequence –> Result into change in genotype & phenotype.
o Change in chromosome –>Results in abnormalities or aberration which is common in cancer cells.
o Change in single base pair in DNA – POINT MUTATION.
●Example – sickle cell anemia.
o Insertion or deletion of base pair of DNA -FRAME-SHIFT MUTATION
37. PEDIGREE ANALYSIS
o To study heritability of certain characteristic features in families.
o Study of the family history about inheritance of a particular trait provides an alternative to breeding experiments.
o Such an analysis of traits in a several of generations of a family is called the PEDIGREE ANALYSIS.
o In the pedigree analysis the inheritance of a particular trait is represented in the family tree over generations.
o In human genetics, pedigree study provides a strong tool, which is utilised to trace the inheritance of a specific trait, abnormality or disease.
38. MENDELIAN DISORDERS
o It is due to alteration or mutation in single gene.
o Mendelian disorders are either dominant or recessive
o Dominant mutation express in both homozygous & heterozygous
o Recessive mutation only express in homozygous state.
o For Sex Linked Inheritance
If disorder is present on X chromosome & recessive male progeny will suffer more from disease
Reason is Y chromosome can’t supress the expression of gene as Normal X can supress.
Male are Either Normal or Sufferer
Females are Normal, Carrier & Sufferer
39. HEMOPHILIA:
o Sex linked recessive disease (on X chromosome)
o A single protein required for blood clotting is affected
o Person suffering from haemophilia A minor injury can be life threatening
o Here a heterozygous female (XX) Carrier female transmit the disease to son
o If male receive this Xo Becomes haemophilic
o Female needs two X to become haemophilic So rare to show this disease .
o The family pedigree of Queen Victoria shows a number of haemophilic descendents as she was a carrier of the disease.
40. Colour blindness
o Sex linked recessive disorder
41. Why Colour blindness is more seen in females as compare to haemophilia
Female also express colour blindness disease if her father is colour blind & Mother is a carrier whereas in haemophilia ,there is rare chance that a female become haemophilic because haemophilia is life threatening during menstruation so they die in earlier stages. So due to Pre- reproductive phase death the haemophilia sufferer females are rare.
42. SICKLE CELL ANEMIA
o Autosomal linked recessive trait .
o The defect is caused by the substation of glutamic acid by valine at the sixth position of beta globin chain of the haemoglobin molecule.
o It is due to single base substation at the sixth codon of beta globin gene from GAG to GUG. (Substitution gene mutation)
43. PHENYLKETONURIAAutosomal recessive trait (single gene mutation)
o The affected person cannot synthesize enzyme phenyl alanine hydroxylase
o This enzyme converts phenylalanine into tyrosine.
o Phenylalanine accumulated and converted into phenyl pyruvic acid & other derivatives which accumulate in brain resulted into mental retardation.
o This chemical also excreted from urine because of poor absorption by kidney.
o It is also example of pleiotropy -The phenotypic result is mental retardation as well as skin pigmentation & reduction in hair also.
44. THALASSEMIA
o Autosome linked recessive blood disease
o In this disease ,one of the globin chain synthesis is reduced
o Formation of abnormal haemoglobin molecule resulting into anaemia
45. Thalassemia is quantitative problem whereas sickle cell anaemia is qualitative issue
46. Cystic fibrosis is also example of Mendelian disorder
47. CHROMOSOMAL DISORDER:
o It is due to absence or excess or abnormal arrangement of one or more chromosomes.
o Reason is failure of segregation (non – disjunction), Which result into gain or loss of chromosomes in daughter cells.
o It is called aneuploidy.
48. DOWN’S SYNDROME :
o 2n =47 (one extra chromosome with 21st pair i.e. trisomy )
o This disorder was described by Langdon Down.
o Symptoms :
●short statured with small head,
● furrowed tongue,
●partially open mouth,
●broad palm with characteristic palm crease
●psychomotor and mental retardness.
49. KLINEFELTER SYNDROME
o It is aneuploidy condition (Trisomy)
o Three sex chromosomes. (XXY)
o Such an individual has overall masculine development but also show gynaecomastia i.e. feminine development (Breast)
o Such individuals are sterile.
50. TURNER’S SYNDROME
o This is also aneuploidy condition (Monosomy)
o Single sex chromosome (44 +XO )
o Such females are sterile with rudimentary ovaries
o Secondary sexual characters are not developed in Turner’s syndrome.
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