CLASS 12TH BIOLOGY CHAPTER- 5 PRINCIPLES OF INHERITANCE AND VARIATION NCERT QUICK REVISION NOTES FOR NEET AND CBSE EXAM
CHAPTER 5: PRINCIPLES OF INHERITANCE AND VARIATION
Heredity and variation Mendelian inheritance; deviations
from Mendelism - incomplete dominance, co-dominance, multiple alleles and inheritance of blood groups, pleiotropy: elementary idea. Chapter of polygenic inheritance: chromosome theory of inheritance; chromosomes and genes; Sex determination in humans, birds and honey bee; linkage and crossing over; sex linked inheritance - haemophilia, colour blindness; Mendelian disorders in humans - thalassemia; chromosomal disorders in humans; Down's syndrome, Turner's and Klinefelter's syndromes.
Phenotype: The observable or external characteristics of an organism. Genotype: The genetic constitution of an organism.
Monohybrid cross: A cross between two individuals of species, considering the inheritance of single pair of contrasting character, e.g. a cross between pure tall (TT) and Dwarf (tt)
Dihybrid cross: A cross between two individuals of a species, considering the inheritance of two pairs of contrasting traits/characters e.g., a cross between Round and Yellow (RRYY) and wrinkled and green (rryy) seed.
Aneuploidy: The phenomenon of gain or loss of one or more chromosome(s) that results due to failure of separation of homologous pair of chromosomes during meiosis.
Trisomy: The condition in which a particular chromosome is present in three copies in a diploid cell/nucleus.
Male heterogamety: When male produces two different types of gametes/sperms e.g. In human beings X and Y.
Female Heterogamety: When female produces two different types of gametes/ova, e.g., female bird produces Z and W gametes.
Gene- It is a segment of DNA called cistron and the unit of in heritance which is carried from parent by a gamete.
Genome-The entire genetic set of a prokaryote or virus or the haploid genetic set of a eukaryote..
Gene Pool-An aggregate of all genes and their alleles, present in and interbreeding population.
Test Cross- A cross between an individual of unknown genotype and recessive parent. It is used to test whether an individual is homozygous (pure) or heterozygous (hybrid). It is also used as test for linkage.
Heredity-Also called inheritance is the process of transmission of genetic characters (traits) from parents to their offsprings.
Gregor Mendel Conducted controlled breeding experiment on garden pea (Pisum sativum) with a single trait. It is called monohybrid cross.
Law of Segregation (law of purity of gametes): The members of allelic pair that remained together in the parent, segregate/separate during gamete formation and only one of the factors enters a gamete.
Law of Independent Assortment: In the inheritance of two pairs of contrasting characters (dihybrid cross) the factors of each pair of characters segregate independently of the factors of the other pair of characters.
Test Cross: When offspring or individual with dominant phenotype, whose genotype is not known, is crossed with an individual which is homozygous recessive for the trait, this cross is known as test cross.
Test cross is done to determine whether the individual parent exhibiting dominant traits is homozygous or heterozygous.
Flower colour is → Violet (Dominant phenotype, Genotype is unknown) Genotype may be WW or Ww
Example:
In incomplete dominance, phenotypic ratio is equal to the genotypic ratio.
MULTIPLE ALLELISM:
It is a phenomenon in which a single character is governed by more than two alleles.
Example:
• ABO blood groups are controlled by gene I
• 'I' has three alleles-IA, IB, and i
IA and IB alleles produce slightly different form of sugar present on plasma membrane of red blood cells.
• In allele 'i' do not produce any sugar.
• In any diploid individual only two alleles can be found. So multiple alleles can be detected only in a population.
Co-dominance: The alleles which do not show dominance recessive relationship and are able to express themselves independently when present together are called co-dominant alleles and this phenomenon is known as codominance. Example: Human blood groups.
There are 3 different alleles, 6 different genotypes control 4 different type of sugar Phenotypes:
In humans, blood group AB shows co-dominance as both the alleles I and IB express themselves fully in presence of each other.
Chromosomal Theory of Inheritance: Proposed by Sutton and Boveri. The pairing and separation of a pair of chromosomes would lead to the segregation of a pair of factors they carried. They united the knowledge of segregation with Mendelian principles.
• Linkage- is the tendency of genes on chromosome to remain together. Linked genes occur on the same chromosome.
• They lie in linear sequence on the chromosome - There is a tendency to maintain the parental combination of genes except for accessional choosers.
• Strength of linkage between genes is inversely proportional to the distance between the two.
Recombination: is the generation of non-parental gene combinations to the offsprings. Tightly linked genes show very low recombination frequency. Loosely linked genes show higher recombination frequency. The frequency of recombination between gene pairs on the same chromosome is a measure of distance between genes and is used to map the position of genes on the chromosome.
Linkage and Recombination.
T.H. Morgan carried out several dihybrid crosses in Drosophila melanogastor. Two of them are given below:
Cross-I: Yellow-bodied and white eyed females crossed with brown-bodied, red eyed males (wild type) F, Progeny
¶ F, Progeny intercrossed and F, generation ratio deviated from 9:3:3:1 (two genes didn't segregate independently)
¶ The Parental combinations were 98.7% and recombinats were 1.3%
Conclusion: The two genes (body colour and eye-colour) are tightly linked; results in less crossing over and less no. of non-parental progeny.
Cross-II: White bodied female with miniature wings and yellow-bodied male with normal wings (wild type) were crossed.
¶ The parental combinatios were 62.8% while the recombinants were 37.2%.
Conclusion: The two genes (body colour & wing's size) are loosely linked; results in more crossing over and more no. of non-parental progeny, The seven character: Mandel Studied in garden pea, had their genes located on different (non-homolohous) ehromusomes or far apart on the same chromosome that they got separated by crossing over. So he was lucky that he could not note linkage and propose law of independent assortment.
CHROMOSOMAL BASIS OF SEX DETERMINATION :
(i) XX-XY type - Female homogametic i.e. XX and male heterogametic i.e. XY in Drosophila, humans.
(ii) XX-XO type-All eggs bear additional X chromosome, Males have only one X chromosome besides autosomes whereas females have a pair of X chromosomes e.g., grasshoppers.
(iii) ZW-ZZ type-The females are heterogametic and have one Z and one W chromosome. The males are homogametic with a pair of Z chromosomes besides autosomes e.g., birds..
(iv) ZO-ZZ type-Females are heterogametic and produce 2 types of eggs (A+Z) and (A+O). The males are homogametic with all the sperms having (A+Z) e.g. moths and butterflies. here A autosome.
Sex determination in honey Bee : In Honey bee fertilized eggs develop into female (Queen or Worker) While unfertilized egg develops into male (drone) by parthenogensis, the males have half no. of chromosomes than a female. The males are haploid (16-chromosomes), females are diploid (32-chromosomes).
THERE ARE THREE TYPES OF INDIVIDUALS:
A record of inheritance of certain genetic traits for two or more generation presented in the form of diagram or family tree is called pedigree.
USEFULNESS OF PEDIGREE ANALYSIS
1. It is useful for genetic counsellors to advice intending couples about the possibility of having children with genetic defects like haemophilia, thalassemia etc.
2. It is helpful to study certain genetic trait and find out the possibility or absence or presence of that trait in homozygous or heterozygous condition in a particular individual.
3. It can indicate the harms a marriage between close relatives, may cause.
MENDELIAN DISORDERS
These are mainly determined by a alternation or mutation in single genes.
1. Haemophilia: Sex linked recessive disease which is transmitted from unaffected carriers female to male progeny. A single protein is affected which is a part of the cascade of proteins involved in the clotting of blood.
XhY= affected male
XhX = carrier female
The heterozygous female for haemophila may transmits the disease to her sons. The possibility of a female suffering from the disease is extremely rare (only when the mother of the female is a carrier is Xh X and father is haemophilic i.e. Xh Y.
2. Sickle-cell anaemia: This is an autosome linked recessive trait. This defect is caused by substitution of glutamic acid by valine at the 6th position of the beta globin chain of the haemoglobin molecule. The mutant Hb molecule undergoes polymerisation under low oxygen tension which results change in shape of RBC from biconcave disc to elongated sickle like structure. The disease is controlled by a pair of allele, HbA and Hbs
• HbA HbA Normal
• Hbs Hbs sufferer
• Hb Hbs. Apparently unaffected/carriers
Thalassemia: Thalassemia is autosome linked recessive disease. This disorder caused by defers in the synthesis of globin chain. Thalassemia is of three types-Alpha Thalassemia, Beta Thalassemia and delta(s).
¶ In alpha Thalassemia production of alpha globin chain is affected. This Thalassemia is controlled by genes HBA1 and HBA2 located on chromosome 16th of each parent. Thalassemia occurs due to mutation or deletion of one or more of the four genes.
¶ In Beta Thalassemia production of (B-globin chain is affected, this thalassemia is controlled by gene HBB located on 11th chromosome of each parent. It occurs due to one or both HBB genes
¶ Thalassemia very few globin is synthesized and is quantitative problem whereas in sickle cell anaemia there is a synthesis of incorrectly functioning globin and is a qualitative disorder.
Delta Thalassemia- It is caused due to defective allele of HBD gene present on chromosome II that farms delta chain of heamoglobius. The effect of this thalassemia is minor as the adults have about 3% harmoglobius consisting of a and of chains. These are caused due to absence or excess of one or more chromosomes.
Colour blindness: Colour blindness is sex-linked recessive trait in which a person fails to distinguish red and green colour. The gene for normal vision is dominant. The normal genes and its recessive alleles are carried by X-chromosome.
XcXc -Colour blind female.
X Xc -Carrier female
Xc Y - Colour blind male
Y Chromosome of male do not carry any gene for certain vision.
INHERITANCE PATTERN IN COLOUR BLINDNESS
CHROMOSOMAL DISORDER:
1. Down's syndrome: Trisomy of chromosomes number 21 (2n + 1) Affected individual is short statured with small round head, furrowed tongue, partially open month, broad palm. Physical, psychomotor and mentaly development is retarded.
2. Klinefelter's syndrome: extra copy of X chromosome; karyotype XXY. Affected individual has overall masculine development with famine characters like gynaecomastia (development of breast) and is sterile 44 autosomes + xxy-47 chromosomes
3. Turner's syndrome: has absence of one X chromosome i.e. 45 with XO. Affected females are sterile with rudimentary ovaries and lack secondary sexual characters. 44 autosomes + x = 45 chromosomes
PLEIOTROPY
The ability of a gene to have multiple phenotypic effects because it influences a number of characters simultaneously is known as pleiotropy. The gene having a multiple phenotypic effect because of its ability to control expression of a number of characters is called pleiotropic gene. E.g. in Garden Pea, the gene which controls the flower colour also controls the colour of seedcoat and presence of red spot in the leaf axis.The disorder phenylketonuria shows pleiotropy.
POLYGENIC INHERITANCE
It is a type of inheritance controlled by three or more genes in which the dominant alleles have cumulative effect with each dominant allele expressing a part of the trait, the full being shown only when all the dominant alleles are present. E.g., Kernel colour in wheat, skin colour in human beings, height in humans, cob length in maize etc. In polygenic inheritance, a cross between two pure breeding parents produces an intermediate trait in F₁. In F2 generation, apart from the two parental types, there are several intermediates (gradiations. show a bell shaped curve). F₁ hybrid form 8 kinds of gametes in each sex giving 64 combination in F2 having 7 phenotypes.
POLYGENIC INHERITANCE SKIN TONE
3. loci: each has two possible alleles: Aa, Bb, Ce, each capital allele adds one unit of darkness, each lower case allele adds nothing. Parents produce F1 offsprings with intermediate tone.
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