Ch-9, Heredity and Evolution (notes)
GENETICS for NTSE, BHU, Olympiad, Board exams
Animal Cell Diagram
⚫️ CELL NUCLEUS (Brain of the Cell)
➡️ a large organelle in eukaryotic organisms which protects the majority of DNA within each cell. The spherical nucleus occupies 10% volume of a cell.
➡️ Functions :
☣️ controls all metabolic activities of the cell
☣️ regulates cell division
☣️ transmits hereditary traits from parent to offspring
⚫️ NUCLEOLUS
➡️ a membrane-less organelle within the nucleus that manufactures ribosomes, the cell's protein-producing structures.
⚫️ NUCLEAR ENVELOPE
➡️ the double-layered membrane that encloses the contents of the nucleus during most of the cell's life cycle.
⚫️ NUCLEAR PORES
➡️ nuclear envelope is perforated with holes called nuclear pores, which regulates the passage of molecules between the nucleus and the cytoplasm permitting some (like building blocks of DNA and RNA, and molecules providing energy for constructing genetic material) to pass through the membrane, but not the others [during cell division].
⚫️ CHROMATIN
➡️ the fine thread-like coiled filaments made up of a complex of DNA + proteins (called histones), found floating in the cytoplasm.
➡️ Functions :
☣️ to package long DNA molecules into more compact, denser structures called Chromosomes.
⚫️ CHROMOSOME (word coined by Heinrich Waldeyer in 1888)
➡️ The chromosomes are highly condensed and coiled chromatin fibres, packed with DNA that forms the genetic material.
➡️ Functions/ Role :
☣️ capable of self reproduction
☣️ passes on traits to their offsprings
☣️ physical basis of heredity
☣️ control 'protein synthesis', cell division, cell repair, cell growth and differentiation
➡️ Chromosomes generally occur in pairs, thus called Diploid (2n) , but gametes contain a single set of chromosomes, thus called Haploid (n).
➡️ Chromosomes = Autosomes + Allosomes (sex chromosomes/hetero-chromosomes).
➡️ Autosomes : Contain genes that determine the somatic (body) characters. Male and Female have equal number of autosomes.
Types of DNA in autosomes - (1) atDNA (2) auDNA.
➡️ Allosomes : are responsible for determining the sex of an individual. There are two types of allosomes :
(1) Human male -> X Y (X chromosome and Y chromosome)
(2) Human female -> X X (both X chromosomes)
➡️ In human beings, each cell normally contains 23 pairs (total: 46) of chromosomes = 22 pairs of autosomes + 1 pair out of two allosomes.
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⚫️ KARYOTYPE - number, size and shape of chromosomes.
⚫️ KARYOGRAM/IDIOGRAM - the diagrammatic representation of Karyotype of a species; consists of all the metaphasic chromosomes arranged in homologous pairs, ascending to descending length, thickness, position of centromere, shape, etc., with the sex chromosome placed at the end.
Structure of DNA
⚫️ DNA is the hereditary material as it contains the genetic information. (most important constituent of chromosome).
➡️ DNA (deoxy ribonucleic acid) is a large molecule consisting of millions of 'nucleotides' . Hence, DNA is called a polynucleotide.
⚫️ Composition of a DNA nucleotide
1» a sugar molecule - deoxy ribose sugar (C5H10O4)
2» a nitrogenous base:
(a) Purines -> adenine (A) and guanine (G)
(b) Pyrimidines -> cytosine (C) and thymine (T)
3» a phosphate group
⚫️ James Watson and Francis Crick's 3D model of DNA (Features) :
1▶️DNA molecule consists of 2 polynucleotide (spiral) chains.
2▶️These chains form a DOUBLE-HELIX structure with two strands which run anti-parallel to one another.
3▶️Nitrogenous bases in the centre are linked to sugar-phosphate units which form the "backbone of the DNA".
4▶️Adenine (A) links Thymine with two hydrogen bonds (covalent) (A=T) , cytosine (C) links guanine (G) with three hydrogen bonds (C≡G). This is called "Complementary Base Pairing". These hydrogen bonds between the nitrogenous bases make the DNA molecule stable.
5▶️Each turn of double helix is 34Å. There are 10 base pairs in a complete turn.
6▶️Nucleotides are joined to one another in a chain by covalent bonds between the sugar of one nucleotide and, the phosphate of the next, called as "PHOSPHO-DIESTER LINKAGE".
GENES (word coined by William Johannsen)
⚫️ Genes (earlier factors) are the fundamental, physical and functional units of heredity which transfer characteristics or traits.
➡️ A section of DNA that provides information for one protein is called "gene" for that protein.
➡️ Genes are made up of polynucleotides (sequence of nucleotides in DNA Or RNA on a chromosome which governs the synthesis of a protein (or, gene product) that controls a specific trait of an organism.
Here, light blue colored portion is called as genetic locus where genes are present. B represents dominant allele and b recessive one.
➡️ Functions of genes :
☣️ determines which proteins are to be made in the cell
☣️ transmission of hereditary characters
☣️ determines particular traits- hair colour, eye colour, tallness, blood group, earlobe, etc.
ALLELES
⚫️ An Allele is one of two or more forms of a given gene variant. It is one of two or more versions of the same gene at the same place on a chromosome.
Tt
▪ Gene representing height.
▪ t representing dwarfness (recessive trait)
▪ T representing tallness (dominant trait)
⚫️ Homologous Chromosome are a set of one maternal and one paternal chromosome that pair up with each other inside a cell during fertilisation.
➡️ Features :
☣️ same shape and size
☣️ bears corresponding genes governing the same trait
☣️ equal in length
⚫️ Heterologous Chromosome
➡️ Features :
☣️ Differ in shape, size or function
☣️ Do not belong to the same pair
TRAIT (characters)
⚫️ Trait is a specific characteristic of an organism which is determined genetically.
➡️ It is a recognisable feature of a human being or any other organism like height, complexion (body structure-color), shape and colour of hair, blood type, eye color, shape of nose and chins.
➡️ TRAITS can be determined by genes or environment or more commonly by interactions between them.
➡️ A genetic trait is manageable to segregation analysis rather than quantitative analysis.
GENETIC VARIATION
⚫️ Variation is the degree by which progeny differ from their parents.
➡️ Genetic Variation is the difference in DNA among individuals or the difference between populations.
➡️ Causes :
☣️ mutation
☣️ random mating between organisms
☣️ random fertilisation
☣️ RECOMBINATION (crossing over) between chromatids of homologous chromosome during Meiosis which reshuffles alleles within an organism's offspring.
☣️ Sexual reproduction [very large variation]
Asexual reproduction [very small variation]
➡️ Within a species there is usually a great deal of variation between individuals. The genetic variation of an entire species is often called GENETIC DIVERSITY.
➡️ BREED : a specific group of domestic animals having homogeneous phenotype, homogeneous behavior and/or other characteristics that distinguish it from other organisms of the same species.
➡️ Examples of Genetic Diversity :-
☢️ Different breeds of dogs ➖ German Shepherd, Bulldog, Golden retriever, St. Bernard, Pomeranian puppy
☢️ Different varieties of roses ➖ Red rose, Purple Rose, Grandiflora, Portland Rose, Prickly Wild Rose.
☢️ There are more than 50,000 varieties of rice, and more than 1000 varieties of mangoes found in India.
☢️ There are a list of cattle breeds in India ➖ Bachaur, Gangatiri, Gir, Kenkatha, Pulikulam, Sahiwal.
SIGNIFICANCE OF ACCUMULATION OF VARIATIONS
⚫️ Reproduction of organisms produces variations, during successive generations which get accumulated in the organisms. Its significance is visible only if it continues to be inherited by progeny for several generations.
➡️ Advantages :
☣️ It allows some individuals within a population to adopt to the changing environment, increases the chance of its survival in a changing environment. [Some new alleles increase an organism's ability to survive and reproduce.
☣️ It is a necessity for organic evolution. [Number of successful variations is maximised by the sexual reproduction.]
Question. Only variations that confer advantage to an individual organism will survive in a population. Do you agree with this statement? Why or why not?
Answer :- Yes ,only those variations that confer advantage to an individual organism will survive in a population.
Suppose, there is a population of red beetles in the green bushes and a colour variation arises during sexual reproduction, so, that one beetle is now green in colour. This variation offers advantage of survival [camouflage to crow] and hence its population will increase. If, however, the variation had produced a blue-coloured beetle, then this colour couldn't offer any survival advantage because blue beetles in green bushes could be easily spotted by a crow and eaten by it.
⚫️ GENOME
➡️ all genetic material of an organism.
⚫️ GENOTYPE
➡️ It is an organism's set of heritable genes that can be passed down from parents to offspring.
➡️ It is an individual's collection of genes.
➡️ Genotype shows the genetic constitution of an organism. Eg. TT, Tt, to where T and t are alleles. Thus, the genotype of a tall plant could be TT and Tt ,whereas that of a dwarf plant is tt.
⚫️ PHENOTYPE
➡️ It is the visible physical characteristic of an organism which is determined by its Genotype.
Eg. TT, Tt results in tall phenotype, whereas tt results in a dwarf phenotype.
➡️ Phenotype is the set of observable characteristics of an individual resulting from the interaction of its genotype with the environment.
HEREDITY
⚫️ It is the transmission of characters from the parents to their offsprings.
INHERITANCE
⚫️ It is the process by which, heredity takes place or genetic information is passed on from parent to offspring.
GENETICS (word coined by bateson)
⚫️ It is the branch of biology that deals with the genes, genetic variation and heredity of living organisms.
MENDELIAN GENETICS
Contribution of Gregor Mendel - Father of Genetics
⚫️ Gregor Johann Mendel [20 July 1822 - 6 Jan 1884] was the first scientist to make a systematic study of patterns of inheritance which involved the transfer of characteristics from parents to progeny.
⚫️ He was an Austrian Monk who discovered the basic principles of heredity through his experiments, which are the foundation for modern genetics.
⚫️ Born to a farmer family in Silesian of Czechoslovakia. After finishing his high school at 18, he entered the Augustinian Monastery at Brunn as a priest. From there he went to the University of Vienna for training in physics, mathematics and natural science.
⚫️ Abbott Mendel returned to St. Thomas Abbey (Konigskloster) in 1854 and continued to work as a priest and teach in high school.
⚫️ He conducted his experiments on the garden pea plant in the monastery for about 9 years from 1856 to 1865. He had worked on nearly 10,000 pea plants of 34 different varieties.
Question. Why did Mendel choose pea plants for conducting his experiments on Inheritance?
Answer :- Mendel's investigations into Inheritance patterns, it was for the first time that statistical analysis and mathematical logic were applied to problems in biology. So, his results pointed to general rules of inheritance rather than being unsubstantiated ideas. The credibility of his data was due to the large sampling size.
💫 He chose pea plants [Pisum sativum] for studying inheritance because :-
1🔊 Pea plants had 7 pairs of deeply defined contrasting characters [or, opposing traits] for his study.
2🔊 It is naturally self -pollinating and, so is very easy to raise pure breeding individuals [PUREBRED : an organism that always passes down certain phenotypic traits to its offspring of many generations. ]
3🔊 It has short life-span as it is an annual and so, it was possible to follow several generations.
4🔊 It is easy to cross-pollinate.
5🔊 The flowers are bisexual.
Mendel's contribution to rules for the Inheritance of Traits [Mendelian Inheritance]
1🔥MONOHYBRID INHERITANCE & LAW OF SEGREGATION
⚫️ Crosses involving inheritance of only one pair of contrasting traits are called as Monohybrid Crosses.
For example, it is a cross between two forms of a single trait like a cross between tall and dwarf plant.
Mendel cross-bred the pure-bred pea plants (dwarf and tall) and noted their occurrence in the progeny of succeeding generations.
⚫️ HYBRIDISATION EXPERIMENT :-
Question. How do Mendel 's experiments show that traits may be dominant or recessive ?
Answer :- In monohybrid cross, Mendel said that trait of dwarfness of one of the parent pea plant hadn't been lost, it was merely suppressed in F1 generation to re-emerge in F2 generation. Mendel called the repressive trait of 'dwarfness' as 'recessive trait'and the expressed trait of 'tallness'as the 'dominant'.
Question. How do Mendel 's experiments show that traits are inherited independently ?
Answer :- In monohybrid cross, All the pea plants produced from the hybrid tall parents of F1 [Filial(child)1 ] generation, were either tall or dwarf, none were of medium height.
🌠 This is because, if the traits of tallness or dwarfness had blended then medium-sized pea plants would have been produced.
🌠Genes located on different chromosomes will be inherited independently w.r.t. each other. Mendel observed that, when lead with more than one trait were crossed, the progeny did not always match the parents.
This is because, different traits are inherited independently.
⚫️ LAW OF SEGREGATION
➡️ It is also known as "Law of Purity of Gametes".
➡️ During gamete formation or "gametogenesis", the alleles for each gene segregate [separate] from each other such that each gamete formed, carries only one allele for each gene.
➡️ This means that the alleles do not mix (blend) in the hybrids. [Non-mixing of alleles]
➡️ A homozygous parent produces all gametes that are similar, while a heterozygous one, produces two kind of gametes ,each having one allele with equal proportion [50-50%].
Consider segregation of one pair of genes "Tt", then 50% [half] of gametes have allele T, and the other 50% have allele "t".
⚫️ MENDEL'S HYPOTHESIS (to explain his results)
1🔷🔶 'Factors' are passed on from one generation to another, now to referred to as genes. These factors occur in pairs and may be alike as in pure breeding tall plants [TT] and pure-breeding dwarf plants [tt], then called "Homozygous", if unlike (different) then called "Heterozygous".
2🔷🔶 Two factors making up a pair of opposing traits are called "alleles", one member [gamete] of each pair is contributed by one parent.
3🔷🔶 When two factors for alternative expression of a trait are brought together by fertilisation. The trait which expresses itself, called as "Dominant" and that which is concealed called the "Recessive".
4🔷🔶 The two alleles for each character segregate (separate) during gamete production.
⚫️ LAW OF DOMINANCE
➡️ If there are two alleles coding for the same trait and one is "dominant", it will express itself in the progeny (organism), while the other won't.
➡️ In a cross of parents that are pure for contrasting traits, only one (dominant) form of the trait will appear in the next generation (F1).
➡️ Dominant alleles are expressed exclusively in the phenotype of a "heterozygote", while recessive traits are expressed only if the organism is "homozygous" for the recessive allele.
⚫️ INCOMPLETE DOMINANCE (Semi/Partial Dominance, Intermediate Mendelian Principle)
➡️ It has variations from Mendel's principles. Carl Correns actually rediscovered it.
➡️ It is when a dominant allele doesn't completely mask (repress) the effects of a recessive allele, and the organism's resulting phenotype shows a "blending" of both alleles.
Examples : Mirabilis jalapa [4 O' clock flower]
Antirrhinium plant [Snapdragons/Dogflower]
Andalusian fowl [a chicken breed]
1🔥DIHYBRID INHERITANCE & LAW OF INDEPENDENT ASSORTMENT
⚫️ It involves the inheritance of two pairs of contrasting characters at the same time, which were chosen by Mendel : shape and colour of seeds, round-yellow seeds and wrinkled-green seeds.
⚫️ HYBRIDISATION EXPERIMENT :-
⚫️ LAW OF INDEPENDENT ASSORTMENT (OR, MIXTURE)
➡️ In the inheritance, of two or more pairs of traits in a cross simultaneously, the genes responsible for each pair of traits are distributed (assorted) independently to the gametes.
➡️ Separate genes for separate traits are passed independently of one another from parents to offspring.
➡️ The presence of new combinations - round green and wrinkled yellow, suggests that the genes for the shape of the seed and colour of the seed are assorted independently.
ENVIRONMENTAL SEX DETERMINATION
⚫️ ESD is the establishment of sex (non-genetically) influenced by environmental factors like light intensity and photoperiod, temperature, water and nutrient availability, and pheromones emitted by surrounding plants or animals.
➡️ There are no sex chromosome; coding for both sexes are on autosomes.
Examples :
🔅Crustaceans
FOR FURTHER NOTES, STAY TUNED😊😊😊😊🔱🔱
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