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Genetics and its evolution with Mendelian inheritanceDr. Manoj Joshi

History of GeneticsWhat is genetics ?Thebranchofbiologythatdealswithheredity,especiallythemechanismsofhereditarytransmissionandthevariationofinheritedcharacteristicsamongsimilarorrelatedorganisms.

People have known about inheritance for a long time by .

Children resemble their parentsDomestication of animals and plants, selective breeding for good characteristicsSumerian horse breeding recordsEgyptian data palm breedingBible and hemophilia

Old IdeasDespite knowing about inheritance in general, a number of incorrect ideas had to be generated and overcome before modern genetics could arise.

All life comes from other life. Living organisms are not spontaneously generated from non-living material. (Big exception: origin of life.)Species concept: offspring arise only when two members of the same species mate. Monstrous hybrids dont exist.Organisms develop by expressing information carried in their hereditary material. The environment cant alter the hereditary material in a directed fashion.

Mid 1800s DiscoveriesThree major events in the mid-1800s led directly to the development of modern genetics.

1859: Charles Darwin publishes The Origin of Species, which describes the theory of evolution by natural selection. This theory requires heredity to work.1866: Gregor Mendel publishes Experiments in Plant Hybridization, which lays out the basic theory of genetics. It is widely ignored until 1900.1871: Friedrich Miescher isolates nucleic acid from pus cells.

Major Events in the 20th Century1900: rediscovery of Mendels work by Robert Correns, Hugo de Vries, and Erich von Tschermak .

1902: Archibald Garrod discovers that alkaptonuria, a human disease, has a genetic basis.

1904: Gregory Bateson discovers linkage between genes. Also coins the word genetics.

1910: Thomas Hunt Morgan proves that genes are located on the chromosomes (using Drosophila).

1918: R. A. Fisher begins the study of quantitative genetics by partitioning phenotypic variance into a genetic and an environmental component.

1926: Hermann J. Muller shows that X-rays induce mutations.

1944: Oswald Avery, Colin MacLeod and Maclyn McCarty show that DNA can transform bacteria, demonstrating that DNA is the hereditary material.

1953: James Watson and Francis Crick determine the structure of the DNA molecule, which leads directly to knowledge of how it replicates

1966: Marshall Nirenberg solves the genetic code, showing that 3 DNA bases code for one amino acid.

1972: Stanley Cohen and Herbert Boyer combine DNA from two different species in vitro, then transform it into bacterial cells: first DNA cloning.

2001: Sequence of the entire human genome is announced.

1953 James Watson and Francis Crick publish the double helix model for DNAs chemical structure

1860s Mendels work on peas allows the conclusion that traits are inherited through discrete units passed from one generation to the next

1870s Friedrich Miescher describes nucleic acids

1910 Thomas Morgans work on fruitflies demonstrates that genes lie on chromosomes

1940s Barbara McClintock describes mobile genetic elements in maize

1958 Crick proposes the central dogma for biological information flow: that DNA makes RNA makes protein1909 The word gene coined by Danish botanist Wilhelm Johannsen

1944 Oswald Avery shows in bacteria that nucleic acids are the transforming principle

1977 Phillip Sharp and Richard Roberts find that protein-coding genes are carried in segments

2001 initial results from the Human Genome Project published

Genetics history and key concepts

Gregor MendelFather of Genetics3 Laws from the 1860s

Principles of genetics were developed in the mid 19th century by Gregor Mendel an Austrian Monk.

Gregor developed these principles without ANY scientific equipment - only his mind!

Gregor experimented with pea plants, by crossing various strains and observing the characteristics of their offspring.

History contStudied the following characteristics:Pea color (Green, yellow)Pea shape (round, wrinkled) Pea shape (round, wrinkled)Flower color (purple, white)Plant height (tall, short)

Law of DominanceEvery gene has two alleles that can code for a trait.One allele is dominant, meaning it will always show.One allele is recessive, meaning it will be masked by the presence of the dominant allele.Hybrids will always show the dominant phenotype. Ex: PP = purplepp = whitePp = purple

Mendelian Genetics

TECHNIQUE

EXPERIMENTMendel crossed two pure strains of pea plants with purple and white flowers and discovered that the first filial generation were all purple

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Law of SegregationMendel arrived at this conclusion by performing monohybrid crosses. These were cross-pollination experiments with pea plants that differed in one trait, for example height of pea plant. The alleles for a trait separate when gametes are formed. These alleles can then randomly united at fertilization.

Anaphase I of MeiosisAnaphase II of Meiosis

Mendelian Genetics 3:1 ratios

Traits must comein discrete units now called genes

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Mendelian Genetics in humans

1st generation(grandparents)2nd generation(parents, aunts,and uncles)3rd generation(two sisters)Widows peakNo widows peakA dominant Mendelian traitWwwwWwWwwwwwwwwwWwWwwwWWWwor

Attached earlobe1st generation(grandparents)2nd generation(parents, aunts,and uncles)3rd generation(two sisters)Free earlobeA recessive Mendelian traitFfFfFfFfFfffFfffffffffFF /orFFFfMany Mendeliantraits some cause disease e.g. cystic fibrosis

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Law of Independent AssortmentMendel began to wonder what would happen if he studied plants that differed in two traits (dihybrid). Data showed that traits produced by dominant factors do not have to appear together in offspring.Ex: A green seed pod (dominant) could appear in a white-flowering pea plant (recessive).In other words, dominant traits dont have to travel together when traits are passed from parents to offspring.Mendels law of independent assortment applies only to traits carried on different chromosomes, i.e.unlinked genes independent assortment occurs as a result of the alignment of homologues during metaphase I, determining which maternal and paternal chromosomes assort to each daughter cell each pair of alleles separates independently of every other pair of unlinked alleles during gamete formation

Dihybrid Crosses:a cross that shows the possible offspring for two traitsFur Color:B: Black b: WhiteCoat Texture:R: Roughr: SmoothBbRr x BbRr

BRbRbrbRBrBRbrBr

Then, find the possible genotypes of the offspring

Dihybrid Crosses:a cross that shows the possible offspring for two traitsFur Color:B: Black b: WhiteCoat Texture:R: Roughr: SmoothBbRr x BbRr

BRbRbrbRBrBRbrBrBBRRBbRRBbRrBBRrBBrrBbRrBbrrBbRRBbRrbbRRbbRrBbRrBbrrbbRrbbrrBBRr

BRbRbrbRBrBRbrBrBBRRBbRRBbRrBBRrBBrrBbRrBbrrBbRRBbRrbbRRbbRrBbRrBbrrbbRrbbrrBBRrHow many of the offspring would have a black, rough coat?How many of the offspring would have a black, smooth coat?How many of the offspring would have a white, rough coat?How many of the offspring would have a white, smooth coat?

Fur Color:B: Black b: WhiteCoat Texture:R: Roughr: Smooth

BRbRbrbRBrBRbrBrBBRRBbRRBbRrBBRrBBrrBbRrBbrrBbRRBbRrbbRRbbRrBbRrBbrrbbRrbbrrBBRrHow many of the offspring would have black, rough coat?How many of the offspring would have a black, smooth coat?How many of the offspring would have a white, rough coat?How many of the offspring would have a white, smooth coat?

Fur Color:B: Black b: WhiteCoat Texture:R: Roughr: SmoothPhenotypic Ratio9:3:3:1

More PracticeOn the back of your notesIn pea plants, yellow seeds (Y) are dominant over green seeds (y), and rounded peas (R) are dominant over wrinkled peas (r).

Cross a plant that is heterozygous for both traits with a plant that is homozygous recessive for both traits. Draw a Punnett square to show all possible offspring, and determine the genotypic and phenotypic ratios.

YyRr X yyrr

YRyRyryryryryrYrYyRryyRryyrrYyRrYyrryyRryyrrYyRrYyrryyRryyrrYyRrYyrryyRryyrrYyrr

Non-Mendelian inheritanceInheritance of traits (phenotypes) is often more complex than predicted by simple Mendelian genetics

Polygenic traits - those determined by more than one gene, vary in the population along a continuum The sum of the effects of all the genes that contribute to the phenotype (e.g. height, skin colour)

Multifactorial traits - those that depend on the environment as well as the genotype The sum of the effects of all the genes and the environmental factors that contribute to the phenotype (e.g. height, skin colour)

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Genetic VocabularyGenetics: The scientific study of heredityGenes: Point on a chromosome that controls the trait.Allele: Alternate forms of a gene/factor. A or aGenotype: combination of alleles an organism has. (genetic traits)Phenotype: How an organism appears. (physical traits)Dominant: An allele which is expressed (masks the other).Recessive: An allele which is present but remains unexpressed (masked)Homozygous: Both alleles for a trait are the same.Heterozygous: The organism's alleles for a trait are different.

Genetic VocabularyProbability : The mathematical chance that an event will happen.Meiosis :The cell division that produces sex cells.Mutation : A change in the type or order of the bases in an organism DNA: deletion, insertion or substitution.Natural Selection : The process by which organisms with favorable traits survive and reproduce at a higher rate than organisms without favorable traits.Evolution :The process by which population accumulate inherited changes over time.

Punnett Squares Genetic problems can be easily solved using a tool called a Punnett square.Tool for calculating genetic probabilities

A Punnett square

Punnett Squares Vocabulary

Punnett Squares : The chart used to show the possible ways genes are combined when passed from parents to offspring.Dominant genes are UPPER CASE (T)Recessive genes are lower case (t)Each parent has two genes for a trait. (TT), (Tt) or (tt).There can be a possible four combinations for each cross.

Punnett Squares Vocabulary ContGENOTYPE: The gene combinationHomozygous Pure dominate or pure recessiveHeterozygous Hybrid a mixed

PHEOTYPE :The physical appearanceDominate traitRecessive traitCombination of two traits (incomplete dominance)

THANKSfor kind attention