## shreehari499 one year ago ***Genetics- The Basics*** I’ve seen a lot of people in OS asking for help on genetics, and this has prompted me to give a tutorial on it. Genetics is one of the most complicated or difficult topic for a lot of people, but once a person is thorough with its basics, it is just a walk on the park..:p . In this Tutorial, I’ll provide the basics of Genetics, which, I think, will help a lot of users. I will divide the Tutorial into 3 sessions. Later on, I’ll solve some problem on it so that The Term ‘Genetics’ will not give you any more fears. Thanks!

1. shreehari499

$$\color{red}{\Huge\text{SESSION 1}}$$

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I will make it into points for your easy understanding..:)

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Let’s first go with some commonly used terms in Genetics

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$$\color{blue}{\Huge\text{So what is Genetics?}}$$ • Genetics is the Study of inheritance, heredity and variation of characters or in simpler words, the Study of genes and chromosomes.

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$$\color{red}{\Huge\text{Inheritance}}$$ • Inheritance is the transmission of characters from parents to progeny.

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$$\color{orange}{\Huge\text{Variation}}$$ • It is the Difference in characteristics between parents and offspring.

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$$\color{yellow}{\Huge\text{Alleles}}$$ • They are the alternative forms of a gene. For E.g. T (tall) and t (dwarf) are two alleles of a gene responsible for the character height.

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Two Allelic Conditions i.e. $$\color{green}{\Huge\text{Homozygous and Heterozygous}}$$ The condition in which chromosome carries similar alleles for a character is called homozygous condition. Ex TT or tt represents homozygous tall or short The condition in which chromosome carries dissimilar alleles for a character is called heterozygous condition. Ex Tt for heterozygous tall (Why tall and not short?) You will understand it in the subsequent section!)

9. shreehari499

$$\color{blue}{\Huge\text{Mendel Experiments and Laws}}$$ • Gregor Johann Mendel was born in 1822 in Heinzendorf, which was a part of Czechoslovakia. He began his genetic experiments on garden pea in 1856 in the garden at the monastery. • He studied only one character at a time. • He used all available techniques to avoid cross pollination by undesirable pollen grains. • He applied mathematics and statistics to analyze the results obtained by him. • I suggest you to refer to this link for more information on his experiments • His Monohybrid ( Single Character) cross will be drawn by me over here..

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If you have gone throug the link, here's Mendel's Monohybrid cross:|dw:1439531592796:dw|

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|dw:1439531627222:dw|

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So the phenotypic ratio is : Tall:Dwarf = 3:1

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Mendel’s Laws 1. $$\color{red}{\\{The Law of Dominance}}$$ • Characters are controlled by discrete units called factors. • Factors occur in pairs. • In a dissimilar pair of factors one member of the pair dominates (dominant) the other (recessive) • This showed that in a heterozygous pair Tt, T dominated over t hence making it Tall

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2. $$\color{red}{\\{The Law of Segregation}}$$ • During gamete formation, the factors (alleles) of a character pair present in parents segregate from each other such that a gamete receives only one of the 2 factors 3. $$\color{red}{\\{ Law of Independent Assortment of Genes }}$$ • It states that ‘when more than one pair of characters are involved in a cross, factor pairs independently segregate from the other pair of characters’.

16. shreehari499

There are certain exceptions from Mendel’s Laws. They will be mentioned here too: Before that there are three important terms to be noted: (IF you’ve gone through the link, the ratios will be easy for you) 1. Phenotype: Physical (Visible) expression of an individual. 2. Genotype: Genetic constitution of an individual. 3. Punnett square (Checker board): A grid that enables to calculate the results of simple genetic crosses.

17. shreehari499

And one more thing, there is something called Dihybrid Crossing. Mendel had conducted a monohybrid cross( where there was only one character involved- as I had drawn earlier) So in a dihybrid cross, there are two characters involved. I will draw it to you and explain it..:)

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Here we take 2 seeds i.e, a round yellow one and a wrinkled green one. The characters round and yellow are dominant while wrinkled and green are recessive. We cross it as done earlier. Round yellow can be represented as RRYY and wrinkled green as rryy

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|dw:1439532494400:dw|

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Thats the F1 generation

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For the F2 generation, we will need a Punnet's Square.

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Self crossing I will attach this diagram for the F2 generation

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|dw:1439532740250:dw|

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The phenotypic ratio is: RoungYellow:RoundGreen:Wrinkledyellow:Wrinkledgreen = 9:3:3:1

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$$\color{black}{\\{ Exceptionsto Mendel’s Laws }}$$ 1) Codominance: A rare exception to Mendel’s Laws, codominance is particularly seen in human blood groups where, in AB blood group, both A as well as B are dominant 2) Multiple Allelism: Here more than two alleles govern the same character. Again the Human Blood group can be given as an example Do you think people were happy with Mendel discovery alone.Nah! We will continue on The Chromosomal Theory Of Inheritance In The Next Session..:) And by the way, if you find any Mistakes or Inefficiency in my Tutorial, you are welcome to comment on it over here so that I will be able to rectify the mistakes and improve the next session. And thanks again!

26. Miracrown

Hey. Thanks for this, very helpful! :)

27. nincompoop

great tutorial free course book http://www.nature.com/scitable/ebooks/cntNm-3/126469954#bookContentViewAreaDivID

28. shreehari499

Thanks for the link. It's a very good. I really appreciate that @nincompoop

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In a dissimilar pair of factors one member of the pair dominates (dominant) the other (recessive)