TLDR;
Alright, folks! This session is all about cracking genetics, focusing on the chapter "Principles of Inheritance and Variation." We're starting with Mendelian genetics, moving through terminologies, and aiming to cover as much ground as possible. The goal is to make short notes, revise concepts, and tackle questions effectively. Repetition is key to mastering this chapter, which can fetch you around 5-6 questions in NEET.
- Genetics is the study of heredity and variation.
- Mendel is the father of genetics.
- Repetition is key to cracking NEET.
Introduction to Genetics [0:22]
The session is an introduction to genetics, specifically the chapter on "Principles of Inheritance and Variation." The educator, C. Pauja, emphasizes that this chapter is crucial for NEET, potentially contributing five to six questions. The plan is to start with Mendelian genetics and then move to post-Mendelian genetics, covering exceptions to Mendel's laws. The educator highlights the importance of attending the full session and making short notes.
Why Study Genetics? [1:35]
Missing the lecture means missing out on potentially 24 marks. Even if you've studied it before, this is a good revision. It's better to study with guidance than to spend four days revising on your own. Live sessions are more effective for learning. Repetition is key to cracking NEET. Overconfidence in biology can be a pitfall.
How to Study Effectively [2:37]
Even with old notes, scribble on a page to keep your brain active. Disable app notifications to dedicate the next 3 hours to biology. Bring a pen, paper, and focus. Distraction-free environment is important. The goal is to understand the chapter well enough to answer any question. A practice session can be arranged if needed.
Challenge Accepted [4:34]
The first challenge is to attend the entire session. Repetition is what helps crack NEET. Overconfidence in biology can be a problem.
What is Genetics? [6:42]
Genetics is a branch that deals with heredity and variation. Heredity is the transmission of characters from parents to offspring, a process called inheritance. Variation refers to the differences between progeny. These differences arise due to crossing over and mutations during gamete formation.
Basic Terminology [10:23]
Cells contain a nucleus, which houses the genetic material, DNA. The length of DNA is about 2.2 meters. During cell division, DNA replicates in the S phase, leading to chromosomes. Humans have 46 chromosomes, or two sets of 23. Homologous chromosomes are similar in size, genes, and banding patterns. Humans have 23 pairs of homologous chromosomes.
Genes, Genotypes and Phenotypes [13:56]
DNA contains genes, which are functional segments coding for proteins. The genetic constitution is called the genotype, while observable characters are the phenotype. Cells with one set of chromosomes are haploid, and those with two sets are diploid. Most body cells are diploid, except for gametes. Genes are located on chromosomes at specific locations called loci.
Alleles and Combinations [20:50]
Chromosomes have genes at specific locations, and homologous chromosomes have the same type of genes. Genes can have alternate forms called alleles. Alleles can be dominant or recessive. Combinations of alleles can be homozygous dominant, homozygous recessive, or heterozygous. Homozygous conditions are also called pure lines. Hemizygous condition occurs when a gene doesn't have a pair.
Mendelian Genetics: The Real Chapter [26:32]
The real chapter begins with Mendelian genetics. Mendel is the father of genetics, Betson is the father of modern genetics, and Thomas Hunt Morgan is the father of experimental genetics. Mendel worked on garden peas for seven years, conducting hybridization experiments.
Mendel: The Man and His Work [27:25]
Mendel, born in 1822, worked on garden peas (Pisum sativum) from 1856 to 1863, performing hybridization experiments. His work wasn't recognized until 1900. Thomas Hunt Morgan worked on fruit flies and gave the concept of Linkage and Recombination.
Why the Pea Plant? [30:52]
Mendel chose the pea plant because it has distinctive characters, produces a large number of seeds, completes its life cycle in one season, has true breeding varieties, and is bisexual, allowing for both self-pollination and cross-pollination.
Mendel's Success and Unrecognized Work [37:20]
Mendel's success was due to his logical approach, application of statistics, record-keeping, large sample size, theoretical explanations, and testing of explanations. However, his work remained unrecognized due to communication barriers, lack of internet, and the scientific community's reluctance to accept mathematical tools in biology.
Rediscovery of Mendel's Work [43:03]
Mendel's work was rediscovered by three scientists: Carl Correns, Tshermark, and Hugo de Vries, who republished his work in Flora.
Pea Plant Basics [46:42]
The pea plant, Pisum sativum, has a diploid condition of 2n = 14. Mendel initially started with 34 varieties but narrowed it down to 14 true breeding varieties, which means 7 characters and 7 pairs of pure line varieties. Each character has two traits. The chromosomes involved were 1, 4, 5, and 7, with the fourth chromosome having the maximum characters.
Traits and Genotypes [51:01]
The seven characters studied by Mendel include seed shape, seed color, flower color, pod color, pod shape, flower position, and stem height. Each character has a dominant and recessive trait. The traits are denoted by letters, with dominant traits using capital letters and recessive traits using lowercase letters.
Dominant and Recessive Traits [54:38]
Dominant traits can be expressed in both heterozygous and homozygous conditions, while recessive traits are expressed only in homozygous conditions.
Monohybrid Cross [58:39]
Monohybrid cross involves studying one character, like stem height. The steps include selecting pure line parents, crossing them to get the F1 generation, and then selfing the F1 generation. The genotypic ratio is 1:2:1, and the phenotypic ratio is 3:1.
Monohybrid Cross: Ratios and Observations [1:03:40]
In a monohybrid cross, the phenotypic ratio is 3:1, and the genotypic ratio is 1:2:1. The quantity of dominant alleles doesn't affect the expression; it's a qualitative game. F1 progeny resembles only one parent and doesn't match any parent genotypically.
Dihybrid Cross: Two Genes in Action [1:10:43]
Dihybrid cross involves studying two genes. Parents differ in two characters, with the rest being the same. For example, seed shape and seed color. The steps are the same as in a monohybrid cross.
Dihybrid Cross: Punnett Square and Phenotypes [1:16:11]
A dihybrid cross involves a 4x4 Punnett square, resulting in 16 offspring. There are four types of phenotypes: round and yellow, round and green, wrinkled and yellow, and wrinkled and green. The phenotypic ratio is 9:3:3:1.
Dihybrid Cross: Genotypic Ratio and Fork Line Method [1:24:20]
The phenotypic ratio of a dihybrid cross is a multiplication of two monohybrid crosses. To find the genotypic ratio, the fork-line method is used. The genotypic ratio is 1:2:1:2:4:2:1:2:1.
Dihybrid Cross: Numerical Problems [1:31:52]
A question is presented involving a cross between round yellow and wrinkled green plants. The task is to find the number of homozygous recessive, homozygous dominant, and heterozygous individuals for both traits if the total number of seeds is 1000. The fork-line method is used to solve the problem.
Parental Combinations and Recombinants [1:38:09]
In a dihybrid cross, parental combinations are 9+1, while recombinants are 3+3.
Formulas for Gametes and Progeny [1:39:13]
The formula for the number of gametes is 2^n, where n is the number of heterozygous traits. The formula for the number of progeny is the number of gametes of parent one multiplied by the number of gametes of parent two.
Mendel's Laws: Dominance and Segregation [1:42:52]
Mendel gave three laws. Two are based on monohybrid cross: law of dominance and law of segregation. One is based on dihybrid cross: law of independent assortment. Law of dominance explains the expression of only one parental character in the F1 generation and both in the F2 generation. It also explains the 3:1 ratio in F2.
Law of Dominance and Exceptions [1:45:27]
Law of dominance is not a universal law. Exceptions include incomplete dominance and codominance. Law of segregation states that alleles do not show any blending and both characters are recovered in the F2 generation. This is also called the law of purity of gametes.
Law of Independent Assortment [1:49:28]
Law of independent assortment states that when two characters are together, they do not interfere with each other. This is applicable only on those characters which are present on different chromosomes. Linkage is the exception to this law.
Back Cross and Test Cross [1:56:49]
Back cross is when an offspring is crossed with either of the parents. Test cross is when F1 is crossed with the recessive parent. This is used to detect the genotype of a phenotypically dominant plant. If all offspring are tall, the genotype is homozygous dominant. If 50% are tall and 50% are dwarf, the genotype is heterozygous.
Incomplete Dominance [2:07:04]
Incomplete dominance occurs when the dominant allele is incompletely dominant over the recessive one. The dominant allele is unable to express completely. Example: flower color in snapdragons. The F1 generation is pink, and the genotypic and phenotypic ratios are both 1:2:1.
Incomplete Dominance: More Examples [2:11:54]
Other examples of incomplete dominance include flower color in Mirabilis jalapa, coat color in shorthorn cattle, and starch synthesis in pea plants. Dominance is not an autonomous feature of a gene; it depends on the product and the phenotype being studied.
Concept of Dominance [2:19:04]
The concept of dominance involves an original, functional, and dominant allele that produces an expression. Over time, evolution and mutation can lead to different outcomes. The protein can be the same but less efficient, leading to the same phenotype. Or, the protein can be non-functional, affecting the phenotype and leading to a recessive allele.
Codominance: Equal Expression [2:24:57]
Codominance is when two different dominant alleles express equally. Example: coat color in cattle. The F1 generation has a roan color, with patches of red and white. The genotypic and phenotypic ratios are both 1:2:1.
Codominance: Blood Groups [2:29:14]
Another example of codominance is blood groups, controlled by the I gene. The I gene has three alleles: IA, IB, and iO. IA and IB are dominant, while iO is recessive. The different combinations of these alleles result in different blood groups.
ABO Blood Group System [2:34:37]
The ABO blood group system is not an example of codominance, but the AB blood group is. The different blood groups are due to different antigens on the surface of red blood cells.
Multiple Allelism [2:36:16]
Multiple allelism is when one gene has more than two alleles. This can only be detected in a population. The formulas for the number of genotypes and phenotypes are n(n+1)/2 and n+1, respectively, where n is the number of alleles.
Background of Blood Groups [2:42:15]
The different blood groups are due to different sugars attached to the H antigen on the surface of red blood cells. The type of sugar is controlled by the I gene. If the IA gene is present, the enzyme N-acetylgalactosaminyltransferase is produced, which attaches N-acetylgalactosamine to the H antigen, resulting in the A antigen. If the IB gene is present, the enzyme galactosyltransferase is produced, which attaches galactose to the H antigen, resulting in the B antigen. If neither gene is present, the H antigen remains unchanged, resulting in the O blood group.
