CBSE Class 12 Biology || Principles of Inheritance and Variation Part 1 || Full Chapter || Mendel's Laws of Heredity and Chromosomal Theory of Inheritance Introduction - Gregor Mendel's experiments with pea plants revealed fundamental principles of heredity. A monohybrid cross involves crossing parents differing in a single trait (e.g., height). Parental generation (P): The original parents. First filial generation (F1): The offspring of the P generation. Second filial generation (F2): The offspring of the self-pollinated F1 generation. Mendel's Law of Dominance - In a monohybrid cross, only one form of the trait (the dominant form) appears in the F1 generation. Both forms of the trait appear in the F2 generation in a 3:1 ratio ( dominant to recessive ). Example: In Mendel's pea plant experiment, tallness (T) was dominant over dwarfness (t). All F1 plants were tall (Tt), and the F2 generation showed a 3:1 ratio of tall to dwarf plants. Mendel's Law of Segregation - Alleles ( different forms of a gene ) do not blend but segregate from each other during gamete formation . Each gamete receives only one allele for each gene. Both parental traits reappear in the F2 generation, even if one is masked in the F1 generation. Example: In the pea plant example, the Tt heterozygotes produce gametes with either T or t, leading to the 3:1 ratio in the F2 generation. Punnett Square - A graphical representation used to predict the genotypes and phenotypes of offspring in a genetic cross. Shows the possible combinations of alleles from each parent. Helps calculate the probability of different genotypes and phenotypes. Example: A cross between TT and tt parents results in all Tt offspring (F1). Self-pollination of Tt plants (F1) results in a 1:2:1 genotypic ratio (TT:Tt:tt) and a 3:1 phenotypic ratio (tall:dwarf). The genotypic ratio can be represented by the binomial expansion $(a+b)^2 a = \frac{1}{2} b = \frac{1}{2}$. Test Cross - A cross between an individual of unknown genotype and a homozygous recessive individual. Used to determine the genotype of the unknown individual. Example: Crossing a tall pea plant (unknown genotype) with a dwarf plant (tt) can reveal if the tall plant is TT or Tt. Chromosomal Disorders - Caused by changes in chromosome number or structure. Aneuploidy: Gain or loss of one or more chromosomes (e.g., trisomy - 2n+1, monosomy - 2n-1). Polyploidy: More than two haploid sets of chromosomes (e.g., triploidy, tetraploidy). Examples of disorders: Down syndrome (trisomy 21): Extra copy of chromosome 21. Klinefelter syndrome (XXY): Extra X chromosome in males. Turner syndrome (XO): Missing X chromosome in females. Changes in chromosome structure include deletions, duplications, and translocations. Incomplete Dominance - Neither allele is completely dominant over the other. Heterozygotes show an intermediate phenotype. Genotypic and phenotypic ratios are the same (1:2:1). Example: Snapdragon flower color: Red (RR) x white (rr) = pink (Rr) in F1. Codominance - Both alleles are fully expressed in heterozygotes. The phenotype shows traits from both alleles. Example: AB blood type: Individuals with IAIB genotype express both A and B antigens on their red blood cells. Multiple Alleles - More than two alleles for a single gene exist in a population. Example: ABO blood group system: Three alleles (IA, IB, i) determine blood type. Pleiotropy - A single gene influences multiple phenotypic traits. Example: A gene affecting starch synthesis in pea plants also influences seed size and shape. Mendel's Law of Independent Assortment - During gamete formation, the segregation of alleles for one gene is independent of the segregation of alleles for another gene. Applies to dihybrid crosses ( crossing parents differing in two traits ). Results in a 9:3:3:1 phenotypic ratio in the F2 generation. Example: In a dihybrid cross involving pea plants with yellow round seeds (YYRR) and green wrinkled seeds (yyrr), the F2 generation shows a 9:3:3:1 ratio of yellow round: yellow wrinkled: green round: green wrinkled seeds. Rediscovery of Mendel's Work and the Chromosomal Theory of Inheritance - Mendel's work was not initially recognized due to limitations in communication, the novelty of his statistical approach, and the inability to explain the physical basis of inheritance. Rediscovered in 1900 by DeVries, Correns, and von Tschermak. Sutton-Boveri chromosome theory: Genes are located on chromosomes, and the behavior of chromosomes during meiosis explains Mendel's laws. Chromosomes segregate independently during meiosis I. Key Takeaways Mendel's laws of dominance and segregation explain the basic patterns of inheritance. Punnett squares are useful tools for predicting offspring genotypes and phenotypes. Incomplete dominance, codominance, multiple alleles, and pleiotropy represent variations on Mendel's basic principles. The chromosomal theory of inheritance integrates Mendel's work with the behavior of chromosomes during meiosis. CBSE Class 12 Biology || Principles of Inheritance and Variation Part 1 || Full Chapter || Gregor Mendel's Experiments and Laws of Inheritance - Monohybrid Cross: - A cross between parents differing in a single trait (e.g., tall vs. dwarf pea plants). Parental (P) Generation: The original parents in a cross. First Filial (F1) Generation: Offspring of the P generation. Mendel observed that all F1 plants were tall (dominant trait). Second Filial (F2) Generation: Offspring of the self-pollinated F1 generation. Showed a 3:1 ratio of tall to dwarf plants. Law of Dominance: - In a monohybrid cross, only the dominant trait appears in the F1 generation. Both traits reappear in a 3:1 ratio in the F2 generation. Genes and Alleles: - Genes are represented by letters (e.g., T for tall, t for dwarf). Alleles are alternative forms of a gene (e.g., T and t are alleles for height). Genotype: Genetic makeup of an organism (e.g., TT, Tt, tt). Phenotype: Observable characteristics of an organism (e.g., tall, dwarf). Homozygous: Having two identical alleles for a trait (e.g., TT, tt). Heterozygous: Having two different alleles for a trait (e.g., Tt). Dominant: Trait that is expressed in the heterozygote. Recessive: Trait that is masked in the heterozygote. Law of Segregation: - Alleles segregate during gamete formation, so each gamete carries only one allele for each gene. Punnett Square: - A diagram used to predict the genotypes and phenotypes of offspring in a genetic cross. Illustrates the probabilities of different genotypes and phenotypes in offspring. Test Cross: - A cross between an individual of unknown genotype and a homozygous recessive individual to determine the unknown genotype. Chromosomal Disorders - Type of Disorder Description Examples Changes in Chromosomal Number Gain or loss of chromosomes. Down's Syndrome (Trisomy 21), Klinefelter's Syndrome (XXY), Turner's Syndrome (XO) Aneuploidy One or more chromosomes gained or lost. Trisomy (2n+1), Monosomy (2n-1) Polyploidy More than two haploid sets of chromosomes. Triploidy (3n), Tetraploidy (4n) Other Patterns of Inheritance - Incomplete Dominance: - A heterozygote shows a phenotype intermediate between the two homozygotes (e.g., pink snapdragon flowers from red and white parents). Genotypic and phenotypic ratios are the same (1:2:1). Codominance: - Both alleles are fully expressed in the heterozygote (e.g., AB blood type). Multiple Alleles: - More than two alleles exist for a gene (e.g., ABO blood group system with IA, IB, and i alleles). Pleiotropy: - A single gene affects multiple phenotypic traits (e.g., a gene affecting both starch synthesis and seed shape in peas). Dihybrid Cross and Law of Independent Assortment - Dihybrid Cross: A cross between parents differing in two traits. Law of Independent Assortment: - During gamete formation, the segregation of alleles for one gene is independent of the segregation of alleles for another gene. Leads to a 9:3:3:1 phenotypic ratio in the F2 generation of a dihybrid cross. Mendel's Work and the Chromosomal Theory of Inheritance - Mendel's work was initially overlooked due to poor communication, unconventional statistical methods, and the inability to explain the physical basis of inheritance. Chromosomal Theory of Inheritance: - Genes are located on chromosomes, and the behavior of chromosomes during meiosis explains Mendel's laws. Sutton and Boveri's work supported Mendel's findings. Likely Exam Questions: Explain Mendel's laws of inheritance with examples. Describe the different patterns of inheritance beyond Mendelian dominance. What is a test cross, and how is it used? Explain the chromosomal theory of inheritance. Compare and contrast monohybrid and dihybrid crosses. Describe the symptoms and genetic basis of Down's syndrome, Klinefelter's syndrome, and Turner's syndrome. Final Recap: Mendel's experiments established the fundamental principles of inheritance: the laws of dominance and segregation. However, inheritance patterns can be more complex than simple dominance, as demonstrated by incomplete dominance, codominance, multiple alleles, and pleiotropy. The chromosomal theory of inheritance integrates Mendel's laws with the behavior of chromosomes during meiosis, providing a complete picture of how traits are inherited. CBSE Class 12 Biology || Principles of Inheritance and Variation Part 1 || Full Chapter || Monohybrid cross: A breeding experiment between two organisms that differ in only one trait. Mendel's experiments with tall and dwarf pea plants are an example. P generation (Parental generation): The first generation in a breeding experiment; the parents. In Mendel's experiments, these were the original tall and dwarf pea plants. F1 generation (First filial generation): The first generation of offspring resulting from a cross between the P generation. In Mendel's pea plant experiments, all F1 plants were tall. F2 generation (Second filial generation): The second generation of offspring, resulting from a self-cross of the F1 generation. Mendel observed a 3:1 ratio of tall to dwarf plants in the F2 generation. Alleles: Different forms of a gene that occupy the same locus (position) on homologous chromosomes. For example, capital T (tall) and small t (dwarf) are alleles for the height gene in pea plants. Genotype: The genetic makeup of an organism, represented by the combination of alleles. Examples include TT, Tt, and tt for pea plant height. Phenotype: The observable characteristics of an organism, determined by its genotype and environmental factors. Examples include tall and dwarf for pea plant height. Homozygous: Having two identical alleles for a particular gene (e.g., TT or tt). Heterozygous: Having two different alleles for a particular gene (e.g., Tt). Dominant: An allele that expresses its phenotypic effect even when heterozygous with a recessive allele. In Mendel's pea plants, tallness (T) was dominant over dwarfness (t). Recessive: An allele whose phenotypic effect is masked by a dominant allele when heterozygous. In Mendel's pea plants, dwarfness (t) was recessive to tallness (T). Law of Dominance: Mendel's law stating that in a monohybrid cross, one trait (the dominant trait) will be expressed over the other (the recessive trait) in the F1 generation. Law of Segregation: Mendel's law stating that during gamete formation, the two alleles for each gene separate, so each gamete receives only one allele. Punnett Square: A graphical representation used to predict the genotypes and phenotypes of offspring from a genetic cross. Test cross: A cross between an individual with an unknown genotype and a homozygous recessive individual to determine the unknown genotype. Incomplete dominance: A form of inheritance where the heterozygote displays an intermediate phenotype between the two homozygous phenotypes. The pink snapdragon flowers are an example. Codominance: A form of inheritance where both alleles are fully expressed in the heterozygote. The AB blood type is an example. Multiple alleles: The existence of more than two alleles for a single gene within a population (e.g., the three alleles for the human ABO blood group system: IA, IB, and i). Pleiotropy: A single gene influencing multiple phenotypic traits. The example given was a gene affecting both starch synthesis and seed shape in pea plants. Dihybrid cross: A breeding experiment between organisms differing in two traits. Law of Independent Assortment: Mendel's law stating that during gamete formation, the segregation of alleles for one gene is independent of the segregation of alleles for another gene. Aneuploidy: A chromosomal abnormality where there is an abnormal number of chromosomes (either fewer or more than the normal diploid number). Trisomy: A type of aneuploidy where there are three copies of a particular chromosome (2n+1). Down syndrome (trisomy 21) is an example. Monosomy: A type of aneuploidy where there is only one copy of a particular chromosome (2n-1). Turner syndrome is an example. Polyploidy: A chromosomal abnormality where there are more than two complete sets of chromosomes. Chromosomal disorders: Genetic disorders caused by changes in chromosome number or structure. Down syndrome, Klinefelter syndrome, and Turner syndrome are examples. Karyotype: A visual representation of the complete set of chromosomes in a cell. Meiosis: A type of cell division that reduces the number of chromosomes in half, producing gametes (sperm and egg cells). Metaphase plate: The equatorial plane of the cell where chromosomes align during metaphase of cell division. Homologous chromosomes: Pairs of chromosomes that carry the same genes, one inherited from each parent. Sutton-Boveri chromosome theory: The theory that genes are located on chromosomes, and that the behavior of chromosomes during meiosis explains Mendel's laws of inheritance. Down's Syndrome, also known as Trisomy 21, is a chromosomal disorder. It occurs when an individual has an extra copy of the 21st chromosome. ( ) This means that instead of the usual two copies of this chromosome, there are three. The presence of this extra genetic material leads to a range of characteristic physical and developmental symptoms. ( ) These can include: A swollen face Bulging and slanting eyes A small mouth A protruding and furrowed tongue Short stature A small, round head A broad palm with a single crease Variations in physical, psychomotor, and mental development ( ) This disorder was first described by Dr. Langdon Down, a British doctor, which is why it is named Down's Syndrome. ( ) Are there any other chromosomal disorders you would like to know more about? This is a homozygous bull white in color with a genotype capital W capital W and this is a homozygous go-[MUSIC] in color with a genotype capital R capital R What color will their cough be in the F1 generation well it isn't white or red and neither is it a color in between the gaff is wrong colored.27:39It's goat is a mix of red and white hair. Therefore in this case the F1 generation doesn't resemble either of the parents as in dominance. No. is it in between as in incomplete dominance displayed was codominance. GUU dominance refers to a situation where a heterozygous organism has a phenotype that demonstrates traits from both dominant as well as recessive genes.28:16A COU dominant trait is not blended but is independently and equally expressed. Another widely known example of codominance in human beings is the blood system which is used to determine human blood type. The plasma membrane of the red blood cells or the RBC is present in human blood has sugar polymers that protrude from its surface.28:52It is these sugar polymers that determined the specificity of the major blood types. For example the difference between blood types A and B lie in a single sugar unit that protrudes from the end of a carbohydrate chain of a glycoprotein or glycol lipid on the plasma membrane of an RBC.29:16It is the gene I that is responsible for determining the type of sugar produced by the RBC's. This gene has three different alleles, namely capital I A, capital IB and small I [MUSIC] of these believes Capital I A and capital IB produce sugars that are slightly different from each other, while the allele small I doesn't produce any sugar. such a cross between plants that differ in two traits or characters is known as a dihybrid cross. This experiment conducted by Mendel resulted in a pea plant that produced yellow and round seeds in the F1 generation here. And you guess the dominant shape and color. obviously, the round shape is dominant over the wrinkled shape and the yellow color dominant over green. Mendel noticed that these results were identical to the results he had obtained when he had carried out Mana hybrid crosses between plants with round and wrinkled seeds and those with yellow and green seeds. Now in Mendel's dihybrid experiment, let's denote the dominant yellow seed color with the genotypic symbol capital Y, and the recessive green seed color with small Y. Likewise, let's denote the round seed as capital R and the wrinkled seed as small R. The genotype of the parents can therefore be denoted as capital R, capital R, capital Y, capital Y, and small R small R, small Y, small Y on crossing the klan's the gametes capital R, capital Y, and small our small Y unites to produce yellow and round seeds. with the genotype capital R, small R, capital Y, small Y, in the F1 generation, Mendel further self-pollinated the plants obtained in the F1 generation to produce F2 generation plants. This time he found that three--fourths of the seeds were yellow, while one-fourth was cream. Similarly, he noticed that three--fourths of the seeds were round, while one--fourth was wrinkled. Therefore the yellow and green color had segregated in the ratio trees to one similar to the results obtained in a monohybrid cross and the round and wrinkled shape had segregated in the same ratio. 3 is to 1. Mendel also found that the phenotypic ratio in the F2 hybrid was 9 is to 3 is to 3 is to 1. That is there were 9 round yellow seeds, 3 round green seeds, 3 wrinkled yellow seeds, and 1 wrinkled green seed Mendel had got the same phenotypic ratio. in several dihybrid experiments that he had conducted. this ratio nine is to 3 is to 3 is to 1 can be derived as The Punnett Square is a diagrammatic tool that plays a significant role in genetics by helping to predict the possible genotypes and phenotypes of offspring resulting from a genetic cross. It was devised by the British geneticist Reginald C. Punnett. ( ) Essentially, a Punnett Square is a graphical representation that allows us to calculate the probability of all potential genotypes for the progeny. ( ) It visually organizes the possible combinations of alleles from each parent. By using a Punnett Square, one can: Determine the potential allelic composition of zygotes (the fertilized eggs). ( ) Understand the independent segregation of genes during meiosis, which is the process of forming gametes (sperm and eggs). ( ) Calculate the expected genotypic ratio (the proportion of different genetic makeups) and phenotypic ratio (the proportion of different observable traits) among the offspring. ( , ) For example, in a simple Mendelian cross, if you know the genotypes of the parents, you can set up a Punnett Square to see all the possible genetic combinations for their offspring and the likelihood of each. This makes it a very useful tool in predicting the outcome of breeding experiments. ( ) Would you like to see an example of how a Punnett Square is used for a specific genetic cross? all cases. That is, Mendel's laws were not universal in occurrence. To understand this better let's further study the experiment conducted on the slab dragon plants. When a true breeding or a pure line red flowered snapdragon with genotype capital R capital R was crossed with a true breeding quite flowered snapdragon with genotype small R small R. The results were quite different from what was seen in the case of the peak lands the flowers of the F1 hybrid were pink and the genotype was capital are smaller. That is the F1 hybrid flowers had a phenotype that resembled neither of the parents but was in between the two. Now the flowers of the F1 hybrid were self-pollinated and the genotypic ratios obtained in the F2 hybrid were 1 is to 2 is to 1. The same result obtained by Mendel in his experiments on peers. however the phenotypic ratios obtained in the snapdragon plant you can see is also 1 is to 2 is to 1, which is different from the ratio 3 is to 1. Dominant is two recessive ratio in the P plant so in the case of incomplete dominance both genotypic and phenotypic ratios are the same that is 1 is to 2 is In Snapdragon plants, the phenomenon of incomplete dominance is observed for flower color. When a true-breeding red-flowered Snapdragon (genotype RR) is crossed with a true-breeding white-flowered Snapdragon (genotype rr), the F1 generation offspring have pink flowers. The genotype of these pink-flowered F1 hybrids is Rr. ( ) This occurs because in incomplete dominance, the dominant allele (R for red color) does not completely mask the effect of the recessive allele (r for white color) in the heterozygous state (Rr). Instead, the heterozygote exhibits a phenotype that is intermediate between the two homozygous phenotypes. In this case, the intermediate phenotype is pink. ( ) The gene for flower color controls the amount of pigment produced. Having two dominant alleles (RR) results in heavy pigment production, leading to red flowers. Having two recessive alleles (rr) results in no pigment production (or a different pigment pathway), leading to white flowers. Having one dominant and one recessive allele (Rr) results in the production of about half the amount of pigment compared to the RR genotype, which creates the pink shade. ( ) Therefore, the genotype for a Snapdragon plant with pink flowers, according to this model of incomplete dominance, is Rr. Would you like to explore how the F2 generation from these pink-flowered Snapdragons would look?