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PRINCIPLES OF INHERITANCE AND VARIATION class 12 - Presentation
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• Gregor Mendel conducted hybridisation experiments on garden peas (1856-1863) to propose inheritance laws. • His experiments applied statistical analysis and mathematical logic to biology problems. • Large sampling size and confirmation of inferences from successive generations confirmed his results. • Mendel's research focused on opposing traits in garden pea plants, establishing a basic framework of inheritance rules. • He conducted artificial pollination/cross pollination experiments using several true-breeding pea lines.
• Mendel selected 14 true-breeding pea plant varieties, selecting contrasting traits like smooth or wrinkled seeds, yellow or green seeds, inflated or constricted green or yellow pods, and tall or dwarf plants.
• Mendel crossed tall and dwarf pea plants to study gene inheritance. • He collected seeds from the first hybrid generation, the Filial1 progeny (F1). • All F1 progeny plants were tall, like one of its parents, and none were dwarf. • In the Filial2 generation, some offspring were 'dwarf', a character not seen in the F1 generation. • The tall and dwarf traits were identical to their parental type and did not show any blending. • Similar results were obtained with other traits, with only one of the parental traits expressed in the F1 generation and both traits expressed in the F2 stage in the proportion 3:1.
• Mendel proposed that something was being stably passed down from parent to offspring through the gametes over successive generations. • He proposed that in a true breeding, tall or dwarf pea variety, the allelic pair of genes for height are identical or homozygous, TT and tt, respectively. • He proposed that in a pair of dissimilar factors, one dominates the other, hence the dominant factor. • The Tt plant is heterozygous for genes controlling one character (height), making it a monohybrid. Monohybrid Cross and Its Impact on Plants • The cross between tall TT and dwarf tt plants is a monohybrid.
• The recessive parental trait is expressed without blending in the F2 generation. • The alleles of the parental pair separate or segregate during meiosis, resulting in only one allele being transmitted to a gamete. • During fertilisation, the two alleles, T from one parent and t from the other, are united to produce zygotes with one T allele and one t allele. • The hybrids are heterozygous as they contain alleles expressing contrasting traits. • The Punnett Square, developed by British geneticist Reginald C. Punnett, shows the production of gametes by the parents, the formation of the zygotes, the F1 and F2 plants.
• The F1 plants of genotype Tt are self-pollinated, producing gametes of the genotype T and t in equal proportion. • When fertilisation occurs, pollen grains of genotype T have a 50% chance to pollinate eggs of genotype T and genotype t. • The resultant zygotes can be of the genotypes TT, Tt, or tt. • At F2, 3/4th of the plants are tall, with some being TT while others are Tt. • The character 'T' tall is expressed within the genopytic pair Tt, indicating dominance over the other allele 'dwarf'.
• Mendel proposed two principles: the First Law or Law of Dominance and the Second Law or Law of Segregation. • The First Law states that characters are controlled by discrete units called factors. • Factors occur in pairs and one member of the pair dominates the other. • The Law of Dominance explains the expression of only one of the parental characters in a monohybrid cross in the F1 and both in the F2.
4.2.2 Law of Segregation • • Alleles do not blend, resulting in both characters being recovered in F2 generation. • Parents contain two alleles during gamete formation, but factors segregate, affecting gamete distribution. • Homozygous parents produce similar gametes, while heterozygous ones produce two types of gametes with equal proportions.
Understanding Dominance in Peas Experiments • Experiments on peas showed that the F1 gene sometimes had a phenotype that didn't resemble either parent. • The color inheritance in the dog flower (snapdragon or Antirrhinum sp.) illustrates incomplete dominance. • In a cross between true-breeding red-flowered (RR) and true-breeding white-flowered plants (rr), the F1 (Rr) was pink. • When the F1 was self-pollinated, the F2 resulted in a ratio of 1 (RR) Red: 2 (Rr) Pink: 1 (rr) White. Concept of Dominance • Dominance is the information contained in a gene to express a particular trait.
• In a diploid organism, there are two copies of each gene, i.e., a pair of alleles. • One of these alleles may be different due to changes that modify the information that particular allele contains. Example of Dominance • An example of a gene containing information for producing an enzyme. • The modified allele could be responsible for producing a normal/less efficient enzyme, a non-functional enzyme, or no enzyme at all. • The phenotype/trait is dependent on the functioning of the unmodified allele. • In this example, the recessive trait is seen due to non-functional enzyme or because no enzyme is produced.
Dominance and Co-Dominance • Dominance refers to the F1 generation resembling both parents. Example of Co-Dominance • Red blood cells' ABO blood grouping is determined by the gene I. • The gene has three alleles: I A, I B, and i. • I A and I B are completely dominant over I B, while I B expresses I A. • When I A and I B are present together, they both express their own types of sugars due to co-dominance. Phenotypes and Multiple Alleles • Multiple alleles can be found only when population studies are conducted. • A single gene product may produce more than one effect.
Example of Dominance • Starch synthesis in pea seeds is controlled by one gene with two alleles (B and b). • B B homozygotes produce large starch grains, while B b homozygotes produce smaller grains. • If starch grain size is considered as the phenotype, the alleles show incomplete dominance. • Dominance is not an autonomous feature of a gene or its product, but depends on the gene product and the production of a particular phenotype.
• Mendel crossed pea plants with different characteristics, such as yellow and green seeds and round and wrinkled seeds. • The resulting seeds had yellow color and round shape, with yellow being dominant over green and round shape dominating over wrinkled. • The genotypic symbols used for the parent plants were Y for dominant yellow seed colour, y for recessive green seed colour, R for round shaped seeds, and r for wrinkled seed shape. • The gametes RY and ry unite on fertilisation to produce the F1 hybrid RrYy. • When self-hybridizing F1 plants, 3/4th of F2 plants had yellow seeds and 1/4th had green.
• Yellow and green color segregated in a 3:1 ratio, similar to a monohybrid cross.
• Mendel observed a 9:3:3:1 ratio in dihybrid crosses, resulting in phenotypes like round, yellow, wrinkled, yellow, round, green, and wrinkled, green. • This ratio can be derived as a combination of 3 yellow:1 green, with 3 round:1 wrinkled. • Mendel's Law of Independent Assortment states that when two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair. • The Punnett square can be used to understand the independent segregation of two pairs of genes during meiosis and the production of eggs and pollen in the F1 RrYy plant.
• The four genotypes of gametes (four types of pollen and four types of eggs) are RY, Ry, rY, and ry, each with a frequency of 25% or 1/4th of the total gametes produced. • The composition of the zygotes that give rise to the F2 plants can be determined by writing down the four types of eggs and pollen on the sides of a Punnett square.
• Mendel observed a 9:3:3:1 ratio in dihybrid crosses, resulting in phenotypes like round, yellow, wrinkled, yellow, round, green, and wrinkled, green. • This ratio can be derived as a combination of 3 yellow:1 green, with 3 round:1 wrinkled. • Mendel's Law of Independent Assortment states that when two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair. • The Punnett square can be used to understand the independent segregation of two pairs of genes during meiosis and the production of eggs and pollen in the F1 RrYy plant.
• The four genotypes of gametes (four types of pollen and four types of eggs) are RY, Ry, rY, and ry, each with a frequency of 25% or 1/4th of the total gametes produced. • The composition of the zygotes that give rise to the F2 plants can be determined by writing down the four types of eggs and pollen on the sides of a Punnett square.
• Morgan conducted dihybrid crosses in Drosophila to study sex-linked genes. • He hybridized yellow-bodied, white-eyed females to brown-bodied, red-eyed males and intercrossed their F1 progeny. • He observed that the two genes did not segregate independently, and the F2 ratio deviated significantly from the 9:3:3:1 ratio. • He observed that when two genes in a hybrid cross were on the same chromosome, the proportion of parental gene combinations was higher than the non-parental type. • Morgan coined the terms linkage and recombination to describe the physical association of genes on a chromosome.
• He found that some genes were very tightly linked and others were loosely linked, showing different levels of recombination. • Alfred Stuartevant used the frequency of recombination between gene pairs on the same chromosome to measure the distance between genes.
• Mendel's studies focused on distinct alternate forms of traits like flower color. • Polygenic traits, which are spread across a gradient, are also prevalent. • These traits are controlled by three or more genes and are influenced by the environment. • Human skin color is a prime example of a polygenic trait. • The phenotype reflects the contribution of each allele, indicating that the effect of each allele is additive. • The genotype with all dominant alleles (AABBCC) has the darkest skin color, while the genotype with all recessive alleles (aabbcc) has the lightest.
• The number of each type of allele in the genotype determines the individual's skin color.
4.5 PLEIOTROPY • • A single gene can exhibit multiple phenotypic expression. • Pleiotropy is a result of a gene's effect on metabolic pathways. • An example is the human disease phenylketonuria, caused by a single gene mutation. • This disease manifests through mental retardation and reduced hair and skin pigmentation.
• The genetic/chromosomal basis of sex determination was first understood through experiments in insects. • Henking (1891) identified a nuclear structure in sperm that 50% received, while the other 50% did not. This structure was named the X body. • Other scientists later concluded that the 'X body' was a chromosome, hence the name X-chromosome. • In many insects, the sex determination mechanism is of the XO type, where all eggs bear an additional X-chromosome. • In some insects and mammals, the XY type of sex determination is observed, where both males and females have the same number of chromosomes.
• In humans and Drosophila, males have one X and one Y chromosome, while females have a pair of X-chromosomes. • In some organisms, like birds, the total number of chromosomes is the same in both males and females, but two different types of gametes are produced by females. • The two different sex chromosomes of a female bird are designated as the Z and W chromosomes.
• The sex determining mechanism in humans is XY type. • Out of 23 chromosome pairs, 22 are identical in both males and females. • Females have a pair of X-chromosomes, while males have X and Y chromosomes. • Males produce two types of gametes: 50% carry X-chromosome and 50% have Y-chromosome. • Females produce only one type of ovum with an X-chromosome. • Fertilization of ovum with X-chromosome zygote develops into a female (XX), while fertilisation with Y-chromosome sperm results in a male offspring. • The genetic makeup of sperm determines the sex of the child.
• Society often blames women for giving birth to female children, leading to ostracisation and ill-treatment.
4.6.2 Sex Determination in Honey Bee • • Sex determination in honey bees is based on the number of chromosome sets received. • Female offspring develop as queens or workers, while unfertilised eggs develop as males through parthenogenesis. • Males have half the number of chromosomes as females, resulting in a haplodiploid sex-determination system. • Males produce sperms through mitosis, but lack a father, limiting sons to grandfathers and grandsons. • Questions remain about the sex-determination mechanism in honey bees and whether sperm or egg is responsible for chick sex.
4.7 MUTATION • • Mutation alters DNA sequences, causing changes in an organism's genotype and phenotype. • It's a phenomenon that leads to variation in DNA, not just recombination. • Chromosomal aberrations, where genes are located on chromosomes, are common in cancer cells. • Point mutation, where a single base pair of DNA changes, is another mutation. • Frame-shift mutations, caused by deletions and insertions of base pairs of DNA, are a classic example. • Mutagens, chemical and physical factors, can induce mutations, including UV radiation.
• The concept of disorders being inherited has been prevalent in human society since ancient times. • The rediscovery of Mendel's work led to the analysis of inheritance patterns in human traits. • Pedigree analysis is a method of studying traits in multiple generations of a family. • This analysis represents the inheritance of a trait in the family tree over generations. • Pedigree study is a powerful tool in human genetics to trace the inheritance of a specific trait, abnormality, or disease. • DNA, the carrier of genetic information, is transmitted from one generation to the next without any change or alteration.
