Gregor Mendel conducted his classical experiments on inheritance using which plant? | NeetCrackers

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Gregor Mendel conducted his classical experiments on inheritance using which plant?

Options

1

Pisum sativum (Garden pea)

2

Datura stramonium

3

Mirabilis jalapa (Four o'clock)

4

Antirrhinum majus (Snapdragon)

Correct Answer

Pisum sativum (Garden pea)

Solution

1

Mendel (1856-1863): experiments in monastery garden in Brno (Moravia).

Plant used: Pisum sativum (Garden Pea)

2

7 pairs of contrasting characters studied. Published results in 1866.

Rediscovered in 1900 by de Vries, Correns, and Tschermak.

Answer: Pisum sativum (Garden pea)

Mendel used Pisum sativum (Garden Pea)
7 characters, monastery in Brno, 1856-1863, published 1866

Theory: Genetics

1. Mendel's Choice of Pea Plant

Gregor Mendel (1822-1884): Austrian monk, father of genetics. Experimented 1856-1863 at the Augustinian monastery of St. Thomas, Brno (then Brunn), Moravia (now Czech Republic). Plant: Pisum sativum (Garden pea). Reasons for choosing pea: Short generation time (1 year). Many offspring per cross. 7 distinct pairs of contrasting characters (easy to score). Natural self-pollination in closed flowers (cleistogamy) = easy to maintain true-breeding lines. Can be artificially cross-pollinated by emasculation + manual pollen transfer. Easy to grow in greenhouse. 34 varieties from seed suppliers available in Brno. Published: "Experiments on Plant Hybrids" (Versuche uber Pflanzenhybriden) in 1866, Proceedings of Brno Natural History Society. Largely ignored for 34 years.

2. The 7 Characters Mendel Studied

Seed shape: round (R_) dominant over wrinkled (rr). Round = starch synthesis normal; wrinkled = rugosus gene mutation causes abnormal starch/sucrose. Seed colour: yellow (I_) dominant over green (ii). Endosperm pigmentation. Seed coat/flower colour: grey/brown with purple flowers (A_) dominant over white (aa). Pod shape: inflated/full (V_) dominant over constricted (vv). Pod colour: green (Gp_) dominant over yellow (gpgp). Flower position: axial/distributed along stem (Fa_) dominant over terminal/at top (fafa). Plant height: tall (Le_) dominant over dwarf (lele). Key: Mendel chose genes on different chromosomes (or far apart) - allows independent assortment. The 7 characters happen to correspond to 7 of the 14 pea chromosomes.

3. Monohybrid Cross and Law of Segregation

P: TT (tall) x tt (dwarf). F1: all Tt (tall) - dominance. F2 (self-F1): TT:Tt:tt = 1:2:1 (genotypic). T_:tt = 3:1 (phenotypic). Key observations: F1 all dominant. F2 shows both parental traits in 3:1 ratio. Recessive trait reappears in F2 (was not lost or blended). Mendel's explanation (Law of Segregation): factors (alleles) occur in pairs. The two factors separate during gamete formation. Each gamete receives one factor. This exactly matches chromosome behaviour in meiosis (Sutton-Boveri chromosome theory, 1902).

4. Dihybrid Cross and Law of Independent Assortment

P: RRYY (round, yellow) x rryy (wrinkled, green). F1: all RrYy (round, yellow). F2: 9 R_Y_ : 3 R_yy : 3 rrY_ : 1 rryy = 9:3:3:1. Mendel's explanation: each pair of factors assorts independently of other pairs during gamete formation = Law of Independent Assortment (2nd Law). Requires: genes on different chromosomes (or very far apart on same chromosome). If linked: deviates from 9:3:3:1.

5. Incomplete Dominance - Mirabilis jalapa

Mirabilis jalapa (4 o'clock flower): RR = red. Rr = pink. rr = white. F2: 1 red : 2 pink : 1 white (phenotypic = genotypic). No allele completely dominant. Mechanism: R allele produces enzyme for pigment. One copy (Rr) produces less enzyme than two copies (RR) = intermediate phenotype (pink, less pigment). Antirrhinum majus (snapdragon): similar. CW CW = white. CR CR = red. CR CW = pink. Also shows incomplete dominance. F2: 1:2:1. Codominance: both alleles expressed. ABO blood groups (IA, IB codominant over i). Sickle cell: HbA/HbS shows both normal and sickle haemoglobin.

6. Chromosomal Basis of Inheritance

Sutton (1902): chromosomes carry genes. Evidence: genes on chromosomes, chromosomes segregate in meiosis (mirrors allele segregation), chromosomes assort independently in meiosis I (mirrors independent assortment). Morgan (Nobel 1933): Drosophila melanogaster experiments. Confirmed chromosome theory. Discovered: sex-linked traits (white eye X-linked). Genetic linkage (genes on same chromosome). Crossing over explains separation of linked genes. Chromosomal maps: map unit = 1% recombination frequency = 1 centiMorgan (cM). Morgan's students mapped first chromosome map. Drosophila advantages: 4 chromosome pairs, giant salivary gland chromosomes (visible bands), short generation time, many offspring, cheap to maintain, known mutations available.

7. Multiple Alleles - ABO Blood Groups

ABO blood groups: 3 alleles (I^A, I^B, i) at one locus (chromosome 9). I^A and I^B codominant. i is recessive. Genotype-phenotype: I^A I^A or I^A i = Group A (A antigen, anti-B antibody). I^B I^B or I^B i = Group B (B antigen, anti-A antibody). I^A I^B = Group AB (both antigens, no antibodies). ii = Group O (no antigens, both anti-A and anti-B antibodies). Blood transfusion: must match ABO and Rh factor. AB = universal recipient (no antibodies). O negative = universal donor (no antigens + Rh-). Forensic use: blood typing to identify suspects (historical; DNA fingerprinting now preferred). Inheritance problems: if both parents are Group A (I^A i), child can be Group O (ii probability = 1/4).

8. Chromosomal Aberrations

Numerical: Aneuploidy: monosomy (2n-1), trisomy (2n+1). Down syndrome: trisomy 21, 47 chromosomes. Risk increases with maternal age. Features: intellectual disability, epicanthal folds, single palmar crease, heart defects, increased leukemia risk. Turner syndrome: 45,X (45,XO). Female, short, webbed neck, infertile, streak gonads, no secondary sex characters. Klinefelter: 47,XXY. Male, infertile, gynecomastia, tall, learning difficulties. Edward: trisomy 18. Patau: trisomy 13. XYY: tall male, otherwise normal. XXX: superfemale. Non-disjunction: failure of chromosomes to separate during meiosis I or II. Polyploidy: triploid (3n), tetraploid (4n). Common in plants. Allopolyploidy: from different species. Wheat = hexaploid (6n). Banana = triploid (seedless). Colchicine inhibits spindle formation, causes polyploidy.

Frequently Asked Questions

1. Why are Mendel's experiments considered a landmark in biology? ⌄

Mendel's genius was in: (1) Choosing the right experimental organism (pea) with clear contrasting characters. (2) Using quantitative methods and counting large numbers of offspring (statistical approach unusual in 1860s biology). (3) Following traits for multiple generations (F1, F2, test cross). (4) Mathematical analysis of results (1/4, 3/4 ratios). (5) Controlling experiments by preventing contamination (closed pea flowers + careful emasculation for crosses). Before Mendel: blending inheritance theory (traits blend in hybrids, never separate). Mendel showed traits are particulate (do not blend). His work was rediscovered in 1900 (de Vries, Correns, Tschermak independently), launching the science of genetics.

2. Why was Mendel's work ignored for 34 years after publication? ⌄

Several reasons: (1) Published in an obscure local journal (Proceedings of Brno Natural History Society, not a major scientific publication). (2) Written in German (limited international audience). (3) Used mathematical/statistical approach unusual for naturalists of his era. (4) Contradicted prevailing blending inheritance theory. (5) Darwin (who Mendel admired) was the dominant figure in biology - Darwin used blending inheritance in his pangenesis theory. Mendel actually read Darwin and annotated his copy. If Darwin had read Mendel: would have provided the hereditary mechanism that Darwin's evolution theory lacked (the problem of genetic dilution). (6) Mendel himself became Abbot of the monastery after 1868, had less time for science. Died in 1884 without recognition. 1900 rediscovery: 3 researchers independently found same results then searched literature and found Mendel had done it all in 1866.

3. What is incomplete dominance and how does it differ from codominance? ⌄

Incomplete dominance: heterozygote has INTERMEDIATE phenotype between two homozygotes. Rr = PINK (between RR red and rr white). Neither allele completely masks the other. Biochemical basis: one allele produces functional enzyme/protein, one allele is non-functional. Half the normal amount of functional protein produces intermediate phenotype. Codominance: heterozygote expresses BOTH alleles FULLY, not an intermediate. AB blood group: BOTH A and B antigens present on RBCs. HbA/HbS sickle cell carrier: BOTH normal (HbA) and sickle (HbS) haemoglobin molecules present. Key distinction: incomplete dominance = intermediate phenotype; codominance = both phenotypes present simultaneously. Both are non-Mendelian in the sense that 3:1 F2 ratio is replaced by 1:2:1 ratio (phenotypic = genotypic in both cases).

4. What is the significance of test crosses in genetics? ⌄

Test cross: crossing individual with unknown genotype with homozygous recessive parent (genotype aa for monohybrid; aabb for dihybrid). Purpose: reveals gametes produced by unknown parent. If unknown is AA: all offspring dominant (all Aa). If unknown is Aa: 1/2 dominant (Aa) + 1/2 recessive (aa) = 1:1 ratio. This directly reveals the gamete types of the heterozygote (50% A and 50% a). Test cross is still widely used today to: determine genotype of individuals with dominant phenotype, analyse linkage (recombination frequency between linked genes), assess hybrid purity in plant breeding, and in forensic genetics (establish parentage patterns).

5. How did Mendel's work relate to Darwin's theory of evolution? ⌄

Darwin (1859) proposed natural selection as mechanism of evolution but did not know the mechanism of heredity. He proposed pangenesis (1868) - hereditary particles "gemmules" from all body parts collect in gonads - but this did not explain variation or particulate inheritance. Problem: under blending inheritance (prevailing view), any variant would be diluted to extinction in a few generations (Fleeming Jenkin's blending argument against Darwin). Mendel's particulate inheritance solves this: alleles do not blend; recessive alleles preserved in heterozygotes and can reappear in later generations. The Modern Synthesis (1930s-1940s): Ronald Fisher, J.B.S. Haldane, Sewall Wright, Theodosius Dobzhansky, and Ernst Mayr united Mendelian genetics with Darwinian evolution. Mathematical population genetics showed how Mendelian variation + natural selection = evolution. This is the Neo-Darwinian synthesis, also called the Modern Synthesis.

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