Principles of Inheritance and Variation Study Guide

Foundations of Molecular Biology and Inheritance

• Conceptual Overview: The inheritance patterns identified by Gregor Mendel established an initial understanding of heredity, though the physical nature of the 'factors' (genes) determining phenotypes was originally unclear.

• Biological Focus: For the century following Mendel, biology focused on the structure of genetic material and the structural basis of converting genotype to phenotype.

• Major Scientific Contributors: The field of molecular biology was built through contributions from scientists including:

  • James Watson

  • Francis Crick

  • Marshall Nirenberg

  • Har Gobind Khorana

  • The Kornbergs (Arthur Kornberg and Roger Kornberg)

  • Seymour Benzer

  • Jacques Monod

  • Sydney Brenner

• Evolutionary Context: Parallel to genetics, the mechanism of evolution was investigated. Modern understanding of evolution is now enriched by molecular genetics, structural biology, and bioinformatics.

Biographies of Watson and Crick

• James Dewey Watson:

  • Birth: Born in Chicago on April 6, 1928.

  • Education: Received a B.Sc. in Zoology in 1947. Developed a serious interest in genetics from bird-watching.

  • Graduate Work: Received a Ph.D. in 1950 from Indiana University, Bloomington, studying the effects of hard X-rays on bacteriophage multiplication.

  • DNA Discovery: Proposed the complementary double-helical configuration in early March 1953 alongside Crick.

• Francis Harry Compton Crick:

  • Birth: Born June 8, 1916, at Northampton, England.

  • Education: Studied physics at University College, London (B.Sc. in 1937). Completed his Ph.D. in 1954 on the thesis "X-ray Diffraction: Polypeptides and Proteins."

  • Career and Honors: Proposed the double-helical structure and replication scheme for DNA in 1953. Became an F.R.S. (Fellow of the Royal Society) in 1959.

• Shared Honors: Watson and Crick jointly received the John Collins Warren Prize (1959), the Lasker Award (1960), the Research Corporation Prize (1962), and the Nobel Prize (1962).

Introduction to Genetics and Heredity

• Definitions:

  • Genetics: The branch of biology dealing with inheritance and the variation of characters from parents to offspring.

  • Inheritance: The process by which characters are passed from parent to progeny; the basis of heredity.

  • Variation: The degree to which progeny differ from their parents.

• Historical Knowledge: Humans recognized by $8000-1000\,B.C.$ that variation was linked to sexual reproduction. They used artificial selection and domestication to exploit natural variations.

  • Example: The Sahiwal cows in Punjab were developed from ancestral wild cows.

Mendel's Laws of Inheritance

• Gregor Mendel: Conducted hybridization experiments on garden peas ($Pisum\,sativum$) for seven years ($1856-1863$).

• Methodological Innovations:

  • First application of statistical analysis and mathematical logic to biological problems.

  • Large sampling size for data credibility.

  • Confirmation of inferences across successive generations to establish general rules.

• Experimental Design: Mendel studied characters with two opposing traits (e.g., tall vs. dwarf).

• True-Breeding Lines: These are lines that show stable trait inheritance and expression for several generations through continuous self-pollination. Mendel selected 14 true-breeding pea plant varieties (7 pairs of contrasting traits).

• Seven Characters Studied:

  1. Stem height: Tall/dwarf

  2. Flower colour: Violet/white

  3. Flower position: Axial/terminal

  4. Pod shape: Inflated/constricted

  5. Pod colour: Green/yellow

  6. Seed shape: Round/wrinkled

  7. Seed colour: Yellow/green

Inheritance of One Gene (Monohybrid Cross)

• The Cross: Mendel crossed tall and dwarf plants. The seeds produced generated the First Hybrid Generation, or Filial 1 ($F_1$).

• $F_1$ Results: All plants were tall. There were no dwarf plants and no blending (in-between heights).

• $F_2$ Results (Self-pollination of $F_1$): Both tall and dwarf traits appeared.

  • $3/4$ of the plants were tall.

  • $1/4$ of the plants were dwarf.

  • Phenotypic Ratio: $3:1$.

  • Genotypic Ratio: $1:2:1$ ($TT:Tt:tt$).

• Proposed "Factors": Mendel proposed that "factors" (now called genes) are units of inheritance passed down unchanged through gametes.

• Alleles: Different forms of the same gene coding for contrasting traits.

• Dominance and Recessiveness:

  • Dominant Factor: The trait expressed in the $F_1$ heterozygote ($T$).

  • Recessive Factor: The trait suppressed in the $F_1$ but reappearing in the $F_2$ ($t$).

• Terminology:

  • Homozygous: Allelic pairs are identical ($TT$ or $tt$).

  • Heterozygous: Allelic pairs are dissimilar ($Tt$).

  • Genotype: The genetic makeup (e.g., $TT$, $Tt$).

  • Phenotype: The observable physical expression (e.g., tall, dwarf).

Mathematical Logic in Genetics

• Law of Segregation: During gamete formation, alleles separate so that each gamete receives only one allele. This is a random process ($50\%$ chance for either allele).

• Punnett Square: Developed by British geneticist Reginald C. Punnett. It is a graphical representation used to calculate the probability of all possible genotypes of offspring.

• Binomial Expression: The genotypic ratio $1/4:1/2:1/4$ is condensable to the form $(ax+by)^2$.

  • Expression: $(1/2T + 1/2t)^2 = 1/4TT + 1/2Tt + 1/4tt$

• Test Cross: To determine the genotype of a plant showing a dominant phenotype (whether it is $TT$ or $Tt$), it is crossed with a recessive ($tt$) parent.

  • If all offspring are tall, the parent was $TT$.

  • If $50\%$ are tall and $50\%$ are dwarf, the parent was $Tt$.

Principles of Inheritance (Formalized Laws)

• First Law: Law of Dominance:

  1. Characters are controlled by discrete units called factors.

  2. Factors occur in pairs.

  3. In a dissimilar pair, one factor dominates the other.

• Second Law: Law of Segregation: Alleles do not show blending. Both characters are recovered in the $F_2$ generation even if one is hidden in the $F_1$. Factors segregate during meiosis.

Deviations from Mendelian Dominance

• Incomplete Dominance: The $F_1$ phenotype is an intermediate between the two parents.

  • Example: Flower color in Snapdragon ($Antirrhinum\,sp.$).

  • Cross: Red ($RR$) x White ($rr$) → $F_1$ Pink ($Rr$).

  • $F_2$ ratio: $1$ Red ($RR$) : $2$ Pink ($Rr$) : $1$ White ($rr$).

  • Explanation: Dominance is not autonomous; it depends on the gene product (enzyme) and its effect on a substrate. If an allele produces a non-functional enzyme or no enzyme, it results in a recessive phenotype.

• Co-dominance: The $F_1$ resembles both parents.

  • Example: ABO blood grouping in humans, controlled by Gene $I$.

  • Three alleles: $I^A$, $I^B$, and $i$.

  • $I^A$ and $I^B$ produce different sugars; $i$ produces no sugar.

  • In an $I^AI^B$ individual, both A and B types of sugars are expressed.

• Multiple Alleles: When more than two alleles govern the same character within a population. ABO blood grouping has 3 alleles and 6 possible genotypes.

Inheritance of Two Genes (Dihybrid Cross)

• Mendel's Dihybrid Cross: Crossing plants with Round Yellow seeds ($RRYY$) and Wrinkled Green seeds ($rryy$).

• $F_1$ Result: All Round Yellow ($RrYy$).

• $F_2$ Result: Phenotypic ratio of $9:3:3:1$.

  • 9 Round-Yellow

  • 3 Wrinked-Yellow

  • 3 Round-Green

  • 1 Wrinkled-Green

• Law of Independent Assortment: When two pairs of traits are combined in a hybrid, the segregation of one pair of characters is independent of the other pair. Four types of gametes are produced ($RY$, $Ry$, $rY$, $ry$), each at a frequency of $25\%$.

Chromosomal Theory of Inheritance

• Rediscovery of Mendel (1900): De Vries, Correns, and von Tschermak independently rediscovered Mendel's work.

• Chromosomes: Visualized by staining as "colored bodies" in the nucleus that double and divide before cell division.

• Sutton and Boveri (1902): Observed that the behavior of chromosomes is parallel to genes.

  • Both occur in pairs.

  • Both segregate during gamete formation.

  • Independent pairs segregate independently.

• Theory: Sutton united chromosomal segregation with Mendelian principles, stating that the pairing and separation of chromosomes lead to the segregation of factors they carry.

Linkage and Recombination

• Thomas Hunt Morgan: Verified the chromosomal theory using fruit flies ($Drosophila\,melanogaster$).

  • Reasons for using Drosophila: Grown on synthetic medium, short life cycle ($2\,weeks$), high progeny count, clear sex differentiation, hereditary variations visible under low power microscope.

• Linkage: The physical association of genes on a chromosome. Genes located on the same chromosome do not assort independently if they are closely linked.

• Recombination: The generation of non-parental gene combinations.

• Experimental Data:

  • Yellow-bodied, white-eyed females x Brown-bodied, red-eyed males.

  • Tightly linked genes: White and yellow ($1.3\%$ recombination).

  • Loosely linked genes: White and miniature wing ($37.2\%$ recombination).

• Genetic Mapping: Alfred Sturtevant used recombination frequency to measure distances between genes and map their positions. This is used today in whole-genome sequencing.

Polygenic Inheritance and Pleiotropy

• Polygenic Inheritance: Traits controlled by three or more genes, showing a gradient of variation rather than distinct alternatives.

  • Example: Human skin color controlled by genes $A, B, C$. The effect is additive ($AABBCC$ is darkest; $aabbcc$ is lightest).

• Pleiotropy: A single gene influencing multiple phenotypic traits, often by affecting metabolic pathways.

  • Example: Phenylketonuria ($PKU$) results from a single gene mutation in the enzyme phenylalanine hydroxylase, manifesting as mental retardation and reduced skin/hair pigmentation.

  • Example: Starch synthesis in pea seeds. One gene ($B, b$) affects both starch grain size and seed shape.

Sex Determination Mechanisms

• Henking (1891): Discovered the "X body" in sperm of some insects.

• XO Type: Males have only one X-chromosome ($XO$); females have a pair ($XX$). Example: Grasshopper.

• XY Type: Males have one X and one smaller Y chromosome ($XY$); females have two X chromosomes ($XX$). Seen in mammals (humans) and Drosophila.

• Male Heterogamety: Males produce two types of gametes (either X/O or X/Y).

• Female Heterogamety: Females produce two types of gametes. Example: Birds (Females are $ZW$, males are $ZZ$).

• Human Sex Determination: Humans have 23 pairs of chromosomes. 22 pairs are autosomes. The 23rd pair determines sex ($XX$ for female, $XY$ for male). The genetic makeup of the sperm determines the sex of the child with a $50\%$ probability.

• Honey Bee Sex Determination: Haplodiploid system.

  • Females: Diploid ($32\,chromosomes$); develop from fertilized eggs.

  • Males (Drones): Haploid ($16\,chromosomes$); develop from unfertilized eggs via parthenogenesis.

  • Males produce sperm by mitosis and have no fathers or sons, but possess grandfathers and grandsons.

Mutation

• Definition: Alteration of DNA sequences resulting in changes in genotype and phenotype.

• Types:

  • Chromosomal Aberrations: Deletions, insertions, or duplications of DNA segments. Common in cancer cells.

  • Point Mutation: Change in a single base pair. Example: Sickle-cell anemia.

  • Frame-shift Mutation: Caused by deletions and insertions of base pairs.

• Mutagens: Factors inducing mutations, such as physical agents (UV radiation) or chemical agents.

Genetic Disorders

• Pedigree Analysis: Study of family history to trace the inheritance of specific traits/abnormalities across generations. Standard symbols represent genders, mating, and affected individuals.

• Mendelian Disorders: Caused by mutation in a single gene.

  • Haemophilia: X-linked recessive. Affects the blood clotting cascade, leading to non-stop bleeding. Queen Victoria was a carrier.

  • Sickle-cell Anemia: Autosomal recessive ($Hb^A, Hb^S$). Caused by the substitution of Glutamic acid ($Glu$) by Valine ($Val$) at the sixth position of the beta globin chain (CAG to GUG). RBCs become sickle-shaped under low oxygen.

  • Phenylketonuria: Autosomal recessive. Lack of phenylalanine hydroxylase converts phenylalanine to phenylpyruvic acid, causing mental retardation.

  • Color Blindness: X-linked recessive. Defect in red or green cones. Occurs in $8\%$ of males and $0.4\%$ of females.

  • Thalassemia: Autosomal recessive blood disease. Reduced synthesis of globin chains ($\alpha$ or $\beta$).

    • $\alpha\,Thalassemia$: Genes $HBA1$ and $HBA2$ on chromosome 16.

    • $\beta\,Thalassemia$: Gene $HBB$ on chromosome 11.

• Chromosomal Disorders: Caused by absence or excess of chromosomes.

  • Aneuploidy: Gain/loss of a chromosome due to failure of chromatid segregation.

    • Down’s Syndrome: Trisomy 21. Characterized by short stature, furrowed tongue, and mental retardation. First described by Langdon Down ($1866$).

    • Klinefelter’s Syndrome: $47, XXY$. Masculine development with feminine features (Gynaecomastia). Sterile males.

    • Turner’s Syndrome: $45, XO$. Absence of one X chromosome. Sterile females with rudimentary ovaries and lack of secondary sexual characters.

  • Polyploidy: Increase in whole sets of chromosomes due to failure of cytokinesis. Common in plants.

Questions & Discussion

• What are the advantages of selecting pea plants for experiments? Mendel selected them because of clear contrasting traits, true-breeding lines, and the ability to control crosses easily.

• How does dominance differ from incomplete dominance? In dominance, one trait completely masks another in the $F_1$. In incomplete dominance, the $F_1$ is an intermediate (e.g., pink flowers).

• What determines the sex of a child in humans? It is determined by the sperm: if the sperm carries an X chromosome, the child is female ($XX$); if it carries a Y, the child is male ($XY$).

• Why are pedigree analyses useful for humans? Because controlled crosses cannot be performed on humans, pedigree charts allow scientists to track the inheritance of genetic disorders through family history.

• What is the difference between Thalassemia and Sickle-cell anemia? Thalassemia is a quantitative problem (too few globin molecules produced), whereas sickle-cell anemia is a qualitative problem (incorrectly functioning globin molecules).