Biology Topic 10: Inherited Change and Inheritance Study Notes

Scope and Coverage of Inherited Change and Inheritance

• The study of inherited change and inheritance encompasses several critical biological domains focusing on how genetic information is transmitted and expressed. The scope includes:

  • Meiosis: The process of reduction division forming haploid cells.

  • Genetics: The fundamental study of genes and heredity.

  • Genotype and Phenotype: Analyzing how the genetic makeup of an organism (genotype) affects its observable characteristics (phenotype).

  • Inheriting Genes: The mechanisms of transferring genetic material.

  • Multiple Alleles: Inheritance patterns involving more than two variants of a gene.

  • Sex Inheritance and Sex Linkage: How biological sex is determined and how genes on sex chromosomes are inherited.

  • Dihybrid Crosses: Tracking the inheritance of two different traits simultaneously.

  • Mutations: Changes in the DNA sequence.

  • Environmental Factors: Understanding how the environment interacts with the genotype to influence phenotype.

Learning Outcomes and Fundamental Concepts

• Passage of Information: Student must understand the movement of information from parents to offspring, including:

  • Haploid ($n$): A cell containing one complete set of chromosomes. In humans, the haploid number is $23$.

  • Diploid ($2n$): A cell containing two complete sets of chromosomes. In humans, this is $46$.

  • Homologous Pairs: Two chromosomes in a pair where one is typically inherited from the mother and one from the father. They have the same shape, carry the same genes in the same positions (loci), and possess centromeres in identical locations.

  • Meiotic Behavior: The movement and interaction of chromosomes in plant and animal cells during division.

• Role of Genes in Phenotypes: Understanding how genes determine characteristics through:

  • Key Terminology: Locus (the specific position of a gene on a chromosome), allele (different forms of a gene), homozygous (two identical alleles), and heterozygous (two different alleles).

  • Genetic Diagrams: Interpreting monohybrid and dihybrid crosses.

  • Protein Relationship: The connection between genes, the proteins they code for, and the resulting phenotype.

  • Gibberellin: Specifically focusing on its role in stem elongation through gene activation.

• Gene Control Mechanisms: Understanding the regulation of genetic expression, including:

  • Structural vs. Regulatory Genes: Structural genes code for proteins that function within the cell; regulatory genes code for proteins that control the expression of other genes.

  • Prokaryotic Protein Production: Control mechanisms in simpler organisms.

  • Transcription Factors: Proteins that bind to DNA to initiate or inhibit gene expression.

  • DELLA Proteins: How Gibberellin triggers the breakdown of DELLA protein repressors to activate genes.

The Mechanism of Meiosis

• Meiosis as Reduction Division:

  • It is a nuclear division that reduces the chromosome number from diploid ($2n$) to haploid ($n$).

  • This ensures that when sperm and egg fuse during fertilization, the resulting zygote has the correct diploid number. Without this reduction, the chromosome number would double every generation.

• Stages of Meiosis:

  • Meiosis consists of two distinct divisions: Meiosis I and Meiosis II.

  • Meiosis I: Homologous chromosomes pair up (synapse) and then separate.

    1. Prophase I: DNA condenses, and chromosomes become visible. Crossing over occurs here.

    2. Metaphase I: Homologous chromosome pairs align along the equator of the spindle.

    3. Anaphase I: Whole homologous chromosomes are pulled apart to opposite poles.

    4. Telophase I: Two groups of chromosomes reach the poles, the nucleus reforms, and the cytoplasm begins to pinch in.

  • Meiosis II: Sister chromatids separate, similar to mitosis.

    1. Prophase II: DNA condenses again; two groups of chromosomes are visible.

    2. Metaphase II: Chromosomes line up in a single file along the spindle equator.

    3. Anaphase II: Chromatids are pulled apart from the middle toward opposite poles.

    4. Telophase II: Four groups of chromosomes form, the cytoplasm divides, and four unique haploid cells are produced.

• Cytokinesis in Meiosis:

  • In animal cells: The cell surface membrane folds inward to create a cleavage furrow, which contracts to divide the cytoplasm.

  • In plant cells: Vesicles from the Golgi apparatus gather at the spindle equator and merge to form a new cell surface membrane.

Sources of Genetic Variation

• Natural Selection and Diversity: Genetically diverse offspring are advantageous for natural selection. Meiosis provides several mechanisms for this:

  • Crossing Over: The exchange of alleles between non-sister chromatids of homologous chromosomes. This creates new combinations of alleles on the resulting chromosomes.

  • Independent Assortment: The random orientation of homologous pairs during Metaphase I and chromatids during Metaphase II leads to different combinations of alleles in gametes.

  • Random Fusion of Gametes: During fertilization, any male gamete can fuse with any female gamete. This random pairing creates unique zygotes, ensuring that individuals from successive sexual reproductions are almost never genetically identical.

Key Genetics Terminology and Allele Types

• Gene: A length of DNA coding for a single polypeptide or protein.

• Dominant Allele: A variant that expresses itself strongly even if only one copy is present (e.g., $AA$ or $Aa$).

• Recessive Allele: A variant whose effect is masked by a dominant allele; it is only expressed if two copies are present ($aa$).

• Co-dominant Alleles: Two or more alleles are dominant, causing both to be expressed in the phenotype (e.g., red and white patches in flowers).

• Linkage:

  • Genetic Linkage: Genes located close together on the same chromosome tend to be inherited together.

  • Autosomal Linkage: Genes on non-sex chromosomes (autosomes) that stay together in the original parental combination.

• Test Cross: A method used to discover the genotype of an individual by crossing them with a known recessive parent and observing the phenotypic ratio of the offspring.

• Filial Generations: $F1$ is the first generation from the parents; $F2$ is the generation obtained by crossing $F1$ individuals.

Punnett Squares and Inheritance Patterns

• Construction Steps:

  1. Determine parental genotypes.

  2. Assign letters (Upper case for dominant, lower case for recessive).

  3. Split parental alleles and place them on the outside of the square.

  4. Fill the internal squares to predict offspring combinations.

  5. Always write the dominant allele before the recessive allele (e.g., $Aa$).

• Examples and Ratios:

  • Dominance: If $B$ (blue) is dominant to $b$ (white), a cross between two heterozygotes ($Bb \times Bb$) results in a probability of $3/4$ ($75\%$) blue flowers and $1/4$ ($25\%$) white flowers.

  • Multiple Alleles: The ABO blood-type system. Alleles $IA$ and $IB$ are co-dominant, and both are dominant over allele $i$ (which codes for no antigens).

  • Sex Linkage: Genes found on the $X$ chromosome but not the $Y$. Because males are $XY$, they only need one copy of a recessive X-linked allele to express the trait.

Relationship Between Genes, Proteins, and Phenotypes

• Central Pathway: A gene codes for mRNA, which is translated into a protein. That protein then directly or indirectly determines the observable phenotype.

• Case Study 1: Albinism ($TYR$ Gene):

  • The $TYR$ gene is on chromosome $11$.

  • It codes for the enzyme tyrosinase, which converts tyrosine into the pigment melanin.

  • A recessive allele causes a lack of tyrosinase, resulting in no melanin and phenotypes characterized by pale skin, hair, and pink or pale blue irises.

• Case Study 2: Sickle Cell Anaemia ($HBB$ Gene):

  • The $HBB$ gene on chromosome $11$ codes for the $\beta$-globin polypeptide in haemoglobin.

  • A single base change in the DNA results in an amino acid substitution, creating abnormal $\beta$-globin.

  • This changes the red blood cell shape. Homozygous individuals produce only sickle-cell haemoglobin; heterozygous individuals produce both normal and abnormal haemoglobin.

• Case Study 3: Haemophilia ($F8$ Gene):

  • The $F8$ gene codes for factor VIII, a blood-clotting protein.

  • Abnormal alleles lead to deficient or non-existent factor VIII production.

  • This is a sex-linked condition because the $F8$ gene is located on the $X$ chromosome.

• Case Study 4: Huntington’s Disease ($HTT$ Gene):

  • Caused by abnormal alleles of the $HTT$ gene (dominant over normal alleles).

  • The gene codes for the protein Huntingtin, involved in neuronal development.

  • Disease occurs when there is a high number of repeated $CAG$ triplets in the nucleotide sequence.

Gene Control: Gibberellin and the Lac Operon

• Stem Elongation (The $Le$ Gene):

  • Height is controlled by the $Le$ gene. The dominant $Le$ allele produces an enzyme used to create active Gibberellin ($GA1$).

  • Active Gibberellin stimulates cell division and elongation.

  • The recessive $le$ allele differs by a single nucleotide/amino acids, producing a non-functional enzyme. Plants homozygous for $le$ are dwarfs because they cannot produce active gibberellin.

• Types of Enzymes in Gene Control:

  • Repressible Enzymes: Synthesized normally until a repressor protein binds to an operator, stopping transcription.

  • Inducible Enzymes: Synthesized only when their substrate is present. The substrate induces transcription to start.

• The Lac Operon (Prokaryotes):

  • A cluster of genes controlled by one promoter, regulating lactase production.

  • Regulatory gene ($lacI$) codes for a lac repressor protein.

  • The repressor has two binding sites: one for the operator and one for lactose.

  • Without lactose: Repressor binds to the operator, preventing transcription of structural genes.

  • With lactose: Lactose binds to the repressor, distorting its shape so it can no longer bind to the operator. Transcription begins.

• Transcription Factors:

  • Proteins that control transcription by binding to specific DNA regions.

  • They ensure genes are expressed in the correct cells, at the correct times, and at the correct levels. Roughly $10\%$ of human genes code for these factors.

• Gibberellin and DELLA Proteins:

  • Gibberellin controls seed germination by stimulating amylase synthesis.

  • In the absence of Gibberellin, DELLA proteins bind to transcription factor PIF, preventing it from binding to the amylase gene promoter.

  • When Gibberellin is present, it binds to a receptor and enzyme that initiates the breakdown of DELLA.

  • This releases PIF, which then binds to the promoter, starting the transcription of the amylase gene.

Questions & Discussion

• Q1. What are haploid and diploid cells?

  • Response: Haploid ($n$) cells contain one set of chromosomes; diploid ($2n$) cells contain two complete sets.

• Q2. What are homologous chromosomes?

  • Response: Pairs of chromosomes (one maternal, one paternal) with the same genes at the same loci.

• Q3. What is reduction division?

  • Response: Division (meiosis) where the chromosome number is halved from diploid to haploid.

• Q4. What are the different stages in meiosis?

  • Response: Prophase I/II, Metaphase I/II, Anaphase I/II, and Telophase I/II.

• Q4 (Repeated). What are homozygous and heterozygous?

  • Response: Homozygous means having two identical alleles at a locus; heterozygous means having two different alleles.

• Q5. What are the different types of alleles?

  • Response: Dominant, recessive, and co-dominant.

• Q6. What is the role of gibberellin in stem elongation?

  • Response: It stimulates cell division and elongation by activating genes via the breakdown of DELLA proteins.

• Q7. What is the difference between structural and regulatory genes?

  • Response: Structural genes code for proteins with specific cellular functions; regulatory genes code for proteins (like transcription factors) that control other genes.

• Q8. What is the difference between repressible and inducible enzymes?

  • Response: Repressible enzymes are turned off by a repressor; inducible enzymes are turned on by the presence of a substrate.

Historical Note: The Punnett Square

• The Punnett square was devised by mathematician Reginald Crundall Punnett in $1905$.

• He created this tool following Gregor Mendel's experiments with yellow and green peas to provide a visual and mathematical way to predict genetic outcomes.