Chromosomal Inheritance and Genetic Linkage

Chapter 13: Chromosomal Inheritance

Part 1: Genetic Linkage

  • Genetic Linkage: Refers to how genes positioned close to each other on the same chromosome inherit together, differently than genes that are far apart or on different chromosomes.
  • Recombination Frequencies: Utilize recombination data to assess if traits are genetically linked and to determine the proximity of these genes on the chromosome.

Thomas Hunt Morgan & Linkage

  • Significance of Morgan's Work: Thomas Hunt Morgan was pivotal in demonstrating that genes are located on chromosomes.
  • White Eye Mutation in Drosophila: Morgan showed that the white eye mutant allele was inherited in conjunction with sex, illustrating sex linkage as the gene was found on the X chromosome.
  • First Proof of Genetic Linkage: Through studies on fruit flies (Drosophila melanogaster), Morgan provided concrete evidence for the link between genetics and chromosomes in heredity.

Unlinked Genes – Mendel’s Peas

  • Definition of Unlinked Genes: Genes located on different chromosomes or far apart on the same chromosome; they assort independently during meiosis, yielding many gametic combinations.
  • Recombination & Independent Assortment: Mendel observed offspring phenotypic combinations that differed from the P generation.
    • P Generation Gametes:
    • Homozygous recessive (yyrr): Produces gametes: yr
    • Heterozygous (YyRr): Produces gametes: YR, Yr, yR, yr
    • Parental-type Offspring: Combination of parental gametes that do not show recombination:
    • Recombinant Offspring: Offspring with different phenotypes, indicating genetic recombination occurred.
    • F₂ Generation Phenotypic Ratio: The resulting ratio of 9:3:3:1 from an F₁ X F₁ cross-fertilization.

Gene Linkage – Morgan’s Flies

  • Linked Genes Defined: Genes that are located close together on the same chromosome and are typically inherited together, not assorting independently. This reduces the number of possible gametic combinations.
  • Experiments Conducted: Morgan's fruit fly experiments aimed to explore how gene linkage influences inheritance of traits.

Experiment Summary: Morgan’s Crossbreeding

  • P Generation:
    • Wild type (gray body, normal wings) = b+ b+ vg+ vg+
    • Double mutant (black body, vestigial wings) = bb vg vg
  • F₁ Dihybrid Testcross: Crossed wild-type F₁ dihybrid with double mutants to establish patterns of inheritance.
  • Observed Offspring Ratios: Morgan calculated ratios based on offspring phenotypes to support linkage hypotheses.
    • Expected 1:1:1:1 ratio if on different chromosomes. Actual observations: ( 965 : 944 : 206 : 185 ) indicated parental pairings predominated, supporting link.

Recombination and Crossing Over

  • Recombinant Offspring: Display new trait combinations, indicating crossing over during meiosis.
  • Recombination Frequency: When 50% of offspring are recombinants, the recombination frequency is evaluated as being connected to physical distance between genes on a chromosome.
    • Morgan's Conclusion: Linked genes exhibit recombination frequencies of less than 50%. Linkage can be occasionally broken by crossing over.

Example of Recombination Calculation

  1. Experimental Cross: True-breeding normal-winged & eyed flies crossed with curled-winged, eyeless flies.
  2. Offspring Counts: 628 normal eyes and wings, 116 normal wings and eyeless, 655 curled-wings and eyeless, 101 curled wings and normal.
  3. Total Offspring: 1500.
  4. Recombinants: Add 116 + 101 = 217.
  5. Frequency Formula: ( ext{Recombination Frequency} = \frac{217}{1500} imes 100 \approx 14.5\% )

Linkage Maps and Recombination Data

  • Expressing Frequencies: Recombination frequencies expressed in Centimorgans (cM), aids in generating genetic and linkage maps.
  • Linkage Map: An ordered list of genetic loci based on recombination frequencies, not indicating exact physical distances.
  • Distance Correlation: How far genes are on a chromosome influences the likelihood of recombination.

Part 2: Nondisjunction, Chromosomal Abnormalities

  • Nondisjunction Effects: Abnormal chromosome counts result from homologous chromosomes or sister chromatids failing to separate during meiosis.
  • Aneuploidy: Condition resulting from fertilization of abnormal gametes, leading to offspring with an abnormal number of chromosomes.
  • Polyploidy: Presence of more than 2 sets of chromosomes in a cell.

Alterations of Chromosome Structure

  • Types of Structural Changes:
    • Deletion: Loss of a chromosomal segment.
    • Duplication: Repetition of chromosomal segments.
    • Inversion: Reversal of segments within a chromosome.
    • Translocation: Movement of chromosomal segments between non-homologous chromosomes; reciprocal translocations involve exchange between two chromosomes.
  • Pathological Implications: Certain cancers linked to translocations.

Notable Chromosomal Disorders

  • Trisomy 21 (Down Syndrome): Most frequent chromosomal anomaly, characterized by distinct facial features, cardiovascular problems, and social behavior conditions.
  • Turner Syndrome: Affects females with missing X chromosome, leading to infertility and organ malformations.
  • Klinefelter Syndrome: Males with an extra X chromosome (XXY), often undiagnosed until fertility issues arise. Treatment available via testosterone therapy.
  • Jacobs Syndrome: Caused by an additional Y chromosome in males (XYY), associated with behavioral issues and learning disabilities.

This guide encompasses intricate details of chromosomal inheritance, genetic linkage, and chromosomal abnormalities, providing a comprehensive reference for further study.