Chapter 13: Modern Understanding of Genetics:

13.1 Chromosomal Theory and Genetic Linkage:

Chromosomal Theory of Inheritance:

• Proposed by Sutton and Boveri (1902)

• Genes are located on chromosomes, which are the vehicles for inheritance

• Evidence supporting this theory:

  • Chromosomes exist in pairs (like Mendel’s factors)

  • Homologous chromosomes separate independently during meiosis

  • Gametes carry half the chromosome number of body cells

  • Fertilization restores the diploid state, allowing for the mixture of genetic material from both parents and contributing to the genetic diversity of the offspring

• Mendel’s laws are explained by how chromosomes behave in meiosis

• Morgan confirmed genes are on chromosomes using fruit flies

Genetic Linkage & Recombination:

• Linked genes are close together on the same chromosome and are usually inherited together

• Mendel’s law of independent assortment applies to genes on different chromosomes or far apart on the same chromosome (linked genes violate this law)

• Sometimes, linked genes swap parts during meiosis (crossing over)

• Crossing over creates genetic diversity

Key point: Closer genes are more likely to be inherited together; farther genes are more likely to recombine.

Genes closer together = less likely to be separated by crossing over

Genetic Mapping

• Scientists can map genes on chromosomes based on how often they cross over

• Recombination frequency is measured in centimorgans (cM) and used to map genes on chromosomes

• Farther apart genes = higher chance of recombination

• 1% recombination = 1cM = 1 map unit (m.u.)

Three-point Test Cross:

• Used to determine gene order and distances between three genes

• Double crossover offspring = least frequent

Recombination frequency = Recombinant progeny / Total progeny (progeny means offspring)

This ratio helps determine genetic linkage and the distance between genes on a chromosome.

Key point: Recombination frequency tells us how far apart genes are on a chromosome; this information helps determine the genetic linkage and can assist in identifying traits associated with specific genetic locations.

Homologous Recombination/Crossing over:

• First observed by Frans Janssen (1909) via chiasmata

• Occurs during Prophase l of meiosis

• Homologous chromosomes exchange segments, creating nonparental (recombinant) genotypes

Parental types: Offspring inherit the same allele combination as the parents

Recombinant types: New allele combinations caused by crossing over

Three Points Test Cross:

It involves examining the offspring from a cross between an individual heterozygous for three traits and an individual homozygous recessive for those same traits. This allows researchers to infer the arrangement of genes.

Why will recombinants make you understand gene linkage better? The number of recombinants you get is the map distance unit.

SNPs

• Single-nucleotide polymorphisms

• Affect a single base of a gene locus, leading to variations in DNA sequences between individuals.

• These variations can influence phenotypic traits and are crucial for studying genetic diversity within populations.

13.2 Chromosomal Basis of Inherited Disorders:

Karyotype & Karyogram:

• Karyotype: Number and appearance of chromosomes

• Karyogram: Image of an individual’s chromosomes arranged by size

• Used to detect chromosomal abnormalities

Banding patterns, centromere positions, and chromosome size help in identification

Nondisjunction:

• Failure of chromosomes to separate during Meiosis

• It can result in aneuploidy, which is a gain or loss of a chromosome

  • Monosomy - loss of chromosome

  • Trisomy - gain of chromosome

  • In all but a few cases, do not survive

• Nondisjunction of sex chromosomes:

  • If the mother is older, nondisjunction can occur more frequently

  • Do not generally experience severe developmental abnormalities

  • Individuals have somewhat abnormal features, but often reach maturity and in some cases may be fertile

Chromosomal disorder:

• Can result from an abnormal number of chromosomes (ex: extra or missing)

• Common disorders include:

  • Down Syndrome (Trisomy 21): Extra chromosome 21

  • Turner Syndrome (XO): Only one X chromosome is present

  • Klinefelter Syndrome (XXY): Extra X in males

Key point: Mistakes in chromosome separation (nondisjunction) during meiosis can cause disorders such as these, leading to an abnormal number of chromosomes in offspring.

Structural Chromosome Abnormalities:

• Deletions - part of a chromosome is missing

• Duplications - extra copies of a gene

• Inversions - gene order is reversed

  • Chromosome segment flips 180 degrees and reinserts

  • Can be: Pericentric (includes centromere) + Paracentric (does not include centromere)

• Translocations: segment of one chromosome moves to another non-homologous chromosome

  • Can disrupt gene regulation or cause diseases like leukemia

Key point: Structure changes can lead to disease or developmental issues

Mendel’s 3-Stage Method:

1. Step One: Produce True-Breeding Plants:

   • Mendel self-fertilized plants for several generations until the offspring always showed the same trait

   • These were homozygous for a specific trait (ex: all purple flowers or all white flowers)

   • He did this for 7 different traits, each with 2 contrasting forms (ex: round vs wrinkled seeds)

   Purpose: To make sure the parental plants were pure and predictable for their traits and to analyze how these traits were passed on to the next generation through dominant and recessive alleles.

2. Step two: Cross-Fertilize True-Breeding Plants (P Generation)

   • He crossed plants that had opposite traits (ex: purple flowers x white flowers)

   • This created the F1 generation (first filial generation)

Observation: All F1 offspring showed only one trait — the dominant one

• For example, all F1 plants had purple flowers even though one parent had white flowers, confirming that the purple trait was dominant over the white trait.

3. Step three: Allow the F1 to Self-Fertilize ——> Observe F2

   • Mendel, let the F1 hybrids self-pollinate, creating the F2 generation

   • He counted thousands of plants and traits

Observation: The recessive trait reappeared in about 1 out of 4 plants, confirming the 3:1 ratio (3 dominant:1 recessive)

Key Discovery: 3:1 is actually 1:2:1

• Mendel went deeper and realized:

  • of the 3 purple plants:

    • 1 was true-breeding (PP)

    • 2 were hybrids (Pp)

    • The 1 white plants was true-breeding recessive (pp)

Conclusion for Mendel’s Experiments:

1. Traits are inherited as discrete units (genes):

   • they do not blend — they stay intact across generations

2. Each organism inherits two alleles for each gene:

   • one from each parents

3. Some alleles are dominant, some are recessive:

   • dominant alleles mask recessive ones in the phenotype

4. Law of Segregation:

   • during gamete formation, the two alleles for a trait seperate from each other, resulting in offspring (gamete) inherit one allele from each parent

5. Traits reappear in predictable ratios:

   • this allows for mathematical prediction of inheritance

Principle of Independent Assortment:

• Genes for different traits sort independently during gamete formation

• That’s why round seeds don’t always come with yellow color — they shuffle randomly

Happens during metaphase 1 of meiosis, when chromosomes line up randomly

Mnemonic: Traits travel their own paths

Probability Rules:

• Rule of Addition:

  • Used when events cannot happen at the same time (mutually exclusive)

  • Pp or pP? —> ¼ + ¼ = ½ chance of hybrid

Polygenic Inheritance:

• Many genes control one trait

• Example: height, skin color

• One mutation —> many symptoms

Pleiotropy:

• One gene affects multiple traits

• Example: sickle cell, cystic fibrosis

• One Mutation —> many symptoms

Mnemonic: Pleiotropy: Plenty of problems from one gene

Q1: What is a dihybrid cross?
A1: A cross that examines the inheritance of two different traits at the same time.

Q2: What genotype do you get from RRYY × rryy?
A2: RrYy — heterozygous for both traits.

Q3: What phenotypes show up in the F₂ generation of a dihybrid cross?
A3: Round Yellow, Round Green, Wrinkled Yellow, Wrinkled Green

Q4: What is the phenotypic ratio of a dihybrid F₂ generation?
A4: 9:3:3:1

Q5: What does the principle of independent assortment state?
A5: Alleles for different traits sort independently during gamete formation.

Q6: When does independent assortment occur?
A6: During Metaphase I of meiosis.

Q7: What is the rule of addition?
A7: Add probabilities of mutually exclusive events (either/or).

Q8: What is the rule of multiplication?
A8: Multiply probabilities of independent events (this AND that).

Q9: What is a testcross?
A9: A cross between an organism with a dominant phenotype and a homozygous recessive to determine the genotype.

Q10: What does it mean if all offspring show the dominant trait in a testcross?
A10: The parent is likely homozygous dominant.

Q11: What is polygenic inheritance?
A11: A trait controlled by many genes, showing continuous variation (e.g., height).

Q12: What is pleiotropy?
A12: A single gene that affects multiple traits or systems (e.g., sickle cell disease).

Q13: What are multiple alleles?
A13: More than 2 alleles for a gene exist in the population (e.g., IA, IB, i for blood type).

Q14: What is incomplete dominance?
A14: A form of inheritance where the heterozygote is a blend of both traits (e.g., pink flowers from red × white).

Q15: What is codominance?
A15: Both alleles are fully expressed in the heterozygote (e.g., type AB blood).

Q16: Which blood type shows codominance?
A16: AB blood type (IAIB)

Q17: Which blood type is recessive?
A17: O blood type (ii)

Q18: What are the three alleles for blood type?
A18: IA, IB, i

Q19: How many alleles can a person have for one gene?
A19: Only 2, even if more exist in the population.

Q 20: Why will recombinants make you understand gene linkage better?

A20: Recombinants help illustrate the concept of gene linkage by demonstrating how alleles can be shuffled during meiosis, revealing how closely genes are located on a chromosome.

Questions for Review:

• What does Mendel’s law explain?

  • Explained by how chromosomes behave in meiosis

• 1. Q: Who proposed the Chromosomal Theory of Inheritance?
  A: Sutton and Boveri in 1902.

  2. Q: What structure carries genes according to the Chromosomal Theory?
  A: Chromosomes.

  3. Q: What organism did Morgan use to confirm genes are located on chromosomes?
  A: Fruit flies (Drosophila melanogaster).

  4. Q: What does a recombination frequency of 1% represent?
  A: 1 centimorgan (cM) or 1 map unit.

  5. Q: What does the law of segregation state?
  A: Alleles separate during gamete formation, so each gamete receives one allele.

  6. Q: What is a dihybrid cross?
  A: A cross that studies the inheritance of two traits at once.

  7. Q: What is the phenotypic ratio of a dihybrid F₂ generation?
  A: 9:3:3:1 (if genes assort independently).

  8. Q: What does "linked genes" mean?
  A: Genes located close together on the same chromosome and inherited together.

  9. Q: When does crossing over occur?
  A: During Prophase I of meiosis.

  10. Q: What increases the likelihood of recombination between two genes?
  A: Greater physical distance between them on a chromosome.

11. Q: Define karyotype.
A: The number and appearance of chromosomes in a cell.

12. Q: What is aneuploidy?
A: An abnormal number of chromosomes.

13. Q: Give an example of a trisomy disorder.
A: Down Syndrome (Trisomy 21).

14. Q: What is nondisjunction?
A: The failure of chromosomes to separate properly during meiosis.

15. Q: What is the key consequence of nondisjunction?
A: It can lead to disorders like trisomy or monosomy.

16. Q: What is pleiotropy?
A: One gene affecting multiple traits.

17. Q: What’s a simple mnemonic to remember pleiotropy?
A: “Pleiotropy = Plenty of problems.”

18. Q: Define SNP.
A: Single-nucleotide polymorphism — a DNA variation at one base pair.

19. Q: What is the purpose of a three-point test cross?
A: To determine the gene order and distance between three genes.

20. Q: Which offspring are least common in a three-point test cross?
A: Double crossover offspring.

21. Q: Why does Mendel’s law of independent assortment not apply to linked genes?
A: Because linked genes tend to be inherited together.

22. Q: What does a karyogram show?
A: A visual image of an individual's chromosomes arranged by size.

23. Q: What’s the genotype of offspring from a cross between RRYY × rryy?
A: RrYy.

24. Q: What are parental types in recombination?
A: Offspring with the same allele combination as the parents.

25. Q: How can structural chromosome changes like translocations affect health?
A: They can disrupt gene regulation and lead to diseases like leukemia.

Q: In a test cross for two characteristics (Figure 13.3), can the predicted frequency of recombinant offspring be 60 percent? Why or why not?
A: No. The maximum frequency of recombination between two genes is 50%, which occurs when the genes are on different chromosomes or very far apart on the same chromosome. A 60% recombination frequency is not possible because it exceeds the maximum expected value.

2. Q: Figure 13.4 — Which of the following statements is true?
Answer: a. Recombination of the body color and red/cinnabar eye alleles will occur more frequently than recombination of the alleles for wing length and aristae length.
(Explanation: Genes that are farther apart are more likely to recombine; the figure likely shows greater distance between body color and eye color loci.)

Review Questions:

3. Q: Which of the following statements about nondisjunction is true?
A: b. Nondisjunction occurring during meiosis II results in 50 percent normal gametes.

4. Q: X-linked recessive traits in humans (or in Drosophila) are observed ________.
A: a. in more males than females

5. Q: The first suggestion that chromosomes may physically exchange segments came from the microscopic identification of ________.
A: c. chiasmata

6. Q: Which recombination frequency corresponds to independent assortment and the absence of linkage?
A: c. 0.50

7. Q: Which recombination frequency corresponds to perfect linkage and violates the law of independent assortment?
A: a. 0

8. Q: Which of the following codes describes position 12 on the long arm of chromosome 13?
A: b. 13q12
(“q” = long arm, “p” = short arm)

9. Q: In agriculture, polyploid crops (like coffee, strawberries, or bananas) tend to produce ________.
A: c. larger yields

10. Q: Assume a pericentric inversion occurred in one of two homologs prior to meiosis. What structure would these homologs assume in order to pair accurately?
A: c. loop

11. Q: The genotype XXY corresponds to:
A: a. Klinefelter syndrome

12. Q: Abnormalities in the number of X chromosomes tend to have milder effects than autosomal abnormalities because of ________.
A: d. X inactivation

13. Q: By definition, a pericentric inversion includes the ________.
A: a. centromere

Critical Thinking Questions:

14. Q: Explain how the Chromosomal Theory of Inheritance helped to advance our understanding of genetics.
A: The Chromosomal Theory of Inheritance confirmed that genes are located on chromosomes and that these chromosomes are passed from parents to offspring through gametes. It connected Mendel’s laws to physical structures observed in cells, providing a molecular basis for inheritance and helping explain phenomena like independent assortment and segregation.

15. Q: Using diagrams, illustrate how nondisjunction can result in an aneuploid zygote.
A: (Since I can't draw here, here's a verbal answer, but I can provide a diagram if you’d like!)
During meiosis I, homologous chromosomes may fail to separate, or during meiosis II, sister chromatids may fail to separate. This results in gametes with either an extra chromosome (n+1) or one missing (n–1). If such a gamete fuses with a normal one during fertilization, the resulting zygote will have an abnormal number of chromosomes — this is called aneuploidy (e.g., trisomy or monosomy).