16.04 Genetic Variation
1. Understanding Genetic Variation
- Definition:
- Genetic Variation: The differences in alleles among individuals within a population, leading to diverse traits in offspring.
- Importance:
- Drives evolution and natural selection.
- Enables populations to adapt to changing environments.
- Increases survival and reproductive success through diverse traits.
2. Key Terms
| Term | Definition |
|---|---|
| Locus (plural: loci) | The specific physical location of a gene on a chromosome. |
| Allele | Different forms of a gene that can produce variations in a trait. |
| Homologous Chromosomes | Pairs of chromosomes containing the same genes at the same loci but possibly different alleles. |
| Gamete | A reproductive cell (sperm or egg) containing half the genetic information of an organism. |
| Chiasmata | Points where homologous chromosomes exchange genetic material during crossing over. |
3. Example Organism: Drosophila melanogaster (Fruit Fly)
- Chromosome Pairs: 5 pairs of chromosomes.
- Example Loci on Chromosome 2:
- Antenna Length:
- Allele E: Long antennae
- Allele e: Short antennae
- Body Color:
- Allele G: Grey
- Allele g: Ebony
- Wing Length:
- Allele W: Normal wings
- Allele w: Vestigial wings
- Eye Color:
- Allele R: Red eyes
- Allele r: Brown eyes
- Antenna Length:
4. Mechanisms Producing Genetic Variation
Genetic variation arises through three primary mechanisms:
- Crossing Over
- Independent Assortment
- Random Fertilization
4.1. Crossing Over
- Definition:
- A process during Prophase I of meiosis where homologous chromosomes exchange segments of their chromatids at points called chiasmata.
- Process:
- Pairing: Homologous chromosomes pair up during Prophase I.
- Exchange: Segments of chromatids are exchanged between non-sister chromatids.
- Result: New combinations of alleles on each chromosome, increasing genetic diversity.
- Example: Crossing Over in Drosophila melanogaster
- Original Chromosome Pair:
- Chromosome 1: Alleles E (long antennae) and R (red eyes)
- Chromosome 2: Alleles e (short antennae) and r (brown eyes)
- Without Crossing Over:
- Gametes Produced:
- Gamete 1: E and R
- Gamete 2: e and r
- Gametes Produced:
- With Crossing Over:
- Chromosome 1: E and r
- Chromosome 2: e and R
- Gametes Produced:
- Gamete 1: E and R
- Gamete 2: e and r
- Gamete 3: E and r
- Gamete 4: e and R
- Original Chromosome Pair:
- Significance:
- Generates new allele combinations, enhancing genetic diversity.
4.2. Independent Assortment
- Definition:
- The random distribution of homologous chromosome pairs during Metaphase I of meiosis, leading to varied combinations of chromosomes—and thus alleles—in gametes.
- Process:
- Alignment: Homologous chromosome pairs line up randomly at the cell’s equator during Metaphase I.
- Separation: Each pair separates independently during Anaphase I.
- Result: Random combination of maternal and paternal chromosomes in each gamete.
- Example: Two Chromosome Pairs
- Chromosome Pair 1: Alleles A/a
- Chromosome Pair 2: Alleles D/d
- Possible Orientations:
- Orientation 1: A and D toward one pole; a and d toward the opposite pole.
- Orientation 2: A and d toward one pole; a and D toward the opposite pole.
- Resulting Gametes:
- Gamete 1: A and D
- Gamete 2: A and d
- Gamete 3: a and D
- Gamete 4: a and d
- Calculation of Genetic Variability:
- Formula: Number of combinations = 2^n, where n is the haploid number of chromosome pairs.
- Example (Humans):
- Haploid Number (n): 23
- Possible Combinations: 2^23 = 8,388,608
- Significance:
- Contributes significantly to the genetic diversity of offspring.
- When combined with crossing over, it exponentially increases genetic variability.
4.3. Random Fertilization
- Definition:
- The random fusion of male and female gametes during fertilization, further increasing genetic variation.
- Process:
- Diverse Gametes: Due to crossing over and independent assortment, each gamete is genetically unique.
- Random Pairing: Any sperm can fertilize any egg, leading to countless possible zygote combinations.
- Example:
- Humans: With approximately 8.4 million gamete combinations from independent assortment alone, random fertilization results in around 70 trillion (8.4 million × 8.4 million) potential genetic combinations.
- Significance:
- Ensures each offspring has a unique genetic makeup.
- Enhances population diversity, providing a robust genetic pool for adaptation and evolution.
5. Summary of Genetic Variation Sources
| Source of Variation | Process | Effect |
|---|---|---|
| Crossing Over | Exchange of alleles between chromatids during Prophase I of meiosis | New allele combinations on each chromosome |
| Independent Assortment | Random alignment and separation of homologous chromosomes during Metaphase I and Anaphase I | Unique combinations of chromosomes in gametes |
| Random Fertilization | Any sperm can fuse with any egg | Diverse genetic combinations in zygotes |
- Overall Significance: These mechanisms collectively ensure extensive genetic variation in sexually reproducing populations, providing the raw material for natural selection and evolution.
6. Diagrams:
- Figure 1:Drosophila melanogaster Chromosome Structure
- Description: Diagram showing homologous chromosome pairs with different alleles at specific loci.
- Figure 2: Crossing Over Process
- Description: Illustration of homologous chromosomes exchanging segments during Prophase I.
- Figure 3: Independent Assortment
- Description: Diagram depicting random alignment of homologous chromosome pairs during Metaphase I.
- Figure 4: Gamete Formation with and without Crossing Over
- Description: Comparison of gametes produced with and without crossing over events.
7. Discussion Questions
- Explain how crossing over contributes to genetic variation.
- Answer: Crossing over exchanges genetic material between homologous chromosomes during Prophase I of meiosis, creating new allele combinations on each chromosome. This results in gametes with unique genetic profiles, increasing diversity in offspring.
- Describe the process of independent assortment and its impact on genetic diversity.
- Answer: Independent assortment occurs during Metaphase I of meiosis when homologous chromosome pairs align randomly at the cell’s equator. This random alignment leads to the separation of different chromosome pairs independently of one another, resulting in numerous possible combinations of chromosomes in gametes and thereby enhancing genetic diversity.
- Why is random fertilization important for genetic variation?
- Answer: Random fertilization ensures that any sperm can fertilize any egg, combining two unique sets of genetic information. This unpredictability further increases the number of possible genetic combinations in offspring, contributing significantly to genetic variation within a population.
Practice Questions
Question 1
Define genetic variation and explain why it is important for a population.
Answer 1
Definition of Genetic Variation: Genetic variation refers to the differences in alleles among individuals within a population, leading to diverse traits in offspring.
Importance of Genetic Variation:
- Drives Evolution and Natural Selection: Genetic variation provides the raw material for evolution. Natural selection acts on these variations, favoring beneficial traits that enhance survival and reproduction.
- Enables Adaptation to Changing Environments: Diverse traits within a population increase the likelihood that some individuals possess characteristics suited to new or altered environmental conditions, facilitating adaptation.
- Increases Survival and Reproductive Success: A variety of traits can improve the overall fitness of the population, as different traits may confer advantages under different circumstances, enhancing the population’s resilience and reproductive success.
Marking Scheme for Question 1
| Criteria | Marks |
|---|---|
| Definition of genetic variation | 2 |
| – Correctly defines genetic variation | |
| Importance of genetic variation | 3 |
| – Drives evolution and natural selection | |
| – Enables adaptation to changing environments | |
| – Increases survival and reproductive success | |
| Total | 5 |
Question 2
Match the following key terms with their correct definitions.
Terms: a) Locus
b) Allele
c) Homologous Chromosomes
d) Gamete
e) Chiasmata
Definitions:
- Different forms of a gene that can produce variations in a trait.
- The specific physical location of a gene on a chromosome.
- Pairs of chromosomes containing the same genes at the same loci but possibly different alleles.
- A reproductive cell containing half the genetic information of an organism.
- Points where homologous chromosomes exchange genetic material during crossing over.
Answer 2
| Terms | Definitions |
|---|---|
| a) Locus | 2. The specific physical location of a gene on a chromosome. |
| b) Allele | 1. Different forms of a gene that can produce variations in a trait. |
| c) Homologous Chromosomes | 3. Pairs of chromosomes containing the same genes at the same loci but possibly different alleles. |
| d) Gamete | 4. A reproductive cell containing half the genetic information of an organism. |
| e) Chiasmata | 5. Points where homologous chromosomes exchange genetic material during crossing over. |
Marking Scheme for Question 2
| Correct Match | Marks |
|---|---|
| a) – 2 | 1 |
| b) – 1 | 1 |
| c) – 3 | 1 |
| d) – 4 | 1 |
| e) – 5 | 1 |
| Total | 5 |
Each correct match is awarded 1 mark.
Question 3
Explain the process of crossing over and its significance in genetic variation.
Answer 3
Process of Crossing Over:
- Pairing of Homologous Chromosomes: During Prophase I of meiosis, homologous chromosomes pair up closely.
- Exchange of Genetic Material: At points called chiasmata, non-sister chromatids exchange segments of their genetic material.
- Formation of New Allele Combinations: This exchange results in chromosomes that have new combinations of alleles, different from those in the parent chromosomes.
Significance of Crossing Over:
- Increases Genetic Diversity: By creating new allele combinations, crossing over ensures that each gamete is genetically unique.
- Enhances Evolutionary Potential: Greater genetic diversity within a population provides more variation for natural selection to act upon, facilitating adaptation and evolution.
Marking Scheme for Question 3
| Criteria | Marks |
|---|---|
| Definition of crossing over | 1 |
| Description of the process | 2 |
| – Pairing of homologous chromosomes | |
| – Exchange of chromatids | |
| – Resulting in new allele combinations | |
| Significance in genetic variation | 2 |
| – Increases genetic diversity | |
| – Enhances evolutionary potential | |
| Total | 5 |
Question 4
Using Drosophila melanogaster, describe how crossing over can result in new allele combinations for antenna length and body color.
Answer 4
Original Allele Arrangement:
- Chromosome 1: Allele E (long antennae) and R (red eyes)
- Chromosome 2: Allele e (short antennae) and r (brown eyes)
Process of Crossing Over:
- Pairing of Homologous Chromosomes: During Prophase I, homologous chromosomes in Drosophila melanogaster pair up.
- Exchange of Genetic Material: Segments containing the alleles for antenna length and body color are exchanged between non-sister chromatids at chiasmata.
New Allele Combinations After Crossing Over:
- Chromosome 1: E (long antennae) and r (brown eyes)
- Chromosome 2: e (short antennae) and R (red eyes)
Resulting Gametes:
- Gamete 1: E and R
- Gamete 2: e and r
- Gamete 3: E and r
- Gamete 4: e and R
This exchange creates new combinations of alleles, leading to increased genetic diversity in the offspring.
Marking Scheme for Question 4
| Criteria | Marks |
|---|---|
| Reference to Drosophila melanogaster | 0.5 |
| Original allele arrangement | 1 |
| Description of crossing over process | 1 |
| New allele combinations post crossing over | 2 |
| Listing resulting gametes | 1.5 |
| Total | 6 |
Partial marks awarded for incomplete but relevant information.
Question 5
What is independent assortment and how does it contribute to genetic diversity? Provide an example using two chromosome pairs.
Answer 5
Definition of Independent Assortment: Independent assortment is the random distribution of homologous chromosome pairs during Metaphase I of meiosis, leading to varied combinations of chromosomes—and thus alleles—in gametes.
Contribution to Genetic Diversity:
- Random Alignment: Homologous chromosome pairs align randomly at the cell’s equator, ensuring that each gamete receives a mix of maternal and paternal chromosomes.
- Increased Combinations: This randomness results in numerous possible genetic combinations in gametes, enhancing the overall genetic diversity of the population.
Example Using Two Chromosome Pairs:
- Chromosome Pair 1: Alleles A and a
- Chromosome Pair 2: Alleles D and d
Possible Orientations:
- Orientation 1: A and D toward one pole; a and d toward the opposite pole.
- Orientation 2: A and d toward one pole; a and D toward the opposite pole.
Resulting Gametes:
- Gamete 1: A and D
- Gamete 2: A and d
- Gamete 3: a and D
- Gamete 4: a and d
This example illustrates how independent assortment of two chromosome pairs can produce four different gamete combinations, thereby increasing genetic diversity.
Marking Scheme for Question 5
| Criteria | Marks |
|---|---|
| Definition of independent assortment | 1 |
| Explanation of contribution to diversity | 2 |
| Example using two chromosome pairs | 3 |
| – Description of chromosome pairs | |
| – Possible orientations | |
| – Resulting gametes | |
| Total | 6 |
Each component must be addressed for full marks.
Question 6
Calculate the number of possible gamete combinations in a human considering independent assortment alone.
Answer 6
Calculation:
- Haploid Number (n) in Humans: 23
- Number of Combinations: Using the formula 2^n
- 223=8,388,6082^{23} = 8,388,608223=8,388,608 possible gamete combinations
Therefore, a human can produce approximately 8,388,608 different gametes through independent assortment alone.
Marking Scheme for Question 6
| Criteria | Marks |
|---|---|
| State haploid number for humans (23) | 1 |
| Apply formula 2^n correctly | 1 |
| Provide correct final number (8,388,608) | 1 |
| Total | 3 |
Full marks awarded for accurate calculation and correct final answer.
Question 7
Describe the process of random fertilization and its role in increasing genetic variation.
Answer 7
Process of Random Fertilization:
- Diverse Gametes: Due to mechanisms like crossing over and independent assortment, each gamete produced is genetically unique.
- Random Pairing: Any sperm cell can fuse with any egg cell during fertilization, regardless of their genetic makeup.
- Formation of Zygote: The fusion results in a zygote with a unique combination of genetic information from both parents.
Role in Increasing Genetic Variation:
- Enhanced Combinations: Random fertilization combines two unique sets of alleles, exponentially increasing the number of possible genetic combinations in the offspring.
- Unique Genetic Makeup: Each offspring inherits a distinct genetic profile, contributing to the overall genetic diversity within the population.
- Population Diversity: This diversity provides a robust genetic pool, enhancing the population’s ability to adapt to environmental changes and resist diseases.
Marking Scheme for Question 7
| Criteria | Marks |
|---|---|
| Definition of random fertilization | 1 |
| Description of the process | 2 |
| – Diverse gametes due to crossing over and independent assortment | |
| – Random pairing of gametes | |
| Explanation of role in genetic variation | 2 |
| – Increased combinations | |
| – Unique genetic makeup of offspring | |
| Total | 5 |
All parts must be covered for full marks.
Question 8
In the context of Drosophila melanogaster, list the alleles for antenna length and body color on chromosome 2.
Answer 8
Antenna Length on Chromosome 2:
- E: Long antennae
- e: Short antennae
Body Color on Chromosome 2:
- G: Grey
- g: Ebony
Marking Scheme for Question 8
| Criteria | Marks |
|---|---|
| List alleles for antenna length | 1 |
| – E: Long antennae | |
| – e: Short antennae | |
| List alleles for body color | 1 |
| – G: Grey | |
| – g: Ebony | |
| Total | 2 |
Each correct allele listed for both traits awards 0.5 marks.
Question 9
Explain how genetic variation enables populations to adapt to changing environments.
Answer 9
Genetic Variation: Genetic variation refers to the differences in alleles among individuals within a population, resulting in diverse traits.
Adaptation to Changing Environments:
- Range of Traits: A population with high genetic variation possesses a wide array of traits, some of which may be advantageous under new environmental conditions.
- Natural Selection: Environmental changes may favor individuals with certain traits. Those individuals are more likely to survive and reproduce, passing their advantageous alleles to the next generation.
- Evolution of the Population: Over time, beneficial traits become more common within the population, enhancing its ability to thrive in the altered environment.
Overall Significance: Genetic variation provides the necessary diversity for natural selection to act upon, ensuring that populations can continuously adapt and evolve in response to environmental challenges, thereby increasing their chances of survival and reproductive success.
Marking Scheme for Question 9
| Criteria | Marks |
|---|---|
| Define genetic variation | 1 |
| Explain adaptation to environments | 2 |
| – Range of traits providing diverse responses | |
| – Natural selection favoring advantageous traits | |
| Link to natural selection and evolution | 1 |
| Overall significance for population | 1 |
| Total | 5 |
Each component must be thoroughly addressed for full marks.
Question 10
Differentiate between crossing over and independent assortment as mechanisms of genetic variation.
Answer 10
Crossing Over:
- Definition: Crossing over is the exchange of genetic material between homologous chromosomes during Prophase I of meiosis.
- Mechanism: It involves the recombination of alleles on the same chromosome, resulting in new allele combinations within a single chromosome.
- Outcome: Creates genetic diversity by producing chromosomes with novel allele combinations that were not present in the parent.
Independent Assortment:
- Definition: Independent assortment is the random distribution of homologous chromosome pairs during Metaphase I of meiosis.
- Mechanism: It involves the separation of different chromosome pairs independently of one another, leading to various combinations of chromosomes in the gametes.
- Outcome: Increases genetic diversity by producing gametes with different combinations of chromosomes, each containing a unique set of alleles.
Key Differences:
- Location in Meiosis: Crossing over occurs during Prophase I, while independent assortment occurs during Metaphase I.
- Type of Genetic Variation: Crossing over affects the genetic makeup within a single chromosome, whereas independent assortment affects the combination of different chromosomes in gametes.
- Resulting Diversity: Crossing over creates new allele combinations on the same chromosome, while independent assortment generates diverse combinations of entire chromosomes.
Marking Scheme for Question 10
| Criteria | Marks |
|---|---|
| Define crossing over | 1 |
| Define independent assortment | 1 |
| Differentiate mechanisms | 2 |
| – Crossing over involves recombination of alleles on the same chromosome | |
| – Independent assortment involves random distribution of different chromosomes | |
| Total | 4 |
Complete differentiation between the two mechanisms is required for full marks.