Chapter 8: Origins of Genetic Variation Flashcards
8.1 Origins of genetic variation
What is a major source of variation?
Meiosis processes and Mutation
8.1 Origins of genetic variation
What is independent assortement?
- Chromosome pairs from both parents are distributed randomly into gametes.
- Results in new combinations of alleles and increases genetic variation.
8.1 Origins of genetic variation
What is crossing over?
- Occurs during meiosis when chromatids exchange segments at the chiasmata.
- Produces new allele combinations, further increasing genetic diversity.
8.2 Transfer of genetic information
Define mutation and the different types of mutations?
- A permanent change in DNA sequence.
Types of Mutations:
* Point Mutations: Affect one or a few nucleotides (substitutions, insertions, deletions).
* Chromosomal Mutations: Involve structural changes in chromosomes.
* Chromosome Number Mutations: Gain or loss of entire chromosomes, often due to meiotic errors.
8.2 Transfer of genetic information
How can random fertilisation act as a form of genetic variation?
- Male and female gametes fuse randomly, leading to unique genetic combinations.
- Males produce more gametes than females, increasing the randomness.
- In humans, multiple sperm compete to fertilise an ovum, ensuring variation.
8.2 Transfer of genetic information
Define the following: Phenotype, genotype, chromosomes, alleles, homozygous, heterozygous, dominant and recessive
- Phenotype: Physical characteristics of an organism, determined by genes and the environment.
- Genotype: Genetic makeup of an organism, inherited from both parents.
- Chromosomes: Contain genes; humans have 46 chromosomes (23 pairs).
- Alleles: Different versions of a gene.
- Homozygous: Two identical alleles for a characteristic (TT or tt).
- Heterozygous: Two different alleles for a characteristic (Tt).
- Dominant Allele: Expressed even if only one copy is present (capital letter, e.g., “T”).
- Recessive Allele: Only expressed if two copies are present (lowercase letter, e.g., “t”).
8.2 Transfer of genetic information
Describe monohybrid cross
Involves the inheritance of one characteristic controlled by a single gene.
Punnett Squares: Used to predict offspring genotypes and phenotypes.
F₁ Generation: Offspring of two homozygous parents (one dominant, one recessive) → all heterozygous (Tt).
F₂ Generation: Crossing two F₁ individuals results in a 3:1 phenotypic ratio (dominant:recessive).
8.2 Transfer of genetic information
What are polygenic traits?
Traits (e.g., eye color, height) that are controlled by multiple genes.
Polygenic inheritance leads to a wide range of phenotypes.
8.2 Transfer of genetic information
What are issues with conducting the test cross to determine alleles?
Theoretical genetic ratios may not always appear due to:
Chance: Random combination of alleles in gametes.
Small Sample Size: Fewer offspring make ratios less reliable.
Inefficient Sampling: Some embryos may not survive.
Large sample sizes improve accuracy in observing expected ratios.
8.2 Transfer of genetic information
What is Mendel’s first law?
- Each trait is controlled by two alleles, one from each parent.
- Alleles separate during gamete formation.
8.2 Transfer of genetic information
What is Mendel’s second law?
- Different traits are inherited independently (exceptions exist for linked genes).
- This is also referred to as the “law of independent assortement”
8.2 Transfer of genetic information
What is codominance in terms of multiple alleles expresssed?
- Some traits have more than two possible alleles.
- Example: ABO blood group system (A, B, and O alleles).
- Even though an individual can only inherit two alleles, multiple alleles exist in a population.
- Codominance: when two dominant allele are both expressed alongside each other
In blood, A and B are both dominant while O is recessive.
8.2 Transfer of genetic information
Describe dihybrid inheritance
The inheritance of two non-interacting (unlinked) genes following Mendel’s law of independent assortment.
8.2 Transfer of genetic information
What is gene linkage?
Gene linkage occurs when alleles of two genes are located on the same chromosome and inherited together as a unit.
8.2 Transfer of genetic information
How can gene linkage affect the inheritance?
- Genes on the same chromosome do not assort independently unless crossing over occurs.
- The closer the genes are, the less likely they will be separated by recombination.
- Evidence of Linkage: When recombinant phenotypes appear less frequently than expected, it suggests the genes are linked.
8.2 Transfer of genetic information
What is the chi-squared test?
- Used to compare observed vs. expected ratios.
- Helps determine if deviations are due to chance or a biological factor.
8.2 Transfer of genetic information
What causes abnormal ratios in dihybrid inheritance?
- In dihybrid inheritance, we usually assume that genes assort independently.
- However, sometimes, expected ratios (9:3:3:1) do not appear in F₂ crosses.
- This happens when two genes are located on the same chromosome and are inherited together. This is called gene linkage.
8.2 Transfer of genetic information
What is the formula for the chi squared test?
8.2 Transfer of genetic information
Describe the example of gene linkage in drosophila?
Parental Generation:
* BBLL (broad abdomen, long wings) × bbll (narrow abdomen, vestigial wings).
F₁ Generation:
* All offspring are BbLl (heterozygous for both traits), showing dominant phenotypes (broad abdomen, long wings).
* Expected F₂ Ratio (if independent assortment occurs): 9:3:3:1
* Actual F₂ Ratio: 3:1, showing that the genes are inherited together rather than assorting independently.
8.2 Transfer of genetic information
What is a chromosome mapping?
Determines the distance between linked genes based on the frequency of recombination.
8.2 Transfer of genetic information
Define linked genes
- Inherited as a unit, meaning fewer recombinant offspring.
- The closer the genes are, the lower the crossover frequency.
8.2 Transfer of genetic information
What is the formula of the crossover value?
8.2 Transfer of genetic information
How does distance of the genes determine crossover value?
Genes close together → few recombination events → low crossover value.
Genes farther apart → more recombination events → high crossover value.
8.2 Transfer of genetic information
What is sex linkage?
Some traits are linked to genes found on sex chromosomes (X and Y) rather than autosomes.
8.2 Transfer of genetic information
Why are males more succeptible to sex linked disorders?
- Females: XX, Males: XY.
- Males inherit their X chromosome from their mother and Y chromosome from their father.
- Since males have only one X chromosome, X-linked recessive traits appear more often in males.
8.2 Transfer of genetic information
What was the experiment involving sex linkage and drosophila?
Drosophila Eye Color Experiment (by Thomas Morgan in the early 20th century):
Eye color is X-linked.
Red eyes (dominant) vs. White eyes (recessive).
Male flies inherit their eye color from their mother because they only have one X chromosome.
Other X-linked traits in humans:
Hemophilia (blood clotting disorder).
Color blindness.
8.2 Transfer of genetic information
Why are females often carriers?
Females (XX) can be carriers without expressing the disorder. (One X can mask the recessive allele on another X)
8.2 Transfer of genetic information
Describe what causes sickle cell anemia
- Haemoglobin Structure (responsible for oxygen transport in the blood) is affected by multiple genes.
- Mutations in any of these genes can disrupt oxygen transport, leading to sickle cell anemia.
8.2 Transfer of genetic information
What is albinism?
- A condition where melanin pigment is not produced due to a mutation in a single gene.
- Recessive trait: Both parents can be carriers without showing symptoms.
- Affects skin, hair, and eye pigmentation.
- Vision problems and higher risk of skin cancer due to lack of protective pigment.
8.2 Transfer of genetic information
What causes albinism?
Albinism is caused by mutations in genes that produce melanin.
8.2 Transfer of genetic information
What is the purpose of a pedigree diagram?
Pedigree diagrams show how traits are inherited across multiple generations in a family.
* Helps track genetic conditions over time.
* Predicts carrier status for recessive mutations.
* Identifies sex-linked traits by distinguishing between male and female inheritance patterns.
8.2 Transfer of genetic information
What does each parent pass on in terms of sex chromosomes?
- Mothers always pass an X chromosome to their offspring.
- Fathers pass an X chromosome to daughters and a Y chromosome to sons.
8.2 Transfer of genetic information
What causes red green colourblindness and is it sex linked?
- Caused by mutations in genes that code for light-sensitive cells (cones) in the retina.
- These genes are located on the X chromosome, making the condition sex-linked.
8.2 Transfer of genetic information
What are the symptoms of haemophilia?
Caused by a defect in a protein required for blood clotting (Factor VIII deficiency).
Symptoms:
Excessive bleeding from minor injuries.
Internal bleeding in joints and muscles.
Potentially fatal if untreated.
8.2 Transfer of genetic information
What is haemophilia?
A sex-linked blood disorder where Factor VIII deficiency prevents normal blood clotting.
8.3 Gene pools
What is a population?
A population is a group of individuals of the same species occupying a particular habitat.
8.3 Gene pools
What is a gene pool?
The total sum of all the alleles in a population at a given time.
8.3 Gene pools
Define genetic variation in terms of a gene pool
- A gene pool contains multiple alleles for a particular trait.
- Evolution is a permanent change in allele frequencies over time.
- Some alleles become more common, while others become less common or disappear.
8.3 Gene pools
What is allele frequency?
Allele frequency refers to how often a particular allele appears in a population. Changes in allele frequency occur due to natural selection and adaptation.
8.3 Gene pools
What is used to calculate normal allele frequency?
Measured as a decimal fraction (0 to 1).
* The dominant allele frequency is represented by “p”.
* The recessive allele frequency is represented by “q”.
The total of both must always equal 1:
* p+q=1
8.3 Gene pools
Outline the Hardy-Weinberg equation
A theoretical model stating that allele frequencies remain constant in a population that is not evolving (under specific conditions).
If allele frequencies change over time, the population is evolving.
Mathematical Expression:
p^2+2pq+q^2=1
Where:
p² = frequency of homozygous dominant (AA) individuals
2pq = frequency of heterozygous (Aa) individuals
q² = frequency of homozygous recessive (aa) individuals
8.3 Gene pools
What makes the Hardy Weinberg equation useful?
Recessive phenotypes are easier to observe.
By measuring the frequency of recessive individuals (q²), we can calculate other allele frequencies in the population.
8.3 Gene pools
To ensure that Hardy-Weinberg can be used, what assumptions are made?
- No mutations.
- Random mating.
- Large population size.
- No migration (population is isolated).
- No selection pressure (all individuals are equally likely to reproduce).
8.3 Gene pools
What two equations are required to calculate the frequency of p and q?
8.3 Gene pools
How do the following 5 factors affect the Hardy Weinberg equilibrium?
a) mutation
b) non-random mating
c) population of varying sizes
d) isolation
e) selection pressure
Mutations:
Introduce new alleles by altering genetic material.
Most are neutral or harmful, but some may confer advantages and influence allele frequencies.
Non-Random Mating:
Random mating maintains constant allele frequencies.
Non-random mating (e.g., sexual selection) increases the probability of certain alleles being passed on.
Populations of Varying Sizes:
Small populations: Genetic drift significantly impacts allele frequencies, leading to a loss of rare alleles.
Large populations: Allele frequencies are stable, with less influence from random events.
Isolation:
Prevents gene flow, maintaining allele frequencies.
Migration disrupts equilibrium by introducing or altering allele frequencies.
Selection Pressure:
Alleles that provide a survival or reproductive advantage increase in frequency over time.
8.3 Gene pools
What is a population bottleneck?
A dramatic reduction in population size due to environmental disasters, diseases, overhunting, or habitat destruction.
8.3 Gene pools
What are the consequences of a population bottle neck?
Reduction in Gene Pool: Many alleles are lost, shrinking the gene pool and reducing genetic diversity.
Vulnerability: The small remaining population is more susceptible to the complete loss of certain alleles, potentially magnifying the effects of mutations or genetic drift.
Over time, the population may recover in size but have a significantly altered genetic composition, often distinct from the original population.
8.3 Gene pools
Describe the founder effect and its main impacts
The loss of genetic variation that occurs when a small group of individuals leaves a larger population to form a new population.
The new population’s allele frequencies are unlikely to match those of the original population.
Rare alleles in the founder members may become amplified in the new population.
8.3 Gene pools
What are the consequences of the founder effect?
- Reduced genetic diversity in the new population.
- Specific alleles may dominate, even if they were rare in the original population.
8.3 Gene pools
Describe stabilizing selection
Definition: Reduces variation in a population by selecting for phenotypes close to the average and against extreme variations.
Outcome: Allele frequencies stabilize over time as the environment favors certain traits.
8.3 Gene pools
Describe directional selection
Favors one extreme phenotype due to environmental pressure, leading to a shift in allele frequencies toward that trait.
Outcome: Shifts in allele frequencies over generations, adapting the population to new conditions.
8.3 Gene pools
Describe disruptive selection
Definition: Favors individuals with extreme traits over those with intermediate traits, leading to increased diversity.
Outcome: The population evolves into subgroups with distinct traits, potentially leading to speciation.
8.3 Gene pools
What is genetic drift and its effects?
Definition:
Random changes in allele frequencies in a population, not caused by natural selection.
Occurs when certain alleles are passed on by chance.
Particularly significant in small populations where alleles can be lost entirely.
Mechanism:
Allele frequencies may increase or decrease due to random reproduction, not because they confer an advantage.
Can lead to a reduction in genetic diversity over time.
Effects:
Genetic drift plays an important role in bottlenecked populations or populations affected by the founder effect.
