Mendelian Genetics Quiz

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Questions and Answers

What does Mendel's Law of Segregation describe?

How alleles for a single trait separate during gamete formation.

Match the following generations from a monohybrid cross with their characteristics:

Parental generation = RR × rr F1 generation = All Rr F2 generation = 3:1 phenotypic ratio (round: wrinkled)

Alleles segregate during mitosis.

False

In a dihybrid cross, the phenotypic ratio of the F2 generation is _____ for round yellow, round green, wrinkled yellow, and wrinkled green.

9:3:3:1 Signup and view all the answers

What is overdominance?

The heterozygote has a superior or more exaggerated phenotype than either homozygote. Signup and view all the answers

What are isoalleles?

Alleles that have similar phenotypic effects. Signup and view all the answers

Which of the following is an example of a recessive lethal gene?

Tay-Sachs disease Signup and view all the answers

What does penetrance refer to?

The proportion of individuals with a specific genotype that express the expected phenotype Signup and view all the answers

Incomplete linkage occurs when genes produce only parental combinations.

False Signup and view all the answers

What is the significance of crossing-over?

It produces new combinations of traits. Signup and view all the answers

Which of these factors can influence crossing-over frequency?

All of the above Signup and view all the answers

What effect does X-ray irradiation have on crossing over?

Increases crossing over near centromere Signup and view all the answers

How does age affect the rate of crossing over?

Older age increases the rate of crossing over Signup and view all the answers

What are gametes formed by single crossing-over called?

Single cross-over gametes Signup and view all the answers

Single crossing-over is more frequent than double crossing-over.

True Signup and view all the answers

How many chiasmata are formed in double crossing-over?

Two chiasmata Signup and view all the answers

What is multiple crossing-over?

Crossing-over occurs at more than two points on the same chromosome pair Signup and view all the answers

Who first constructed a chromosome map?

Sturtevant Signup and view all the answers

What is the primary purpose of gene mapping?

Determine the linear order of genes on chromosomes Signup and view all the answers

What units are used to express genetic distance?

Centimorgan (cM) Signup and view all the answers

What does one map unit indicate?

The linkage distance that yields 1% recombination Signup and view all the answers

What technique is used in gene mapping?

FISH (Fluorescent In Situ Hybridization) Signup and view all the answers

What is chromosome painting?

Direct visualization of specific chromosomes using hybridized labeled whole chromosome probes Signup and view all the answers

What effect does crossing-over have on genetic variation?

It increases variation Signup and view all the answers

Study Notes

Gene Segregation and Interaction

Law of Segregation: Alleles for a single trait separate during gamete formation.
Key Observations from Mendel's Experiments:
- Organisms carry two alleles for each trait.
- Alleles segregate during meiosis.
- Alleles recombine during fertilization.
Monohybrid Cross:
- Parental generation: RR (round seeds) × rr (wrinkled seeds).
- F1 generation: All Rr (round seeds).
- F2 generation: 3:1 phenotypic ratio (round: wrinkled).

Law of Independent Assortment

Alleles of different genes segregate independently of one another during gamete formation.
Dihybrid Cross:
- Parents: RRYY (round yellow) × rryy (wrinkled green)
- F1 generation: All RrYy (round yellow)
- F2 generation: 9:3:3:1 phenotypic ratio (round yellow: round green: wrinkled yellow: wrinkled green).

Chromosomal Basis of Mendelian Laws

Segregation: Homologous chromosomes separate during meiosis I.
Independent Assortment: Chromosomes carrying different genes assort independently when forming gametes.

Dominance Relationships

Incomplete Dominance: Heterozygote phenotype is intermediate between homozygous phenotypes.
- Example: In Mirabilis jalapa, red (RR) and white (rr) flowers produce pink (Rr) flowers.
Overdominance: Heterozygote has a superior phenotype than either homozygote.
- Example: Drosophila heterozygote (w*/w) produces more fluorescent pigments than homozygous wild type (w*/w*) or mutant (w/w).
Co-Dominance: Both alleles in a heterozygote are fully expressed.
- Example: MN blood groups, a heterozygote (MN) expresses both M and N antigens.

Multiple Alleles

More than two alleles exist for a gene in a population.
- Example: ABO blood group in humans, which has three alleles (A, B, O).
Isoalleles: Alleles with similar phenotypic effects.
Mutant Isoalleles: Produce a variation from the normal phenotype.
Normal Isoalleles: Produce the typical phenotype.

Lethal Genes

Recessive Lethals: Lethal when in the homozygous recessive condition.
- Example: Tay-Sachs disease is lethal in homozygous individuals.
Dominant Lethals: Lethal in both homozygous and heterozygous states.
- Example: Huntington’s disease, symptoms cause death in heterozygous individuals.
Penetrance of Lethal Genes: Describes the extent to which a lethal gene is expressed.
Environmental Influence on Lethal Genes:
- Conditional Lethal: Lethal only under specific environmental conditions.
  - Example: Temperature-sensitive mutations in Drosophila are lethal at restrictive temperatures.

Modifier Genes

Genes that change the phenotypic effects of other genes.
- Enhancement: Increases the effect of the main gene.
- Dilution: Reduces the effect of the main gene.
- Suppressors: Mask or suppress the expression of mutant alleles.

Gene Interactions

Interactions between two or more genes influencing a single trait.
- Novel Phenotypes: Interaction between alleles produces new phenotypes.
  - Example: Comb shape in chickens has four phenotypes.
- Recessive Epistasis: A recessive allele at one locus masks the effects of another gene.
  - Example: Coat color in mice (9:3:4 ratio), where the cc genotype results in albinism.
- Dominant Epistasis: A dominant allele at one locus masks the effects of another gene.
  - Example: Summer squash color (12:3:1 ratio) or fowl feather color (13:3 ratio).
- Complementary Genes: Two genes must have dominant alleles for a particular phenotype to be expressed.
  - Example: Flower color in peas (9:7 ratio).
- Duplicate Genes: Either of two genes can produce the same phenotype.
  - Example: Seed capsule shape in Shepherd’s purse (15:1 ratio).

Pseudoalleles

Closely linked genes produce new phenotypes through recombination.
- Lewis Effect or Position Effect: Phenotype is influenced by genotype and allele position on the chromosome.

Environmental Influence on Gene Expression

Environment can significantly influence gene expression.
- Penetrance: Proportion of individuals with a specific genotype that express the expected phenotype.
  - Example: Polydactyly may not be expressed in individuals with the dominant allele.
- Expressivity: Degree to which a genotype is expressed in individuals with the same genotype.
  - Example: Individuals with polydactyly may have varying sizes of extra digits.
- Pleiotropy: A single gene affects multiple traits.
  - Example: Phenylketonuria (PKU) affects brain development and skin pigmentation.
- Phenocopy: Environmental condition mimics a genetic phenotype.
  - Example: Thalidomide exposure can cause limb malformations.
- External Environmental Factors:
  - Temperature: Coat color in Siamese cats and Himalayan rabbits.
  - Light: Light is needed for chlorophyll production in plants.
  - Nutrition: Nutritional factors can influence the expression of genes.
  - Maternal Relations: Blood type incompatibilities between mother and fetus.
- Internal Environmental Factors:
  - Age: Traits may manifest later in life, such as baldness.
  - Sex: Traits such as milk production are limited to one sex.
  - Substrates: Reactions largely depend on substrates the organism synthesizes.

Twin Studies

Compare traits between identical (monozygotic) and fraternal (dizygotic) twins.
- Concordant: Twins share the same trait.
- Discordant: One twin expresses the trait, the other does not.
Twin concordance helps determine the roles of environment and heredity.

Probability and Statistical Testing

Product Law: Probability of two independent events occurring together is the product of their individual probabilities.
Sum Law: Probability of one of two mutually exclusive events occurring is the sum of their probabilities.
Level of Significance: Degree to which observed data differ from expected results.
Chi-Square Test: Statistical method comparing observed genetic ratios with expected ratios: χ² = Σ (O - E)² / E
Binomial Distributions: Calculate the probability of specific combinations of genotypes.
Normal Distribution: Distribution of genetic traits in a population follows a bell-shaped curve as sample sizes increase.

Linkage and Recombination

Introduction: Genes located on different chromosomes are not linked.
- They assort independently, showing the classic 9:3:3:1 inheritance pattern.
Genes on the same chromosome do not assort independently:
- They are linked.
Linkage groups: All the linked genes on a chromosome.
Strength of Linkage: Directly proportional to the distance between genes.
Discovery of Genetic Linkage:
- William Bateson and Reginald Punnett (1905) studied sweet peas, particularly flower color and pollen shape.
- They observed that linked alleles did not follow the 9:3:3:1 pattern and supported the idea that they were linked.

Why Linkage?

Linkage groups organize genes for coordinated transmission from one generation to the next.

Types of Linkage

Complete Linkage: Genes are consistently inherited together in their parental combinations.
Incomplete Linkage: Genes produce some percentage of non-parental combinations (due to occasional breakage during crossing over).

Significance of Linkage

Reduces the chance of recombination and helps maintain parental characters.
Makes it difficult for breeders to combine desirable characters in one variety.

Autosomal and Sex Linkage

Autosomal Linkage: Linked genes are present on autosomes.
Sex Linkage: Linked genes are present on sex chromosomes.

Crossing-Over and the Inheritance of Linked Genes

Crossing-over: Exchange of DNA segments between homologous chromosomes during prophase I of meiosis.
- It can separate linked genes, creating recombinant gametes.
Parental Gametes: Maintain the original linkage of genes.
Recombinant Gametes: Original linkage is undone due to crossing over.

Significance of Crossing-Over

Produces new combinations of traits.
Provides genetic variation and plays a role in breeding.

Theories of Crossing-Over

Contact-First Theory: Inner two chromatids touch first, break, and then re-unite.
Breakage-First Theory: Chromatids break first, then the broken segments re-unite to form new combinations.

Factors Affecting Crossing-Over

Sex: Crossing over is often reduced in male mammals.
Mutation: Mutations can reduce crossing over.
Temperature: High or low temperatures can increase crossing over.
X-ray Effect: X-ray irradiation can increase crossing over near centromere.
Age: Older age can increase the rate of crossing-over.

Types of Crossing-Over

Single Crossing-Over: Only one chromatid of each chromosome is involved and one chiasma is formed.
- Produces single cross-over gametes.
Double Crossing-Over: Two or more chromatids are involved and two or more chiasmata are formed.
- Produces double cross-over gametes.
Multiple Crossing-Over: More than two chiasmata are formed.

Multiple Crossing-Over

Occurs when more than two chiasmata are formed on a chromosome pair.
Can be triple (3 chiasmata), quadruple (4 chiasmata), and so on.
A rare phenomenon.

Crossing-Over

Provides evidence for the linear arrangement of genes on chromosomes.
Allows for chromosome mapping.
Creates new gene combinations, resulting in variations in offspring.

Chromosome Mapping

Sturtevant (1913) constructed the first chromosome map, demonstrating the position of genes on the X chromosome.
Also known as genetic maps.

Gene Mapping

Involves analysing gene loci on the genome, revealing the linear order of genes on chromosomes.
Two types: Physical Maps and Genetic Maps.

Physical Maps

Assign loci to chromosomes based on:
Somatic cell hybrid panels
In situ hybridization
Comparative mapping.
Coordinate using chromosome regions or bands.
Measure distances between loci in kilobases.

Genetic Maps

Constructed by studying meiotic recombination through linkage analysis.
Reveal the genetic distance of loci, based on the frequency of crossing-over during recombination.
Provide an ordered array of sequence tagged sites along chromosomes.

Genetic Distance

Measured in centimorgans (cM).
1 cM equals 1% crossing over, approximately equivalent to 1000 kb.
Two loci with a 1% recombination rate are 1 cM apart.

Map Unit

Represents the linkage distance producing 1% recombination.

Linkage Group

When mapping numerous genes in a species, genes are observed to occur in linkage groups.
Each linkage group corresponds to a chromosome pair.

FISH (Fluorescent In Situ Hybridization)

A rapid and effective gene mapping technique.
Uses probes labeled with fluorochrome dyes, emitting different colors under UV light.
Probe location is visualized using an epifluorescence microscope.

Chromosome Painting

A FISH technique application for visualizing specific chromosomes.
Utilizes whole chromosome specific probes called "paints".
Allows direct visualization of specific chromosomes using fluorescent detection.

Chromosome Painting Applications

Improves accuracy in cytogenetic studies.
Identifies species-specific chromosomes in somatic cell hybrids.
Detects chromosomal rearrangements or abnormalities.

Recap: Linkage

The tendency for genes on a chromosome to stay together and be passed on as a unit to the next generation.
Results in more parental types of offspring, as genes are less likely to separate.
Strength of linkage increases with closer gene proximity.
Helps maintain newly improved varieties.

Recap: Crossing-Over

Exchange of genes or chromosome parts, breaking established linkage and forming new combinations.
Produces recombination.
Frequency of crossing-over decreases with genes being closely placed.
Acts as a source of variation for new varieties.

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Description

Test your knowledge on Mendel's laws and genetic concepts such as segregation, crossing-over, and chiasmata. This quiz covers key principles from Mendelian genetics and their implications on hereditary traits. Match generations from monohybrid crosses with their characteristics and explore advanced topics like overdominance and penetrance.