AP Biology Chapter 14: Mendel and Genes

Questions and Answers

Explain the concept of blending, and then describe how Mendel's 'particulate' gene hypothesis was different.

Blending is the idea that the genetic material of the two parents mix in such a way that over generations a uniform population arises. Mendel's hypothesis states that parents pass on discrete heritable units, genes, that retain their separate identities in offspring.

Explain how using pea plants allowed Mendel to control mating.

The reproductive parts of pea plants are enclosed in a flower, which hinders cross-pollination, allowing mostly self-fertilization. Mendel manipulated this by removing immature stamens and cross-pollinating plants.

What is the difference between a character and a trait?

A character is a heritable factor that varies among individuals, while traits are the variants of a character.

Define P generation, F₁ generation, and F₂ generation.

P generation refers to true breeding parents in a genetic cross. F₁ generation is the offspring of the P generation, and F₂ generation is the offspring of the F₁ generation. Signup and view all the answers

Explain how Mendel's simple cross of purple and white flowers refuted blending.

If blending were true, the F₁ hybrids would have pale purple flowers, but all F₁ offspring had purple flowers. The white flower trait reappeared in the F₂ generation. Signup and view all the answers

Explain how Mendel's simple cross determined dominant and recessive characteristics.

Mendel reasoned that the heritable factor for white flowers did not disappear in the F₁ plants but was masked by the purple flower factor, establishing purple as dominant and white as recessive. Signup and view all the answers

In sexually reproducing organisms, why are there exactly two chromosomes in each homologous pair?

One chromosome is paternal, and one is maternal. Signup and view all the answers

Describe each of Mendel's four concepts in his model.

<ol> <li>Alternative versions of genes cause variation in inherited characters. 2. For each character, every organism inherits two copies of a gene, one from each parent. 3. If the two alleles differ, one, the dominant allele, dictates appearance; the other, the recessive allele, does not. 4. (Law of segregation) The two alleles separate during gamete formation, ending in different gametes.</li> </ol> Signup and view all the answers

What is the F₂ phenotypic and genotypic ratio for a PP x pp cross?

Phenotypic ratio: 3:1, Genotypic ratio: 1:2:1. Signup and view all the answers

Which generation is completely heterozygous in a genetic cross?

F₁ generation. Signup and view all the answers

Which generation has both heterozygous and homozygous offspring?

F₂ generation. Signup and view all the answers

Demonstrate how to determine if a tall pea plant is homozygous tall or heterozygous tall using a test cross.

Breed the tall plant with a short homozygous plant. If all F₁ offspring are tall, the mystery plant is homozygous; if 1/2 are tall and 1/2 are short, it is heterozygous. Signup and view all the answers

Explain the difference between a monohybrid cross and a dihybrid cross.

A monohybrid cross studies one character, while a dihybrid cross studies two characters. Signup and view all the answers

Explain Mendel's Law of Independent Assortment.

Each pair of alleles segregates independently during gamete formation. Signup and view all the answers

In probability, what is an independent event?

An independent event is one whose outcome is unaffected by previous trials. Signup and view all the answers

State the multiplication rule and give an original example.

The multiplication rule states that the probability of two independent events occurring together is the product of their probabilities. For example, the probability of two coin tosses resulting in heads is $1/2 * 1/2 = 1/4.$ Signup and view all the answers

State the addition rule and give an original example.

The addition rule states that the probability of any two mutually exclusive events occurring is the sum of their individual probabilities. Signup and view all the answers

What is the probability that a couple will have a girl, a boy, a girl, and a boy in this specific order?

1/16. Signup and view all the answers

Explain how incomplete dominance is different from complete dominance, and give an example of incomplete dominance.

In incomplete dominance, neither allele is completely dominant, leading to a blended phenotype. An example is a cross between red and white flowers producing pink offspring. Signup and view all the answers

Compare and contrast codominance with incomplete dominance.

In codominance, two alleles are equally dominant and affect the phenotype distinctly, while in incomplete dominance, the phenotype is a blend. An example of codominance is blood type AB. Signup and view all the answers

Explain why dominant alleles are not necessarily more common than recessive alleles in the gene pool.

Natural selection affects the commonness of alleles, regardless of dominance. For example, polydactyly is dominant but not common. Signup and view all the answers

Explain what is meant when a gene is said to have multiple alleles.

A gene with multiple alleles has more than two allelic forms, as seen in the ABO blood groups. Signup and view all the answers

What is pleiotropy? Explain why this is important in diseases like cystic fibrosis and sickle-cell disease.

Pleiotropy is when a gene has multiple phenotypic effects, significant in diseases like cystic fibrosis and sickle-cell disease that have multiple symptoms from one defective gene. Signup and view all the answers

Explain epistasis.

Epistasis occurs when the phenotypic effects of one gene alter the effects of another gene. Signup and view all the answers

Explain why the dihybrid cross has four yellow Labrador retrievers instead of the three predicted by Mendel's work.

The E/e gene is epistatic to the B/b gene, affecting the phenotypic ratio. Signup and view all the answers

Why is height a good example of polygenic inheritance?

Height is influenced by two or more genes, which have an additive effect on the phenotype. Signup and view all the answers

Quantitative variation usually indicates _________.

polygenic inheritance. Signup and view all the answers

Using the terms norm of reaction and multifactorial, explain the potential influence of the environment on phenotypic expression.

Norms of reaction are broadest for polygenic characters, with both genetic and environmental factors contributing to the characters' quantitative nature. Signup and view all the answers

In the pedigree, explain why you know the genotype of one female in the third generation, but are unsure of the other.

The presence of a free earlobe could indicate either FF or Ff genotype, while the recessive trait indicates one genotype. Signup and view all the answers

Describe what is medically important to know about the behavior of recessive alleles.

Many genetic disorders are inherited as simple recessive traits and can vary in severity. Signup and view all the answers

Provide information for cystic fibrosis, sickle-cell disease, achondroplasia, and Huntington's disease.

a) Cystic fibrosis is a recessive disorder causing excessive mucus. b) Sickle-cell disease is a recessive disorder changing red blood cells. c) Achondroplasia is a form of dwarfism caused by a dominant allele. d) Huntington's disease is a dominant disorder causing nervous system degeneration. Signup and view all the answers

Explain the three main steps in amniocentesis.

<ol> <li>Sample amniotic fluid starting at week 14-16. 2. Perform tests on fluid or cultured cells. 3. Culture fetal cells for karyotyping.</li> </ol> Signup and view all the answers

What are the strengths and weaknesses of each fetal test?

Strength of amniocentesis: collects amniotic fluid, providing more information. Weakness: cells cultured for weeks. Strength of CVS: allows immediate karyotyping. Weakness: no amniotic fluid collected. Signup and view all the answers

What are the symptoms of phenylketonuria (PKU) and how is newborn screening used to identify it?

PKU symptoms include inability to metabolize phenylalanine, leading to intellectual disability. It can be detected at birth using biochemical tests. Signup and view all the answers

Study Notes

Mendel and the Gene Idea

Blending vs. Particulate Hypothesis: Blending suggests genetic material from parents mixes, leading to uniformity over generations. Mendel proposed that discrete units (genes) are passed on, maintaining their identities.
Pea Plant Experimentation: Pea plants have enclosed reproductive structures, enabling Mendel to control mating by preventing self-fertilization and ensuring known parentage.
Character vs. Trait: A character is a heritable feature (e.g., flower color) while traits are variants of a character (e.g., purple or white flowers).
Generational Definitions:
- P generation: True breeding parent generation.
- F₁ generation: First filial offspring generation.
- F₂ generation: Second filial offspring generation from F₁.
Mendel's Flower Cross Findings:
- Refuted blending since all F₁ hybrids had purple flowers, and white flowers reappeared in F₂, indicating dominance and recessiveness.
- Demonstrated that the white trait was masked but not lost in F₁.
- Showed the value of observing multiple generations for uncovering patterns of inheritance.
Chromosomes in Homologous Pairs: Each homologous pair consists of one paternal and one maternal chromosome, ensuring genetic diversity.
Mendel's Four Concepts:
- Alternative gene versions cause variation.
- Each organism inherits two gene copies, one from each parent.
- Dominant alleles determine appearance; recessive alleles do not affect phenotype visibly.
- Law of segregation: allele pairs separate during gamete formation (observable in meiosis).
Genotypic and Phenotypic Ratios: In a PP x pp cross, the phenotypic ratio is 3:1, and genotypic ratio is 1:2:1. F₁ generation is entirely heterozygous, while F₂ generation contains both heterozygous and homozygous offspring.
Testcross Methodology: A tall plant's genotype (homozygous or heterozygous) can be determined by crossing with a homozygous dwarf; resulting offspring indicate the parent plant's alleles.
Monohybrid vs. Dihybrid Cross: Monohybrid involves one trait study; dihybrid involves two traits, such as flower color and seed shape.
Law of Independent Assortment: Allele segregation occurs independently during gamete formation, allowing for diverse genetic combinations.
Independent Events in Probability: Outcomes of trials do not affect each other, allowing for independent probability calculations.
Multiplication Rule: The probability of multiple independent events occurs together by multiplying their probabilities (e.g., two coin tosses).
Addition Rule: The probability of mutually exclusive events occurring is calculated by summing their individual probabilities.
Specific Probability Example: The chance of having a girl, a boy, a girl, and a boy in that order is 1/16.
Incomplete Dominance vs. Complete Dominance: Incomplete dominance produces an intermediate phenotype (e.g., crossing red and white flowers yields pink), while complete dominance does not show variation between heterozygous and homozygous individuals.
Codominance: Two dominant alleles express phenotypically in equal measure (e.g., AB blood type).
Dominant vs. Recessive Allele Prevalence: Dominant alleles are not necessarily more common in a population; their frequency is influenced by natural selection (e.g., polydactyly).
Multiple Alleles in Genetics: Genes can have more than two allelic forms, exemplified by the ABO blood group system.
Pleiotropy: A single gene can affect multiple phenotypic traits, which is significant in diseases such as cystic fibrosis and sickle-cell disease.
Epistasis: The phenotypic expression of one gene can modify or suppress that of another, affecting traits.
Dihybrid Cross Findings: A dihybrid cross may yield unexpected ratios due to epistatic interactions between genes.
Polygenic Inheritance Example: Traits like height arise from the additive effects of multiple genes, resulting in continuous variation.
Norm of Reaction and Multifactorial Traits: The environment influences phenotypes, particularly for polygenic traits, where both genetic and environmental factors interplay.
Pedigree Analysis: Dominant traits may indicate heterozygosity or homozygosity based on phenotype, while recessive traits reveal only one possible genotype.
Recessive Alleles in Genetics: Many disorders are inherited as recessive traits. Understanding their behavior aids in identifying and treating genetic disorders like cystic fibrosis and albinism.
Common Genetic Disorders:
- Cystic Fibrosis: Caused by a recessive allele; leads to mucus buildup and vulnerability to infections.
- Sickle-Cell Disease: Recessive condition due to a single nucleotide mutation affecting hemoglobin.
- Achondroplasia: Dwarfism caused by a dominant allele; prevalent recessive allele.
- Huntington’s Disease: Dominantly inherited condition leading to neurological degeneration.
Prenatal Testing Methods:
- Amniocentesis: Involves fluid sampling from the amniotic sac post-14 weeks; tests for genetic disorders after cell culture.
- Chorionic Villus Sampling (CVS): Involves tissue sampling from the placenta as early as 8 weeks, allowing immediate testing.
Fetal Testing Pros and Cons: Amniocentesis offers extensive additional tests but is time-consuming; CVS provides quicker results but lacks fluid analysis.
Phenylketonuria (PKU): A disorder leading to an inability to metabolize phenylalanine, detectable through routine newborn screening to prevent severe mental disability.

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Explore the key concepts from Chapter 14 of the Campbell Active Reading Guide related to Mendel's theories and the gene idea. This quiz focuses on blending inheritance and Mendel's particulate gene hypothesis, providing a thorough understanding of fundamental genetic principles.