Chapter 2 - Mendelian Patterns of Inheritance
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What did Mendel conclude about the genetic determinants of traits?

  • They are unit factors that are passed intact from generation to generation. (correct)
  • They do not pass from one generation to the next.
  • They can only exist in one form.
  • They are influenced by environmental factors.

    • What is a key feature of alleles during gamete formation?

  • They are duplicated in each gamete.
  • They separate so that each gamete receives only one allele. (correct)
  • They only occur as dominant traits.
  • They remain together in pairs.

    • How many alleles does an individual carry for a given genetic character?

  • Two (correct)
  • Three
  • One
  • Four

    • In Mendel's F2 generation, the ratio of dominant to recessive traits was approximately what?

    <p>3:1</p> Signup and view all the answers

    What does the term 'homozygous' refer to?

    <p>Having two identical copies of a gene.</p> Signup and view all the answers

    What does phenotype refer to in genetics?

    <p>The observable traits of an organism.</p> Signup and view all the answers

    In a cross between two heterozygous tall plants (Tt x Tt), what are the possible genotypes of the offspring?

    <p>TT, Tt, tt</p> Signup and view all the answers

    What does Mendel's Law of Segregation explain?

    <p>How two alleles of a gene separate during gamete formation.</p> Signup and view all the answers

    What term is used to describe the visible characteristics such as the appearance of seeds, pods, or flowers?

    <p>Traits</p> Signup and view all the answers

    In the process of self-fertilization in the garden pea, which gametes are involved?

    <p>Male and female gametes from the same plant</p> Signup and view all the answers

    What is a hybrid offspring generated from cross-fertilization called?

    <p>Monohybrid</p> Signup and view all the answers

    What does the F1 generation demonstrate when true-breeding parents differ in a single character?

    <p>Dominance</p> Signup and view all the answers

    What ratio of plants in the F2 generation typically shows the dominant trait?

    <p>3:1</p> Signup and view all the answers

    What best describes a recessive trait in Mendelian genetics?

    <p>It can be masked in the presence of a dominant trait.</p> Signup and view all the answers

    Which allele notation convention is used to represent dominant and recessive traits?

    <p>Uppercase for dominant and lowercase for recessive</p> Signup and view all the answers

    Which statement best describes the outcome of a single factor cross?

    <p>It only examines the traits of one kind.</p> Signup and view all the answers

    What factors can influence the height of plants?

    <p>Both genes and environmental factors</p> Signup and view all the answers

    In an epistatic gene interaction, what occurs?

    <p>One gene masks the expression of another gene</p> Signup and view all the answers

    Which of the following was observed in Mendel's F2 generation of sweet peas?

    <p>¾ purple and ¼ white flowers</p> Signup and view all the answers

    What was the unexpected finding from Bateson and Punnett's crossing of true-breeding white strains?

    <p>F1 were all purple flowers</p> Signup and view all the answers

    What role do proteins play in gene interactions, such as epistasis?

    <p>Different proteins are involved in the same cellular function</p> Signup and view all the answers

    What is the chance of having an affected son when both parents are considered?

    <p>50%</p> Signup and view all the answers

    In Morgan's Drosophila experiment, what was the primary color of the offspring produced from the P-generation cross?

    <p>Red-eyed</p> Signup and view all the answers

    What does the notation 'Xw+' in Drosophila genetics represent?

    <p>Normal allele</p> Signup and view all the answers

    How many white-eyed females were counted in the F2 generation of Morgan's experiments?

    <p>0</p> Signup and view all the answers

    What typically occurs in the case of mutations that result in recessive alleles?

    <p>Eliminate protein synthesis</p> Signup and view all the answers

    What was the significance of the first trait localized to a specific chromosome in 1910?

    <p>It linked eye color to sex</p> Signup and view all the answers

    In a typical dominant/recessive genetics scenario, what effect does the recessive allele have on the heterozygote phenotype?

    <p>It has no effect on the phenotype</p> Signup and view all the answers

    Which statement best describes a wild-type allele?

    <p>It is prevalent and encodes a functional protein</p> Signup and view all the answers

    What were the genetic results derived from the F1 generation crosses in Morgan's experiments?

    <p>Predominantly red-eyed females</p> Signup and view all the answers

    What is the total number of red-eyed males observed in the F2 generation?

    <p>1,011</p> Signup and view all the answers

    What ratio of flower color was observed in the F2 generation?

    <p>9:7</p> Signup and view all the answers

    Which condition must be met for a plant to exhibit purple flowers?

    <p>At least one dominant allele from each of the genes</p> Signup and view all the answers

    What effect do the cc and pp genotypes have on flower color?

    <p>They result in white flowers due to masking</p> Signup and view all the answers

    What characteristic defines quantitative traits compared to discrete traits?

    <p>They show continuous variation</p> Signup and view all the answers

    How many genes are deduced to be involved in flower color determination based on the described ratios?

    <p>Two genes</p> Signup and view all the answers

    Which of the following traits is NOT typically classified as quantitative?

    <p>Purple flower color</p> Signup and view all the answers

    What is the result of a plant being homozygous for either the c or p allele?

    <p>It produces white flowers regardless of other alleles</p> Signup and view all the answers

    Epistasis refers to which of the following concepts?

    <p>Interactions among different genes affecting phenotypic expressions</p> Signup and view all the answers

    What is the expected phenotypic ratio in the offspring when two heterozygous tall plants (Tt) are crossed?

    <p>3:1</p> Signup and view all the answers

    What is indicated by the presence of a recessive trait in the offspring during a testcross?

    <p>The parent was heterozygous.</p> Signup and view all the answers

    What does Mendel's Law of Independent Assortment state?

    <p>Alleles of different genes segregate independently.</p> Signup and view all the answers

    In a two-factor cross, what ratio would you expect if the genes assort independently?

    <p>9:3:3:1</p> Signup and view all the answers

    What is a key characteristic of pedigrees used in genetics?

    <p>They illustrate traits over several generations in one family.</p> Signup and view all the answers

    What happens when an individual is crossed with a homozygous recessive parent in a testcross?

    <p>Phenotypes of offspring will determine the parent's genotype.</p> Signup and view all the answers

    What is the likelihood of two heterozygous individuals producing a homozygous recessive offspring?

    <p>25%</p> Signup and view all the answers

    Which of the following is true regarding cystic fibrosis and its carriers?

    <p>Carriers are phenotypically normal.</p> Signup and view all the answers

    What is the critical gene affected in cystic fibrosis?

    <p>CFTR</p> Signup and view all the answers

    What is the phenotypic ratio predicted by linked assortment in a two-factor cross?

    <p>3:1</p> Signup and view all the answers

    What is a common characteristic of recessive genetic diseases in humans?

    <p>Unaffected parents can have affected children.</p> Signup and view all the answers

    Which of the following describes Huntington disease?

    <p>It typically manifests later in life.</p> Signup and view all the answers

    In filling a Punnett square, which genotype corresponds to the box combining T and t?

    <p>Tt</p> Signup and view all the answers

    What is the primary result of a testcross?

    <p>Identify the genotype of an individual with a dominant phenotype.</p> Signup and view all the answers

    Study Notes

    BIOL 1P91 Part 3

    • The course is BIOL 1P91, Part 3
    • The instructor is Dr. Lori MacNeil
    • Email: [email protected]
    • Office: MC F229
    • Office hours by appointment

    Interactive Tool

    • A tool called Slido will be used for interactive polls and practice questions during lectures.
    • Students can access Slido from any smartphone, tablet, or laptop.
    • Slido information is available at slido.com and #Meeting.

    Mendelian Patterns of Inheritance

    • This is Chapter 17 of a Biology textbook, likely Brooker | Widmaier | Graham | Stiling, Sixth Edition.
    • The chapter outline includes Mendel's Laws of Inheritance, the Chromosome Theory of Inheritance, Pedigree Analysis of Human Traits, Sex Chromosomes and X-Linked Inheritance, Variations in Inheritance, Gene Interaction, and Genetics and Probability.

    Explaining Inheritance

    • Inheritance is the acquisition of traits passed from parent to offspring.
    • Historically, pangenesis was a theory postulating inheritance through "seeds" produced by body parts.
    • Late 19th-century theories also included that if traits changed over a lifetime, hereditary material would also change, leading to inherited modified traits.
    • The Lamarck's theory of acquired characteristics is described in terms of giraffe's neck—traits acquired during an organism's lifetime can be passed on to future generations.
    • Blending inheritance, another theory which is also refuted, suggests hereditary traits are blended together in offspring, with these traits passed to subsequent generations.

    Gregor Mendel

    • Gregor Mendel is considered the "Father of modern genetics".
    • He was a priest who studied physics and mathematics.
    • His seminal work on pea plants, started in 1856, was published in 1866.
    • His work was largely ignored until rediscovered around 1900.
    • Mendel's studies have revolutionized the understanding of inheritance.

    Garden Pea (Pisum sativum)

    • Garden peas have several advantageous properties for genetic studies:
      • Genetic variation: Many plant varieties with visible differences in characteristics.
      • Easy self-fertilizing: Pollination is facilitated by the tightly enclosed flowers.
      • Easy to make crosses: Manual cross-pollination or hybridization is easily performed.
    • Differences can be seen in shape, color, etc. for traits

    Single-Factor Crosses

    • Follows the inheritance of traits for a single character.
    • P generation: True-breeding parents (e.g., homozygous tall x homozygous dwarf).
    • F₁ generation: Offspring of the P cross.
    • F₂ generation: Offspring obtained by allowing F₁ individuals to self-fertilize.

    The Data (Mendel's Results)

    • Mendel's experiments with peas demonstrated a consistent 3:1 ratio in the F₂ generation (i.e. 3/4 individuals with the dominant trait and 1/4 individuals with the recessive trait).
    • His data showed that traits were not blended but rather retained their individual identities through generations

    Three Important Ideas from Mendel's Work

    1. Traits: Existing in two forms—dominant and recessive
    2. Genes: An individual carries two genes for a character, and each gene has variant forms (alleles).
    3. Segregation: Two alleles of a gene separate during the formation of gametes, passing one allele to each gamete.

    Genotype and Phenotype

    • Genotype: The genetic makeup of an individual for a particular trait or set of traits.
    • Homozygous dominant: Having two identical dominant alleles (e.g., TT)
    • Homozygous recessive: Having two identical recessive alleles (e.g., tt)
    • Heterozygous: Having two different alleles (e.g., Tt)
    • Phenotype: The observable characteristics of an individual.

    Punnett Squares

    • A tool to predict the genotypes and phenotypes of offspring in genetic crosses.
    • Five steps:
      • Write down the genotypes of the parents.
      • Write down the possible gametes for each parent.
      • Create an empty Punnett square.
      • Fill in the possible genotypes of the offspring by combining the alleles of the gametes.
      • Determine the relative proportions of genotypes and phenotypes in the offspring.

    Testcross

    • A genetic cross to determine the genotype of an individual that has a dominant phenotype.
    • An individual with a dominant phenotype is crossed to a homozygous recessive individual—if any recessive offspring result, the parent was heterozygous.

    Two-Factor Crosses

    • Follows the inheritance patterns of two different traits in an organism.
    • Two hypothetical postulates:
      • Linked assortment (traits inherited together): This predicts a 3:1 ratio in the progeny.
      • Independent assortment (alleles independently distributed to gametes): This predicts a 9:3:3:1 ratio in the progeny.
    • Mendel's results validated independent assertion.

    Mendel's Results (Data)

    • The F₁ generation plants are dihybrids meaning that the plants are hybrid with respect to both traits.
    • In F₂ generation results using independent assortment, there is a 9:3:3:1 inheritance pattern.

    Mendel's Law of Independent Assortment

    • The alleles of different genes are independently assorted during gamete formation.

    Pedigree Analysis

    • Pedigree analysis examines the presence of a particular trait across generations in a family.
    • Used to understand the inheritance of genetic diseases and determine whether the mutant allele is dominant or recessive
    • Allows for prediction of the likelihood of and individual being affected with certain traits.

    Example: Cystic Fibrosis

    • ≈3% of Europeans are heterozygous carriers of the recessive CFTR (disease-causing allele).
    • Homozygous individuals exhibit CF symptoms resulting from a mutated CFTR gene (encoding a transmembrane chloride channels).
    • This interferes with chloride ion flow, altering water flow out of cells, producing thick and sticky mucus.

    Human Disease

    • Many human genetic diseases are often recessive, as disease alleles persist in heterozygous carriers, who are unafected.
    • Huntington Disease is an example of a human disease that shows dominant traits:
      • Symptoms occur later in life, often after reproduction.
      • The normal allele encodes a functional nerve protein.
      • The mutant allele forms an abnormal, aggregating protein, disrupting nerve function.

    Pedigree of a Dominant Trait

    • Visual depiction illustrating inheritance of dominant traits through generations.
    • Individuals showing a particular trait are identified in a family pedigree diagram.

    Autosomes vs. Sex Chromosomes

    • Autosomes are chromosome pairs found in both male and female organisms.
    • Sex chromosomes are distinctive, differing between the sexes.
    • Genes on the sex chromosomes present diverse inheritance patterns

    Human Chromosomes

    • A human typically has 46 chromosomes.
    • 22 pairs of autosomes and one pair of sex chromosomes (XX in females, XY in males).
    • The presence of a Y chromosome determines maleness in humans.

    X-Linked Traits

    • X-linked traits are present on the X sex chromosome but not the Y.
    • In humans, the X chromosome is larger and carries more genes than the Y.
    • Females can be homozygous or heterozygous for X-linked genes.
    • Males are hemizygous for X-linked genes (i.e. exhibiting only one copy of each X-linked gene).

    Example: Hemophilia A

    • X-linked recessive disease (more frequent in males).
    • Results from defective clotting proteins—characterized by excessive bleeding.
    • If a heterozygous female and an unaffected male are parents, sons have a 50% chance of having hemophilia; daughters have a 25% chance.

    Morgan's Drosophila Experiments

    • Drosophila melanogaster experiments demonstrated that specific traits (e.g., eye color) are linked to particular chromosomes.
    • Eye color genes were linked to the X chromosome.

    Morgan's Drosophila Crosses

    • These experiments studied the inheritance of traits (e.g., eye color) in fruit flies.
    • Crossing flies with different eye color genes demonstrated some traits are linked across generations.

    Protein Function Explains Dominance

    • Wild-type alleles are prevalent alleles, encoding proteins that function correctly.
    • Mutant alleles are rare, usually arising from a mutation, causing decreased or absent of functional protein products.
    • In many cases, having 50% of the normal protein amount leads to a normal trait expressed while a homozygous recessive trait occurs when there is no or almost no functional protein expressed, resulting in a recessive phenotype.

    Genotype-Phenotype Connection in Simple Mendelian Inheritance

    • The relationship between genotype and phenotype is depicted in a simplified model for a single-gene trait.
    • The amount of functional protein produced directly correlates with the phenotype expressed.

    Some Single-Gene Traits: Do not exhibit a simple dominant/recessive relationship

    • Some traits do not have a simple dominant/recessive relationship between alleles.

    Incomplete Dominance

    • Heterozygous individuals exhibits a phenotype between the two homozygous phenotypes.
    • This is apparent when crossing 2 plants—red and white flowers—producing offspring with pink flowers.

    Codominance

    • A heterozygous trait in which both alleles are expressed simultaneously and resultantly, leading to both phenotypes in the trait.
    • ABO blood types are a well-known example in humans.

    ABO Blood Group

    • Blood type depends on which two alleles are inherited (multiple alleles).
    • Each blood group is characterized by the antigens present on red blood cells and antibodies in the plasma.

    Role of the Environment

    • Phenotype (observable characteristics) can be influenced by the environment.
    • The norm of reaction describes the range of phenotypes seen in individuals with the same genotype under differing environmental conditions.
    • For example, genetically identical plants may grow to different heights in different temperatures/environments.

    Complex Traits

    • Most traits are influenced by multiple genes and environmental factors.
    • Gene interactions (e.g., epistasis) occur when a single trait is controlled by the products of two or more genes.

    Example: Epistasis in Sweet Peas

    • The relationship between genes involved in flower color determination was studied using a breeding experiment.
    • The results suggested that more than one gene is involved in determining flower color.
    • If the genotype doesn't have the dominant allele for one gene, then flowers will be white regardless of the other gene's genotype.

    Enzymatic pathway

    • An example demonstrating how genes work together to produce a color (e.g., purple flower pigment) through a series of enzymatic reactions.

    Epistasis in Sweet Pea

    • The relationship between genes/alleles involved in flower color determination.
    • C allele is dominant to c allele for flower color.
    • P allele is dominant to p allele for flower color.
    • cc genotype masks P, and pp masks C genotypes, leading to white flowers even when dominant alleles exist.

    Polygenic Inheritance

    • Traits exhibiting continuous variation across a phenotypic range.
    • Examples include height, weight, skin color, metabolic rate, and heart size in humans.
    • Polygenic inheritance is determined by multiple genes and influenced by environmental factors.

    Example: Grain Pigmentation in Wheat

    • This demonstrates polygenic inheritance, illustrating how multiple genes influence the continuous variation in grain color in wheat.

    Different Types of Mendelian Inheritance Patterns

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