Module 2 - Genetics

Winter Biology

Textbook Notes: Chapter 14 Textbook Notes, “Mendel and the Gene”

Navigation

• Introduction
• 14.1: Medel’s Experimental System
  • What Questions Was Mendel Trying to Answer?
  • The Garden Pea Served as the First Model Organism in Genetics
• 14.2: Mendel’s Experiments with a Single Trait
  • The Monohybrid Cross: Dominant and Recessive Traits; A Reciprocal Cross; Do Mendel’s Results Hold for Other Traits?
  • Particulate Inheritance: Genes, Alleles, and Genotypes; The Principle of Segregation; Mendel’s Claims to Explain the Monohybrid Cross
• 14.3: Mendel’s Experiments with Two Traits
  • The Dihybrid Cross
  • Using a Testcross to Confirm Predictions
• 14.4: The Chromosome Theory of Inheritance
  • Meiosis Explains Mendel’s Principles
  • Testing the Chromosome Theory of Inheritance: The White-Eyed Mutant; The Discovery of Sex Chromosomes; Sex-Linkage

Introduction

• Gregor Mendel - an Austrian monk that laid the groundwork for the chromosome theory of inheritance.
• Walter Sutton and Theodor Boveri linked the discovery of meiosis with Mendel’s discovery to form the chromosome theory of inheritance.
  • States genes are located on chromosomes, and transmission of chromosomes to daughter cells accounts for patterns of inheritance.
• Launched the theory of genetics, which concerns the inheritance of traits.

14.1: Medel’s Experimental System

• Questions about heredity (inheritance) were a primary concern.
• Trait - any observable characteristic about an individual.

What Questions Was Mendel Trying to Answer?

• What are patterns of the transmission of traits from parents to offspring?
• Two hypotheses at the time:

The Garden Pea Served as the First Model Organism in Genetics

• Mendel chose the garden pea (Pisum sativum) to investigate.
  • Model organism - species used for research b/c it is easy to work with, and conclusions drawn from it may apply to other species.
  • Mendel could control which parents were involved in mating.
  • A polymorphic trait is one that appears commonly in 2+ forms.
    • e.g. purple or white flowers.
• How did Mendel control matings?
  • Garden pea flowers have male and female reproductive structures.
    • Usually undergo self-fertilization - flower’s pollen falls into the female reproductive organ of the same flower.
  • Mendel prevented self-fertilization by removing male reproductive organs before pollen formed, then transferred pollen from another pea plant.
    • This is cross-fertilization, or just a cross.
• What traits did Mendel study?
  • Phenotype - observable traits of an individual.
  • Mendel used peas that differed in seven phenotypes.
  • Pure line - individuals that produce offspring identical to the parents when they are crossed to another member of the same population or are self-fertilized.
    • Predictable pure lines can be used such that a new phenotype is the result of a cross, rather than some random event.
  • Offspring with matings between true-breeding parents that differ in one or more traits: hybrids.

14.2: Mendel’s Experiments with a Single Trait

• In his first set of experiments, Mendel crossed pure liens of garden peas that differed in only one trait.
• Crossed round-seed individuals and wrinkle-seed individuals (from pure lines).
  • Individuals used in the initial cross: parental generation.
  • Progeny (offspring) are F1 generation, “first filial”.
    • Latin Filius and filia mean “son” and “daughter”.
  • Continues as F2 generation, F3, so on.

The Monohybrid Cross: Dominant and Recessive Traits; A Reciprocal Cross; Do Mendel’s Results Hold for Other Traits?

• Mendel took pollen from plants in the round-seed land and placed it on female reproductive organs for plants from the wrinkle-seed line.
  • All seeds produced from this cross were round.
• Result:
  • Traits did not blend together to form an intermediate phenotype, but only the round-seed trait appeared.
    • This contradicts the blending-inheritance hypothesis.
  • The wrinkle-seed trait disappeared.

Dominant and Recessive Traits

• Mendel planted the F1 seeds and allowed the pea plants to self-fertilize when they matured.
  • Each of the F1 plants inherited a genetic determinant for round and wrinkled seeds each.
  • Monohybrid cross: parents that each carry two different genetic determinants for the same trait.
  • Produces a hybrid for a single trait.
• When F2 seeds were analyzed, results were:
  • 5474 round
  • 1850 wrinkled
• Wrinkle-seed shape disappeared in the F1 generation but reappeared in the F2 generation.
• Results:
  • Wrinkled shape is recessive because this phenotype recedes, or seems to be hidden.
  • Round shape is dominant because it dominates over the wrinkle-seed determinant when both are present.
• Genetics: “dominant” and “recessive” refer to which phenotype is observed and which is masked when individuals carry two different genetic determinants for a trait.
• Ratio is 3:1.

A Reciprocal Cross

• Does the male or female parent have a particular genetic determinant?
• Performed a second set of crosses between two pure-line populations.
  • “In reverse” - used pollen from the pure line of wrinkle-seed peas on the round peas.
  • This is a reciprocal cross; set of matings where the mother’s phenotype is the initial cross and the father’s is second, and vice versa.
• Results are identical; all F1 progeny had round seeds.
• Sometimes, reciprocal crosses do produce different results.

Do Mendel’s Results Hold for Other Traits?

• Mendel established results of crosses with 6 other traits.
• Important patterns:
  • F1 progeny only showed the dominant trait.
  • Reciprocal crosses produced the same results.
  • Ratio of dominant to recessive phenotypes in the F2-generation was 3:1.

Particulate Inheritance: Genes, Alleles, and Genotypes; The Principle of Segregation; Mendel’s Claims to Explain the Monohybrid Cross

• Mendel proposed a hypothesis: particulate inheritance.
  • Hereditary determinants for traits to not blend together or become modified through use.
  • Hereditary determinants maintain their integrity and act like discrete, unchanging particles.

Genes, Alleles, and Genotypes

• Gene: a hereditary determinant of a trait.
  • Mendel proposed each individual can have two versions of any gene: alleles.
  • Combination of alleles in an individual: genotype.
• Hypothesis that pea plants have two copies of each gene comes from the need to explain why a trait can disappear in one generation and pop up in another.
  • If each individual carries 2 alleles and one is dominant over another, in a hybrid, the recessive trait will be hidden.
  • When F1 hybrids are crossed, some F2 offspring may inherit two copies of the recessive allele.

The Principle of Segregation

• Two members of each gene pair must segregate (separate) into different gamete cells during the formation of eggs and sperm.
  • Each gamete has one allele of each gene: principle of segregation.
• Modelling the segregation of alleles:
  • R = dominant allele, r = recessive allele.
  • RR and rr are homozygous - always produce offspring with the same phenotype because they are homozygous.
  • Rr and rR are heterozygous - having different alleles.
• Punnett square modeling (assuming heterozygous mother and father):

• This yields a 3:1 ratio in the F2 generation.

Mendel’s Claims to Explain the Monohybrid Cross

1. Peas have two copies of each gene, and thus may have two different alleles of the gene.
2. Genes are particles of inheritance that do not blend together.
3. Each gamete contains one copy of each gene (allele).
4. Males and females contribute equally to the genotype of offspring.
5. Some alleles are more dominant than others.

14.3: Mendel’s Experiments with Two Traits

• Working with one trait allowed Mendel to infer each pea plant had two copies, and the principle of segregation.
• Do alleles of different genes segregate together or independently?

The Dihybrid Cross

• Mendel crossed a pure-line parent that produced round yellow seeds with a pure-line parent that produced wrinkled green seeds.
  • The F1 offspring should be heterozygous for both genes.
    • These offspring are called dihybrids.
  • Mating between dihybrids is a dihybrid cross.
• Mendel had established the allele for yellow seeds (Y) was dominant to the allele for green seeds (y).
• Two possibilities for transmission of genes to offspring:
  1. Alleles for seed shape and color are transmitted independently.
     • Independent assortment hypothesis; the two alleles of each gene are sorted into gametes independently of each other.
  2. Alleles for seed shape and color are transmitted into gametes together.
     • Dependent assortment hypothesis; one particular allele’s transmission is dependent on the transmission of another.
• Predictions of hypotheses:

• Data yielded 9:3:3:1; Mendel hence accepted the principle of independent assortment.
• Seeing what gametes can be produced by segregation and independent assortment is tricky as the number of traits increases.

Using a Testcross to Confirm Predictions

• Mendel did experiments w/ combinations of traits other than seed shape and color and obtained similar results.
  • Each dihybrid cross produced a 9:3:3:1 ratio.
  • Mendel, however, looked for an alternative test to provide more evidence for its correctness.
• An RrYy plant should produce four different types of gametes in equal proportions.
• Testcross - a parent with a dominant phenotype but an unknown phenotype is crossed with a parent that contributes only recessive alleles.
  • The unknown parental genotype can be determined by analyzing the phenotype of offspring.
• Researchers can infer if the other parent is homozygous or heterozygous for the dominant allele.
• If the principle of independent assortment is valid, the testcross should produce four types of offspring if the parent is RrYy and only one type if the tested parent is RRYY.
• Results of Mendel’s testcross confirmed the principle of independent assortment.

14.4: The Chromosome Theory of Inheritance

• Sutton and Boveri realized meiosis accounted for Mendel’s rules.

Meiosis Explains Mendel’s Principles

• Alleles of a gene for seed shape are shown in a particular position (locus or loci - plural) along a certain chromosome.
• Paternal and maternal chromosomes possess alleles at the seed-shape gene locus.
  • If alleles for different genes are located on different chromosomes, they will assort independently of one another in meiosis I.
  • There are two equally likely ways for homologous pairs to line up.
  • Four types of gametes will be produced in equal proportions.
• Mendel’s rules can be explained by postulating genes are located on chromosomes that line up independently before being separated.
• The idea genes were located on chromosomes was based on an observed observation; experiments were needed to confirm that chromosomes carried genes.

Testing the Chromosome Theory of Inheritance: The White-Eyed Mutant; The Discovery of Sex Chromosomes; Sex-Linkage

• The fruit fly Drosophila melanogaster has been the center of genetic studies.
  • Small size
  • Ease of rearing in lab
  • Short generation time (~10 days)
  • Abundant offspring (several hundred per mating)

The White-Eyed Mutant

• A male fly was discovered with white eyes rather than wild-type red eyes.
  • This was likely a mutation - a heritable change in a gene.
  • An individual with a mutation is a mutant.
• Thomas Hunt Morgan’s experiments:

• There is a relationship between the sex of the parent and the inheritance of eye color.

The Discovery of Sex Chromosomes

• Nettie Stevens studied the chromosomes of insects.
• There was a striking difference in chromosome complements of males and females.
• Stevens discovered sex chromosomes (X and Y) and autosomes.
  • Female flies have XX; male flies have XY.

Sex-Linkage and the Chromosome Theory of Inheritance

• The transmission pattern of the X chromosome can explain the results of reciprocal crosses.
• Half the gametes produced by males have an X chromosome and half a Y chromosome.
  • Proposal: the gene for eye color is located on the X chromosome, and the Y chromosome does not carry this gene.
• Terminology:
  • Sex-linked gene - genes located on either sex chromosome.
  • X-linked gene - a gene on the X chromosome.
  • Y-linked gene - a gene on the Y chromosome.
  • Sex-linked inheritance - patterns of inheritance on genes that give different results for reciprocal crosses.
  • X-linked inheritance - when genes for patterns of inheritance that give different results for reciprocal crosses are located on the X chromosome.
  • Y-linked inheritance - when genes for patterns of inheritance that give different results for reciprocal crosses are located on the Y chromosome.
  • Autosomal inheritance - genes on non-sex inheritance.
• These experiments provide strong support for the chromosome theory of inheritance.

Notes on Genetics Video Lectures

Crash Course Biology: Heredity

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• Heredity is the passing of genetic springs from parent to offspring.
• Concept of reproduction from Aristotle: suggested that we are a mixture of our parents’ traits.
• Austrian monk: Gregor Mendel, demonstrated inheritance followed certain laws.
• By crossing pea plants and seeing which got passed on, he came up with a framework for how genes are inherited.
• Chromosomes: the form DNA takes to get passed onf rom parent to child.
• Gene: a section of DNA on a chromosome to determine a trait.
  • Polygenic trait: controlled by multiple genes.
  • Pleitropic trait: a gene that controls many traits are expressed.
  • Mendelian trait: a gene that controls exactly one trait.
• An allele is a version of a gene.
• Any cell that isn’t a sperm or egg cell are somatic diploid cells.
  • Gametes are haploid cells (one set of chromosomes) that, during fertilization, form diploid cells.
• Some cells are polyploid: they have more than one version of each gene.
• When there are two alleles that decide the outcome of a certain trait, one is dominant and the other is recessive.
• Reginald C. Punnett: created the Punnett square to diagram results of crosses.
• Sex-linked inheritance
  • 22 pairs of autosomes, and the 23rd pair is the sex chromosome.
  • Women have two X chromosomes, and men have an X chromosome and a smaller Y chromosome.
  • There may be recessive alleles on the X chromosome that is expressed in males because there is no corresponding allele in the Y chromosome to dominate it.

Introduction to Heredity

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• We had the general sense that children’s traits resembled their parents’ traits.
• Gregor Mendel - father of heredity and genetics.
• When we study classical genetics, we might be able to find what future generations look like.
• An allele: a version of a gene.
  • Heterozygous genotype: the two alleles are different.
  • Homozygous recessive genotype: the two alleles are both recessive.
  • Homozygous dominant genotype: the two alleles are both dominant.
• Genotype: versions of genes, Phenotypes: what is expressed.

Notes on Punnett Squares for Dihybrid Crosses, Independent Assortment, Incomplete Dominance, Codominance, and Multiple Alleles

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• Dihybrid Cross
• Incomplete Dominance
• Codominance
• Multiple Alleles and Independent Assortment

Dihybrid Cross

Mom: BB, Dad: Bb (B allele for brown, b allele for blue)

Genotypic Punnett Square

Phenotypic Punnett Square

Incomplete Dominance

Plant 1: RW, Plant 2: RW (R allele for red, W allele for white).

Genotypic Punnett Square

Phenotypic Punnett Square - RW blends to form pink.

Codominance

Mom: AO, Dad: AB (A, B, and O alleles for blood types)

Genotypic Punnett Square

Phenotypic Punnett Square - O is recessive but A and B are codominant. If they are expressed together, the bloodtype is AB.

Multiple Alleles and Independent Assortment

Mom: BbTt, Dad: BbTt (B allele for brown eyes, b allele for blue eyes, T allele for large teeth, t allele for small teeth)

Independent assortment states that alleles are assorted independently of each other.

Genotypic Punnett Square

Phenotypic Punnett Square