Genetics Notes

Mendel's Experiments and Key Concepts

Blending vs. Particulate Hypothesis

• The blending hypothesis was considered, but Mendel focused on the particulate hypothesis.

• Particulate hypothesis: Parents pass on discrete heritable units (genes).

• Mendel documented this through experiments with garden peas.

Why Peas?

• Many varieties with distinct heritable features (characters) exist.

• Character: Heritable feature (e.g., flower color).

• Trait: Variant of a character (e.g., purple or white flower color).

• Mendel could control mating between plants (cross-pollination).

Mendel's Experimental Design

• He tracked characters with two distinct alternative forms (e.g., tall vs. short).

• He used true-breeding varieties.

• True-breeding: Plants that produce offspring of the same variety when self-pollinated (e.g., PP or pp).

• Hybridization: Mating two contrasting, true-breeding varieties.

• P generation: True-breeding parents.

• F1 generation: Hybrid offspring of the P generation.

• F2 generation: Produced when F1 individuals self-pollinate or cross-pollinate.

Observations and Ratios

• P generation: Showed true-breeding traits (e.g., white and purple flowers).

• F1 generation: Only purple flowers appeared (no white).

• F2 generation: Both purple and white flowers reappeared in a 3:1 ratio (purple to white).

• Mendel's Inference: White color was masked by a dominant purple trait.

Mendel's Model: Four Related Concepts

1. Alternative Versions of Genes (Alleles)

   • Genes account for variations in inherited characters.

   • Alleles: Alternative versions of a gene (e.g., purple flower allele, white flower allele).

   • Each gene resides at a specific locus on a specific chromosome.

   • Example: At a specific locus, an allele either produces an enzyme for purple color or doesn't, resulting in white color.

2. Inheritance of Two Alleles

   • For each character, an organism inherits two alleles, one from each parent.

3. Dominant vs. Recessive Alleles

   • If the two alleles at a locus differ, the dominant allele determines the organism's appearance.

   • The recessive allele has no effect on the phenotype.

4. Law of Segregation

   • The two alleles for a heritable character segregate during gamete formation (egg and sperm).

   • Each gamete receives only one of the two alleles present in the organism.

   • Random fertilization ensures each offspring receives one allele from each parent.

   • The segregation model accounts for the 3:1 ratio in the F2 generation.

   • Punnett squares can be used to predict the outcomes.

Terminology

• Homozygous: Having two identical alleles for a character (e.g., PP or pp).

• Heterozygous: Having two different alleles for a character (e.g., Pp).

• Phenotype: Physical appearance (e.g., purple flowers).

• Genotype: Genetic makeup (e.g., PP, Pp, or pp).

Testcross

• Used to determine if an individual with a dominant phenotype is homozygous dominant or heterozygous.

• Breed the mystery individual with a homozygous recessive individual.

• If any offspring display the recessive phenotype, the mystery parent must be heterozygous.

• If no offspring display the recessive phenotype, the mystery parent is likely homozygous dominant.

Monohybrid Cross

• Monohybrids: Individuals heterozygous for one character (e.g., Pp).

• Monohybrid cross: A cross between two such heterozygotes (Pp \, x \, Pp).

• This involves tracking inheritance patterns for a single trait.

• Punnett square has four squares.

Dihybrid Cross

• Involves tracking inheritance patterns for two characters at the same time.

• Dihybrids are produced in the F1 generation.

• Law of Independent Assortment: Alleles for different traits are inherited independently of each other (e.g., big L doesn't always have to be with big R).

• Punnett square has 16 squares.

Probability Rules

• Multiplication Rule: Used to determine the probability of two or more independent events occurring together. Key idea: events are separate from each other.

• Addition Rule: Was mentioned but not clarified when to use it.

Beyond Simple Mendelian Genetics

• Not all heritable characters are determined as simply as Mendel studied.

• Basic principles still apply to more complex patterns of inheritance.

Complete Dominance

• The phenotypes of the heterozygote and dominant homozygote are identical (e.g., PP and Pp both show the dominant phenotype).

Incomplete Dominance

• The phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties (e.g., a pink flower resulting from a cross between red and white flowers).

Codominance

• Two dominant alleles affect the phenotype in separate, distinguishable ways (e.g., both alleles are expressed in the heterozygote).

Alleles and Nucleotide Sequences

• Alleles are simply variations in a gene's nucleotide sequence.

Recessive Disorders

• Most traits are recessive.

• Memorize dominant ones when mentioned.

• Example: Tay-Sachs disease (recessive).

Epistasis

• A gene at one locus alters the phenotypic expression of a gene at a second locus.

• Example: Coat color in Labrador retrievers.

  • One gene determines pigment color: B for black, b for brown.

  • Another gene determines whether the pigment will be deposited in the hair: E for color, e for no color.

  • If ee is present, the coat will be white, regardless of the B allele.

  • If E is present (Ee or EE), the pigment will be shown.

Quantitative Characters and Polygenic Inheritance

• Quantitative characters: Vary in a population along a continuum (e.g., height, skin color).

• Polygenic inheritance: Multiple genes contribute to one trait, resulting in a large amount of variety; Nurture (the environment) can also affect how certain genes are expressed.

Dominant vs. Recessive Traits in Populations

• The abundance of dominant and recessive alleles doesn't necessarily reflect their prevalence in the population.

Human Genetics and Pedigrees

• Humans are not ideal subjects for genetic research.

• Pedigrees: Family trees that describe the interrelationships of parents and children across generations.

• Used to trace inheritance patterns of particular traits.

Recessive Inheritance and Genetic Disorders

• Many genetic disorders are inherited in a recessive manner.

• Carriers: Heterozygous individuals who carry the recessive allele but are phenotypically normal.

• Example: Cystic fibrosis (recessive).

Dominant Inheritance and Genetic Disorders

• Dominant disorders are less common than recessive disorders.

• Example: Dwarfism, Huntington's disease

Chromosomal Basis of Inheritance

Chromosome Theory of Inheritance

• Genes have specific loci on chromosomes.

• Chromosomes undergo segregation and independent assortment.

Thomas Hunt Morgan's Experiments with Fruit Flies

• Fruit flies (Drosophila melanogaster) are useful for genetic studies.

• Wild type: Normal, common phenotype.

• Mutant type: Less common phenotype.

• Morgan's experiment: Mated male flies with white-eyed mutant female flies.

• He discovered that the inheritance of the eye color gene was related to the sex of the fly.

• Only males had white eyes.

Sex-Linked Genes

• Genes located on the sex chromosomes.

• In humans (and fruit flies), the X chromosome is larger and contains more genes than the Y chromosome.

X-Inactivation in Female Mammals

• One of the two X chromosomes in each cell of a female mammal is randomly inactivated during embryonic development.

• Inactive X condenses into a Barr body.

• If a female is heterozygous for a sex-linked trait, she will be a mosaic for that trait.

Linked Genes

• Genes located close together on the same chromosome tend to be inherited together.

• Example: Body color and wing size in fruit flies.

• Parental types: Offspring with a phenotype matching one of the parental phenotypes.

• Recombinant types (recombinants): Offspring with nonparental phenotypes.

Genetic Recombination and Mapping

• Crossing over: The process that breaks the physical linkage between genes on the same chromosome.

• Recombination frequency: The percentage of recombinant offspring.

• Linked Genes: If the % of recombinant offspring is less than 50%.

• Unlinked Genes: If the % of recombinant offspring is over 50%.

Genetic Maps and Linkage Maps

• Genetic map: An ordered list of the genetic loci along a particular chromosome.

• Linkage map: A genetic map based on recombination frequencies; distances between genes can be expressed as map units (centimorgans).

• Cytogenetic map: Shows the positions with chromosomal features (location of each gene, chromosomal features).

Chromosomal Abnormalities

• Nondisjunction: Failure of chromosomes to separate properly during meiosis.

• Aneuploidy: Results from the fertilization of gametes in which nondisjunction has occurred.

  • Monosomic: Zygote with only one copy of a particular chromosome (2n - 1).

  • Trisomic: Zygote with three copies of a particular chromosome (2n + 1).

• Polyploidy: A condition in which an organism has more than two complete sets of chromosomes (common in plants).

Alterations of Chromosome Structure

• Breakage of a chromosome can lead to four types of changes:

  • Deletion: Loss of a chromosomal segment.

  • Duplication: Repeat a chromosomal segment.

  • Inversion: Reverses a segment within a chromosome.

  • Translocation: Moves a segment from one chromosome to another. (non homologous one)