Genetics Quiz Chapter 5

Genetics Overview

• Genetics is the study of heredity and variations
• Heredity is the transmission of characteristics across generations
• Variation are differences in traits present within members of the same species

Terminologies

• Pedigree: A diagram showing ancestral relationships and transmission of genetic traits across generations in a family
• Proband: The first family member seeking medical attention, leading to the construction of the pedigree
• Autosomal Trait: A trait controlled by alleles located on autosomes (non-sex chromosomes)
• Autosomes are the 22 numbered chromosomes
• Genotype: The gene pair an individual carries for a trait, symbolised as letter pairs
• Dominant alleles are capital letters
• Recessive alleles are lowercase letters
• Homozygous dominant (DD), Heterozygous (Dd), and Homozygous recessive (dd)
• Phenotype: The observable trait determined by an individual's genotype
• Dominant trait requires at least one dominant allele
• Recessive trait requires two recessive alleles
• Phenocopy: A trait expressed due to environmental conditions,not genotype
• Twins:
• Identical (monozygotic): Develop from a single fertilized egg, with subsequent separation of cells
• Fraternal (dizygotic): Develop from separate fertilized eggs

Gregor Johann Mendel

• Mendel laid the foundation of genetics in 1866
• His work on pea plants (Pisum sativum) was pioneering
• His work, unnoticed until 1900, established the concept of genes as hereditary units

Research Method: Crossing Pea Plants

• Mendel's method involved controlled mating between pea plants with specific traits
• He focused on traits (e.g., flower color) that had clear alternative forms (e.g., purple or white)
• Used a true-breeding approach, selecting plants that always produced offspring with the same traits

Advantages of Pea Plants for Genetic Study

• Pea plants have many distinct heritable traits
• Traits could be easily controlled for mating experiments
• Self-pollination was a viable option, in combination with cross-pollination

Mendel's Experimental Approach

• Mendel tracked only traits with alternative forms, like flower colors
• He used true-breeding varieties for initial experiments
• Conducted hybridization (cross-mating, contrasting, true-breeding varieties) to obtain the P generation, parental generation
• F₁ generation was offspring of the P generation
• F₂ generation arose from self- or cross-pollination of F₁ hybrids

Laws of Inheritance

• Law of Segregation: Members of each gene pair are separated during gamete formation, each gamete receiving only one allele
• Law of Independent Assortment: Alleles for different traits are assorted independently
• These reflect the basic rules of probability

Law of Segregation (First Mendelian Law)

• Every trait is governed by a pair of alleles
• Alleles segregate during gamete formation in meiosis
• Each gamete receives one of the alleles

Mendel's Experiment (Following a Single Character)

• Derived the law of segregation
• Individuals are heterozygous
• A cross between such heterozygotes is called monohybrid cross
• In Mendel's experiment with flower color (purple vs. white), he observed a 3:1 ratio in the F₂ generation

Mendel's Experiment (Multiple Characters)

• Led to understanding of independent assortment
• Involves tracking two or more traits simultaneously

Mendel's Model

• Developed hypothesis to explain 3:1 inheritance pattern in offspring
• Included four related concepts: Alternative gene versions are called alleles; Each character has two alleles from each parent; The dominant allele masks recessive allele effects, and the separation of alleles is independent

Concept 1: Alternative Versions of Genes Account for Variations in Inherited Characters

• Alternate gene versions are alleles
• Alleles are alternative forms of a gene

Concept 2: For Each Character, an Organism Inherits Two Alleles, One from Each Parent

• Two alleles are inherited - one from each parent

Concept 3: If Two Alleles Differ, One (Dominant) Determines Appearance; the Other (Recessive) Has No Effect

• Dominant allele masks the recessive allele when present
• The dominant allele determined the expressed trait

Concept 4

• The law of segregation
• Alleles separate during gamete formation
• This creates different possible gametes, influencing traits

Useful Genetic Vocabulary

• An organism with two identical alleles is a homozygote
• Parents which breed true, are homozygous for the traits
• If homozygous dominant are crossed with recessive homozygotes, all the offspring will be heterozygotes

Phenotype and Genotype Ratios

• The observable trait is the phenotype
• The genetic makeup is the genotype
• Phenotypes can share the same phenotype but have different genotypes
• e.g. purple flowers: PP & Pp; white flowers: pp

The Testcross

• Used to determine the genotype of an individual with a dominant phenotype by breeding with a homozygous recessive individual
• If offspring show the recessive phenotype, the mystery parent is heterozygous

Law of Independent Assortment (Second Mendelian Law)

• Alleles for different traits are independently assorted
• Genes located far apart on different chromosomes assort independently

Mendel's Experiments (Involving Two Characters)

• Helped identify the second law of inheritance
• Involves two true-breeding parents with different traits
• Results in hybrid offspring(dihybrids) that carry the characters

The Multiplication and Addition Rules

• Used to determine probability of different outcomes
• The probability of multiple events occurring together is the product of their individual probabilities

Solving Complex Genetics Problems

• Apply the multiplication and addition rules to predict outcomes of crosses involving multiple characters
• Dihybrid or multi-character crosses are equivalent to individual monohybrid crosses

Seatwork (Examples)

• Worked examples in tomatoes to demonstrate the concept of independent assortment
• Various crosses are performed with tomatoes to demonstrate inheritance principles based on phenotype and genotype

Dominant and Recessive Traits in Humans

• Illustrative table of both Dominant & Recessive traits in humans

Dominant and Recessive Traits in Humans

• Illustrative table of various dominant and recessive traits in humans