Mendelian Genetics
Introduction to Mendelian Genetics
Mendel discovered the basic principles of heredity through experiments with garden peas.
Key Laws of Inheritance:
Law of Segregation (law not theory)
Law of Independent Assortment (alone movement)
Concept 14.1: Mendel's scientific approach identified these two fundamental laws of inheritance.
Advantages of Using Pea Plants in Experiments
Peas have several advantageous characteristics:
Multiple varieties available
Short generation time
High number of offspring produced
Controlled mating possible
Mendel started his experiments with true-breeding varieties.
Concept: Mendel's Experimental, Quantitative Approach.
True Breeding: Offspring which have constituently exhibited same phenotype.
Hybridization and Generations in Mendelian Experiments
Typical Experiment Process: Mendel would mate two contrasting true-breeding varieties in a method called hybridization.
P Generation: Parental generation with true-breeding plants… homozygous.
F1 (“Son 1”) Generation: First filial generation resulting from the P generation mating.
F2 Generation: Second filial generation resulting from mating of the F1 generation.
Historical Context of Heredity Explanations
In the 1800s, the prevailing explanation of heredity was the “blending” hypothesis… Which is proven false.
Mendel's Experiments:
Crossed contrasting true-breeding white- and purple-flowered pea plants… resulting in a one lost trait/color.
When F1 Hybrids (F1 (purple) x F1 (purple)) crossed most ended up purple, with some white.
Upon crossing F1 hybrids, Mendel uncovered a ratio in the resulting F2 generation, revealing new inheritance patterns.
~3 purple : 1 white ratio was consistently observed, indicating that the purple trait is dominant over white.
^^^ Phenotype not Genotype!
Findings of Mendel: The Law of Segregation
During gamete formation, two alleles for a specific gene separate (segregate), so only one gamete receives one allele.
Mendel concluded that only the purple flower factor influenced flower color in F1 hybrids.
Definitions:
Purple flower color referred to as dominant.
White flower color referred to as recessive.
Notably, the factor for white flowers was neither diminished nor destroyed as it re-emerged in the F2 generation.
Patterns of Inheritance Observed by Mendel
Mendel noted this pattern of inheritance across six other pea plant traits.
A “heritable factor” identified by Mendel is now termed an allele.
Concept Overview: Mendel’s Model
Model developed to explain the 3:1 inheritance pattern in F2 offspring:
Variations in inherited characters arise from alternative versions of genes (alleles)… specific variation of a gene: is it white or yellow?
Each organism inherits two alleles for each character, one from each parent… alleles may be same or different.
If two alleles at a locus differ, one is the dominant and one is the recessive… Dominant alleles mask the effect of recessive alleles in the phenotype.
Law of Segregation: Alleles for a heritable character segregate during gamete formation, resulting in different gametes.
Punnett Square and 3:1 Ratios
The 3:1 ratio in F2 generation explained via Punnett squares.
Capital letter for dominant allele; lowercase letter for recessive allele.
Letters on sides are based on the genetic make up of the parents, representing the alleles they can pass on to their offspring.
Genetic Vocabulary to Understand Mendelian Genetics
Definitions:
Homozygote: An organism with two identical alleles for a gene (rr or RR)
Homozygous: Refers to the condition of having identical alleles (e.g., AA or aa).
Heterozygote: An organism with two different alleles for a gene.
Heterozygous: Refers to having different alleles (e.g., Aa).
Important Note: Unlike homozygotes, heterozygotes are not true-breeding.
Gene: Sequence of of nucleotides at a specific place, or locus, along a chromosome.
Alleles: The variations of genes that can be exhibied.
Phenotype and Genotype Explained
Definitions:
Phenotype: The observable physical and physiological traits of an organism.
Genotype: The genetic constitution of an individual.
Example: For flower color in pea plants, both genotypes PP and Pp exhibit the same purple phenotype but differ in their genetic makeup / genotype.
3:1 → Monohybrid Cross (one gene cross)
Testcross Methodology (Still Monohybrid testcross)
Individuals with dominant phenotypes (PP or Pp) can be either homozygous dominant or heterozygous.
To determine the genotype, a testcross is utilized:
Involves breeding the individual of unknown genotype with a homozygous recessive individual.
Observation: If any offspring exhibit the recessive phenotype, the mystery parent must be heterozygous.
P_ x pp → Dominant phenotype and unknown genotype x two recessive.
If the result is all purple, then the known is a dominant homozygote (PP), whereas if any white flowers are produced, the known parent must be heterozygous (Pp).
1:1:1:1 Ratio when parent is heterozygous.
Monohybrid Crosses
F1 offspring from testcrosses: termed monohybrids (heterozygous for one character).
A cross between monohybrids is called a monohybrid cross.
Dihybrid offspring: combination of two traits, leading to a dihybrid cross.
3:1 or 1:1 ratio.
The Dihybrid (2 genes) Cross Experiment
2 traits are followed: True breeding → PP__ x pp__
Example outcomes based on Mendel's experiments:
P Generation: YYRR x yyrr
F1 Generation: All YyRr (dihybrid).
Gamete combinations and predicted offspring ratios calculated.
Final outcomes:
Phenotypic ratio of
9:3:3:1 exhibited in F2 generation.
Law of Independent Assortment
Mendel’s findings led to the law of independent assortment: two or more genes assort independently during gamete formation, resulting in genetic variation.
Only applies to genes (allele pairs) located on different chromosomes or very far apart on the same chromosome, as close genes tend to be inherited together due to linkage.
Probability Laws in Mendelian Genetics
Mendel’s laws reflect probability rules applicable to independent events (like coin tosses).
Each allele segregates independently in gametes.
Complex Inheritance Patterns
Concept 14.3 highlights that inheritance patterns can be more complex:
Not all traits are dictated by a single gene with two alleles.
Principles of segregation and independent assortment still apply even in complex scenarios.
Extensions of Mendelian Genetics for a Single Gene
Situations where inheritance deviates from classic Mendelian patterns include:
Non-complete dominance:
Complete dominance: where one allele completely masks another.
Incomplete dominance: F1 Phenotype is between the P phenotype; an intermediate (e.g., red x white = pink).
Even though parents are heterozygous for different traits, the offspring can exhibit a unique color despite lack of “blending”.
Codominance: 2 dominant alleles expressed in phenotype (e.g., AB blood type).
Frequency of Dominant Alleles: not always more common in populations than recessive alleles.
Polydactyly: 1 in 400 U.S. births produces someone with an extra toe or finger. Caused by dominant allele.
Multiple Alleles Present in Populations: Example - ABO blood groups determined by three alleles (IA, IB, i).
I^A, I^B, i - A, B are carbohydrates attached to the surface of red blood cells, which determine the blood type of an individual.
Pleiotropy and Polygenic Inheritance
Pleiotropy: single genes affecting multiple phenotypic traits.
Sickle Cell Disease: Mutation in HBB gene, causing it to be HBs. This causes a sickle shape due to hydrogen bonds in secondary structure, causing pain, organ damage and weakness.
Cystic Fibrosis: multiple symptoms: excessive mucus production
Polygenic Inheritance: multiple genes that independently affect a single trait.
Example: height, affected by over 180 genes; skin pigmentation; moles and more.
Quantitative Characters: those that vary in the population along a continuum.
Epistasis: expression of a gene at one locus alters phenotypic expression at a second locus
Lab color depends on two genes:
B for black and b for brown
E for color and e for no color
BbEe x BbEe = multiple coat colors.
Multifactorial Traits and Nature vs. Nurture
Traits influenced by both genetic (genotype) and environmental factors yield broad phenotypic ranges.
Phenotype range is always broadest for polygenic characters.
Multifactorial Traits: Traits which depend on multiple genes combined with environmental influences… color.
Human Genetics and Mendelian Inheritance Patterns
Humans are not good subjects for genetic research.
Ethics… Ellis Island and Eugenics.
Generational time is very long.
Few offspring
Basic Mendelian genetics endures as foundation of human genetics.
Pedigrees: Visuals cataloging family traits across generations to predict inheritance patterns.
Recessively Inherited Disorders
Disorders typically require homozygosity (aa) for the recessive allele (phenotype shows only in homozygous individuals).
Carriers are heterozygous individuals who carry recessive gene but phenotypically don’t exhibit it (Aa).
Example: Albinism - characterized by lack of pigmentation.
Dominantly Inherited Disorders
Rare dominant alleles causing diseases arise via mutations.
A lot more dangerous than recessively inherited disorders, as only one copy of the dominant allele is needed for the disorder to manifest in the phenotype.
Example: Achondroplasia, a form of dwarfism from a dominant allele.
Multifactorial Disorders
Hydrangeas: Have genetics which allow environment to play a role.
Many diseases, such as heart disease, cancer, alcoholism, ad mental illnesses, have both genetic and environment components.
Genetic testing and counseling… ID carriers, fetal testing, amnio cenctesis, chorionic villus sampling