Genetics Lecture 3: Mendelian Genetics and Alleles

Basic Principles of Heredity

Introduction to Mendelian Genetics

Mendelian genetics forms the foundation of classical genetics, describing how traits are inherited from one generation to the next through discrete units called genes. Johann Gregor Mendel's experiments with garden peas established the basic laws of heredity.

• Gene: A unit of heredity that is transferred from parent to offspring and determines some characteristic of the offspring.

• Allele: Different forms of a gene found at the same locus.

• Genotype: The genetic constitution of an organism.

• Phenotype: The observable characteristics or traits of an organism.

Johann Gregor Mendel and His Experiments

Mendel conducted breeding experiments with garden peas from 1856 to 1863, presenting his results in 1865. His work was published in 1866 but remained largely unnoticed until its recognition in 1900.

• Key Dates:

  • 1856-1863: Breeding experiments

  • 1865: Results presented at a scientific meeting

  • 1866: Paper published

  • 1900: Work recognized

Historical Theories of Inheritance

Blending Inheritance

Blending inheritance was an early theory suggesting that offspring are a 'blend' of parental traits. If true, all members of a species would eventually converge upon a uniform phenotype, and variation would disappear. However, some traits reappeared after several generations, contradicting this theory.

• Example: Crossing a white horse and a black horse produces a gray horse, but some traits reappear in later generations.

Mendel's Experimental System

Garden Pea and Mendel’s Success

Mendel chose the garden pea (Pisum sativum) for its suitability for genetic studies.

• Easy to cultivate

• Produces many progeny

• Relatively short generation time

• Diploid and sexually reproducing

• Parentage can be controlled

• Self-fertilizing (monecious) but can be cross-pollinated

• Different pure-breeding strains available

Self-Fertilization and Cross-Fertilization in Peas

• Self-fertilization: Produces true-breeding generations.

• Cross-fertilization: Achieved by preventing self-fertilization and manually transferring pollen.

Mendelian Laws and Genetic Crosses

Monohybrid Crosses

Monohybrid crosses involve parents that differ in a single trait. Mendel observed that:

• F1 plants inherit genetic factors from both parents, but only one phenotype appears (dominant).

• The two alleles in each parent separate when gametes form; each gamete receives one allele.

• Traits that appear unchanged in F1 are dominant; those that disappear are recessive.

• Alleles separate randomly and with equal probability into gametes.

Punnett Squares

Punnett squares are used to determine the resulting genotypes and phenotypes from genetic crosses.

• Example: Crossing Bb x Bb yields genotypes BB, Bb, and bb.

Backcross and Testcross

• Backcross: Mating between an F1 individual and one of the parental genotypes, often the recessive parent.

• Testcross: Crossing an individual of unknown genotype with a homozygous recessive individual to determine the genotype.

Mendel’s Second Law: Principle of Independent Assortment

Genes at different loci separate independently during gamete formation, provided the genes are on different chromosomes. Dihybrid crosses typically yield a 9:3:3:1 phenotypic ratio.

• Example: Crossing RrYy x rryy and analyzing the resulting phenotypes.

Probability in Genetics

Probability Rules

• Multiplication Rule: The probability of two independent events occurring together is the product of their individual probabilities. Equation: $P(A ext{ and } B) = P(A) imes P(B)$

• Addition Rule: The probability of either of two mutually exclusive events occurring is the sum of their individual probabilities. Equation: $P(A ext{ or } B) = P(A) + P(B)$

Applying Probability to Genetic Crosses

• Break down complex crosses into single-locus crosses.

• Use multiplication rule for independent loci.

• Example: Probability of AaBbccDdEe x AaBbCcddEe yielding aabbccddee: $1/4 imes 1/4 imes 1/2 imes 1/2 imes 1/4 = 1/256$

Sampling Error and Statistical Analysis

Sampling Error

Random events may not yield expected results due to sampling error. The chi-square test evaluates the role of chance in deviations between observed and expected outcomes.

• Chi-square formula: $\chi^2 = \sum \frac{(O - E)^2}{E}$ Where O = observed, E = expected.

• Degrees of freedom (df): Number of outcomes minus one ($df = n - 1$).

• Null hypothesis ($H_0$): No real difference between observed and expected values.

Pedigree Analysis and Human Inheritance

Pedigree Conventions

Pedigrees are diagrams that show the inheritance of traits in families. They reveal patterns of inheritance and help identify carriers and affected individuals.

• Squares represent males; circles represent females.

• Shaded symbols indicate affected individuals.

• Half-shaded symbols indicate carriers (heterozygotes).

Autosomal Recessive Traits

• Traits typically skip generations.

• Appear equally in both sexes.

• Unaffected parents can have affected offspring.

Autosomal Dominant Traits

• Traits almost always appear in each generation.

• Affected individuals have at least one affected parent.

• Appear equally in both sexes.

Representative Human Traits

Recessive Traits	Dominant Traits
Albinism	Achondroplasia
Alkaptonuria	Brachydactyly
Ataxia telangiectasia	Congenital stationary night blindness
Color blindness	Ehler-Danlos syndrome
Cystic fibrosis	Hypotrichosis
Duchenne muscular dystrophy	Huntington disease
Galactosemia	Hypercholesterolemia
Hemophilia	Marfan syndrome
Lesch-Nyhan syndrome	Myotonic dystrophy
Phenylketonuria	Neurofibromatosis
Sickle-cell anemia	Phenylthiocarbamide tasting
Tay-Sachs disease	Porphyria (some forms)

Genetic Syndromes: Syndromic Hearing Loss and Deafness

DFNB1 is an autosomal recessive form of congenital, non-progressive, sensorineural hearing loss. The gene encodes connexin 26, a gap junction protein that forms channels for the transport of nutrients and signaling molecules between neighboring cells.

• Inheritance: Both parents must carry the recessive allele for offspring to be affected.

• Function: Connexin proteins form gap junctions essential for cellular communication in the inner ear.

Additional info: Pedigree analysis and probability calculations are essential tools for predicting inheritance patterns and assessing genetic risk in families.