Biology 101: Intro to Genetics

Biology 101: Intro to Genetics

Gregor Mendel

• Title: The father of genetics.

• Background:

  • An Austrian monk and scientist.

  • Conducted the first systematic study of inheritance using pea plants in the 1860s.

  • Studied biology, physics, and math at the University of Vienna.

  • Returned to the monastery to teach and conduct research.

The Pea Plant

• Reproductive Structures:

  • Pea flowers possess both male (pollen) and female (ovule) gametes, typically undergoing self-fertilization.

  • Mendel manipulated reproduction by removing male structures and performing hand-pollination to control mating.

Traits and Characters

• Definitions:

  • Character: An observable physical feature (e.g., seed shape, seed color, flower color).

  • Trait: A specific form of a character (e.g., wrinkled seeds vs. smooth seeds, purple flowers vs. white flowers).

Mendel’s Experiments

• True-breeding Plants:

  • Identified true-breeding plants for certain traits.

  • Crossing two true-breeding purple flower plants results in solely purple offspring.

  • This method applies to all traits studied.

• Crossing Plants:

  • Crossed true-breeding plants with different traits (e.g., purple flowers and white flowers).

  • This formed the parental generation (P).

Later Generations

• F1 Generation:

  • The offspring from the parental cross are called the F1 generation (first filial generation).

  • Mendel recorded the number of F1 offspring expressing each trait (e.g., white and purple flowers).

• F2 Generation:

  • Crossed F1 generation individuals to produce the F2 generation.

Monohybrid Crosses

• Hybrid Definition:

  • A hybrid is an individual produced by crossing parents with contrasting traits.

  • All F1 generation members were hybrids of the P generation traits.

• Conducting Monohybrid Crosses:

  • Crossed F1 individuals with one another.

  • The term monohybrid refers to examining hybrids for a single trait.

Monohybrid Cross for Seed Shape

• Crossing Results:

  • Repeated monohybrid crosses for 7 traits with similar observations:

  • F1 generation exhibited one parent’s trait while the alternative appeared absent.

  • The absent trait manifested in approximately 1/4 of the F2 generation.

Dominant and Recessive Traits

• Observations:

  • One trait appeared consistently across all F1 plants, termed dominant.

  • The trait that seemed lost in F1 is considered recessive.

  • Ratios in the F2 generation showed dominant to recessive traits as approximately 3:1.

• Question: Which flower color is dominant?

Results of Monohybrid Crosses

• Data Table (Table 12.1):

  • Mendel's results from various trait crosses:

  • Round seeds x Wrinkled seeds: 5,474 dominant to 1,850 recessive, total 7,324 with a ratio of 2.96:1.

  • Yellow seeds x Green seeds: 6,022 dominant to 2,001 recessive, total 8,023 with a ratio of 3.01:1.

  • Purple flowers x White flowers: 705 dominant to 224 recessive, total 929 with a ratio of 3.15:1.

  • Inflated pods x Constricted pods: 882 dominant to 299 recessive, total 1,181 with a ratio of 2.95:1.

  • Green pods x Yellow pods: 428 dominant to 152 recessive, total 580 with a ratio of 2.82:1.

  • Axial flowers × Terminal flowers: 651 dominant to 207 recessive, total 858 with a ratio of 3.14:1.

  • Tall stems x Dwarf stems (1 m): 787 dominant to 277 recessive, total 1,064 with a ratio of 2.84:1 (0.3 m).

Genetic Information in Peas

• Storage of Genetic Information:

  • Peas, like all organisms, store genetic information in DNA within cell nuclei on chromosomes (analogous to volumes in a large book).

  • Each chromosome harbors unique “recipes” for proteins known as genes, determining traits.

• Genes and Chromosomes:

  • A gene is defined as a segment of DNA on a chromosome responsible for encoding a specific protein (e.g., an enzyme that produces a purple pigment).

  • Variations in genes, known as alleles, exist among different individuals (e.g., producing yellow pigment instead of purple).

Inheritance of Traits

• Trait Determination:

  • Traits are inherited through chromosomes from parents.

  • Humans have 46 chromosomes (23 from each parent), arranged in homologous pairs containing identical genes but potentially different alleles.

• Alleles in Genetics:

  • An allele is a specific variant of a gene (e.g., hair color gene with alleles for red, blond, black, brunette).

  • The expressed trait depends on the two alleles—dominant alleles exert greater influence on traits than recessive ones.

Mendel's Findings Reviewed

• Alleles in Pea Plants:

  • Each pea plant has two alleles per gene, one inherited from each parent.

  • Alleles may be identical (homozygous) or differ (heterozygous).

• Phenotype vs Genotype:

  • A phenotype is the observable expression (e.g., white flowers).

  • Genotype is composed of alleles carried by individuals.

• Genotypes Correspondence:

  • In homozygous individuals, phenotypes reflect genotypes (e.g., two white flower alleles reflect a white flower phenotype).

  • In heterozygous cases, dominant alleles mask recessive ones (e.g., a purple flower results from one purple and one white allele).

Transmission of Alleles

• Gamete Formation:

  • Alleles sort into gametes (sperm and egg in humans; pollen and ovule in plants).

  • This process occurs during meiosis, where gametes receive half the parent's chromosome count.

• Mendel’s Law of Segregation:

  • During gamete formation, a person's two alleles segregate.

  • Each gamete gets a single allele, and fertilization restores the species’ diploid state.

Gamete Generation Examples

• Homozygous Gametes:

  • A homozygous dominant (RR) produces gametes all containing the “R” allele.

  • A homozygous recessive (rr) results in all “r” gametes.

• Heterozygous Gametes:

  • An individual with genotype Rr produces two types of gametes: R and r.

Punnett Squares

• Purpose:

  • Punnett squares predict offspring genotypes (and phenotypes) based on parental gametes.

  • Male gametes are listed across the top, female gametes on the side; offspring combinations fill in the square.

• Example of Punnett Square:

  • Given parental generation of RR x rr:

  • F1 Generation results in all Rr individuals.

  • F2 Generation yields a 3:1 ratio in seed phenotypes from a self-pollinating Rr plant.

Dominance Mechanism

• Gene Functionality:

  • Genes influence phenotypes by coding for proteins responsible for specific functions, e.g., purple pigment production.

  • Dominant alleles typically yield a functional protein (purple pigment); recessive alleles may not function (no pigment, leading to white flowers).

  • In heterozygotes, one functioning dominant allele suffices to produce the desired phenotype (e.g., purple).

Transition to Human Inheritance Patterns

• Application of Mendel’s Laws:

  • Mendel's principles apply for tracking human genetics through pedigree analysis over many generations, useful for understanding allele dominance.

Dominant Trait Inheritance Patterns

• Characteristics:

  • Affected individuals harbor at least one dominant allele (e.g., Aa or AA).

  • Affected individuals typically have an affected parent.

  • Approximately 50% of offspring will exhibit the trait if one parent is affected.

Example of Autosomal Dominant Inheritance

• Generational Crossing:

  • 1st Generation (Grandparents): Ww, WW, ww, Ww.

  • 2nd Generation (Parents, Aunts, Uncles): Various combinations producing Ww and ww offspring.

  • 3rd Generation (Sisters): Effects of the widow's peak trait resulting in either WW, Ww, or ww phenotypes.

Autosomal Dominant Genetic Disorders

• Common Conditions:

  • Achondroplasia: A type of dwarfism characterized by abnormal growth.

  • Polydactyly: The presence of extra fingers or toes.

  • Huntington’s Disease: A degenerative neurological disorder that is fatal.

Recessive Trait Inheritance Patterns

• Characteristics:

  • Individuals with recessive traits can have unaffected parents (both carriers Aa).

  • Traits can appear to skip generations with a probability of 1 in 4 offspring exhibiting the trait if both parents are heterozygous.

Example of Autosomal Recessive Inheritance

• Generational Crossing:

  • 1st Generation (Grandparents): Ff, Ff, and ff.

  • 2nd Generation (Parents, Aunts, Uncles): Variations producing FF, Ff, and ff children.

  • 3rd Generation (Sisters): Representation of recessive traits.

Autosomal Recessive Genetic Disorders

• Common Conditions:

  • Tay Sachs Disease: Dysfunctional lysosomes leading to early death.

  • Cystic Fibrosis: Abnormal mucus production triggering severe respiratory infections.

  • Sickle Cell Anemia: Malformation in hemoglobin causing efficient oxygen transport impairment and possible tissue damage.

Tracking Rare Alleles

• Challenges:

  • Rare recessive alleles are difficult to trace as heterozygotes display no symptoms.

  • Unlikely for two carriers to marry, and amongst their offspring, the chance for inheritance is merely 25%.

• Genealogical Studies in Isolated Populations:

  • Geneticists focus on isolated groups (e.g., Amish communities) or geographically confined demographics.

  • Smaller populations enhance the chances of finding carriers who share recessive genes.

Polygenic Inheritance

• Characteristics:

  • Some human traits arise from multiple gene interactions, e.g., hair, skin, and eye colors.

  • More dominant alleles lead to darker coloration by contributing to pigmentation.

Environmental Influence on Traits

• Examples:

  • Traits like human height significantly depend on nutrition and environmental factors.

  • A graphical presentation of men's average heights by year of birth demonstrates changes due to these influences.

Multifactorial Disorders

• Origins of Disease:

  • Many diseases are the result of complex interactions between genetic predispositions and environmental factors.

  • Example: Heart disease, illustrating the interplay between lifestyle and genetic factors.

Genetic Mutations

• Definition:

  • A mutation represents a heritable change in the DNA sequence.

  • Changes can result from DNA damage or errors in replication.

• Long-term Effects:

  • If a mutation leads to an advantageous trait, natural selection can favor its spread throughout the population over generations.