Mendel's Experiments: Genetic Inheritance

• Gregor Mendel is considered the father of modern genetics.
• Mendel's pea plant experiments provided the foundation for understanding inheritance patterns and genetic theories.

Mendel's Experiments: Key Topics

• Mendel began with true-breeding pea plants that consistently produced offspring with the same traits when self-pollinated.
• He tracked how traits were inherited by cross-pollinating true-breeding plants with different traits.
• Mendel used self-fertilization (plant pollinates itself) and cross-fertilization (crossing two different plants) to control genetic outcomes.
• Self-fertilization produced offspring with predictable traits, while cross-fertilization introduced genetic variability.
• Mendel differentiated between genotype (genetic makeup) and phenotype (physical expression of traits).
• Experiments examined genotypes and phenotypes to understand inheritance patterns.
• Punnett squares predict offspring genetic outcomes based on parental genotypes.
• A monohybrid cross (crossing two heterozygous plants for a single trait) yields a 3:1 phenotypic ratio, showing trait dominance.

Mendel's Key Laws

• The Law of Segregation states that two alleles for each gene separate during gamete formation.
• Each gamete carries only one allele for each gene, so offspring inherit one allele from each parent.
• The Law of Independent Assortment states that genes on different chromosomes assort independently during gamete formation.
• Independent assortment creates genetic diversity in offspring by mixing allele combinations.

Dominant and Recessive Alleles

• Mendelian genetics distinguishes between dominant and recessive alleles.
• Dominant alleles (e.g., tallness, T) mask recessive allele (t) effects in heterozygous individuals.
• Recessive traits (e.g., shortness) appear only if both alleles are recessive (tt).
• Dominant alleles are expressed in the phenotype with only one copy present; a heterozygous individual (Tt) will display the dominant trait.
• Recessive alleles are only expressed in the phenotype if both copies of the gene are recessive; a homozygous recessive individual (tt) will be short.

Genotypic and Phenotypic Ratios

• The genotypic ratio is the relative frequency of different genotypes.
• The phenotypic ratio is the frequency of observable traits.
• Punnett squares predict genotypic and phenotypic ratios, key to understanding inheritance patterns.
• In a monohybrid cross (Tt x Tt), the phenotypic ratio is 3:1 (3 tall plants to 1 short plant).
• The genotypic ratio in a monohybrid cross is 1:2:1 (1 homozygous dominant, 2 heterozygous, 1 homozygous recessive).
• A dihybrid cross (AaBb x AaBb) involves two traits.
• The phenotypic ratio in a dihybrid cross is 9:3:3:1, reflecting independent assortment of alleles.

Human Genetics: Pedigrees and Inheritance Patterns

• Human genetics involves understanding how traits are passed through generations.
• Pedigree analysis is a tool for studying inheritance in families using standardized symbols.
• Pedigrees show trait inheritance patterns across generations, using filled symbols for affected individuals and empty symbols for unaffected.
• Pedigrees help understand how genetic disorders pass down and predict inheritance likelihood.
• In recessive inheritance, a genetic disorder appears if an individual inherits two copies of the mutated allele (e.g., cystic fibrosis).
• Dominant disorders can appear with just one copy of the mutant allele.
• X-linked traits are inherited differently in males (XY) and females (XX).
• Males need only one copy of a mutant allele on the X chromosome to express the trait.
• Females need two copies of the mutant allele to express an X-linked recessive trait.

Chromosomal Behavior and Meiosis

• Meiosis forms gametes (sperm and egg cells) and is central to genetic inheritance.
• Chromosome behavior during meiosis helps explain Mendelian inheritance principles.
• The synaptonemal complex forms during Prophase I of meiosis.
• It helps homologous chromosomes pair and align for recombination.
• Genes on different chromosomes assort independently during meiosis, contributing to genetic diversity.
• Independent assortment is evident in dihybrid crosses or multiple gene interactions.

Genetic Recombination and Linkage

• Genetic recombination and linked genes relate to exam questions.
• Genetic recombination occurs during meiosis when chromosomes exchange DNA segments, creating new allele combinations.
• Linked genes, located close together on the same chromosome, tend to be inherited together.
• Genes far apart on the chromosome are more likely to assort independently.

Sex Determination Systems

• Organisms use different sex determination systems.
• In the X-Y system (humans), males are XY and females are XX.
• The Y chromosome and the SRY gene determine male development.
• In the X-0 system (insects), sex is determined by the ratio of X chromosomes to autosomes.
• Males have one X chromosome (XO), while females have two (XX).
• In the Z-W system (birds, reptiles), males are ZZ and females are ZW.
• This system is the reverse of the X-Y system, where males are homogametic (ZZ) and females are heterogametic (ZW).
• In the haplodiploid system (bees), males are haploid (from unfertilized eggs) and females are diploid (from fertilized eggs).
• This system plays a role in bee colony social structure.

Genetic Calculations and Probability

• Understanding genetic probabilities predicts genetic cross outcomes.
• Punnett squares predict genotypic and phenotypic ratios.
• Multiplication and forked-line methods can be used for complex genetic problems.
• A test cross determines if an organism with a dominant phenotype is homozygous dominant or heterozygous.
• It involves crossing the organism with a homozygous recessive individual.

Genetic Disorders and Pedigree Analysis

• Cystic fibrosis is inherited recessively.
• Pedigree analysis traces cystic fibrosis inheritance and determines offspring inheritance likelihood.