Chp. 15 - Exam 3 Bio 190

Chapter 15 - Transmission of Genetic Information from Parents to Offspring I: Patterns That Follow Mendel's Laws

• Mendel's laws of inheritance are outlined
• Chromosome theory of inheritance is explained
• Pedigree analysis of human traits is discussed
• Variations in inheritance patterns and their molecular basis are examined
• Sex chromosomes and X-linked inheritance patterns are described

Section 15.1 Learning Outcomes

• Advantages of using garden peas to study inheritance are listed
• The difference between dominant and recessive traits is described
• Genotype and phenotype are distinguished
• The outcome of genetic crosses using a Punnett square is predicted
• Mendel's law of segregation and law of independent assortment are stated

15.1 Mendel's Laws of Inheritance

• Parents and offspring often have striking similarities, traits are passed down from parents to offspring
• Observations of chromosome transmission during mitosis and meiosis provided evidence for particulate inheritance
• Particulate inheritance proposes that hereditary traits are transmitted in discrete units (genes)
• A gene is a unit of heredity

15.1 Mendel's Laws of Inheritance - Many Different Patterns Exist

• Simple Mendelian inheritance: Traits are determined by a pair of alleles, dominant allele masks the recessive allele.
• Incomplete dominance: Heterozygote displays an intermediate phenotype to homozygotes.
• X-linked inheritance: Traits are located on the X chromosome, appearing more in males than females.
• Epigenetic inheritance: Genes are silenced during gamete or embryo development, impacting traits during an individual's lifetime
• Linkage: Multiple genes are transmitted as a unit, linked genes do not sort independently.
• Extranuclear inheritance: Traits inherited from the mother, located in mitochondrial or chloroplast genomes.

15.1 Mendel's Laws of Inheritance

• In 1856, Gregor Mendel began using pea plants to study inheritance over 8 years.
• A trait is an identifiable characteristic (e.g., flower color, seed color), a variant for a character

15.1 Mendel's Laws of Inheritance - Mendel Chose the Garden Pea

• Pea plants are usually self-fertilizing
• Stamens form male gametes and ovules form female gametes, facilitating cross-pollination

15.1 Mendel's Laws of Inheritance - By Following the Inheritance Pattern of Single Traits

• Mendel studied inheritance patterns of plants differing in a single character (single-factor or monohybrid crosses)
• "True-breeding" plants were crossed
• All offspring were monohybrids (heterozygous) showing the dominant trait
• Mating monohybrids revealed the recessive trait

15.1 Mendel's Laws of Inheritance - By Following the Inheritance Pattern of Single Traits (continued)

• Mendel named the trait displayed in the F1 generation the dominant trait
• The trait masked in the F1 generation and reappearing in the F2 generation is the recessive trait
• Two "particles" (alleles) generate each trait
• With one dominant allele present, the dominant trait appears

15.1 Mendel's Laws of Inheritance - By Following the Inheritance Pattern of Single Traits (continued)

• Alleles are inherited in pairs, one maternal and one paternal, at each gene locus
• Mendel's law of segregation: Alleles of a gene separate and segregate during gamete formation, with each gamete receiving only one allele.

15.1 Mendel's Laws of Inheritance - By Following the Inheritance Pattern of Single Traits (continued)

• After gamete formation, they unite in fertilization
• Depending on gamete genotypes, different genotypes and phenotypes are observed in the offspring.
• A Punnett square is a visual representation of possible offspring phenotypes and genotypes.

15.1 Mendel's Laws of Inheritance - Analyzing the Inheritance Pattern of Two Characters

• Mendel asked if separation of alleles at one locus influences the separation of alleles at another locus.
• The Punnett square with 4 gametes from each parent results in 16 possible genotype outcomes

15.2 Chromosome Theory of Inheritance

• At Mendel's time, the nature and location of genes were unknown.
• Later scientists used microscopes, observing dividing cells, and identified chromosomes as carriers of hereditary information.

15.2 Chromosome Theory of Inheritance (continued)

• Chromosomes carry DNA, the genetic material, and genes are found within the chromosomes
• Chromosomes are replicated and passed from parents to offspring.
• Diploid cells have 2 sets of homologous chromosomes (maternal and paternal), each carrying a full complement of genes.
• Meiosis: Homologous chromosomes segregate into different daughter nuclei.
• Gametes are haploid cells that combine during fertilization to form a diploid cell, each gamete transmits one set of chromosomes

15.2 Chromosome Theory of Inheritance - Law of Segregation

• A gene's locus is its position on a chromosome
• Each member of a homologous pair carries an allele for the same gene at the same locus
• The pairing and segregation of homologous chromosomes during meiosis explains Mendel's law of segregation

15.2 Chromosome Theory of Inheritance - Law of Segregation (continued)

• Mendel's law of segregation: Two alleles of a gene separate during gamete formation, ensuring each gamete receives only one allele

15.3 Pedigree Analysis of Human Traits

• Human geneticists analyze pedigrees (family trees) to determine inheritance patterns
• Pedigree analysis determines if a mutant allele is dominant or recessive and predicts the likelihood of individuals being affected
• Wild-type alleles are common; mutant alleles are rare.
• Most genes display autosomal inheritance patterns
• Genes on sex chromosomes have distinct inheritance patterns

15.3 Pedigree Analysis of Human Traits (continued)

• Pedigrees trace diseases like cystic fibrosis (autosomal recessive), showing affected individuals and unaffected individuals
• Heterozygotes (carriers) can produce affected offspring
• Pedigrees can also trace dominant disorders such as Huntington disease (autosomal dominant). Key trait is every affected person has an affected parent.

15.4 Variations in Inheritance Patterns & Molecular Basis

• Mendelian inheritance describes inheritance patterns of genes that segregate and assort independently
• In simple Mendelian inheritance, alleles are either dominant or recessive
• Understanding protein function at a molecular level explains variations in inheritance patterns

15.4 Variations in Inheritance Patterns & Molecular Basis (continued)

• Different inheritance patterns exist (e.g., incomplete dominance, X-linked, epigenetic inheritance, linkage, extranuclear inheritance)
• Protein function at the molecular level explains the phenomenon of dominance
• Sometimes a dominant allele need only a single copy for sufficient protein function, heterozygotes display a dominant phenotype
• Gene regulation can increase expression level for heterozygotes

15.4 Variations in Inheritance Patterns & Molecular Basis (continued) - Incomplete Dominance

• Incomplete dominance occurs when the heterozygote shows an intermediate phenotype
• In the case of a red-flowered plant crossed with a white-flowered plant, the heterozygote will have a pink phenotype
• The genotype and phenotype ratios for F2 generation are both 1:2:1

15.4 Variations in Inheritance Patterns & Molecular Basis (continued)- Environmental Role in Phenotype

• Genotype provides a plan for the phenotype
• Environment provides nutrients and energy for phenotype execution
• The norm of reaction is the phenotypic range that individuals with a particular genotype exhibit under different environmental conditions
• Genetically identical plants can exhibit different heights under different temperature conditions

15.5 Sex Chromosomes and X-Linked Inheritance Patterns

• Sex chromosomes determine the sex of individuals, differing between males and females
• Sex chromosomes present in many (but not all) species with 2 sexes.
• X-linked genes are located on the X chromosome.
• Males are hemizygous for X-linked genes; they have only one copy of each X-linked gene (one X chromosome)
• X-linked recessive traits are more likely to appear in males.

Explore the fundamental principles of genetic inheritance as outlined by Mendel. This quiz covers Mendel's laws, chromosome theory, and the significance of genomic patterns in humans. Test your understanding of dominant vs. recessive traits, and learn how to predict genetic crosses using Punnett squares.