Ch+23+new+FP-module 8

Mendel's Laws

• Mendel's Laws: Principles governing inheritance as observed in one-trait and two-trait crosses.

• Variations to Mendel: Exploration of non-Mendelian inheritance patterns including X-linkage.

History and Genetics

• Early Theories of Inheritance:

  • Hippocrates' Pangenesis: Theory proposing that particles from all body parts contribute to reproductive organs, suggesting inheritance of acquired characteristics.

  • Blending Inheritance Theory: The idea that offspring inherit blended traits from parents, leading to intermediate characteristics.

Mendel and The Peas

• Gregor Mendel: Recognized as the Father of modern genetics due to his systematic approach and mathematical analysis of inheritance.

• Pea Plants as Subjects:

  • Benefits included short generation times, large offspring numbers, distinct varieties, and controllable matings.

Mendel's Pea Breeding Experiments

• Studied 7 Traits: Each trait had two distinct forms (phenotypes).

  • Generational Definitions:

    • P Generation: Parental generation.

    • F1 Generation: First filial generation offspring.

    • F2 Generation: Offspring of the F1 generation.

• Hybrid Offspring: Result from crossing two different parental types.

Mendel’s Experimental Results

• Consistent Patterns: In F1 generation, hybrids show only one of the parents’ traits.

• Monohybrid Cross:

  • In F2 generation, ratio typically shows 3:1 (dominant to recessive traits).

Mendel's Inferences

• Alleles: Different versions of genes;

  • Homozygous: Individuals with two identical alleles.

  • Heterozygous: Individuals with two different alleles.

• Dominance:

  • Dominant alleles determine phenotype appearance, while recessive alleles do not show unless homozygous.

• Law of Segregation: Alleles segregate during gamete formation.

Alleles and Homologous Chromosomes

• Understanding Alleles: Different alleles occupy the same locus on homologous chromosomes.

• Meiosis and Allele Segregation: Homologous chromosomes separate during Meiosis I, reflecting Mendelian patterns.

Genetics Terms and Shorthand

• Dominant vs. Recessive Alleles:

  • Dominant traits are represented by uppercase letters (e.g., A) and recessive by lowercase letters (e.g., a).

• Phenotype vs. Genotype:

  • Phenotype: Observable traits.

  • Genotype: Genetic makeup, which may include combinations like AA (dominant), Aa (heterozygous), or aa (recessive).

Law of Segregation and Gamete Formation

• Main Inheritance Principle:

  • Traits separate during gamete formation due to chromosome movement in meiosis.

  • Gametes must only have one allele from each parent (e.g., A or a, but not Aa).

Practice! - Understanding Traits

• Traits to Assess:

  • Ear lobes (attached/unattached), tongue rolling (can/can't), widow’s peak, freckles, polydactyly.

  • Engage in assessments to determine your own phenotype and infer genotype.

Genetics Problem-Solving Guidance

• General Approach:

  • Use consistent letters for alleles, determine parental genotype, predict gametes and offspring using Punnett squares if necessary.

• Genotype Considerations: Given phenotypes can lead to two genotype possibilities for dominant traits.

One Trait Crosses

• Monohybrid Cross Example: Illustrated by crossing true-breeding purple and white flowers to observe F1 generation traits.

Punnett Squares

• Utilization: Representation of possible offspring genotypes from parental gametes.

Understanding Probability in Genetics

• Probability Basics: Ranges from 0 to 1; events must add to 1.

• Example: Gamete possibilities in Aa individuals yield probabilities of allele combinations.

Test Crosses

• Purpose: To determine unknown genotypes of dominant phenotypes through crosses with known recessive genotypes.

• Example Process: Match unknown genotypes with recessive traits to reveal heterozygous or homozygous nature.

Dealing With Two Traits

• Gene Linkage: Determines whether traits are inherited together or separately; linked genes behave differently based on their chromosomal positions during gamete formation.

Law of Independent Assortment

• Pattern Recognition: Explains that different traits segregate independently of each other during gamete formation, allowing for a variety of combinations.

Two-Trait Cross

• Genetic Crosses:

  • Dihybrid kits analyze combinations of traits and yield a 9:3:3:1 ratio in the F2 generation when both traits are heterozygous.

Two Trait Test Cross Example

• Test Cross: Used to identify unknown genotypes based on offspring results through a test against known recessive traits.

Complex Patterns Beyond Mendel’s Explanation

• Real-World Complexity: Mendel's initial simplicity is altered by multi-gene traits, incomplete dominance, codominance, and environmental factors.

Incomplete Dominance

• Trait Expression: Heterozygotes display intermediate traits; e.g., red and white flowers yield pink offspring.

Codominance and Multiple Alleles

• Coexpression of Alleles: Both alleles can manifest simultaneously (e.g., blood type AB).

• Population Level Variation: More than two alleles can exist within populations, but individuals are diploid.

Polygenic Inheritance

• Multiple Genes for Traits: Traits like height and skin color are governed by multiple genes, leading to continuous variation.

Environmental Influences on Genes

• Nurture vs. Nature Debate: Various environmental factors can influence the expression of genes significantly affecting phenotype.

Sex Chromosomes

• Understanding Human Sex chromosomes:

  • Females have XX, males have XY; each contributes genetically different gametes.

Sex-Linked Genes

• Genes on Sex Chromosomes:

  • Typically on the X chromosome, affecting males more due to having one X chromosome.

Sex Linked Problem Example

• Problem Solving: Identifies traits passed through sex-linked inheritance in a cross of different phenotypes.

Sex-Linked Disorders in Humans

• Disorders: Conditions such as hemophilia and color blindness affect individuals based on sex-linked alleles, frequently informally observed in males due to the absence of a second allele.