Study Notes: Mendelian Genetics, Chromosomal Inheritance, and Meiosis

Chapter 13: Meiosis and Sexual Reproduction Life Cycles

Offspring Acquire Genes from Parents by Inheriting Chromosomes

Sexual reproduction involves the transmission of genetic information from parents to offspring through the inheritance of chromosomes. This process ensures genetic continuity and variation in populations.

• Genes: Units of heredity made up of DNA, located on chromosomes.

• Gametes: Reproductive cells (sperm and egg) that transmit genes from one generation to the next.

• Somatic Cells: All body cells except gametes; contain two sets of chromosomes (diploid).

• Fertilization: The fusion of gametes, restoring the diploid number of chromosomes.

• Variation: Offspring differ genetically from their parents and siblings due to the reshuffling of genes during meiosis and fertilization.

Fertilization and Meiosis Alternate in Sexual Life Cycles

Sexual life cycles involve the alternation of meiosis and fertilization, which maintains chromosome number across generations.

• Human Life Cycle: Involves meiosis producing haploid gametes, fertilization restoring diploid state, and mitosis for growth and development.

• Types of Life Cycles:

  • Animals: Gametes are the only haploid cells.

  • Plants and Some Algae: Exhibit alternation of generations with both multicellular diploid and haploid stages.

  • Fungi and Some Protists: Only the zygote is diploid; meiosis produces haploid cells that divide by mitosis.

Meiosis Reduces the Number of Chromosome Sets from Diploid to Haploid

Meiosis is a specialized type of cell division that reduces the chromosome number by half, producing haploid gametes from diploid cells.

• Stages of Meiosis:

  • Meiosis I: Homologous chromosomes separate, reducing chromosome number.

  • Meiosis II: Sister chromatids separate, similar to mitosis.

• Key Events:

  • Synapsis and crossing over during Prophase I increase genetic variation.

  • Independent assortment of chromosomes during Metaphase I.

Genetic Variation Produced in Sexual Life Cycles Contributes to Evolution

Sexual reproduction generates genetic diversity, which is essential for evolution by natural selection.

• Sources of Variation:

  • Independent assortment of chromosomes

  • Crossing over

  • Random fertilization

Chapter 14: Mendel and the Gene Idea

Mendel Used the Scientific Approach to Identify Two Laws of Inheritance

Gregor Mendel's experiments with pea plants established the fundamental principles of heredity, known as the law of segregation and the law of independent assortment.

• Law of Segregation: Each individual has two alleles for each gene, which segregate during gamete formation so that each gamete carries only one allele for each gene.

• Law of Independent Assortment: Genes for different traits can segregate independently during the formation of gametes.

• Key Terms: gene, allele, homozygous, heterozygous, dominant, recessive, genotype, phenotype.

Probability Laws Govern Mendelian Inheritance

Genetic crosses can be analyzed using the rules of probability, including the multiplication and addition rules.

• Multiplication Rule: Probability of two independent events occurring together is the product of their probabilities.

• Addition Rule: Probability of any one of two or more mutually exclusive events is the sum of their probabilities.

• Solving Genetics Problems: Use Punnett squares and probability rules to predict genotypic and phenotypic ratios.

Degrees of Dominance and Extensions to Mendelian Genetics

Not all traits follow simple dominant-recessive inheritance. There are variations such as incomplete dominance, codominance, and multiple alleles.

• Incomplete Dominance: Heterozygotes have an intermediate phenotype.

• Codominance: Both alleles are expressed in the phenotype.

• Multiple Alleles: More than two alleles exist for a gene (e.g., ABO blood groups).

• Pleiotropy: One gene affects multiple phenotypic traits.

• Epistasis: One gene affects the expression of another gene.

• Polygenic Inheritance: Multiple genes independently affect a single trait (e.g., skin color).

Many Human Traits Follow Mendelian Patterns of Inheritance

Some human traits are inherited according to Mendelian principles, including certain genetic disorders.

• Pedigree Analysis: Used to study inheritance patterns in families.

• Recessively Inherited Disorders: Require two copies of the mutant allele (e.g., cystic fibrosis, sickle-cell disease).

• Dominantly Inherited Disorders: Only one copy of the mutant allele is needed (e.g., Huntington's disease).

• Multifactorial Disorders: Influenced by genetic and environmental factors.

Chapter 15: The Chromosomal Basis of Inheritance

Mendelian Inheritance Has Its Physical Basis in the Behavior of Chromosomes

The chromosomal theory of inheritance states that genes are located on chromosomes, which segregate and independently assort during meiosis.

• Key Evidence: Behavior of chromosomes during meiosis parallels Mendel's laws.

• Thomas Hunt Morgan's Experiments: Demonstrated that genes are located on chromosomes using fruit flies (Drosophila melanogaster).

Sex-Linked Genes Exhibit Unique Patterns of Inheritance

Genes located on sex chromosomes (X and Y) show distinctive inheritance patterns, such as X-linked recessive traits being more common in males.

• Examples: Color blindness, hemophilia, Duchenne muscular dystrophy.

• X-Inactivation: In female mammals, one X chromosome is randomly inactivated in each cell.

Linked Genes Tend to Be Inherited Together

Genes located close together on the same chromosome tend to be inherited together, a phenomenon known as genetic linkage.

• Crossing Over: Homologous chromosomes exchange segments during meiosis, producing recombinant chromosomes and increasing genetic variation.

Alterations of Chromosome Number or Structure Cause Some Genetic Disorders

Changes in chromosome number (aneuploidy) or structure (deletions, duplications, inversions, translocations) can lead to genetic disorders.

• Aneuploidy: Abnormal number of chromosomes (e.g., Down syndrome: trisomy 21).

• Structural Alterations: Deletion, duplication, inversion, translocation.

• Sex Chromosome Aneuploidy: Turner syndrome (XO), Klinefelter syndrome (XXY).

Some Inheritance Patterns Are Exceptions to Standard Mendelian Inheritance

Not all genes are inherited in a Mendelian fashion. Some exceptions include genes located in organelles (mitochondria, chloroplasts) and genomic imprinting.

• Genomic Imprinting: Expression of an allele depends on whether it is inherited from the mother or the father.

• Extranuclear Genes: Genes located outside the nucleus, such as in mitochondria, are inherited maternally.

Table: Comparison of Mitosis and Meiosis

Feature	Mitosis	Meiosis
Number of cell divisions	1	2
Number of daughter cells	2	4
Chromosome number in daughter cells	Diploid (2n)	Haploid (n)
Genetic composition	Identical to parent	Genetically unique
Role in organism	Growth, repair, asexual reproduction	Sexual reproduction, genetic variation

Key Equations

• Probability of Independent Events: $P(A \text{ and } B) = P(A) \times P(B)$

• Probability of Mutually Exclusive Events: $P(A \text{ or } B) = P(A) + P(B)$

Additional info: These notes are adapted and expanded from class outlines based on Campbell Biology, covering core concepts in Mendelian genetics, chromosomal inheritance, and meiosis for General Biology students.