Chapter 09 Sexual Reproduction and Meiosis
Asexual Reproduction
Definition: A form of reproduction where offspring are genetically identical to the parent.
Characteristics:
Relatively rare in multicellular organisms.
Predominantly seen in single-celled organisms.
Mechanism: Mitosis is the underlying process for asexual reproduction.
Example: Amoeba reproducing asexually is depicted.
Meiosis and Sexual Reproduction
Definition: Meiosis is a specialized type of cell division essential for sexual reproduction.
Outcome: It generates sperm and egg cells, each genetically unique.
Importance: These specialized sex cells are fundamental to sexual reproduction.
Genetic Diversity and Costs of Sexual Reproduction
Energy Requirement: Attracting a mate and producing sex cells demand considerable energy, which can be costly to the organism.
Value of Sexual Reproduction: Despite the costs, sexual reproduction is advantageous as it can create or maintain genetic diversity.
Homologous Chromosomes
Definition: A homologous pair of chromosomes will have the same size, structure, and genes but can carry different alleles.
Composition: In each pair, one chromosome is inherited from the father and one from the mother.
Example: The A and D alleles from each parent are the same, while the B alleles differ.
Diploid Cells
Definition: Diploid cells contain two sets of homologous chromosomes, contributing to the genetic makeup of sexually reproducing organisms.
Inheritance: One set of chromosomes is inherited from each parent.
Karyotype Analysis: A karyotype displays 46 chromosomes arranged in 23 pairs—22 pairs of homologous autosomes and 1 pair of sex chromosomes (XX for females, XY for males).
Chromosome Alleles in Homologous Chromosomes
Unique Characteristics: Homologous chromosomes carry genes for the same traits, arranged in the same order but containing different alleles.
Inheritance: Each individual inherits one allele per gene from each parent.
Life Cycle Phases
Mitosis and Meiosis: In multicellular organisms, mitosis is used for growth and repair, while meiosis is employed for sexual reproduction.
Germ Cells and Gametes
Definition of Germ Cells: Specialized cells capable of undergoing meiosis, located in the ovaries and testicles.
Production: Germ cells generate gametes (sperm and egg cells).
Characteristics of Gametes:
Gametes are haploid and do not undergo mitosis.
During meiosis, the chromosome number is halved, resulting in unique cells.
Fertilization and Zygotes
Process of Fertilization: Fusion of sperm and egg to form a zygote.
Zygote Characteristics:
The zygote is the first cell of the new organism.
It undergoes mitosis to develop into an adult, maintaining genetic identity with the zygote.
Haploid Gametes and Chromosome Numbers
Chromosome Fusion: In humans, sperm cells (23 chromosomes) fuse with egg cells (23 chromosomes) resulting in a zygote with 46 chromosomes.
Comparison: Meiosis and Mitosis
Both processes include similar phases: Interphase, Prophase, Metaphase, Anaphase, Telophase, and Cytokinesis.
Unique Aspect of Meiosis: PMAT occurs twice, resulting in four cells with unique mixtures of alleles and a halved chromosome count.
Genetic Variability from Meiosis
Uniqueness of Offspring: Offspring can be genetically unique despite originating from the same parents.
Gamete Variation: Chromosomes in gametes are shuffled, resulting in genetic diversity.
Mechanisms of Genetic Shuffling in Meiosis
Crossing Over: During Prophase I, homologous chromosomes align and exchange genetic material, creating new allele combinations.
Independent Assortment: In Metaphase I, chromosome pairs align randomly, producing various combinations of chromosomes.
For humans, the combinations possible are given by (2^n), leading to about 8 million combinations of chromosomes in gametes.
This variability can result in approximately 64 trillion combinations when combining the gametes of a mother and a father.
Random Fertilization
Any egg can combine with any sperm during the mating process, increasing genetic diversity in offspring.
Types of Twins
Monozygotic Twins: Genetically identical twins form when a single embryo splits during development.
Dizygotic Twins: Unique DNA twins arise from two separate eggs fertilized by different sperm; their genetic similarity is akin to that of regular siblings.
Functions of Mitosis and Meiosis
Mitosis: Responsible for adding and replacing identical cells and is the process used for cellular growth and maintenance.
Meiosis: A unique process that produces haploid gametes with distinct DNA, essential for sexual reproduction.
Distinctions of Meiosis:
Two rounds of cell division
Crossing over of genetic material
Independent assortment of chromosomes
Errors in Meiosis
Nondisjunction: A failure of chromosome separation that can result in gametes with incorrect chromosome numbers, leading to potential developmental issues.
Trisomy 21 (Down Syndrome)
Definition: Condition characterized by an extra chromosome 21.
Incidence: Affects about 1 in 700 children; most prevalent chromosome number abnormality.
Characteristics:
Distinctive facial features, typically shorter life expectancy, and varying degrees of developmental delays.
Risks increase with maternal age, particularly after age 35; prenatal screenings recommended.
Nondisjunction Consequences
Nondisjunction can lead to abnormalities in all gametes and can have lethal consequences.
Extra chromosomes may cause birth defects, while missing chromosomes often prevent normal development.
Sex Chromosome Abnormalities and Associated Conditions
Conditions include:
XXX (Triplo-X): 1 in 1500 females
XXY (Klinefelter Syndrome): 1 in 750 males
XYY (Jacobs Syndrome): 1 in 1000 males
XO (Turner Syndrome): 1 in 2000 females
Common Symptoms: Often involve low IQ and failure to develop secondary sexual characteristics.
Review Questions
1.) What are the major differences between Mitosis and Meiosis?
2.) What does Meiosis yield? How many cells? How many chromosomes in each cell?
3.) What is nondisjunction? Why does this occur? What can it cause?
4.) What are advantages to both sexual and asexual reproduction?
5.) What are disadvantages to both sexual and asexual reproduction?