Meiosis is crucial for creating genetic diversity in sexually reproducing organisms. The majority of eukaryotic organisms, whether multicellular or unicellular, rely on meiosis and fertilization as part of their reproduction. Sexual reproduction depends on the fusion of two gametes, each carrying a single set of chromosomes. When gametes combine, they form a zygote—a fertilized egg that restores the diploid state by containing two sets of chromosomes. (Haploid cells have one set of chromosomes, while diploid cells have two.) To maintain a stable chromosome number across generations, diploid cells must undergo a process that reduces their chromosome sets by half before forming gametes; otherwise, each successive fertilization would continuously double the chromosome count. This reduction is achieved through a specialized type of nuclear division essential for sexual reproduction.
By producing haploid gametes, meiosis reshuffles alleles through two processes: independent assortment and crossing over:
Independent assortment randomizes which copy of each homologous chromosome is passed to the offspring, but linkage can limit this mixing for genes on the same chromosome.
During prophase I, homologous chromosomes pair up in a process called synapsis, forming a structure known as a tetrad.
The protein complex known as the synaptonemal complex holds these homologous chromosomes together, facilitating proper alignment. At specific sites along the tetrad, called chiasmata, crossing over occurs, whereby segments of chromatids are exchanged.
Double crossover events can vary in outcome: in a 2-strand double crossover, the chromatids initially exchange segments and then reverse the exchange, leading to no net recombination; in a 3-strand double crossover, one chromatid participates in both exchanges, producing two recombinant chromatids; and in a 4-strand double crossover, all four chromatids are involved, resulting in four recombinant products.
Eukaryotes generally use an XX (female) and XY (male) system, with the Y chromosome carrying few genes. Sex-linked traits reside mostly on the X chromosome. Red-green color blindness, hemophilia, Duchenne muscular dystrophy, and Fragile X syndrome are all examples of inherited conditions found primarily in biological males, though the traits are carried by biological females.
Inheritance can also occur outside of nuclear DNA. Cytoplasmic inheritance refers to the exclusively maternal transmission of organellar DNA (e.g., mitochondria).
A mutation is a change in the DNA sequence independent of normal recombination. It can arise randomly from replication errors or be induced by chemical or physical mutagens.
Types of mutations:
Depending on environmental conditions, mutations may be advantageous (enhancing fitness, such as albino moths in the early Industrial Revolution in England blending in better with ash-covered trees) or deleterious (reducing fitness, such as limb deformations that cause an animal to be less likely to outrun predators).
Mutagens are agents that cause mutations, and carcinogens are mutagens capable of triggering changes that lead to cancer.
Inborn errors of metabolism- Some hereditary metabolic disorders, like Phenylketonuria (PKU), arise from specific mutation-driven enzyme deficiencies.
Separate from natural selection, genetic drift randomly alters allele frequencies within a population due to chance events, sometimes overshadowing adaptive forces.
Bottleneck effect: when an event greatly reduces the population so that a small sample is left to reproduce, traits that were much less common in the larger population but happen to occur in the remaining group often become very common in the future generations as they repopulate
Founder effect: when a small portion separates from a larger group or herd and resettles in a new area, smaller diversity in the traits and genes of the “colonists” results in future generations that show less genetic diversity than the members of the species at large.
Synapsis, crossing over, and genetic diversity
By allowing homologous chromosomes to pair and exchange segments (synapsis and crossing over), meiosis underlies the extensive genetic variation that fuels evolution and contributes to the unique genetic makeup of each individual.
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