While the last subject of the AP Biology curriculum looked at the mechanisms of meiosis, this section focuses on why Meiosis and genetic diversity in a population. We’ll begin by defining genetic variety and examining how it manifests itself at the molecular level. We’ll next look at meiosis and the procedures that allow alleles to be distributed to new daughter cells.
We’ll examine how the Laws of Segregation and Independent Assortment are formed in this way. The first rule governs chromosomal segregation, whereas the second governs how various genes are inherited separately from one another (unless they are linked on the same chromosomes). Finally, we’ll look at how recombination (also known as crossing over) might help to unlink genes and promote genetic diversity in populations.
Meiosis and Genetic Diversity Overview
Consider the tale of bananas in the 1950s to see why enhancing genetic variety via meiosis is so essential. At the time, banana producers mostly produced the Gros Michel species of banana. It was shorter and thicker than the Cavendish kind we know today, but it had a considerably more enticing flavour.
However, since banana producers generally depended on clones of the same banana plant, the banana population lacked genetic variety. As a result, when a banana-eating fungus swept across the Gros Michel crop, there was little genetic variation, and the illness wiped off the whole variety. Cavendish bananas are now solely available in supermarkets.
Meiosis and Genetic Diversity
Process of Meiosis
If banana growers had been foresighted and placed Gros Michel bananas through the meiosis process on a regular basis, the crop may have retained enough genetic variety to defend itself against the banana-eating fungus. Meiosis uses a variety of ways to recombine genetic variations from various lines, resulting in unique and resistant offspring. The AP exam will almost certainly include a question on how meiosis enhances genetic diversity. So join us as we go into the process of meiosis and demonstrate how it boosts genetic diversity in a population.
To comprehend how meiosis improves a population’s genetic variety, we must first comprehend what genetic variants are and how they might aid a population’s survival in the face of continually changing environmental circumstances.
Each gene in diploid organisms has two alleles, one acquired from their mother and the other from their father. These alleles contribute to the organism’s overall phenotype (or appearance). Meiosis and sexual reproduction occur between each generation of organisms, resulting in a mixture of alleles in the progeny. This is how a red and white flower may give birth to pink offspring, and these pink offspring can give birth to kids with all three colour variants.
Let’s look at how this genetic variety manifests itself at the chromosomal level. Homologous chromosomes, often known as homologs, are the chromosomes obtained during fertilisation from the maternal and paternal gametes. Each of these homologous chromosomes has a set of genes that are all located in the same place on each homolog. Keep in mind that only sister chromatids are linked to the centromere following DNA replication, so these homologous chromosomes will never get joined at the centromere.
Let’s think about this in terms of our hypothetical flower colour alleles. Each homolog has an allele from a distinct paternal source if these homologs possess the flower colour gene. A red flower acquires two capital “A” alleles, one from the mother and the other from the father. Each parent gave a white flower two tiny “a” alleles, which means it has two broken copies of the gene and can’t create colour. One functioning “A” allele and one non-functional “a” allele were given to a pink flower.
Keep in mind that this cell will copy its DNA before meiosis. Each chromosome will have two sister chromatids, effectively replicating each allele, resulting in four alleles. These alleles will be separated into distinct gametes during meiosis and may be messed up by the crossing over process, as we shall see.
What is the significance of this for the survival of a species? because shifting environmental circumstances are unpredictably unpredictable. For example, bees love pink flowers, whereas moths prefer white flowers. Bees may prefer pink flowers and, in most cases, are superior pollinators. However, if a chemical wipes out all the bees in an area, the pink blossoms are dead. These plants would all become extinct if they solely reproduced asexually and did not need meiosis to mix up their alleles, much like the banana variety at the beginning of this film!
Let’s take a look at meiosis and the processes that guarantee that genetic variety is maintained in future generations via sexual reproduction.
Assume that the flower-producing plant from the preceding paragraph is a two-chromosome creature. Each chromosome contains two homologs, one from the mother and one from the father of the plant. Each homolog has been reproduced by prophase I of meiosis, and the sister chromatids are linked together at the centromere. The maternal chromosomes are purple, whereas the paternal chromosomes are green in this image.
The homologous chromosomes have aligned on the metaphase plate by the end of metaphase I of meiosis. In this case, chromosome 1 has the blossom colour gene. Because each homologous chromosome has been duplicated, there are four alleles total: one set of maternal alleles and one set of paternal alleles.
These maternal and paternal homologs are separated into distinct cells as meiosis I advance. The sister chromatids in each of these cells are split into their own cells during meiosis II. This mechanism eventually distributes one of the four alleles to each of the four new daughter cells.
While meiosis readily divides these homologous chromosomes and the alleles they contain into distinct cells, the true magic comes when all the genes on all the chromosomes are randomly assigned to daughter cells. During prophase, I, another unique process termed crossing over occurs, in which maternal and paternal homologs exchange genetic material.
Meiosis and Genetic Diversity
Mendel’s Genetic Laws
Mendel’s genetic law of independent assortment and segregation are both derived from the meiosis process. As a monk who loved raising and monitoring pea plants, Mendel had plenty of time on his hands to uncover these principles, simply by counting the phenotypes of the offspring and noticing how frequently certain features showed together.
Scientists have been able to validate and expand on these rules thanks to the development of microscopes and high-throughput genetic sequencing. Let’s look at each of these statutes in detail. Maternal and paternal alleles of a gene are separated into distinct gametes according to the Law of Segregation. As we observed in the previous section, meiosis I separate homologous chromosomes, whereas meiosis II separates alleles on sister chromatids.
The Law of Independent Assortment, on the other hand, asserts that distinct genes coding for different characteristics are inherited separately. The genes for flower colour and plant height, for example, have no effect on each other. In other words, if one gamete obtains the purple flower gene, it has no bearing on whether that same gamete receives the tall or short allele. This rule assures that features are distributed randomly in future generations, resulting in enormous genetic variety.
Genes on the same chromosome are an important exception to this rule. Because they are physically linked, they are referred to as “linked-genes.” However, meiosis has one more trick up its sleeve: crossing over, which can segregate all but the most closely related linked genes.
Crossing over, also known as recombination, takes place during prophase I of meiosis, when homologous chromosomes condense and are pushed toward the metaphase plate. The synaptonemal complex—a collection of RNAs and proteins that keep the two homologous chromosomes together—holds the homologous chromosomes together momentarily while they are condensed and gathered.
Synapsis refers to the binding of homologous chromosomes. Because the two non-sister chromatids that are closest to each other carry the same gene sequence, their structures are very similar. While maternal and paternal homologs may have distinct alleles, alleles are usually just a few nucleotides apart on a chromosome with hundreds of millions of nucleotides.
A chiasma occurs when these two remarkably identical non-sister chromosomes cross over one another. When two non-sister chromatids are crossed over each other in this fashion, a holiday junction might occur. The chiasma generates breaks in comparable spots along each non-sister chromatid, resulting in this four-way DNA structure. The DNA repair enzymes have the potential to mix up the strands of DNA that have been entangled as they attempt to fix this arrangement. The recombinant chromatids are split and end up in different gametes as the synaptonemal complex breaks down and meiosis progresses to completion.
As a result, the alleles ordinarily carried by maternal and paternal chromosomes are mixed together. This is why no kid resembles their parents or grandparents perfectly. Even whether they inherit a complete set of maternal or paternal chromosomes, the process of crossing over assures that certain chromosome-linked characteristics may be recombined between maternal and paternal homologous chromosomes!