# The Significance of Meiosis ## Importance of Genetic Variation - Having genetically different offspring can be **advantageous for natural selection** - Meiosis has several mechanisms that **increase the genetic diversity of gametes** produced - Both crossing over and independent assortment result in **different combinations of alleles** in gametes

Genetic variation in offspring increases the chances of survival in changing environments through natural selection

## Crossing Over

Crossing over is the process by which non-sister chromatids exchange alleles

### The Process - During meiosis I homologous chromosomes **pair up** and are in very close proximity to each other - The non-sister chromatids can **cross over and get entangled** - These crossing points are called **chiasmata** (singular: chiasma) - The entanglement places **stress on the DNA molecules** - As a result of this, a section of chromatid from one chromosome may **break and rejoin** with the chromatid from the other chromosome ### Significance of Crossing Over - This swapping of alleles is significant as it can result in a **new combination of alleles** on the two chromosomes - There is usually at least one, if not more, chiasmata present in each bivalent during meiosis - Crossing over is **more likely to occur further down the chromosome** away from the centromere

The further apart two genes are on a chromosome, the more likely crossing over will occur between them

## Independent Assortment

Independent assortment is the production of different combinations of alleles in daughter cells due to the random alignment of homologous pairs along the equator of the spindle during metaphase I

### The Process - In prophase I homologous chromosomes pair up - In metaphase I they are pulled towards the **equator of the spindle** - Each pair can be arranged with **either chromosome on top** - this is completely random - The orientation of one homologous pair is **independent/unaffected** by the orientation of any other pair - The homologous chromosomes are then **separated and pulled apart** to different poles ### Calculating Possible Combinations - The combination of alleles that end up in each daughter cell depends on how the pairs of homologous chromosomes were lined up - To work out the number of different possible chromosome combinations the formula $2^n$ can be used - Where $n$ corresponds to the number of chromosomes in a haploid cell - For humans this is $2^{23}$ which calculates as **8,324,608 different combinations**

The formula $2^n$ only accounts for independent assortment, not crossing over or random fusion of gametes

The relationship between chromosome number and possible combinations is **exponential**, meaning even small increases in chromosome number lead to dramatic increases in genetic diversity:

### Significance of Independent Assortment - The different combinations of chromosomes in daughter cells **increases genetic variation between gametes** - This means that the proportion of paternal or maternal chromosomes that end up on each side of the equator is **due to chance** ## Random Fusion of Gametes - Meiosis creates genetic variation **between the gametes** produced by an individual through crossing over and independent assortment - This means each gamete carries **substantially different alleles** - During fertilization, **any male gamete can fuse with any female gamete** to form a zygote ### Significance - This random fusion of gametes at fertilization creates **genetic variation between zygotes** as each will have a unique combination of alleles - There is an almost zero chance of individual organisms resulting from successive sexual reproduction being **genetically identical**

Random fusion provides a third source of genetic variation in addition to crossing over and independent assortment

## Summary: Three Sources of Variation in Meiosis | Source | Stage | Effect | |--------|-------|--------| | Crossing over | Prophase I | Exchange of alleles between non-sister chromatids creates new combinations on chromosomes | | Independent assortment | Metaphase I | Random alignment of homologous pairs creates different combinations of maternal and paternal chromosomes | | Random fusion | Fertilization | Any gamete can fuse with any other, creating unique combinations in offspring | ## Additional Source of Variation - Genetic variation can occur on an even smaller scale than chromosomes - **Mutations** can occur within genes during DNA replication - A random mutation that takes place during DNA replication can lead to the production of **new alleles** and increased genetic variation - These mutations provide the raw material for evolution by natural selection

A mutation is a random change to the base sequence of DNA that may result in a new allele

## Why Genetic Variation Matters - Genetic variation within a population increases the likelihood that: - Some individuals will survive environmental changes - The population can adapt to new conditions through natural selection - The species will avoid extinction - Without genetic variation, all individuals would be equally susceptible to the same threats (diseases, climate change, etc.) - Sexual reproduction, through meiosis and random fusion, maintains high levels of genetic variation in populations