Variation of Traits from Sexual Reproduction Overview

Key Concepts

• Variation
• Causes of variation
• Sexual reproduction
• Crossing over
• Independent assortment
• Random fertilization

Recall: 

• What is heredity? 
• What is meiosis? 

Introduction: 

Variation is the main reason why members of the same species, or in the case of humans, even the same families look different from each other. Variations can be seen in every species, including plants and animals. We can see that no two kittens of a cat look similar most of the time. 

Variations are the differences between individuals in a population. It can also be defined as the degree to which differences are seen between parents and their offspring. 

Explanation: 

Variations are caused by various means, which include: 

1. Sexual reproduction 

2. Mutations  

3. Gene flow 

Sexual reproduction: 

Many young organisms, such as the pups shown in the adjacent picture, resemble their parents. Like most animals, each puppy has two parents – a mother and a father. The genetic material that an organism receives from its parents determines how it looks and functions. 

Reproduction is the process by which organisms give birth to their young ones. Sexual reproduction is the process that involves the fusion of two compatible gametes or sex cells. It results in the production of offspring by combining the genetic material of two different cells. 

Half of the genetic material is contained in an egg cell, and the other half is contained in the sperm. Sperms are formed in the male reproductive organs, and eggs are formed in the female reproductive organs. 

A process called fertilization refers to the fusion of a sperm cell and an egg cell. The new cell that forms is called a zygote. A zygote further develops into a new organism. 

The genetic material consists of DNA which is transferred from parents to offspring. DNA contains information required for the growth and function of an organism. The DNA is stored in cells that have a nucleus.  

When cell division occurs, the DNA code is copied and passed to the new cells. In this way, newly formed cells receive the same coded information that was in the original cell. DNA is part of the chromosomes found in a cell’s nucleus. Each chromosome is made up of a long strand of DNA wrapped around proteins, like thread wrapped around a spool 

The large DNA molecule, called a double helix, resembles a twisted ladder. Smaller sugar-phosphate molecules occupy the sides of the ladder. The rungs of the ladder are made of paired nitrogen bases. These pairs fit together like puzzle pieces. 

Variations arising due to sexual reproduction: 

Sexual reproduction results in numerous possibilities of genetic variation. In other words, sexual reproduction gives rise to genetically unique offspring. They differ in characters from both parents as well as from each other. This occurs for several reasons, which are as follows: 

1. When the homologous chromosomes form pairs at the time of prophase I of meiosis I, crossing-over takes place. Crossing-over is referred to as the exchange of genetic material between homologous chromosomes. It creates new combinations of genes on each chromosome. 

2. Homologous chromosomes are randomly distributed among daughter cells during meiosis, and different chromosomes segregate independently of one another. This is referred to as “independent assortment.” It results in gametes with unique chromosomal combinations. 

3. In sexual reproduction, two gametes combine to produce offspring. But which two gametes will it be out of the millions? This is likely to be a matter of chance. It is obviously another source of genetic variation in offspring. This is known as random fertilization. 

When all of these mechanisms work together, the amount of possible variation is remarkable. For example, each human couple has the ability to have about 64 trillion genetically distinct children. No wonder we are all different! 

Crossing over:  

Crossing-over is referred to as the exchange of genetic material between non-sister chromatids of homologous chromosomes that happens during prophase I. Recall that during prophase I, homologous chromosomes line up in pairs, gene-for-gene down their entire length, forming a configuration with four chromatids, known as a tetrad.  

At this point, the chromatids are very close to each other, and some material from two chromatids switch chromosomes; that is, the material breaks off and reattaches at the same position on the homologous chromosome.  

Within the same pair of homologous chromosomes, this exchange of genetic material can occur multiple times, resulting in unique gene combinations. This process is also known as recombination. 

Independent assortment and random fertilization: 

During metaphase I, tetrads migrate to the metaphase plate with the kinetochores on the opposite poles. The homologous chromosomes arrange themselves randomly at the equator. This is the second event that introduces variation into the gametes. In every cell that undergoes meiosis, the orientation of the tetrads differs.  

The number of variations depends upon the number of chromosomes making up a set. At the metaphase plate, there are two possibilities for orientation. As a result, the maximum number of alignments is 2n, where n is the number of chromosomes per set. Given these two events, it is highly unlikely that any two haploid cells resulting from meiosis will have the same genetic composition. 

In humans, there are over 8 million different ways in which the chromosomes can line up during metaphase I of meiosis. It is the specific processes of meiosis that result in four unique haploid cells, resulting in these many combinations.  

This independent assortment, by which the chromosomes are inherited from either the father or the mother, can sort into any gamete, producing the potential for enormous genetic variation. 

Independent assortment along with random fertilization creates more possibilities for genetic variation between any two people than the number of individuals alive today. Sexual reproduction is the random fertilization of a female gamete with the help of a male gamete.  

In humans, over 8 million (223) chromosome combinations exist in the production of gametes in both males and females. Fertilization occurs when a sperm cell with over 8 million chromosome combinations fertilizes an egg cell with over 8 million chromosome combinations. 

Not considering the distinct combinations created by crossing-over, that’s nearly 64 trillion unique combinations. To put it another way, each human couple has the potential to produce a child with approximately 64 trillion different chromosomal combinations!