Genetic variation

Phenotype variation

The observable characteristics of an organism are its phenotype

What is phenotypic variation?

Phenotypic variation is the difference in phenotypes between organisms of the same species

This variation means that the individuals within a population of a species may show a wide range of variation in phenotype

Genetic factors affecting phenotypic variation:

In some cases, phenotypic variation is explained by genetic factors

For example, the four different blood groups observed in human populations are due to different individuals within the population having two of three possible alleles for the single ABO gene

Environmental factors affecting phenotypic variation

In other cases, phenotypic variation is explained by environmental factors

For example, clones of plants with exactly the same genetic information (DNA) will grow to different heights when grown in different environmental conditions

Combination of genetic and environmental factors to explain phenotypic variation:

Phenotypic variation can also be explained by a combination of genetic and environmental factors

For example, the recessive allele that causes sickle cell anaemia has a high frequency in populations where malaria is prevalent due to heterozygous individuals being resistant to malaria

The phenotypic variation of the individuals in a population is determined by the genetic variation within the population and the interaction of the environment on the individuals:

Phenotypic variation = Genetic variation + Environment

Genetic variation:

• Organisms of the same species will have very similar genotypes, but two individuals (even twins) will have differences between their DNA base sequences

• Considering the size of genomes, these differences are small between individuals of the same species

• The small differences in DNA base sequences between individual organisms within a species population is called genetic variation

• Genetic variation is transferred from one generation to the next and it generates phenotypic variation within a species population

How is genetic variation transferred?

• Genetic variation is transferred from one generation to the next and it generates phenotypic variation within a species population

• The primary source of genetic variation is mutation (changes in the DNA base sequence)

• Mutation results in the generation of new alleles

• • The new allele may be advantageous, disadvantageous or have no apparent effect on phenotype

  • New alleles are not always seen in the individual that they first occur in

  • They can remain hidden (not expressed) within a population for several generations before they contribute to phenotypic variation

Genetic variation is also caused by the following processes as they result in a new combination of alleles in a gamete or individual:

• Crossing over of non-sister chromatids during prophase I of meiosis

• Independent assortment of homologous chromosomes during metaphase I of meiosis

• Random fusion of gametes during fertilization

What is crossing over?

• Crossing over is the process by which non-sister chromatids exchange equal lengths of alleles

Process of crossing over

• 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

Why is crossing over significant?

• 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 chiasma present in each bivalent during meiosis but often there are multiple chiasmata

• Crossing over is more likely to occur further down the chromosome away from the centromere

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 different combinations of chromosomes in daughter cells increases genetic variation between gametes

• In prophase I homologous chromosomes pair up and 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

• 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 2ⁿ can be used, where n corresponds to the number of chromosomes in a haploid cell

• For humans this is 2²³ which calculates as 8,324,608 different combinations

Random fertilisation 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

• 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

Sources of genetic variation table

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