Genetic Variation

 Objectives 

The reader will be able to:

o Explain what variation is  

o Distinguish between continuous and discontinuous variation 

o Distinguish between heritable and non-heritable variation. 

o Explain the causes of variation and state their source. 

o Explain the consequences of variation.  

o Explain the term Recombinant DNA technology and state their applications.

 

Variation: Understanding Differences in Characteristics

Variation refers to the differences in characteristics among individuals of the same species. These differences can be due to inherited genes from parents or acquired through environmental factors. Common examples of variation include differences in skin color, hair color, hair curliness, eye color, and sex.


Types of Variation

Variation can be broadly categorized into two main types: Continuous Variation and Discontinuous Variation.


Discontinuous Variation

Discontinuous variation is characterized by distinct, clear-cut differences between traits, with no intermediate forms. An example of discontinuous variation is human blood groups. There are only four types of blood groups (A, B, AB, or O), with no intermediate forms. Other examples include Rhesus factors, sex (male or female), red-green color blindness, hemophilia, and sickle cell anemia.


variation

Continuous Variation

Continuous variation, on the other hand, refers to variation within a species that shows intermediate forms between two extremes. Human height is an example of continuous variation, ranging from the shortest to the tallest individuals, with any height possible between these extremes. Other examples include weight, skin color, intelligence, age, body size, and fingerprint patterns.

variation


Differences Between Continuous and Discontinuous Variation

Continuous variation

Discontinuous variation

Intermediate forms present

No intermediate forms

Controlled by the gene (polygene); polygenic

Controlled by major genes (alternate or multiple alleles); not polygenic 

Controlled by the genes and environment

Entirely genetic (not affected by environmental conditions)

Caused by co-dominance

Caused by complete dominance

Follow normal distribution curve. i.e frequency of characteristics evenly distributed.

Not evenly distributed



Inherited and Non-Heritable Variation

Variation within a species can be inherited (genetics) or acquired (environmental).

Inherited Variations: These variations are passed down from parents to offspring through genes. Examples include eye color, ear shape, blood group, sickle cell, albinism, tongue rolling, and gender.

Non-Heritable Variations: These variations are acquired through environmental influences and are not inherited. Examples include goiter, river blindness, scars, knowledge, and language.


Causes or Sources of Variation

Variation can be caused by environmental factors and genetic factors.

Environmental Causes of Variation

Environmental factors that cause variation include climate, diet, accidents, light intensity, altitude, culture, and lifestyle. For instance, dietary habits can influence body weight, and plants growing in different light conditions may exhibit varying growth patterns.


Genetic Causes of Variation or  Source of Genetic Variation

Genetic variation is essential for the evolution and adaptation of species. It arises from several sources, both genetic and environmental. Understanding these sources helps in studying genetics and evolutionary biology. Here are the main sources of genetic variation:

  • Mutation: Spontaneous changes in genes or chromosomes.
  • Epistasis: Suppression of one gene's effect by another gene.
  • Co-dominance: Both alleles in a heterozygous organism are equally expressed.
  • Crossing Over: Exchange of genetic material between homologous chromosomes during meiosis.
  • Polyploidy: Having more than two sets of chromosomes.
  • Hybridization: Crossing of different species or varieties.
  • Polygenic Characters: Traits controlled by multiple genes.
  • Segregation and Recombination: Separation of homologous chromosomes and their recombination during fertilization.
  • Independent Assortment: Random distribution of genes during gamete formation.
  • Incomplete Dominance: Neither allele completely masks the other.


a. Mutation: A Source of Genetic Variation

Mutations are spontaneous changes in genes or chromosomes that introduce genetic variation. Mutations can be:

1. Gene Mutations 

Alterations in the sequence of nucleotides within a gene. Examples include point mutations leading to conditions like albinism, color blindness, hemophilia, and cystic fibrosis.

  • Point Mutations: Alterations in a single nucleotide base. Examples include sickle cell anemia and cystic fibrosis.
  • Insertions and Deletions: Addition or loss of nucleotide bases, which can lead to frameshift mutations.
  • Duplication: Repetition of a segment of DNA.
  • Inversions: Reversal of a DNA segment.
  • Translocations: Movement of DNA segments between non-homologous chromosomes.

      Examples: Albinism, color blindness, hemophilia.

      Further Reading: National Center for Biotechnology Information: Mutation


2. Chromosomal Mutation

Chromosomal mutations involve changes in the number or structure of chromosomes and can have significant effects on an organism's phenotype and development. These mutations can lead to various genetic disorders and can occur spontaneously or due to environmental factors. Here’s a detailed look into the types, causes, and effects of chromosomal mutations.

Types of Chromosomal Mutations

  1. Structural Chromosomal Mutations

    These mutations involve changes in the structure of a chromosome. They can result in various types of rearrangements:

    • Deletions: A portion of the chromosome is lost. This can result in the loss of several genes, which may lead to genetic disorders. For example, Cri du Chat syndrome is caused by a deletion on chromosome 5.

    • Duplications: A segment of the chromosome is duplicated, resulting in extra genetic material. Duplications can lead to developmental disorders or increased risk of certain cancers. For instance, the duplication of the 21st chromosome is associated with Down syndrome.

    • Inversions: A chromosome segment is reversed 180 degrees. While often harmless, inversions can lead to problems during meiosis, affecting fertility and increasing the risk of genetic abnormalities in offspring. An example is the inversion on chromosome 9, which can be associated with certain reproductive issues.

    • Translocations: Segments of chromosomes are exchanged between non-homologous chromosomes. This can result in diseases such as chronic myeloid leukemia (CML), which is associated with a specific translocation between chromosomes 9 and 22.

  2. Numerical Chromosomal Mutations

    These mutations involve changes in the number of chromosomes. They can be categorized into:

    • Aneuploidy: The gain or loss of one or more chromosomes from the normal diploid number. For instance, Down syndrome (Trisomy 21) is caused by an extra chromosome 21.

    • Polyploidy: The presence of more than two complete sets of chromosomes. Polyploidy is common in plants and can result in larger and more vigorous plants. In animals, it is less common but can occur in some amphibians and reptiles.

Causes of Chromosomal Mutations

  1. Errors During DNA Replication

    Mistakes during DNA replication can lead to structural abnormalities in chromosomes. DNA polymerase errors or incorrect repair of DNA damage can contribute to these mutations.

  2. Environmental Factors

    Exposure to physical or chemical mutagens can cause chromosomal mutations. Examples include:

    • Radiation: X-rays, gamma rays, and UV light can cause breaks in DNA strands, leading to structural chromosome mutations.

    • Chemicals: Certain chemicals, such as those found in tobacco smoke or mustard gas, can induce chromosomal aberrations.

  3. Errors During Cell Division

    Problems during mitosis or meiosis, such as nondisjunction (failure of chromosomes to separate properly), can lead to numerical chromosomal mutations.

Effects of Chromosomal Mutations

  1. Genetic Disorders

    Many chromosomal mutations are associated with genetic disorders. For example:

    • Down Syndrome: Caused by an extra copy of chromosome 21 (Trisomy 21). It results in developmental and intellectual disabilities.

    • Turner Syndrome: Occurs when a female has only one X chromosome (45,X). It results in short stature and infertility.

    • Klinefelter Syndrome: Involves an extra X chromosome in males (47,XXY). It can cause reduced testosterone levels and infertility.

  2. Cancer

    Chromosomal mutations can lead to cancer by disrupting the normal regulation of cell growth and division. For example:

    • Chronic Myeloid Leukemia (CML): Associated with a translocation between chromosomes 9 and 22, resulting in the BCR-ABL fusion protein that promotes cancer cell growth.

    • Burkitt Lymphoma: Linked to a translocation involving chromosome 8 and 14, leading to the overexpression of the MYC oncogene.

  3. Reproductive Issues

    Structural chromosomal mutations can affect fertility. For example, individuals with inversions or translocations may experience difficulties conceiving due to complications during gamete formation.

  4. Developmental Abnormalities

    Chromosomal mutations can result in various developmental abnormalities. For instance, deletions or duplications of chromosomal segments can cause congenital disabilities and developmental delays. 


Detecting Chromosomal Mutations

  1. Karyotyping

    A laboratory technique that involves arranging chromosomes into a standard format to identify numerical and structural abnormalities. It is often used in prenatal screening and cancer diagnosis.

  2. Fluorescence In Situ Hybridization (FISH)

    A technique that uses fluorescent probes to bind specific chromosome regions, allowing for the detection of chromosomal abnormalities at a molecular level.

  3. Chromosome Microarray

    A high-resolution technique that detects chromosomal deletions and duplications across the entire genome, providing detailed information on chromosomal imbalances.

      Further Reading:

 

b. Recombination (Crossing Over)

During meiosis, homologous chromosomes exchange genetic material in a process called crossing over or recombination. This shuffling of genes creates new allele combinations in the offspring.

Examples: Variation in traits like eye color and blood type.

Further Reading: Nature Reviews Genetics: Genetic Recombination


c. Independent Assortment

During meiosis, chromosomes are distributed randomly into gametes, which results in a variety of possible genetic combinations. This independent assortment of chromosomes contributes to genetic diversity.

Examples: Different combinations of alleles for various traits.

Further Reading: National Human Genome Research Institute: Independent Assortment


d. Genetic Drift

Genetic drift refers to random changes in allele frequencies in a population, which can lead to significant genetic variation over time, especially in small populations.

Examples: Founder effect, bottleneck effect.

Further Reading: Evolutionary Biology: Genetic Drift


e. Gene Flow

Gene flow, or gene migration, occurs when individuals from different populations interbreed, introducing new alleles into a population. This movement of genes between populations increases genetic variation.

Examples: Migration of animal populations leading to interbreeding and genetic diversity.

Further Reading: Ecology and Evolutionary Biology: Gene Flow


f. Polyploidy

Polyploidy involves having more than two sets of chromosomes. This condition can occur naturally and result in significant genetic variation, especially in plants.

Examples: Many crop plants like wheat and potatoes.

Further Reading: Plant Science: Polyploidy


g. Hybridization

Hybridization occurs when individuals from different species or varieties interbreed, resulting in offspring with a mix of genetic traits from both parents.

Examples: Mules (horse-donkey hybrids), hybrid crops like hybrids of corn and soybeans.

Further Reading: Hybridization in Biology


h. Epistasis

Epistasis occurs when the expression of one gene is influenced by one or more other genes. This interaction can affect the phenotypic expression of traits.

Examples: The interaction between genes that influence coat color in dogs.

Further Reading: Genetics Society of America: Epistasis


i. Incomplete Dominance and Co-Dominance

  • Incomplete Dominance: Neither allele is completely dominant, resulting in a blend of traits. For example, red and white flowers producing pink offspring.
  • Co-Dominance: Both alleles are equally expressed, such as in the AB blood type where both A and B antigens are present.

Examples: Sickle cell anemia (incomplete dominance), AB blood type (co-dominance).

Further Reading: Genetics Home Reference: Incomplete Dominance



Consequences of Variation

Variation within a population can lead to:

  • Natural Selection: Favorable variations provide a selective advantage, leading to the survival and reproduction of individuals with these traits, while unfavorable variations may lead to extinction.
  • Artificial Selection: Humans apply knowledge of genetics to selectively breed plants and animals with desirable traits.


Application of Variation (Importance of Studying Genetics)

Understanding genetic variation has several practical applications, including:

  • Crime detection
  • Resolving paternity disputes
  • Ensuring compatible blood transfusions
  • Determining blood groups
  • Classifying human races
  • Producing disease-resistant crops
  • Studying fossils and anthropology

For more in-depth information, visit the following resources:


Genetic Engineering: Manipulating Variation

Genetic engineering involves manipulating genes to produce organisms with desirable qualities. Key steps include:

  • Using restriction enzymes to cut DNA at specific locations.
  • Joining DNA fragments with ligases to form desirable DNA molecules.
  • Inserting desired DNA into vectors for replication in host cells.

Applications of genetic engineering include:

  • Diagnosing and treating genetic diseases (gene therapy)
  • Producing single-cell proteins
  • Generating vaccines and interferons
  • Mass-producing human insulin
  • Creating genetically modified foods
  • Improving growth rates
  • Producing organs for transplants

By understanding and utilizing genetic variation, we can enhance our approach to agriculture, medicine, and conservation. For more information on genetic variation and its applications, visit:

These resources provide comprehensive insights into the importance and applications of genetic variation in various fields.


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