ZOOHCC - 501: Principles of Genetics (Theory)
Unit 1: Mendelian Genetics and its Extension
Multiple alleles
Multiple alleles are three or more alternative forms of a single gene that exist within a population. Individual organisms can only have two alleles for a given gene (one from each parent), but multiple alleles are available throughout the population.
Examples of multiple alleles in human populations
A classic example of multiple alleles is the human ABO blood group system. The ABO system is governed by the presence of her three alleles: A, B and O. The A and B alleles are co-dominant. That is, if an individual carries both alleles, both are equally expressed. On the other hand, the O allele is recessive to both the A and B alleles, and individuals with two copies of the O allele have O blood.
Another example of multiple alleles is coat color in rabbits. The gene that determines coat color has four alleles: C (full color), cch (chinchilla), chd (Himalayan) and c (albino). Each allele produces a different phenotype, and allele expression can interact in complex ways to produce variations in coat color.
Multiple alleles can also affect disease susceptibility. For example, the human HLA gene has hundreds of different alleles, and specific combinations of these alleles can increase or decrease an individual's susceptibility to autoimmune diseases such as multiple sclerosis and rheumatoid arthritis. there is.
The presence of multiple alleles within a population allows for greater genetic diversity and adaptation to different environments. However, predicting the outcome of genetic crosses and determining inheritance patterns for specific traits can also be more difficult.
How multiple alleles contribute to genetic diversity
Multiple alleles contribute to genetic diversity by providing different versions of genes within a population. This variation in gene expression allows for the expression of different phenotypic traits within populations, even for traits controlled by a single gene.
For example, her ABO blood group system in humans is determined by her three alleles: A, B, and O. These alleles encode different versions of glycosyltransferase enzymes that add sugars to the surface of erythrocytes. Individuals with the A allele produce a slightly different version of the enzyme than those with the B allele, and individuals with the O allele produce no enzyme at all. Mutations in this ABO gene give rise to different blood types and affect immune responses. People with type A make antibodies against type B, and people with type B make antibodies against type A. A person with type AB blood does not produce either type of antibody, but a person with type O blood produces both types.
Multiple alleles can also affect other traits, such as: B. Animal fur color or plant flower color. In such cases, different alleles produce different pigments or affect pigment distribution, resulting in different colors and patterns. Overall, multiple alleles contribute to genetic diversity by providing different options for expressing specific traits. This diversity allows populations to adapt to changing environments, which can lead to the evolution of new traits over time.
Lethal alleles
A lethal allele is an allele that, when present in the homozygous state, causes death of the organism before reaching maturity. In other words, lethal alleles prevent an organism from developing or surviving, resulting in death.
Lethal alleles can be dominant or recessive. For dominant lethal alleles, organisms with only one copy of the allele may be affected and unable to survive and reproduce. For example, Huntington's disease is a dominantly inherited disorder caused by a lethal allele that causes progressive neurodegeneration. A recessive lethal allele is expressed only if the organism has two copies of the allele. An example of a recessive lethal allele in humans is Tay-Sachs disease, a rare genetic disorder that affects the central nervous system and is fatal in early childhood. Tay-Sachs is caused by a recessive lethal allele that prevents certain fatty substances from being broken down in the brain, resulting in severe neurological symptoms.
In some cases, lethal alleles may persist in the population despite their adverse effects. This is because the allele may be recessive and thus not affect heterozygous individuals. As a result, the allele is passed undetected from generation to generation, allowing two carriers of the allele to inherit two copies and leave lethally affected offspring.
Lethal alleles can pose challenges for geneticists studying the inheritance patterns of specific traits. When studying traits involving multiple genes or complex genetic interactions, it is important to understand the presence of lethal alleles and their effects on inheritance and survival.
The impact of lethal alleles on gene expression and evolution
A lethal allele is a genetic mutation that can be fatal during development or early in life. These mutations can have profound effects on gene expression and evolution. Here are some of the ways in which lethal alleles can affect gene expression and evolution.
Reduced genetic diversity: Lethal alleles reduce the genetic diversity of a population by eliminating specific genetic variations. This may reduce the population's ability to adapt to changing environmental conditions.
Heterozygosity Dominance: Some lethal alleles may have a heterozygosity dominance. That is, individuals with one copy of the lethal allele and one copy of the wild-type allele have a survival advantage over individuals with two copies of the wild-type allele. This may allow lethal alleles to be maintained in low-frequency populations.
Compensatory selection: In some cases, multiple lethal alleles can be maintained within a population through compensatory selection, in which selection favors a balance between different alleles. This helps maintain genetic diversity within the population.
Epistasis: Lethal alleles can interact with other genes and affect gene expression. This is called epistasis and can result in complex patterns of gene expression and genetic interactions. Evolutionary trade-offs: Lethal alleles can produce evolutionary trade-offs, where the advantage of a particular trait comes at the expense of increased mortality. For example, certain alleles that increase resistance to pathogens may also increase susceptibility to other diseases.
Lethal alleles in plants and their effects on crop yield
These mutations can significantly affect yield in different ways.
Reduced genetic diversity: Lethal alleles reduce crop genetic diversity by eliminating specific genetic variations. This reduces the plant's ability to adapt to changing environmental conditions and makes it vulnerable to pests, diseases and climate change.
Reproductive Impairment: Lethal alleles can cause reproductive impairment in plants and reduce the number of viable seeds produced. This can lead to lower yields and less profitability for farmers.
Degradation: Lethal alleles can affect crop quality by altering the plant's chemical composition. For example, some lethal alleles can reduce levels of certain nutrients, making crops less nutritious.
Breeding Challenges: Lethal alleles can pose challenges to plant breeding programs. In some cases, breeders must eliminate lethal alleles from breeding populations to avoid reproductive failure and reduced crop quality. This can be difficult as lethal alleles may be present in wild or unimproved strains with other desirable traits.
Yield variation: Lethal alleles can lead to yield variation within crops. Plants with lethal alleles may exhibit reduced growth or yield, while other plants within the same crop may not be affected. Planning can be difficult.
Lethal alleles and genetic counseling: how to manage risk in human populations.
Genetic Testing: Genetic testing can identify individuals who carry lethal alleles or who are at risk of carrying lethal alleles. This can help individuals and families understand their risk of passing on lethal alleles to their children and make informed decisions about family planning.
Reproductive Options: Genetic counseling can inform individuals and families about their reproductive options, such as in vitro fertilization (IVF) with pre-implantation genetic diagnosis (PGD), adoption, or donor sperm or eggs. These options can help reduce the risk of passing on lethal alleles to offspring.
Carrier Testing: Carrier testing can identify individuals who carry one copy of a lethal allele but do not exhibit any symptoms or negative health effects. This information can help individuals make informed decisions about family planning and reduce the risk of passing on lethal alleles to their children.
Family History: Genetic counseling can help individuals and families understand their family history of lethal alleles and other genetic conditions. This information can help inform decisions about family planning and identify individuals who may benefit from genetic testing or carrier screening.
Support: Genetic counseling can provide emotional support and help individuals and families navigate the complex and often difficult decisions related to family planning and genetic risk.
