Because dominant alleles always have an immediate effect upon the phenotype, they tend to be strongly selected against. People with achondroplasia not only survive to adulthood: they can also reproduce.
When they do, they have a 1 in 2 chance of passing on the allele to the next generation, keeping the allele in the gene pool achondroplasiacs are never homozygous: two alleles is a fatal combination. And as a result, this allele persists in the population. If you feel that you understand the concepts in this tutorial, take this brief quiz. If not, carefully re-read the material above, and then take the quiz.
Which square represents the genotype of an individual who is a carrier for Tay-Sachs? Which square shows the genotype of an individual who would have Tay-Sachs disease? We would expect natural selection to remove alleles with negative effects from a population, and yet many populations include individuals carrying such alleles.
Human populations, for example, generally carry some disease-causing alleles that affect reproduction. So why are these deleterious alleles still around anyway? What keeps natural selection from getting rid of them? There are several possible explanations:. They may be maintained by heterozygote advantage When carrying two copies of an allele is disadvantageous, but carrying only one copy is advantageous, natural selection will not remove the allele from the population — the advantage conferred in its heterozygous state keeps the allele around.
For example, the allele that causes sickle cell anemia is deleterious if you carry two copies of it. But if you only carry one copy of it and live in a place where malaria is common, the allele is advantageous because it confers resistance to malaria.
Fertility can be restored by doubling the number of chromosomes; then every chromosome can find an exact match with which to pair during meiosis. This can happen by chance in nature as in the example of the grass Spartina anglica a tetraploid derivative of a hybrid between a North American species S.
Spartina anglica has colonized large areas of coastal Britain since its origin at the turn of the century. Many crop plants are allopolyploids originating from duplication of the genomes of interspecific hybrids. Examples of allotetraploids among the Brassica vegetables have already been discussed in lab. Wheat, Triticum aestivum and strawberry, Fragaria ananassa are examples of allohexaploids.
More recently the deliberate induction of polyploidy by colchicine treatment has become a standard practice in plant breeding in order to obtain fertile hybrids. Hybrid vigor is stabilized in a plant with two or more genomes between which there is no recombination. Polyploids are also more vigorous than their diploid ancestors, even if there is no mixing of genomes autopolyploids contain multiple sets of a single haploid genome. Another escape from sterility in hybrid plants is apomixis , the production of an embryo from a diploid cell without meiosis and gamete fusion.
This occurs in many hawthorns and brambles and has the same advantages and disadvantages as vegetative reproduction. Seed production is assured and a successful genotype can rapidly invade a suitable habitat but there is little prospect of future variation to adapt to changing circumstances unless the sexual process can be restored. PlantFacts Site Index.
Population genetics "In a large population in which random mating occurs and in the absence of forces that change the proportions of alleles the original ratio of dominant alleles to recessive alleles will be retained from generation to generation" This statement of the Hardy Weinberg Law describes a state of equilibrium in the genetic makeup of populations that is more interesting for its exceptions than the comparitively rare occasions when it occurs.
Exceptions to the rule lead to genetic change, or evolution in populations: When a small population breaks away or gets isolated, some alleles may be lost from the gene pool. Populations can become separated in their breeding as well as geographically. A mutation causing flowering at a different time than in the main population would block the flow of genes between them.
Recessive lethal alleles do not cause death in the heterozygous form because a certain threshold of protein output is maintained. Translocation can be beneficial, for example, the translocation of a part of one chromosome to a different chromosome could link DNA segments in a way that results in a positive effect.
The result is an expanded genome with new genes that may take on new functions, playing a major role in evolution. What is translocation and how could it be beneficial? It is the movement of a part of 1 chromosome to a different chromosome and this can link DNA segments which can result in a positive affect.
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