Heredity
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Heredity, also called inheritance or biological inheritance, is the passing on of
Overview
In humans, eye color is an example of an inherited characteristic: an individual might inherit the "brown-eye trait" from one of the parents.[1] Inherited traits are controlled by genes and the complete set of genes within an organism's genome is called its genotype.[2]
The complete set of observable traits of the structure and behavior of an organism is called its
Heritable traits are known to be passed from one generation to the next via DNA, a molecule that encodes genetic information.[2] DNA is a long polymer that incorporates four types of bases, which are interchangeable. The Nucleic acid sequence (the sequence of bases along a particular DNA molecule) specifies the genetic information: this is comparable to a sequence of letters spelling out a passage of text.[7] Before a cell divides through mitosis, the DNA is copied, so that each of the resulting two cells will inherit the DNA sequence. A portion of a DNA molecule that specifies a single functional unit is called a gene; different genes have different sequences of bases. Within cells, the long strands of DNA form condensed structures called chromosomes. Organisms inherit genetic material from their parents in the form of homologous chromosomes, containing a unique combination of DNA sequences that code for genes. The specific location of a DNA sequence within a chromosome is known as a locus. If the DNA sequence at a particular locus varies between individuals, the different forms of this sequence are called alleles. DNA sequences can change through mutations, producing new alleles. If a mutation occurs within a gene, the new allele may affect the trait that the gene controls, altering the phenotype of the organism.[8]
However, while this simple correspondence between an allele and a trait works in some cases, most traits are more complex and are controlled by multiple interacting genes within and among organisms.[9][10] Developmental biologists suggest that complex interactions in genetic networks and communication among cells can lead to heritable variations that may underlie some of the mechanics in developmental plasticity and canalization.[11]
Recent findings have confirmed important examples of heritable changes that cannot be explained by direct agency of the DNA molecule. These phenomena are classed as
Relation to theory of evolution
When
Darwin's initial model of heredity was adopted by, and then heavily modified by, his cousin
The inheritance of acquired traits was shown to have little basis in the 1880s when August Weismann cut the tails off many generations of mice and found that their offspring continued to develop tails.[25]
History
Scientists in Antiquity had a variety of ideas about heredity: Theophrastus proposed that male flowers caused female flowers to ripen;[26] Hippocrates speculated that "seeds" were produced by various body parts and transmitted to offspring at the time of conception;[27] and Aristotle thought that male and female fluids mixed at conception.[28] Aeschylus, in 458 BC, proposed the male as the parent, with the female as a "nurse for the young life sown within her".[29]
Ancient understandings of heredity transitioned to two debated doctrines in the 18th century. The Doctrine of Epigenesis and the Doctrine of Preformation were two distinct views of the understanding of heredity. The Doctrine of Epigenesis, originated by
During the 18th century, Dutch microscopist Antonie van Leeuwenhoek (1632–1723) discovered "animalcules" in the sperm of humans and other animals.[30] Some scientists speculated they saw a "little man" (homunculus) inside each sperm. These scientists formed a school of thought known as the "spermists". They contended the only contributions of the female to the next generation were the womb in which the homunculus grew, and prenatal influences of the womb.[31] An opposing school of thought, the ovists, believed that the future human was in the egg, and that sperm merely stimulated the growth of the egg. Ovists thought women carried eggs containing boy and girl children, and that the gender of the offspring was determined well before conception.[32]
An early research initiative emerged in 1878 when Alpheus Hyatt led an investigation to study the laws of heredity through compiling data on family phenotypes (nose size, ear shape, etc.) and expression of pathological conditions and abnormal characteristics, particularly with respect to the age of appearance. One of the projects aims was to tabulate data to better understand why certain traits are consistently expressed while others are highly irregular.[33]
Gregor Mendel: father of genetics
The idea of particulate inheritance of genes can be attributed to the
Modern development of genetics and heredity
In the 1930s, work by Fisher and others resulted in a combination of Mendelian and biometric schools into the modern evolutionary synthesis. The modern synthesis bridged the gap between experimental geneticists and naturalists; and between both and palaeontologists, stating that:[36][37]
- All evolutionary phenomena can be explained in a way consistent with known genetic mechanisms and the observational evidence of naturalists.
- Evolution is gradual: small genetic changes, recombination ordered by natural selection. Discontinuities amongst species (or other taxa) are explained as originating gradually through geographical separation and extinction (not saltation).
- Dobzhansky, it was downgraded later as results from ecological genetics were obtained.
- The primacy of population thinking: the genetic diversity carried in natural populations is a key factor in evolution. The strength of natural selection in the wild was greater than expected; the effect of ecological factors such as niche occupation and the significance of barriers to gene flow are all important.
The idea that
Almost all aspects of the synthesis have been challenged at times, with varying degrees of success. There is no doubt, however, that the synthesis was a great landmark in evolutionary biology.[40] It cleared up many confusions, and was directly responsible for stimulating a great deal of research in the post-World War II era.
There is growing evidence that there is transgenerational inheritance of epigenetic changes in humans[42] and other animals.[43]
Common genetic disorders
Types
The description of a mode of biological inheritance consists of three main categories:
- 1. Number of involved loci
- Monogenetic (also called "simple") – one locus
- Oligogenic– few loci
- Polygenetic – many loci
- 2. Involved chromosomes
- Autosomal – loci are not situated on a sex chromosome
- Gonosomal – loci are situated on a sex chromosome
- X-chromosomal – loci are situated on the X-chromosome (the more common case)
- Y-chromosomal – loci are situated on the Y-chromosome
- Mitochondrial – loci are situated on the mitochondrial DNA
- 3. Correlation genotype–phenotype
- Dominant
- Intermediate (also called "codominant")
- Recessive
- Overdominant
- Underdominant
These three categories are part of every exact description of a mode of inheritance in the above order. In addition, more specifications may be added as follows:
- 4. Coincidental and environmental interactions
- Penetrance
- Complete
- Incomplete (percentual number)
- Expressivity
- Invariable
- Variable
- Heritability (in polygenetic and sometimes also in oligogenetic modes of inheritance)
- Maternal or paternal imprinting phenomena (also see epigenetics)
- Penetrance
- 5. Sex-linked interactions
- Sex-linked inheritance (gonosomalloci)
- cryptorchism)
- Inheritance through the maternal line (in case of mitochondrial DNA loci)
- Inheritance through the paternal line (in case of Y-chromosomal loci)
- Sex-linked inheritance (
- 6. Locus–locus interactions
- Epistasis with other loci (e.g., overdominance)
- Gene coupling with other loci (also see crossing over)
- Homozygotous lethal factors
- Semi-lethal factors
Determination and description of a mode of inheritance is also achieved primarily through statistical analysis of pedigree data. In case the involved loci are known, methods of molecular genetics can also be employed.
Dominant and recessive alleles
An allele is said to be dominant if it is always expressed in the appearance of an organism (phenotype) provided that at least one copy of it is present. For example, in peas the allele for green pods, G, is dominant to that for yellow pods, g. Thus pea plants with the pair of alleles either GG (homozygote) or Gg (heterozygote) will have green pods. The allele for yellow pods is recessive. The effects of this allele are only seen when it is present in both chromosomes, gg (homozygote). This derives from Zygosity, the degree to which both copies of a chromosome or gene have the same genetic sequence, in other words, the degree of similarity of the alleles in an organism.
-
Hereditary defects inenzymesare generally inherited in an autosomal fashion because there are more non-X chromosomes than X-chromosomes, and a recessive fashion because the enzymes from the unaffected genes are generally sufficient to prevent symptoms in carriers.
-
On the other hand, hereditary defects in structural proteins (such asdominant-negativeprocess, wherein a mutated gene product adversely affects the non-mutated gene product within the same cell.
See also
- Hard inheritance
- Lamarckism
- Heritability
- Particulate inheritance
- Non-Mendelian inheritance
- Epigenetic inheritance
- Transgenerational epigenetics#Major controversies in the history of inheritance
- Inheritance of acquired characteristics
- Structural inheritance
- Blending inheritance
References
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The article, written by an obscure Moravian monk named Gregor Mendel
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