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The word zygosity may also be used to describe the genetic similarity or dissimilarity of twins. [6] Identical twins are monozygotic, meaning that they develop from one zygote that splits and forms two embryos. Fraternal twins are dizygotic because they develop from two separate oocytes (egg cells) that are fertilized by two separate sperm.
The human twin birth rate in the United States rose 76% from 1980 through 2009, from 9.4 to 16.7 twin sets (18.8 to 33.3 twins) per 1,000 births. [5] The Yoruba people have the highest rate of twinning in the world, at 45–50 twin sets (90–100 twins) per 1,000 live births, [6] [7] [8] possibly because of high consumption of a specific type of yam containing a natural phytoestrogen which may ...
The power of twin designs arises from the fact that twins may be either identical (monozygotic (MZ), i.e. developing from a single fertilized egg and therefore sharing all of their polymorphic alleles) or fraternal (dizygotic (DZ), i.e. developing from two fertilized eggs and therefore sharing on average 50% of their alleles, the same level of genetic similarity found in non-twin siblings).
When this preference is favored, it allows a population to generate and/or maintain polymorphism (genetic variation within a population). The fitness advantage aspect of disassortative mating seems straightforward, but the evolution of selective forces involved in disassortative mating are still largely unknown in natural populations.
In genetics, concordance is the probability that a pair of individuals will both have a certain characteristic (phenotypic trait) given that one of the pair has the characteristic. Concordance can be measured with concordance rates , reflecting the odds of one person having the trait if the other does.
The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individual organism. The sexual fusion of haploid cells is called karyogamy , the result of which is the formation of a diploid cell called the zygote or zygospore.
In reproductive biology, heterogamy is the alternation of differently organized generations, applied to the alternation between parthenogenetic and a sexual generation. [1] [2] This type of heterogamy occurs for example in some aphids.
Among dizygotic or genetically non-identical twin pairs, there was only 1 of 38 (2.6%) pairs where both twins were trans. [9] The significant percentage of identical twin pairs in which both twins are trans and the virtual absence of dizygotic twins (raised in the same family at the same time) in which both were trans would provide evidence ...