HETEROGAMETIC SEX DETERMINATION
It is proposed by Correns. It is based on number and nature of sex chromosomes.
Sex Determination Based on Differences in Sex Chromosomes. In this type of sex determination, both males and females have the same number of chromosomes.
i. XX-XY Method of Sex Determination:
In human beings and insects like Drosophila both females and males have the same number of chromosomes.
The X and Y chromosomes originated in a pair of autosomes, and differentiated as the Y chromosome degenerated progressively. The Y contains few active genes and is composed largely of repetitive DNA sequences. Most Y genes have copies on the X from which they evolved; this includes even the sex-determining gene SRY as well as several genes required for spermatogenesis. The X contains a disproportionate number of genes that affect reproduction and brain function (or both). ( J.A.M. Graves, 2010)
H UMAN chromosomes have been studied for over half a century but knowledge in this field has expanded rapidly since 1956 when Tjio and Levan' reported the finding of 46 chromosomes in cells from human fetal lung cultures, instead of the 48 previously accepted as the normal number for many decades. The sex chromosomes are represented by an XX complex in the normal female and an XY complex in the normal male. (HENDERSON, J. G; 1958)
The X chromosome is present in both males and females and is comprised of a very interesting pattern of ancestor contribution. Men get their only X chromosome from their mother, but women receive one from each parent. Because the X chromosome can still recombine in gamete production, each X chromosome is comprised of DNA that was contributed from both male and female ancestors. Females can pass along an X chromosome that contains information from both their parents, but males can only pass along an X that is a virtual copy of the one they received from their mothers. It is thus the male’s sperm that determines the gender of each offspring in humans and in dogs.
(Christopher, 2011)
It is proposed by Correns. It is based on number and nature of sex chromosomes.
Sex Determination Based on Differences in Sex Chromosomes. In this type of sex determination, both males and females have the same number of chromosomes.
i. XX-XY Method of Sex Determination:
In human beings and insects like Drosophila both females and males have the same number of chromosomes.
The X and Y chromosomes originated in a pair of autosomes, and differentiated as the Y chromosome degenerated progressively. The Y contains few active genes and is composed largely of repetitive DNA sequences. Most Y genes have copies on the X from which they evolved; this includes even the sex-determining gene SRY as well as several genes required for spermatogenesis. The X contains a disproportionate number of genes that affect reproduction and brain function (or both). ( J.A.M. Graves, 2010)
H UMAN chromosomes have been studied for over half a century but knowledge in this field has expanded rapidly since 1956 when Tjio and Levan' reported the finding of 46 chromosomes in cells from human fetal lung cultures, instead of the 48 previously accepted as the normal number for many decades. The sex chromosomes are represented by an XX complex in the normal female and an XY complex in the normal male. (HENDERSON, J. G; 1958)
The X chromosome is present in both males and females and is comprised of a very interesting pattern of ancestor contribution. Men get their only X chromosome from their mother, but women receive one from each parent. Because the X chromosome can still recombine in gamete production, each X chromosome is comprised of DNA that was contributed from both male and female ancestors. Females can pass along an X chromosome that contains information from both their parents, but males can only pass along an X that is a virtual copy of the one they received from their mothers. It is thus the male’s sperm that determines the gender of each offspring in humans and in dogs.
(Christopher, 2011)
Punnett square predicts all possible combinations in a cross of parents.
It is a matrix that shows all of the possible combinations of male and female gametes when
one or more pairs of independent alleles are crossed. Letters representing the male and female gametes are placed along the left side and the top of the matrix, respectively. The genotypes of the offspring produced by each pairing of gametes occupy the cells in the matrix. ( Mueller and young, 2001)
Comparing two XY systems mammals and fruit flies reveals massive differences:
In both systems, XX gives females and XY gives males, but HOW that happens is completely different. In mammals, sex depends on the presence or absence of the Y XXY is male. In flies, it's the number of X chromosomes that matter, while the Y is irrelevant XXY flies are perfectly happy females. And then there's dosage compensation, which makes sure that XY cells and XX cells express the same levels of the genes on the X. In mammals, this is done by shutting down one of the X chromosomes in XX, while flies do it by doubling expression of the X in XY cells. And just for good measure, nematodes do it by reducing expression of both Xs by half in XX individuals.
ii.The ZW method of sex-determination
The ZW sex-determination system is a system that determines the sex of offspring in birds, some fish, and some insects (including butterflies and moths), and some reptiles.
Birds have female heterogamety with Z and W sex chromosomes. These evolved from different autosomal precursor chromosomes than the mammalian X and Y. ( Kiwoong Nam and Hans Ellegren, 2008)
It is a matrix that shows all of the possible combinations of male and female gametes when
one or more pairs of independent alleles are crossed. Letters representing the male and female gametes are placed along the left side and the top of the matrix, respectively. The genotypes of the offspring produced by each pairing of gametes occupy the cells in the matrix. ( Mueller and young, 2001)
Comparing two XY systems mammals and fruit flies reveals massive differences:
In both systems, XX gives females and XY gives males, but HOW that happens is completely different. In mammals, sex depends on the presence or absence of the Y XXY is male. In flies, it's the number of X chromosomes that matter, while the Y is irrelevant XXY flies are perfectly happy females. And then there's dosage compensation, which makes sure that XY cells and XX cells express the same levels of the genes on the X. In mammals, this is done by shutting down one of the X chromosomes in XX, while flies do it by doubling expression of the X in XY cells. And just for good measure, nematodes do it by reducing expression of both Xs by half in XX individuals.
ii.The ZW method of sex-determination
The ZW sex-determination system is a system that determines the sex of offspring in birds, some fish, and some insects (including butterflies and moths), and some reptiles.
Birds have female heterogamety with Z and W sex chromosomes. These evolved from different autosomal precursor chromosomes than the mammalian X and Y. ( Kiwoong Nam and Hans Ellegren, 2008)
In the ZW system it is the ovum that determines the sex of the offspring, in contrast to the XY sex-determination system. The Z chromosome is larger and has more genes, like the X chromosome in the XY system. Well-differentiated Z and W chromosomes are ubiquitous only in birds and snakes. All the hundreds of bird species that were tested display ZW female heterogamety. The Z chromosome is very uniform in size, either the fourth or fifth largest, but the W chromosome varies from virtual homomorphy in the flightless ratites to extreme differentiation in most carinates.
Sex chromosomes in other reptiles are much more variable than those of birds and snakes. Many lizards and a few turtles have heteromorphic sex chromosomes detectable by traditional cytological techniques. Lizards, the largest reptile group, have the most diverse array of sex chromosome systems among the reptiles. Sex chromosomes have been found in 172 of the 953 lizard species karyotyped (of the total 4765 extant lizard species), representing 10 of the 18 lizard families. Male and female heterogamety, as well as multiple chromosomal sex determining systems have been described in several families.
The XY and ZW chromosomes share no genes at all. Male and female heterogamety resulting from XY- and ZW-systems have been thought to evolve independently from a common ancestor with environmental sex determination, or to go through an intermediate stage in which sex is determined by environmental cues. However, there are at least some examples which support the alternative theory that direct ZW⇔XY transitions have occurred -- at least in fish and amphibians, and probably in reptiles. The extraordinary sex chromosome complex in the platypus and the gene bias in mammalian orthologues of the chicken Z provide some support for the hypothesis that such a transition may also have occurred in the early history of mammals. The alternative hypothesis of vertebrate sex chromosome evolution is that the mammalian XY-system may have arisen directly from an ancient reptilian ZW-system, rather than an intermediate or transitory state of environmental sex determination. This hypothesis will be testable with data from the emerging platypus genome sequence and by comparative mapping studies with birds, reptiles and amphibians, which will give new insights into the sex chromosome evolution of vertebrates. (Tariq Ezaz, 2006).
Sex chromosomes in other reptiles are much more variable than those of birds and snakes. Many lizards and a few turtles have heteromorphic sex chromosomes detectable by traditional cytological techniques. Lizards, the largest reptile group, have the most diverse array of sex chromosome systems among the reptiles. Sex chromosomes have been found in 172 of the 953 lizard species karyotyped (of the total 4765 extant lizard species), representing 10 of the 18 lizard families. Male and female heterogamety, as well as multiple chromosomal sex determining systems have been described in several families.
The XY and ZW chromosomes share no genes at all. Male and female heterogamety resulting from XY- and ZW-systems have been thought to evolve independently from a common ancestor with environmental sex determination, or to go through an intermediate stage in which sex is determined by environmental cues. However, there are at least some examples which support the alternative theory that direct ZW⇔XY transitions have occurred -- at least in fish and amphibians, and probably in reptiles. The extraordinary sex chromosome complex in the platypus and the gene bias in mammalian orthologues of the chicken Z provide some support for the hypothesis that such a transition may also have occurred in the early history of mammals. The alternative hypothesis of vertebrate sex chromosome evolution is that the mammalian XY-system may have arisen directly from an ancient reptilian ZW-system, rather than an intermediate or transitory state of environmental sex determination. This hypothesis will be testable with data from the emerging platypus genome sequence and by comparative mapping studies with birds, reptiles and amphibians, which will give new insights into the sex chromosome evolution of vertebrates. (Tariq Ezaz, 2006).
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