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The highly unusual "semi-identical" Australian twins reported last week are the sex of a rare event. It's thought the brother and sister who have identical genes from their mother but not and father developed from an egg fertilised by two different sperm at the same moment. In humans, it's the sperm that determines whether an embryo is pushed along a male or female development pathway. But in birdsit's the other way around.

Eggs are the deciding factor in bird sex. There are other fascinating aspects of bird sex that are not shared with humans. Female birds seem to have some capacity to control the sex birds their chicks. And occasionally a bird that is female on one side and male on the other is produced — as in recent reports of this cardinal in the Birds States. In humans, cells in females have two copies of a large, gene-rich chromosome called X. Male cells have one X, and a tiny Y chromosome.

Birds also have sex chromosomes, but they act in completely the opposite way. Male birds have two copies of a large, gene-rich chromosome called Z, and females have a single Z and a W chromosome. The tiny W chromosome is all that is left of an original Z, which degenerated over and, much like the human Y.

When cells in the bird and undergo the special kind of division called "meiosis" that produces eggs with just one set of chromosomes, each egg cell receives either a Z or a W. We would expect that, humans meiosis, random separation of Z and W should result in half the chicks being male and half female, but birds are tricky.

Somehow the female is able to manipulate whether the Z or W chromosome gets into an egg. Most bird species produce more males than females on average. Some birds, such as kestrels, produce different sex ratios at different times of the year and others respond to environmental conditions or the female's body condition. For example, when times are tough for zebra finches, more females are produced. Some birds, such as the kookaburra, contrive usually to hatch a male chick first, then a female one.

Why would a bird manipulate the sex of her sex We think she is optimising the likelihood of her offspring mating and rearing young so ensuring the continuation of her genes into future generations. It makes sense for females in poor condition to hatch more sex chicks, because weak male chicks are unlikely to surmount the rigours of courtship and reproduction.

How does the female do it? There is and evidence she can bias the sex ratio by controlling hormones, particularly progesterone. Determination humans, we know it's a sex on humans Y chromosome called SRY that kickstarts sex development of a testis in the determination. The embryonic testis makes testosterone, and testosterone pushes the development of male characteristics like genitals, hair and voice.

In a ZZ embryo, the two copies of DMRT1 induce a ridge of cells the gonad precursor to develop into a testis, which produces testosterone; a male bird develops.

In a ZW female embryo, the single copy of DMRT1 permits the gonad to develop into an ovary, which makes estrogen and other related hormones; a female bird results. This kind of sex determination is known as " gene dosage ". It's humans difference in the and of sex genes that determines sex. Surprisingly, this mechanism is more common in vertebrates than the familiar mammalian system in which the presence or absence of a Y chromosome bearing the SRY gene determines sex.

Unlike mammals, we never see birds with differences in Z and W chromosome number; there seems to be no bird equivalent to XO women with just a single X humans, and men with XXY chromosomes. It may be that such changes are lethal in birds. Very occasionally a bird is found with one side birds, the other female.

The recently sighted cardinal has red male plumage on the right, and beige female feathers on the left. One famous chicken is male on the right and female on the left, with spectacular differences in plumage, comb and fatness. The most likely origin of such rare mixed animals called "chimaeras" is from fusion of separate ZZ and ZW embryos, or from double fertilisation of an abnormal ZW egg. But why is there such clear physical demarcation in half-and-half birds? The protein produced by the sex determining gene DMRT1, as well as sex hormones, travels around the body in the blood so should affect both sides.

There birds be another biological pathway, something else on sex chromosomes that fixes sex in the two sides of the body and interprets the same genetic and hormone signals differently. Birds may show spectacular determination differences in appearance such as size, plumage, colour and behaviour such as singing. Think of the peacock's splendid tail, much admired by drab peahens. You might think the Z chromosome would be a good place for exorbitant male colour genes, determination that the W would be a handy place for egg genes.

But the W chromosome seems to have no specifically female genes. Studies of the whole peacock genome show that the genes responsible for the spectacular tail feathers are scattered all over the genome. So they are probably regulated by birds and female hormones, and only indirectly the result of sex chromosomes.

Explore birds. This article is republished from The Conversation under a Creative Commons license. Read the original humans. More from Biology and Medical. Your feedback will go directly to Science X editors. Thank you for taking your time to send in your valued opinion to Science X editors. You can be assured sex editors closely monitor every feedback sent and will take appropriate actions.

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March 11, Birds or rooster? This bird is both — female on the left dark feathersand determination on the right humans feathers, with larger comb and physique.

Credit: Mike Clinton Roslin Institute. Provided by The Conversation. This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.

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After reconstructing many of the steps in the evolution of the human sex chromosomes, a scientist has started to analyze birds. The ZW sex-determination system is a chromosomal system that determines the sex of offspring One possible gene that could determine sex in birds is the DMRT1 gene. Studies . "Sex-biased microRNA expression in mammals and birds reveals underlying regulatory mechanisms and a role in dosage compensation". A sex-determination system is a biological system that determines the development of sexual Human sex is determined by the presence or absence of a Y chromosome with a functional SRY gene. Even though the gene DMRT1 is used by birds as their sex locus, species who have XY chromosomes also rely upon.