[paul.fr]

If i remember correctly (big if), i share 50% of my mothers and fathers DNA. Has it always been this way? Was there a time when the balance was not equal?

If it has always been this way, why has, say the sperm, not somehow managed to get more of it's DNA passed on to the child?

[dkv]

Cross XX and XY chromosomes.
we get XX,XY,XX,XY
Knowing that you are a male.
It can only be XY.
But which guess means that you carry 50% characteristics of father and rest from mother.
Animals can carry Z chromosomes as well.
But is the underlying distribution of X chromosome genes and Y chormosome genes are exactly the same?
I dont know.

[iko]

The Y chromosome is one of the sex-determining chromosomes in humans and most other mammals. In mammals, it contains the gene SRY, which triggers testis development, thus determining maleness.
 Overview

Most mammals have one pair of sex chromosomes in each cell. Males have one Y chromosome and one X chromosome, while females have two X chromosomes. In mammals, the Y chromosome contains the gene that triggers embryonic development as a male. This gene is SRY. Other genes (in addition to SRY) on the Y chromosomes of men and other mammals are needed for normal sperm production.

There are exceptions, however. Among humans, some men have two X's and a Y ("XXY", see Klinefelter's syndrome), or one X and two Y's (see XYY syndrome), and some women have three Xs or a single X (and no Y, "X0", see Turner syndrome). There are other exceptions in which SRY is damaged (leading to an XY female), or copied to the X (leading to an XX male). For related phenomena see Androgen insensitivity syndrome and Intersex.

Many groups of organisms in addition to mammals have Y chromosomes, but these Y chromosomes do not share common ancestry with mammalian Y chromosomes. Such groups include fruit flies (Drosophila melanogaster and relatives), some other insects, some fish, some reptiles, and some plants. In fruit flies, the Y chromosome does not trigger male development. Instead, sex is determined by the number of X chromosomes. So XXY fruit flies are female, and fruit flies with a single X (X0), are male but sterile.

Other organisms have mirror image sex chromosomes: the female is "XY" and the male is "XX", but by convention biologists call a "female Y" a W chromosome and the other a Z chromosome. For example, female birds, snakes, and butterflies have ZW sex chromosomes, and males have ZZ sex chromosomes.


 [ Invalid Attachment ]

A Male ' Y' Chromosone looking typically upright and erect !!

Klinefelter syndrome

http://www.aafp.org/afp/20051201/2259_f1.jpg

Klinefelter syndrome is caused by an additional X chromosome in males (47,XXY). Clinical findings are nonspecific during childhood; thus, the diagnosis commonly is made during adolescence or adulthood in males who have small testes with hypergonadotropic hypogonadism and gynecomastia. Virtually all men with Klinefelter syndrome are infertile.
Epidemiology
Approximately one in 1,000 boys is born with an additional X chromosome-47,XXY, the karyotype that causes Klinefelter syndrome.
This karyotype is detected at or before birth in 10 percent of affected boys, and it is found during adulthood in 25 percent of affected men. Almost all men with a 47,XXY karyotype will be infertile; Klinefelter syndrome accounts for 3 percent of male infertility. Klinefelter syndrome is common in infertile men with oligospermia or azoospermia (5 to 10 percent).

ikod

[paul.fr]

But why has the females egg, never developed to pass on more of the mother's DNA, than the father's? Wouldn't one want to pass on more of their code?

[iko]

But why has the females egg, never developed to pass on more of the mother's DNA, than the father's? Wouldn't one want to pass on more of their code?

More ain't better in this case too.  Quality makes the difference even here.
Passing 'more DNA' doesn't imply to achieve a better result in this context.
Diploid cells need a proper 'balance' of their vital information package (i.e. chromosomes with their genes) to allow a perfect transfer to their progeny (million of new cells) during DNA duplication, chromosome build-up and mitotic division.
Any defect of this delicate system causes troubles.  A fifty-fifty share of the information is mandatory for this type of mechanism.
Infact, abnormal chromosomal assets not only carry heavy defects, but often result in spontaneous abortion.

ikod

http://www.web-books.com/MoBio/Free/Ch8E.htm
http://en.wikipedia.org/wiki/Biological_life_cycle

[_Stefan_]

Is it because genes come in pairs of alleles, so in order to make things split of and recombine neatly each parent donates one of those copies?

Also in a sense each parent does pass on 'more than half' their genes, because each gamete produced is a different set of halves.

Related question: Does genetic recombination occur before/during production of gametes, or does it occur afterwards, once the zygote has formed?

[iko]

Imagine to put together an engine:
You need a certain number of bolts.
If one gives twice the number of screws needed, or half, and the other does the same with nuts...well, they're gonna have troubles.

Related question: Does genetic recombination occur before/during production of gametes, or does it occur afterwards, once the zygote has formed?

The answer is: mostly during production of gametes, in the 'meiosis' phase.

wikizip:  "genetic recombination"

http://en.wikipedia.org/wiki/Genetic_recombination
http://en.wikipedia.org/wiki/Meiosis

http://www.anselm.edu/homepage/jpitocch/genbio/recombvar.JPG

http://www.anselm.edu/homepage/jpitocch/genbio/mitosisnot.html

[_Stefan_]

Thanks Iko.

So do parts of each chromosome pair also cross over during recombination?

[iko]

Yes, Stefan.

The closer 2 genes are on a chromosome, the higher probability to 'segregate' together.

http://en.wikipedia.org/wiki/Crossing_over

http://upload.wikimedia.org/wikipedia/commons/thumb/0/0e/Morgan_crossover_1.jpg/774px-Morgan_crossover_1.jpg

[MayoFlyFarmer]

the original question actually is a good one.  it would make evolutionary sense if one sex of a species found a way to pass on more than 50% of their DNA to its offspring.  my guess is that the result would be simlar to that of inbreeding (lack of diversity) and thus would lead to a decrease in overall fitness.