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40

This is a interesting question and for a long time it was thought that they do not age. In the meantime there are some new papers which say that bacteria do indeed age. Aging can be defined as the accumulation of non-genetic damages (for example oxidative damage to proteins) over time. If too much of these damages are accumulated, the cell will eventually ...


17

Firstly I'll clarify that you are talking about simultaneous hermaphrodites rather than sequential hermaphrodites (1st one sex, then the other e.g. the limpet Patella vulgata). It is perhaps easiest to address the question by countering it and asking why dioecy (2 sex systems/2 gonochoric types e.g. male and female) is better? As you have pointed out there ...


15

When dealing with humans, there are only two Biological genders as defined by the presence or absence of the Y-Chromosome. If the Y-Chromosome is not present, or through some process gets totally deactivated, the human will appear and function as a Female. XX = Female XY = Male XXY = Male (Klinefelter's Syndrome) XYY = Male (Aneuploidy - Normal ...


14

Having descended testes is a derived characteristic within mammals; monotremes and the Afrotheria (including elephants) all retain the ancestral character state (Kleisner, et al., 2010)2. Among those mammals with descended testes, these can be ascrotal or scrotal. Testicular descent is hypothesized to have only occurred once within Mammalia, with the ...


14

You have clearly given this a lot of thought. Unfortunately, as @adam.r said, you are laboring under certain misapprehensions. The quick answer is that each generation does not "improve" on the last. That is a common misconception. In a bit more detail: First of all, your copying metaphor is a bad one. There was no "perfect original", I expand on this ...


12

The Fisher's principle is not applicable to the fetuses because it has been formulated for parental expenditure and basically states that the ratio of male to female parents (implying that both parties have reached the age of fertility) will tend to 1:1. There are several mechanisms that we can use and that are mentioned in the canonical paper by James ...


12

Inbreeding depression, "the reduced survival and fertility of offspring of related individuals" (quoting the linked article), is a well-known and well-understood biological effect. It does, indeed, affect humans. The problem is that recessive mutations become more likely to affect the survival of the offspring of relatives. Imagine that you have a mutation ...


11

I thought that the only difference between the male and female reproductive cell's DNA code is the X-Y chromosome that determines gender, not differences that can define the species of the offspring. Why does this happen? Well that is not the only difference, there are certain characters that are only transmitted by one of the parents. First of all you ...


11

Interfamilial hybrids have never, to my knowledge, been recorded occurring naturally (without human intervention). In plants, somatic inter-familial hybrids have been produced for a wide variety of species pairs in the lab (e.g. between carrot and barley; Kisaka et al. 1997). In animals, there are some historical reports of hybrids between chickens ...


11

Sperm are cheaper than eggs According to Bateman's Principle the males of a species are (typically) more able to produce a greater variance in the number of offspring. This is because generally the males can produce many gametes and achieve many matings whereas females are limited by the number of eggs, resources, and time lost to pregnancy. Therefore, ...


10

Rice and Salt$^1$ bred fruit flies for 35 generations and from one line of flies created two groups that were isolated from each other reproductively. They could not interbreed because they no longer bred in the same environment. Depending on one's definition of 'species' this could be a case of artificial speciation. $^1$Rice WR, Salt GW (1988), Speciation ...


9

The blood of the fetus does not mix with the blood of the mother. Instead, the placenta provides a system where the two separate blood streams flow past each other with thin separation allowing nutrients to flow between the two streams but the not the blood cells and other large components.


8

There is a good open access review about the effects of different hormones on sexual arousal. In most instances, the hormone is acting on the brain, which signals the information to gonads and genitals via nerves (i.e., the hypothalamic-pituitary-gonadal axis). However, in a study in which I have participated, we found a more direct and probably ancient ...


8

I want to know can +ve and -ve blood group of a couple could be a cause of miscarriage in pregnancy? Yes. In extreme cases, it can. You are talking about Rh Incompatibility, and can become a big issue when the mother has Rh(-) blood and the father has Rh(+) blood (such as the case with your cousin and his wife). What could have happened (but definitely ...


8

The concept of a human/chimp hybrid is called a humanzee. No humanzees have ever been recorded despite the fact that sexual intercourse between humans and chimps has been recorded on several occasions. This suggests that male-human/female-chimpanzee is not capable of producing viable offspring, at the very least. As to why, several reasons will contribute: ...


7

From the standpoint of the skeletal system, spaying is definitely not good for her health. Removal of the ovaries mimics what happens at menopause, when circulating estrogen levels fall. Estrogen is necessary for maintenance of bone mineralization. Without estrogen, bones start to demineralize (the same process that happens in human females after menopause, ...


7

Daniel's answer excellently explains the issue in an understandable fashion, though you may have been looking for a more cell biological approach. According to Immunological aspects of sperm receptors on the zona pellucida of mammalian eggs, 1976, Dunbar & Shivers, the egg cell has a receptor construction which is necessary for the sperm to attach. If ...


7

The biological environment in every cell and tissue of your body is an extremely complex, tightly controlled system. There are tens of thousands of genes in a typical eukaryotic genome, and depending on tissue types and environmental conditions, these genes are turned on and off in a very controlled manner. There is regulation at every point in the flow of ...


7

A common problem for any woman who has experienced pregnancy and childbirth is damage to the levator ani muscle, which comprises a portion of the "pelvic floor." from here This can cause a number of symptoms, including urinary incontinence (due to an increase in pressure on the bladder from less effective muscular support), and possible prolapse of the ...


7

The following paper(1) studies the trends in male to female ratios among newborns over the period 1950-1994 in 29 countries (20 major European countries, USA, Canada, Japan, and others). In all countries at any time point, the male to female ratio was always higher than one. In some countries (Mexico and a few northern eastern European countries) the male ...


7

This isn't so precisely focused on tortoises, but a general theory in evolutionary biology for why some animals live longer is K vs r selection theory. The idea here is that animals will make a sort of evolutionary 'choice' and configure themselves to breed as numerously and quickly as they can. This is called 'r' selection, named after the constant that ...


7

Maternal inheritance of mitochondrial DNA is very well conserved, although some species, such as some mussels, show paternal inheritance. As for why or what the advantage is, some of it is due to basic logistics: sperm cells have ~100-1000 mitochondria, egg cells have 105-106, so male contributions are largely washed out. Plus, most mitochondria in sperm ...


7

The frequency rises with maternal age due to a peculiarity of meoisis in female mammals. Meiosis is originated in the fetal ovary, arresting at metaphase I with the homologous chromosomes aligned for segregation. Cells remain in this state until the time of ovulation, often decades later in humans. The longer cells remain in the arrested ...


6

I think you might be confusing sex and gender. The terms are often used interchangeably, but strictly speaking, they have different biological meanings. Sex refers to the biological categorization based on genetics, reproductive organs, or similar things, whereas gender is based on social identity. For humans, there are only two sex chromosomes, X and Y, ...


6

The propensity for heterozygotic twins seems to be driven primarily by genetics, with additional factors playing a role (http://152.98.160.29/contents/p/staff/CV162Lewis_UQ_Copy.pdf', info site): hormones ("Mothers of fraternal twins tend to be taller, and have earlier and shorter menstrual cycles") ethnic background (which is really genetic) - African ...


6

Meiosis is the type of cell division responsible for the diversification of genetic material among egg and sperm cells. The diversity comes primarily from crossing over (Prophase I) and the cell divisions (Telophase I & II) later on in the process. Meiosis begins with one diploid cell containing two copies of each chromosome—one from the organism's ...


5

there is some evidence that the female reproductive system may also do some gender based selection and skew the chances of one or another outcome depending upon environment, if I recall the snake work correctly. I found a recent (2002?) review: West SA, Reece SE, Sheldon BC. 2002. Sex ratios. Heredity 88: 117–24. Furthermore, in some of these cases ...


5

How are such species are defined, and at what point dogs stop being dogs anymore? This is a bit like the is-Pluto-a-planet-discussion. A group of scientists have to come together and hold a big conference. You have a few principles that you want to adhere to and then it's big groups of people making decisions.


5

here I found an answer. Not sure how accurate is is, though. Data seems to be from the US Here is a rewrite of parts of that article: 266 days before birth, all we have is a fertilized egg. (~33% chance of living birth). The odds are calculated using data from in-vitro fertilization, and the next stage is 66%. In in-vitro, some 50% of the eggs cannot ...


5

Very very very very small. Next to impossible, millions of sperm cells start the journey and only a very small fraction of the sperm produced make it as far as the egg if "inserted properly." The vagina is a very hostile environment - to prevent infection - and this means very few sperm make it far enough. We produce in the order of 200-300 million sperm and ...



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