Hot answers tagged reproduction
15
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 ...
13
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 ...
12
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 ...
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
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 ...
8
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 ...
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 ...
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
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 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.
6
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 ...
6
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 ...
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
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, ...
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
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 ...
5
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 ...
4
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.
4
The cloaca, which is the common opening of the urinary, excretory, and reproductive systems, is present in birds as well as in non-avian reptiles (and thus presumably dinosaurs), amphibians, and monotremes (e.g., duck-billed platypus). To answer your first question, yes, this condition does seem to be universal for those groups mentioned above.
To answer ...
4
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, ...
4
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 ...
4
In automixy the meiotic cells give rise to diploid offsprings. This can happen by diploidization of the haploid cell (1n->2n), which will produce homozygotes or endomitosis prior to meiosis (4n->2n) which produces heterozygotes. Examples:
Cnemidophorus uniparens : 4n->2n
Sphyrna tiburo: 1n->2n
I don't know of any case where there is fusion ...
3
EvoDevo is not my field, but I will try to give you some pointers.
Mammals as vertebrates, start out as fertilized oocytes, transform into a rapidly dividing lump of cells with eventual polarity through numerous gradients of numerous chemical messengers, and form a neural tube.
The forming body is ever more segmented through the use of gradients and ...
3
Using a somatic cell in an a ovum is what is typically done in the process of cloning. It was the same process used to create Dolly the Sheep. What you're asking about is something very different:
Consider that somatic cells are properly diploid. Whereas an ovum itself just contains a haploid number of chromosomes. In typical sexual reproduction the sperm ...
3
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 ...
3
I happen to have seen a talk by an anthropologist who was working on this (i can't reference them here i'm sorry to say - forgotten her name). I can only give an example from their work...
If you look at old world and new world primates, there is a large difference between the gestation time. If you look at the table in the link, lemurs have half the ...
3
There is no evidence that postnatal hormone exposure can change the sexual sexual preference of an individual. However atypical hormonal stimulation of the fetus can induce homosexuality.
Homosexuality does not develop without a variety of social factors which are up-to-date mostly unexplained. But can develop without hormonal stimulation.
As real life ...
3
Joan Strassman's work is probably the route to go for this.
The short of your answer is that several things mediate who ends up where in the slug:
Cheaters are limited from exploiting other
clones by high relatedness, kin discrimination, pleiotropy, noble
resistance, and lottery-like role assignment.
Here's the most relevant paper:
Strassmann, J. ...
Only top voted, non community-wiki answers of a minimum length are eligible
