There are multiple ways our bodies ensure that we have a unique set of DNA that differs from our parents. For starters, you inherit two copies of each chromosome—one copy from your mom and one copy from your dad. This means that your genome all of your DNA is already different because it contains chromosomes from both of your parents. This can also help explain why two siblings appear to have different genetic ancestry , since they may get different chromosomes from their parents.
But there is more to the story than the combination of chromosome pairs. The actual sequence of DNA on each of the chromosomes is unique due in part to recombination. To understand this, we have to talk about the process of making gametes , which are also known as sperm or egg cells.
One of the traits that makes gametes different from all other cells in your body is they only have one copy of each chromosome,making a total of 23 chromosomes; this is in contrast to most other cells in the body which have two copies of every chromosome, giving them a total of The body does this to make sure that a fertilized egg has the right amount of chromosomes—23 donated from the egg, and 23 donated from the sperm to give a total of Gametes are made from a special kind of cell dedicated to producing either sperm or eggs.
Take the case of men and women and sex chromosomes. Females have two X chromosomes, while males have an X and a Y. The Y chromosome makes up perhaps 1 percent of the DNA. But there is very little correspondence between the Y and the other chromosomes, including the X. In other words, the DNA of a human male differs as much from that of a female as either does from a chimpanzee of the same sex. What does this mean? Most of the differences that we notice are caused by a very tiny fraction of our DNA.
So there is plenty of room for genetic differences among us. Some noteworthy evolutionary changes in human beings have occurred relatively rapidly, despite the slow overall rate of change at the DNA level. The difference between the skin color of Africans and Europeans probably evolved in less than fifty thousand years, an adaptation to differences in climate.
Still more rapid were changes in genes that confer resistance to malaria in Africa and Mediterranean regions; it only took between four and eight thousand years for the new genes to evolve. What genetic analysis reveals is that some of the genetic changes that seem so significant to us depended on a very tiny fraction of our DNA.
But, as I said, this tiny fraction is still a very large number of bases. No two human beings are alike in the traits they possess. Some are tall, others are short; some are stocky, others thin; some are gifted musically, others tone deaf; some are athletic, others awkward; some are outgoing, others introverted; some are intelligent, others stupid; some can write great poetry or music, most cannot.
And so on. To understand our differences, we need to consider not just DNA, but its cellular products as well. This area of study is new, but it is progressing rapidly. The emphasis is changing from DNA sequences to genes.
A gene is a stretch of DNA, usually several thousand base pairs long. The function of most genes is to produce proteins. The genome sequencing project has revealed that we humans have thirty to forty thousand genes. But since a gene often produces more than one kind of protein, sometimes producing different kinds for different body parts, the number of kinds of protein is more like one hundred thousand. We share a number of genes with chimpanzees, genes that make us primates rather than elephants or worms.
Evolutionary scientists believe that many of the differences that we observe between ourselves and chimpanzees involve changes in the amount rather than in the nature of gene products. Human beings and chimpanzees share proteins that produce body hair and brains, but in chimpanzees these proteins produce more hair and less brains.
Why this should be so is still far from being fully understood. But this is a research area that is advancing very rapidly, and there are good genetic leads to be followed up. Of course, not every human difference has a genetic cause. Many are environmental, or are the result of interactions between genes and environment.
Even genetically identical twins develop into distinct individuals. The ability to learn a language is largely innate, built into the nervous system of all normal people, as demonstrated so beautifully in the effortless way in which young children learn to speak.
But the particular language any individual learns obviously depends on the social setting. Mozart was a great composer partly because of his genes and partly because of his training. Ramanujan had a great talent for mathematics, but without his being exposed to a textbook — not a very good one, by the way — he could never have made his astounding discoveries.
Michael Jordan has a talent for basketball, but it would never have developed had he grown up among the Inuits. Just as there are great differences among individuals, there are average differences, usually much smaller, between groups. Italians and Swedes differ in hair color. Sometimes the differences are more conspicuous, such as the contrasting skin color and hair shape of Africans and Europeans. But, for the most part, group differences are small and largely overshadowed by individual differences.
Biologists think of races of animals as groups that started as one, but later split and became separated, usually by a geographical barrier. As the two groups evolve independently, they gradually diverge genetically. The divergences will occur more quickly if the separate environments differ, but they will occur in any case since different mutations will inevitably occur in the two populations, and some of them will persist.
This is most apparent in island populations, where each island is separate and there is no migration between them. Each one has its own characteristic types. In much of the animal world, however, and also in the human species, complete isolation is very rare. The genetic uniformity of geographical groups is constantly being destroyed by migration between them.
In particular, the major geographical groups — African, European, and Asian — are mixed, and this is especially true in the United States, which is something of a melting pot. But the concept itself is unambiguous, and I believe that the word has a clear meaning to most people.
Genetics Awareness Project. Home What is Genetics? What is Genetics? Can a genetic test I buy at the store or online tell me my risk for certain diseases? I have heard that scientists have cloned humans. Is that true? How many cells each twin originates from may determine how genetically different they are. More uneven splits of the embryo may lead to a greater number of gene differences in the twins. By Tina Hesman Saey January 22, at am. Brain Concussion patients should avoid screen time for first two days By Kathiann Kowalski November 10, Tech Did Facebook put Instagram profits ahead of teen safety?
By Kathiann Kowalski November 1, Sorry, your blog cannot share posts by e-mail.
0コメント