How Sexual Reproduction Creates Genetic Variation
Sexual reproduction increases genetic variation in offspring, which in turn increases the genetic variability in species. You can see the effects of this genetic variability if you look at the children in a large family and note how each person is unique. Imagine this kind of variability expanded to include all the families you know (not to mention all the families of all the sexually reproducing organisms on Earth), and you begin to get a feel for the dramatic genetic impact of sexual reproduction.
DNA polymerase occasionally makes uncorrected mistakes when copying a cell’s genetic information during DNA replication. These mistakes are called spontaneous mutations, and they introduce changes into the genetic code. In addition, exposure of cells to mutagens (environmental agents, such as X-rays and certain chemicals that cause changes in DNA) can increase the number of mutations that occur in cells. When changes occur in a cell that produces gametes, future generations are affected.
When homologous chromosomes come together during prophase I of meiosis, they exchange bits of DNA with each other. This crossing-over results in new gene combinations and new chances for variety. Crossing-over is one way of explaining how a person can have red hair from his mother’s father and a prominent chin from his mother’s mother. After crossing-over, these two genes from two different people wound up together on the same chromosome in the person’s mother and got handed down together.
Independent assortment occurs when homologous chromosomes separate during anaphase I of meiosis. When the homologous pairs of chromosomes line up in metaphase I, each pair lines up independently from the other pairs.
So, the way the pairs are oriented during meiosis in one cell is different from the way they’re oriented in another cell. When the homologous chromosomes separate, many different combinations of homologous chromosomes can travel together toward the same side of the cell. How many different combinations of homologous chromosomes are possible in a human cell undergoing meiosis? Oh, just 223 — that’s 8,388,608 to be exact. Now you can begin to see why even large families can have many unique children.
Fertilization presents yet another opportunity for genetic diversity. Imagine millions of genetically different sperm swimming toward an egg. Fertilization is random, so the sperm that wins the race in one fertilization event is going to be different than the sperm that wins the next race. And, of course, each egg is genetically different too.
So, fertilization produces random combinations of genetically diverse sperm and eggs, creating virtually unlimited possibilities for variation. That’s why every human being who has ever been born — and ever will be born — is genetically unique. Well, almost. Genetically identical twins can develop from the same fertilized egg, but even they can have subtle differences due to development.
Nothing’s perfect, even in the cellular world, which is why sometimes meiosis doesn’t occur quite right. When chromosomes don’t separate the way they’re supposed to, that’s called nondisjunction. The point of meiosis is to reduce the number of chromosomes from diploid to haploid, something that normally happens when homologous chromosomes separate from each other during anaphase I. Occasionally, however, a pair of chromosomes finds it just too hard to separate, and both members of the pair end up in the same gamete.
What happens next isn’t pretty. Two of the final four cells resulting from the meiotic process are missing a chromosome as well as the genes that chromosome carries. This condition usually means the cells are doomed to die. Each of the other two cells has an additional chromosome, along with the genetic material it carries. Well, that should be great for these cells, shouldn’t it? It should mean they’ll have an increased chance for genetic variation, and that’s a good thing, right?
Wrong! An extra chromosome is like an extra letter from the IRS. It’s not something to hope for. Many times these overendowed cells simply die, and that’s the end of the story. But sometimes they survive and go on to become sperm or egg cells. The real tragedy, then, is when an abnormal cell goes on to unite with a normal cell. When that happens, the resulting zygote (and offspring) has a trisomy, which is three of one kind of chromosome rather than the normal two.
Here’s the real problem with this scenario: All the cells that develop by mitosis to create the new individual will be trisomic (meaning they’ll have that extra chromosome). One possible abnormality occurring from an extra chromosome is Down syndrome, a condition that often results in some mental and developmental impairment and premature aging.
Most people with Down syndrome have an extra copy of Chromosome 21. If an egg with two number 21 chromosomes is fertilized with a normal sperm cell with just one number 21, the resulting offspring has 47 chromosomes (24 + 23 = 47), and Down syndrome occurs.
You probably already know that the mother’s age is a factor in the creation of genetic abnormalities such as Down syndrome, but do you know why? The answer probably has something to do with the fact that egg formation begins so early in human development. Meiosis begins in the fetal stage for females and then the future egg cells hang out in the ovaries until puberty, when one per month restarts meiosis in preparation for fertilization.
If a cell has been waiting its turn for 40 or 45 years, it’s pretty darned old — in cellular terms at least. (Aging gametes aren’t such an issue for males because they don’t start making sperm until after puberty. Meiosis is a continuous process for them, producing new cells all the time.)
Pink and blue chromosomes
Human males and females are different in many ways, but their chromosomes look remarkably similar. If you compared karyotypes for males and females, the first 22 pairs of chromosomes would look the same. The only difference would be in that 23rd pair, where females would have two long chromosomes and males would have one long and one short. How can such a little difference in chromosomes make such a big difference in biology?
In many organisms — including humans and fruit flies — the sex of an individual is determined by specific sex chromosomes, which scientists refer to as the X and Y chromosomes. The 23 pairs of human chromosomes can be divided into 22 pairs of autosomes, chromosomes that aren’t involved in the determination of sex, and one pair of sex chromosomes. Men and women have the same types of genes on the 22 autosomes and on the X chromosome.
But only guys get a special gene, located on the Y chromosome, that jump-starts the formation of testes in boy fetuses when they’re about 6 weeks old. After the testes form, they produce testosterone, and that turns on the development of additional male characteristics. The Y chromosome is smaller than all the other chromosomes, but it packs one powerful little gene!