As a result, two gametes virtually never have exactly the same combination of chromosomes. Each chromosome contains dozens to thousands of different genes. The total possible combination of alleles for those genes in humans is approximately 70,,,, This is trillions of times more combinations than the number of people who have ever lived. This accounts for the fact that nearly everyone, except monozygotic twins , is genetically unique.
While homologous pairs of chromosomes are independently assorted in meiosis, the genes that they contain are also independently assorted only if they are part of different chromosomes.
Genes in the same chromosome are passed on together as a unit. Such genes are said to be linked. For example, the "A" and "B" alleles in the illustration below will both be passed on together if the lower chromosome is inherited.
Similarly, "a" and "b" are linked in the other chromosome. Genetic linkage continues as homologous chromosomes separate in the formation of sex cells Linked genes most likely account for such phenomena as red hair being strongly associated with light complexioned skin among humans.
If you inherit one of these traits, you will most likely inherit the other. Genetic linkage of this sort can be naturally ended. During the first division of meiosis, sections near the ends of chromosomes commonly intertwine and exchange parts of their chromatids with the other chromosome of their homologous pair. This process of sections breaking and reconnecting onto a different chromosome is called crossing-over. In the example shown below, "A" and "B" are unlinked by this process.
Crossing-over unlinks alleles of genes as homologous chromosomes separate in the formation of sex cells Crossing-over usually results in a partial recombination , or creation of combinations of alleles in chromosomes not present in either parent. While in mitosis, genes are generally transferred faithfully from one cellular generation to the next; in meiosis and subsequent sexual reproduction , genes get mixed up. Sexual reproduction actually expands the variety created by meiosis, because it combines the different varieties of parental genotypes.
Thus, because of independent assortment, recombination, and sexual reproduction, there are trillions of possible genotypes in the human species.
During cell division, chromosomes sometimes disappear. This occurs when there is some aberration in the centromere , and spindle fibers cannot attach to the chromosome to segregate it to distal poles of the cell. Consequently, the lost chromosome never properly groups with others into a new nuclear envelope , and it is left in the cytoplasm , where it will not be transcribed.
Also, chromosomes don't always separate equally into daughter cells. This sometimes happens in mitosis, when sister chromatids fail to separate during anaphase. One daughter cell thus ends up with more chromosomes in its nucleus than the other. Likewise, abnormal separation can occur in meiosis when homologous pairs fail to separate during anaphase I. This also results in daughter cells with different numbers of chromosomes. The phenomenon of unequal separation in meiosis is called nondisjunction.
If nondisjunction causes a missing chromosome in a haploid gamete, the diploid zygote it forms with another gamete will contain only one copy of that chromosome from the other parent, a condition known as monosomy. Conversely, if nondisjunction causes a homologous pair to travel together into the same gamete, the resulting zygote will have three copies, a condition known as trisomy Figure 3.
The term " aneuploidy " applies to any of these conditions that cause an unexpected chromosome number in a daughter cell. Aneuploidy can also occur in humans. For instance, the underlying causes of Klinefelter's syndrome and Turner's syndrome are errors in sex chromosome number, and Down syndrome is caused by trisomy of chromosome However, the severity of phenotypic abnormalities can vary among different types of aneuploidy.
In addition, aneuploidy is rarely transferred to subsequent generations, because this condition impairs the production of gametes. Overall, the inheritance of odd chromosome number arises from errors in segregation during chromosome replication.
Often, it is these very exceptions or modifications of expected patterns in mitosis and meiosis that enrich our understanding of how the transfer of chromosomes is regulated from one generation to the next.
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Genetics and Statistical Analysis. Thomas Hunt Morgan and Sex Linkage. Developing the Chromosome Theory. Genetic Recombination. Gregor Mendel and the Principles of Inheritance. Mitosis, Meiosis, and Inheritance. Multifactorial Inheritance and Genetic Disease. Non-nuclear Genes and Their Inheritance. Polygenic Inheritance and Gene Mapping. Sex Chromosomes and Sex Determination. Sex Determination in Honeybees. Test Crosses.
Biological Complexity and Integrative Levels of Organization. Genetics of Dog Breeding. Human Evolutionary Tree. Mendelian Ratios and Lethal Genes. Environmental Influences on Gene Expression. Epistasis: Gene Interaction and Phenotype Effects. Genetic Dominance: Genotype-Phenotype Relationships. Not really. We don't know all of our genes yet and even if we did, we still wouldn't know which mutations would cause a change nor which would be lethal.
Perhaps one day we can figure it out but not yet. Suffice it to say that the number of possible people is still really, really big. And of course that isn't the whole story. We have completely ignored all of the DNA outside of the gene. And this is where most of our DNA is -- something like This DNA isn't all "junk" either.
Among other things, it is responsible for figuring out when a gene should be turned on and off. What this means is that changes out there can make people different as well.
For example, the ability to drink milk as an adult is the result of a mutation in some of the DNA found outside of genes. All mammals can digest the sugar in milk lactose as babies. Most lose that ability as adult. Mammals lose their ability to drink milk because the lactase gene gets shut off.
It is the DNA outside of the gene that determines when it is on and when it is off. Lactose tolerant adults have a mutation in that DNA so that the gene doesn't shut off as an adult. Now I would argue that two people who were identical except for their ability to digest milk as an adult would be two different people. So DNA outside of genes is important too. And we know even less about that DNA than we do about genes so that predicting which bases are important out there is even more difficult.
This still isn't enough. There can be differences that are not due to changes at the A, G, C, and T's level. Sometimes what mom eats can affect our DNA for life. Scientists at Duke University showed that pregnant mice that received Vitamin B12, folic acid, choline and betaine gave birth to babies with brown coats.
Pregnant mice that did not receive these supplements gave birth to mice with yellow coats. The difference wasn't because the mom who had yellow mice was low on these supplements. Why did diet affect the hair color of the pups? The extra supplements caused a specific gene, called Agouti, to be turned down. This is what caused the coat color to change -- the bases of the Agouti gene didn't change, just how the gene was used by the mouse pups changed.
Think about this like a light bulb on a dimmer switch. The light bulb is the gene and the dimmer switch is the DNA outside of the gene.
The way a dimmer switch works is that it controls the amount of electricity that gets to the light bulb. A dim and a bright bulb are both the same bulb -- it is just how they are used that is different. Same thing with a gene. The sequences outside act like dimmer switches.
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