The Chromosomes are the nuclear DNA in humans. We have 23 pairs of chromosomes. 22 autosomes that are labeled 1 to 22 from the (historically) longest to the shortest and occur in pairs. And the Allosome (or Sex) chromosomes X and Y that, in some combination, form a final pair in the most common case. Mitochondria is the only DNA that is (a) NOT a chromosome and (b) exists outside the cell nucleus in humans.
What can be confusing is the term strand. Some say a DNA strand forms a chromosome. And so they refer to like strands being the similar copies of a chromosome in the autosomes or possibly X in biological females. Others consider a strand to be each side of the double helix. So the strands are bonded together to form the double helix and thus a chromosome. That latter is the more formal and proper use of the term. There is an analogy here to the word "twin" where it refers to the singular person of a known pair. But you can have "identical" twins and "fraternal" twins. Pairs of like chromosomes are "fraternal" in this context. The two strands that form a chromosome are "identical" (albeit a negative or mirror image of each other). To avoid confusion and improper use of more strict biological terms, we try to use the term strand when referring to a single side of the double helix. So two strands form a DNA molecule which is either a chromosome or non-nuclear mitochondria.
A chromosome structure is not really important to understand genetic genealogy. Suffice it to say there are genes that are areas defined to code or develop specific proteins, and there are inter-gene (formerly called junk) areas between. The ends of the chromosome are termed telomere and are like end caps that are known to slowly degrade and shorten over time. The centromere is a region where the chromosome attach with sister chromatids during replication before being pulled apart, We only mention these two locations and the structure because they are highly volatile and generally avoided during the test of DNA markers.
For the most part, in our cells, we have pairs of chromosomes: one from our mother and one from our father. They are more properly termed homologous chromosomes but that knowledge is not necessary for understanding here. The chromosomes in the pair do not match each other identically. (With the caveat that 99.999% of the DNA between any two people is the same; so identical is a very relative term here.) Due to recombination, we more rarely share an exact copy of a chromosome with a sibling or even a parent. The only exception is identical twins from the same sac that usually have (near) identical DNA throughout. Although we always get 1/2 our chromosomes from a parent, that parent may not give us half from each of their parents (i.e. your grandparents). So although you (normally) have four grandparents, you may only have 20% of your paternal grandfather's DNA and 30% of your paternal grandmothers. The amount of your paternal or maternal grandparents DNA will always add up to 50% as you get 50% of your DNA from each parent, always. The variance of how much you inherit from which grandparent starts to widen quickly with great grandparents and each generation beyond. It is a statistical chance with a wide variance (standard deviation).
The autosomes range in approximate size from 250 million base-pairs (for chromosome 1) down to 50 million base-pairs (for chromosome 22). This represents from 280 cM to 80 cM; the more common unit of measure in genetics. X and Y are approximately 155 and 60 million base-pairs; respectively with the X coming in around 195 cM. The Y chromosome is never really compared and reported as matching segments like the other chromosomes. This because it is so nearly identical as passed down, to start with, and because the ends of the Y chromosome actually recombine with the X. Biology is messy and such nuances are found if you desire to look. (For example, part of a male's Y SNPs are reported as the second value in the pair of his X result. Thus giving rise to what appears as a second X chromosome in the male tester.)
The sex cells are haploid or contain only one of each chromosome type. Normal cells are diploid and contain the pairs of each chromosome. 23 pairs or 46 chromosomes. So haploid cells contain 23 chromosomes and about 3.2 billion base-pairs. Diploid cells contain 46 chromosomes and about 6.4 billion base-pairs. The haploid nucleus has around 3,700 cM of DNA in length. The diploid around 7.4 cM. The amount depends on who calculates and how — centimorgan determination varies. What is key here is that most genetic genealogy testing is covering testing diploid cells and returns around 700,000 SNP value pairs. One value for each chromosome. So, on average, this is about one allele of every 10,000 base pairs. Nowhere near a full-sequence coverage. But as most of our DNA is identical, it is good enough for our use here. We should add that Sequencing testing, now becoming common for yDNA testing, has a much greater coverage and leading to the discovery of many more SNP's. SNP)s are believed to occur roughly every 300 base-pairs on average and thus have approximately 10 million SNPs in the haploid and thus 20 million in the diploid. It is not known how STRs figure into that approximate estimate of SNP calculation.
When most discussion mentions the Human Genome, it is referring to the haploid or a single instance of each type of a chromosome (the caveat being it usually includes both the X and Y chromosome). When many regurgitate numbers, they tend to do so for the haploid Human Genome (unknowingly, mistakenly) and not the diploid that is actually being tested in genetic genealogy.
One last thing to cover. We mentioned a chromosome consists of two strands. These strands are the opposite of each other as each rung in the double-helix ladder consists of a base-pair and its opposite. It is important to know which strand you are talking about. So they define them as forward / sense / positive / 3'-5' strand and the opposite reverse / anti-sense / negative / 5'-3'strand. The reference model will have an A base-pair value on the forward strand and a T on the reverse one. A variant that represents an A to T transition is thus defined as being on the forward strand. We usually always try to use this orientation. But sometimes the industry uses the opposite for some markers.
What can be confusing is the term strand. Some say a DNA strand forms a chromosome. And so they refer to like strands being the similar copies of a chromosome in the autosomes or possibly X in biological females. Others consider a strand to be each side of the double helix. So the strands are bonded together to form the double helix and thus a chromosome. That latter is the more formal and proper use of the term. There is an analogy here to the word "twin" where it refers to the singular person of a known pair. But you can have "identical" twins and "fraternal" twins. Pairs of like chromosomes are "fraternal" in this context. The two strands that form a chromosome are "identical" (albeit a negative or mirror image of each other). To avoid confusion and improper use of more strict biological terms, we try to use the term strand when referring to a single side of the double helix. So two strands form a DNA molecule which is either a chromosome or non-nuclear mitochondria.
A chromosome structure is not really important to understand genetic genealogy. Suffice it to say there are genes that are areas defined to code or develop specific proteins, and there are inter-gene (formerly called junk) areas between. The ends of the chromosome are termed telomere and are like end caps that are known to slowly degrade and shorten over time. The centromere is a region where the chromosome attach with sister chromatids during replication before being pulled apart, We only mention these two locations and the structure because they are highly volatile and generally avoided during the test of DNA markers.
For the most part, in our cells, we have pairs of chromosomes: one from our mother and one from our father. They are more properly termed homologous chromosomes but that knowledge is not necessary for understanding here. The chromosomes in the pair do not match each other identically. (With the caveat that 99.999% of the DNA between any two people is the same; so identical is a very relative term here.) Due to recombination, we more rarely share an exact copy of a chromosome with a sibling or even a parent. The only exception is identical twins from the same sac that usually have (near) identical DNA throughout. Although we always get 1/2 our chromosomes from a parent, that parent may not give us half from each of their parents (i.e. your grandparents). So although you (normally) have four grandparents, you may only have 20% of your paternal grandfather's DNA and 30% of your paternal grandmothers. The amount of your paternal or maternal grandparents DNA will always add up to 50% as you get 50% of your DNA from each parent, always. The variance of how much you inherit from which grandparent starts to widen quickly with great grandparents and each generation beyond. It is a statistical chance with a wide variance (standard deviation).
The autosomes range in approximate size from 250 million base-pairs (for chromosome 1) down to 50 million base-pairs (for chromosome 22). This represents from 280 cM to 80 cM; the more common unit of measure in genetics. X and Y are approximately 155 and 60 million base-pairs; respectively with the X coming in around 195 cM. The Y chromosome is never really compared and reported as matching segments like the other chromosomes. This because it is so nearly identical as passed down, to start with, and because the ends of the Y chromosome actually recombine with the X. Biology is messy and such nuances are found if you desire to look. (For example, part of a male's Y SNPs are reported as the second value in the pair of his X result. Thus giving rise to what appears as a second X chromosome in the male tester.)
The sex cells are haploid or contain only one of each chromosome type. Normal cells are diploid and contain the pairs of each chromosome. 23 pairs or 46 chromosomes. So haploid cells contain 23 chromosomes and about 3.2 billion base-pairs. Diploid cells contain 46 chromosomes and about 6.4 billion base-pairs. The haploid nucleus has around 3,700 cM of DNA in length. The diploid around 7.4 cM. The amount depends on who calculates and how — centimorgan determination varies. What is key here is that most genetic genealogy testing is covering testing diploid cells and returns around 700,000 SNP value pairs. One value for each chromosome. So, on average, this is about one allele of every 10,000 base pairs. Nowhere near a full-sequence coverage. But as most of our DNA is identical, it is good enough for our use here. We should add that Sequencing testing, now becoming common for yDNA testing, has a much greater coverage and leading to the discovery of many more SNP's. SNP)s are believed to occur roughly every 300 base-pairs on average and thus have approximately 10 million SNPs in the haploid and thus 20 million in the diploid. It is not known how STRs figure into that approximate estimate of SNP calculation.
When most discussion mentions the Human Genome, it is referring to the haploid or a single instance of each type of a chromosome (the caveat being it usually includes both the X and Y chromosome). When many regurgitate numbers, they tend to do so for the haploid Human Genome (unknowingly, mistakenly) and not the diploid that is actually being tested in genetic genealogy.
One last thing to cover. We mentioned a chromosome consists of two strands. These strands are the opposite of each other as each rung in the double-helix ladder consists of a base-pair and its opposite. It is important to know which strand you are talking about. So they define them as forward / sense / positive / 3'-5' strand and the opposite reverse / anti-sense / negative / 5'-3'strand. The reference model will have an A base-pair value on the forward strand and a T on the reverse one. A variant that represents an A to T transition is thus defined as being on the forward strand. We usually always try to use this orientation. But sometimes the industry uses the opposite for some markers.
See Also
DNA, Allosome, AutosomesExternal References
- Khan Academy Chromosomes, Chromatids and Chromatin, part of High School Biology course
- Wikipedia Chromosome, Chromatid, and Homologous Chromosome
- NCBI Reference to the book "The Cell" section on Chromosome and Chromatid