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Chromosomal Inheritance

When the cell is not dividing, and the DNA is actively being used for expressing genes, it is loosely packed as chromatin.

Chromosomes are long strands of DNA that, in eukaryotes, are wrapped around proteins called histones. The DNA is tightly wound up, allowing for it to be easily moved during cell division.

As humans, we have 23 pairs of chromosomes (so 46 total), with one set from each parent. We have 22 autosomes (non-sex chromosomes) and 1 pair of sex chromosomes (XX or XY).

The basis of chromosomal inheritance is the concept that genes are located on chromosomes at particular locations known as genetic loci. However, the location of genes on chromosomes can lead to non-Mendelian patterns.

Sex-Linked

So far, what we have discussed the inheritance of traits on autosomes. When genes are found on a sex chromosome, the sex of the parent - as well as the sex of the offspring - have an impact on the inheritance of the trait.

In humans, and many other species, this means that the genes are found either on an X or a Y chromosome.

If it's on the Y chromosome, things are fairly simple. Males are XY and females are XX, meaning that if it's a Y-linked trait, then dad will pass it on to his sons, and none of his daughters. You can look at the Punnett Square to the right here as a model of how this works.

When a trait is X-linked, it can be either dominant or recessive. Because males only have one X chromosome, and they always pass a Y to their sons, a dad can never pass it to his daughter. In addition, males are hemizygous and don't have a second X to mask the trait. Meaning either way, if a male has the affected X-linked allele, they will show it. Females, on the other hand, can be heterozygous. So if it's an X-linked recessive trait, you can have carriers - females that have the allele but don't express the phenotype. In this situation, the trait is more common in males than females, and will often appear to skip a generation.

Linkage

Mendel's Law of Independent Assortment tells us that the chance of inheriting one allele does not depend on how likely we are to inherit any other allele. When genes are on the same chromosome, this is because of crossing over in Prophase I of meiosis.

However, if genes are particularly close together, they are less likely to cross over. When this happens, the genes are more likely to be inherited together than not, violating independent assortment. This is known as linkage, and the two genes can be considered linked.

If you look at the above image, the shaded space behind the chromosomes is where crossing over must happen in order for the genes to be separated. While it's not entirely random where crossing over occurs, and there are crossing over hotspots, it is clear that the bottom chromosomes - where the genes are closer - has a much lower chance of the genes being separated.

We can use how often crossing over (aka recombination) occurs to build a gene map. The recombination frequency (r.f. = [# recombinants / total offspring] x 100%) is equal to one centimorgan (cM) or one "map unit". These are relative units that tell us how far apart genes are on a chromosome.

Let's work through an example problem of how to solve this.

Professor Kukui is studying exeggutor genetics. He is looking at the traits for neck length, presence of a tail, and leaf color. Long necks (N) are dominant over short necks (n), tails (T) are dominant to not having a tail (t), and green leaves (L) are dominant to red leaves (l).

Professor Kukui does three test crosses, checking for linkage between the traits, and gets the following results:

NnTt x nntt

235 long-neck / tail

18 long-neck / no tail

230 short-neck / no tail

17 short-neck / tail

TtLl x ttll

54 tail / green leaves

270 tail / red leaves

274 no tail / green leaves

42 no tail / red leaves

NnLl x nnll

154 long-neck / green leaves

15 long-neck / red leaves

13 short-neck / green leaves

168 short-neck / red leaves

Determine the relative distance between each gene, as well as their order on the chromosome.

Click here to see the work/answer!

Ok, so how do we solve that? First, we need to find the recombination frequencies.

Let's start with the first cross (NnTt x nntt). In order to find the recombination frequency, we need to find the number of recombinants. As crossing over happens with less frequency for linked genes, the recombinants won't occurs as often as other combinations. In other words, it's the two phenotypes that occur the least! So let's plug into the equation

(18 + 17) / (235 + 18 + 230 + 17) = .07 * 100% = 7%

This means there is a 7cM distance between the N genetic locus and the T genetic locus.

If we do this for the other two we get:

T & L: (54 + 42) / (54 + 270 + 274 + 42) = .15 * 100% = 15% = 15cM between T and L

N & L: (15 + 13) / (154 + 15 + 13 + 168) = .08 * 100% = 8% = 8cM between N & L

Now that we have the distances, it's almost like a puzzle putting them in order. See how they fit so that all numbers add up properly. I usually start with the biggest distance as my starting "outer edges" and then see where things fit from there, but you don't have to. Just see how everything gits together.

|----7---- | ---- 8 ----|

T ------- N --------- L

| ----------15--------|

You should get something like above, with the order TNL. T and N are 7cM apart, N and L are 8cM apart, and T and L are 15cM apart. 7+8 = 15 so it all fits!

Nonnuclear DNA

While the vast majority of our DNA is found inside of our nuclear, not all of it is. If you think back to the evidence for endosymbiosis, the mitochondria and chloroplast contain their own, circular chromosomes. This DNA does have some genes on it, and you can inherit certain conditions from them. In the large majority of cases, these organelles are inherited solely from the egg cell. In other words, mitochondrial traits (and chloroplast ones in plants) tend to be inherited by all of a mother's kids, while a father will not pass it on to any.

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