Mapping Genes - Online Tutor, Practice Problems & Exam Prep

Gene mapping involves determining the locations of genes on chromosomes using recombination frequencies. The formula for calculating recombination frequency (r f) is Number of recombinant offspringTotal offspring × 100. A frequency less than 50% indicates linked genes, while 50% suggests independent assortment. Modern mapping techniques include genomic sequencing and markers like single nucleotide polymorphisms (SNPs) and microsatellites, enhancing our understanding of gene locations and functions.

Mapping Genes

Finally, we're discussing mapping in this whole chapter, but this is the video where I'm going to tell you how to determine the locations of genes on a chromosome. Traditionally, the best way to map gene loci is by using recombination frequencies. These are the frequencies of recombinant offspring produced by a cross. These offspring are mixes of the parents, not exact phenotypic replicas, but combinations. So, the formula used here reflects map distance, but you can replace that with recombination frequency if you prefer or if that's what your professor uses. Essentially, they're the exact same thing, either rf, which stands for recombination frequency, or map distance. The formula is expressed as:

number of recombinant offspring total number of offspring × 100

This calculation provides the ratio, which when multiplied by 100, gives you the percentage.

I'm going to revisit Morgan's cross. I covered this in the crossing over section previously. If you're confused about how Morgan determined these ratios, I suggest revisiting that video which explains this cross in great detail. Briefly, Morgan crossed homozygous dominant with homozygous recessive, produced two gametes, and thus an F1 generation which was heterozygous dominant for both traits. He then crossed that F1 with a tester and obtained results, which I will summarize:

• Genotypes
• Phenotypes
• Offspring totals
• Types: Parental types resemble the parents, and recombinant types do not.

Using the recombination formula, we find the number of recombinants (151 plus 154) over the total number (2839), multiplied by 100, giving us 10.7%. This value represents both the recombination frequency and the map distance, which we measure in map units (m.u.). The significance of this 10.7 m.u. is that it represents the physical distance on the chromosome, suggesting that the two genes are 10.7 map units apart.

An important concept to remember is that the closer two genes are, the less likely they are to cross over and recombine. Think of this like sitting closely next to a friend in a hallway; it's less likely for someone to sit between you. Conversely, if you are sitting far apart, crossover events are more likely to occur between you.

In exam scenarios, if you're given a recombination frequency of, say, 33%, determine whether the genes are linked. Genes linked have a recombination frequency less than 50%. If the frequency is close to or equal to 50%, linkage is unlikely, indicating independent assortment and ensuring equal allele distribution. Never will the frequency exceed 50% because that would indicate unequal assorting, which contradicts Mendelian genetics.

Remember, linkage occurs if it's less than 50%; it does not occur if equal to 50%, and it will never be greater. So let's have fun with this in mind!

Modern Mapping

Okay. So now, let's talk about modern mapping. A lot of times, I get asked, you know, okay, this is great. I know how to do this with fruit flies, right? But is this actually real life? Do people answer is that they do sometimes. I'm sure people do, but that there are new types of gene loci maps. Now, the type you're familiar with, these are the recombination maps. Please use the recombination frequencies. These are the types that I explained to you. Now, there are physical maps, and this actually uses the genomic sequence. And how do you get this sequence? Well, you have to sequence the DNA, which is more advanced technology than they had during Morgan's time. And so, these are physical maps. You sequence the DNA, you have the nucleotides, and then you use those nucleotides to determine where the genes are located on the chromosome.

So you have to sequence the entire chromosome, or the genome of the organism to get the actual locations. Let me back up. And so, you can also map via certain genomic markers. These are also times where I use genomic sequencing. So, you can use single nucleotide polymorphisms. These are single nucleotide changes. And you can say, okay. Well, I know there are some SNPs here and there, or that's what you call them, SNPs. And say, I know that there are some SNPs here, some SNPs there. So let me use these SNP locations to determine where the gene is. You can use a restriction fragment length polymorphism (RFLPs), and these are DNA sequences that restriction enzymes cut. If you don't remember or have never heard of a restriction enzyme, all it is is a protein that cuts DNA at a specific sequence. So you can take the genomic DNA. You can cut it at specific sequences. You can then sequence that further to look at, you know, where the cuts were made, and how you can use that to determine gene locations.

And then you also have microsatellites, and these are short repetitive sequences found throughout the genome. There's a lot of repetitive DNA in our genome, and these are present in genes, and they're present in regulatory regions, they're present in, sort of, what's considered junk DNA or non-coding regions. They're present throughout the entire genome, and you use them to learn information about where genes are located. Genes with these sequences can be very easily identified through gene sequencing, because you just look for where the repeats are.

So, these modern mapping methods are much more commonly used today, depending on what you, kind of, need and what the scientists are looking for. But understanding traditional methods, they're super important to understanding the concepts of, like, how you identify where genes are on a chromosome, and recombinants, and parental types, and all the things that you learned. So, with that, let's unify.

The genetic distances between three genes (ab, nm, kf) were determined using a two-point mapping cross. Determine the sequence of the three genes. 

• A. Ab – nm – kf
• B. Ab – kf – nm
• C. Kf – ab – nm

True or False:Recombination frequencies are never greater than 50%

• A. True
• B. False

Using the following data collected from a test cross, calculate the recombination frequency. 

• A. 15.9%
• B. 10.3%
• C. 40.5%
• D. 32.7%

Here’s what students ask on this topic:

What is the formula for calculating recombination frequency in gene mapping?

The formula for calculating recombination frequency (r_f) in gene mapping is:

$\frac{\mathrm{Number\; of\; recombinant\; offspring}}{\mathrm{Total\; offspring}}\times \; 100$

This formula helps determine the percentage of recombinant offspring produced in a cross. The recombination frequency is directly related to the map distance between genes on a chromosome. A frequency less than 50% indicates that the genes are linked, while a frequency of 50% suggests independent assortment.

How do you determine if genes are linked or not using recombination frequency?

To determine if genes are linked, you use the recombination frequency. If the recombination frequency is less than 50%, the genes are likely linked, meaning they are located close to each other on the same chromosome. If the recombination frequency is 50%, the genes are not linked and are undergoing independent assortment, indicating they are on different chromosomes or far apart on the same chromosome. Recombination frequencies greater than 50% do not occur because independent assortment equally distributes alleles, capping the frequency at 50%.

What are modern methods of gene mapping?

Modern methods of gene mapping include genomic sequencing and the use of genomic markers. Genomic sequencing involves determining the DNA sequence of an organism's genome to locate genes. Genomic markers such as single nucleotide polymorphisms (SNPs), restriction fragment length polymorphisms (RFLPs), and microsatellites are also used. SNPs are single nucleotide changes, RFLPs are DNA sequences cut by restriction enzymes, and microsatellites are short repetitive sequences found throughout the genome. These methods provide more precise and detailed information about gene locations compared to traditional recombination maps.

Why is it important to understand traditional gene mapping methods?

Understanding traditional gene mapping methods is crucial because they form the foundation of genetic research and education. Traditional methods, such as using recombination frequencies to determine gene loci, help students grasp fundamental concepts like linkage, recombination, and independent assortment. These concepts are essential for understanding more advanced topics in genetics and modern mapping techniques. Additionally, traditional methods provide historical context and illustrate the evolution of genetic research, highlighting the progress made with modern technologies.

What is the significance of a recombination frequency of 10.7%?

A recombination frequency of 10.7% indicates that the two genes in question are 10.7 map units apart on the chromosome. This means that 10.7% of the offspring from a cross will be recombinant, showing new combinations of traits not found in the parents. The map distance, measured in map units (m.u.), directly correlates with the physical distance between genes on a chromosome. The closer the genes are, the lower the recombination frequency, and vice versa. This information is crucial for constructing genetic maps and understanding gene linkage.