1. A Population Genetics Case Study: The Cheetah

CheetahWhen chasing down prey, cheetahs have a top sprinting speeds of up to 60 miles (100 kilometers) per hour. It’s hard to imagine a better adapted animal.

But even apart from human-caused problems related to the destruction of their habitat, cheetahs are in trouble. For one thing, cheetahs have extremely low fertility. Cheetahs have always been difficult to breed in zoos and in wildlife parks. Part of that difficulty results from cheetah social behavior (see http://nationalzoo.si.edu/SCBI/endangeredspecies/cheetah/ for more information). But in addition, studies of cheetah sperm (collected by investigators who were trying to figure out why it was so hard to breed captive cheetahs in zoos and wildlife parks), have shown that

  1. Up to 79% of the sperm in any cheetah sperm sample are abnormal.
  2. Cheetah sperm count is low (only 25 million per ejaculate, much lower than in other species of cats).

The underlying problem is that cheetahs have extremely low genetic variability. It’s almost as if all cheetahs were members of the same family. They have much less genetic variation than other wild cats The Cheetah in Genetic Peril, O’Brien, Wildt, and Bush. As we’ll see, this genetic uniformity could put the cheetah at risk of extinction. With very little variation, their ability to adapt to environmental change is limited.

To understand the cheetah’s genetic situation, we need to look at how genes are distributed and how genes change in populations. This field is known as population genetics. Let’s start.

2. Interactive Reading: The Gene Pool, Fixed Alleles, Variable Alleles

To increase your retention of what you’re reading, I’ve pulled out a few words from the passage below. Drag them back into the text as you read.

[qwiz style=”width: 500px; border: 1px solid black;” qrecord_id=”sciencemusicvideosMeister1961-Pop-gen: Gene Pool, Fixed Alleles IR”]

[h]The Gene Pool, Fixed Alleles, and Variable Alleles

[i]A gene pool

[q labels = “top”]

The first concept for us to understand is the gene pool. The gene pool is the total of all the ________ for all of the genes in a population. To understand this concept, start by thinking of yourself. It took about 20,000 genes to spell out how to make your body, and to tell your body’s cells how to behave. You inherited these 20,000 genes from your __________, receiving one copy from your mother, and a second from your father.

Now, you’re not the only person who has these ___________ genes. You ________ your genes from your parents, so they have them. If you have siblings, they have these genes, too. So do your cousins. In fact, every human being has the ______ set of genes.

[l]20,000

[fx] No. Please try again.

[f*] Excellent!

[l]alleles

[fx] No, that’s not correct. Please try again.

[f*] Good!

[l]inherited

[fx] No, that’s not correct. Please try again.

[f*] Good!

[l]parents

[fx] No, that’s not correct. Please try again.

[f*] Good!

[l]same

[fx] No, that’s not correct. Please try again.

[f*] Good!

[q labels = “top”]

Two homologous chromosomes, one from the mother, and one from the father. At the locus for gene “A,” both alleles are the the same. At the locus for gene “B,” the two alleles are different (“b” and “B”)

Put together, all of these genes can be thought of as a cookbook. Each of the 20,000 genes is a different _________: one recipe may be for a muscle protein. A second one may be for a digestive enzyme. To develop from a zygote, and for your body to properly function, you need to have two _______ of each of these 20,000 recipes. But those recipes aren’t ___________. In the same way as your recipe for tomato sauce might be slightly different from mine, it’s also possible that your mother’s genetic recipe for making a particular enzyme might be slightly different from your father’s. In genetics, these _________ versions of a gene are known as alleles.

You can also think of this in terms of where genes are on chromosomes. A gene’s physical location on a chromosome is called its locus. At any locus, the DNA is going to code for the protein that this gene is responsible for. But the specific nucleotide ___________ (the DNA _________) might be different.

[l]alternative

[fx] No. Please try again.

[f*] Excellent!

[l]bases

[fx] No. Please try again.

[f*] Excellent!

[l]copies

[fx] No. Please try again.

[f*] Great!

[l]identical

[fx] No. Please try again.

[f*] Good!

[l]recipe

[fx] No. Please try again.

[f*] Great!

[l]sequence

[fx] No. Please try again.

[f*] Excellent!

[!!!]+second page START COPY HERE[/!!!]

[q labels = “top”]

Now, zoom out from thinking about your own genes to thinking about the genes of an entire population. Every member of that _________ will have the same genes. But there will be a variety of _______. On a very small scale, in a population with only five individuals and five genes, you can think about it like this.

gene locus 1 gene locus 2 gene locus 3 gene locus 4 gene locus 5
Individual 1 allele 1a only allele 2a only allele 3b only allele 4a only allele 5a only
Individual 2 alleles 1a and 1b allele 2a only allele 3c only allele 4a only allele 5b only
Individual 3 allele 1b only allele 2a only allele 3a only allele 4a only alleles 5a and 5b
Individual 4 allele 1a only allele 2a only allele 3b and 3c allele 4a only alleles 5a and 5b
Individual 5 alleles 1a and 1b allele 2a only allele 3b only allele 4a only  allele 5b only

Focus first on gene locus 1. If you follow the column downward, you can see that it has two alleles, allele 1a and allele 1b. By contrast, gene 2 has only one allele. Genes that have only one allele in an entire population are known as fixed alleles. Gene 3 has three alleles, the kind of situation that you’d find with the alleles for the ABO _______ type system, which also has three alleles (A, B, and o). Gene 4 is also a ________ allele.

Now look at the individuals. Individual 1 happens to be ____________ for all five of the genes. But individual two is heterozygous for gene 1, and homozygous for the other genes.

[l]alleles

[fx] No. Please try again.

[f*] Good!

[l]blood

[fx] No, that’s not correct. Please try again.

[f*] Correct!

[l]fixed

[fx] No, that’s not correct. Please try again.

[f*] Great!

[l]homozygous

[fx] No, that’s not correct. Please try again.

[f*] Correct!

[l]population

[fx] No, that’s not correct. Please try again.

[f*] Correct!

[x]Great job. Now continue on to the quiz below, which will help you master the concepts you just read about. Or, if you’d like to read this material again, click below.

[restart]

[/qwiz]

3. Checking Understanding

In this tutorial, we started by looking at cheetahs and their low genetic variability. We followed that by examining four concepts

  1. population genetics
  2. alleles
  3. gene pools
  4. fixed alleles

The quiz below will test you on your mastery of these concepts.

[qwiz random = “true” style=”width: 600px; border: 3px solid black;” qrecord_id=”sciencemusicvideosMeister1961-Pop-gen: Key Terms 2019″]

[h]Population genetics key terms

[i]A gene pool

[!!!!!!] question 1 +++++++++[/!!!!!!]

[q topic = “alleles in populations”] The study of genes in populations is called

[c*] population genetics

[c] population science

[c] molecular genetics

[f] Correct. The study of genes in populations is called ‘population genetics.’

[f] No. ‘Population science’ is too broad. Choose an answer that includes both ‘genetics’ and ‘population’

[f] No. ‘Molecular biology’ focuses on the molecules of heredity, such as DNA. Choose an answer that includes both ‘genetics’ and ‘population’

[!!!!!!] question 2 +++++++++[/!!!!!!]

[q topic = “alleles in populations”] An alternative version of a gene is called a(n)

[c] gene-alt

[c*] allele

[c] gene variant.

[f] No. You made a sensible choice, but that’s not the term in use. The ‘al’ that starts the word ‘alternative’ is a clue to the answer.

[f] Correct. ‘Allele’ is an alternative version of a gene.

[f] No. You made a sensible choice, but that’s not the term in use. The ‘al’ that starts the word ‘alternative’ is a clue to the answer.

[!!!!!!] question 3 +++++++++[/!!!!!!]

[q topic = “alleles in populations”] Alleles can be thought of as different ‘recipes’ for the same

[c] trait

[c] protein

[c] gene

[c*] all of the above

[f] No. Your answer is incomplete. Alleles are recipes for different versions of the same trait. But there are other equally correct ways to think about what these genetic recipes are for. Choose a different answer next time.

[f] No. Your answer is incomplete. Alleles are recipes for different versions of the same protein. But there are other equally correct ways to think about what these genetic recipes are for. Choose a different answer next time.

[f] No. Your answer is incomplete. Alleles are recipes for different versions of the same gene. But there are other equally correct ways to think about what these genetic recipes are for. Choose a different answer next time.

[f] Correct. Alleles are recipes for different versions of the same gene. Genes code for proteins. And proteins are the most important factor behind an organism’s traits (either directly, or through controlling traits through enzymes).

[!!!!!!] question 4 +++++++++[/!!!!!!]

[q topic = “alleles in populations”] The total of all the alleles for all of the genes in a population is known as its

[c] genetic diversity index.

[c] combined allele ensemble.

[c*] gene pool.

[f] No. Think of all of the genes in a population as water in a pool, and you’ll be able to figure out the answer.

[f] No. This would be a pretty good description, but it’s not the one in use. Think of all of the genes in a population as water in a pool, and you’ll be able to figure out the answer.

[f] Correct. The ‘gene pool’ is the total of all the alleles for all of the genes in a population.

[!!!!!!] question 5 +++++++++[/!!!!!!]

[q topic = “alleles in populations”] The diagram below represents a gene pool for a population of five individuals with five genes. Is individual 4 homozygous or heterozygous for gene 3?

[c] homozygous

[c*] heterozygous

[f] No. To be homozygous, both of the alleles that the individual possesses has to be the same. Individual 4 has two different alleles for gene 3 (3b and 3c). Next time, remember that the prefix ‘homo’ means ‘the same,’ while the prefix ‘hetero’ means ‘different.’

[f] Correct. Individual 4 has two different alleles for gene 3 (alleles 3b and 3c), making him or her heterozygous for that gene.

[!!!!!!] question 6 +++++++++[/!!!!!!]

[q topic = “alleles in populations”] The diagram below represents a gene pool for a population of five individuals with five genes. If we use the symbol ‘A’ to represent gene 1, which of the following makes sense as a representation of individual 2’s genotype for that gene?

[c] AA

[c*] Aa

[c] aa

[f] No. ‘Individual 2 is heterozygote for gene 1 (he or she has two different alleles). ‘AA’ would be the genotype of an individual who was homozygous dominant. Think of a representation that would show those two alleles as being different.

[f] Correct. Individual 2 has two different alleles for gene 1. That makes him or her a heterozygote, and you would represent this as ‘Aa.’

[f] No. Individual 2 is heterozygote for gene 1 (he or she has two different alleles). ‘aa’ would be the genotype of an individual who was homozygous recessive. Think of a representation that would show those two alleles as being different.

[!!!!!!] question 7 +++++++++[/!!!!!!]

[q topic = “alleles in populations”] The diagram below represents a gene pool for a population of five individuals with five genes. Is gene 5 a fixed allele?

[c] Yes. Gene 5 is a fixed allele.

[c*] No. Gene 5 is NOT a fixed allele.

[f] Incorrect. ‘Fixed’ allele means that there’s only one allele in the entire population for that gene. If you look at the ‘gene 5’ column, you can see that there are TWO alleles (alleles 5a and 5b).

[f] Correct. If you look at the ‘gene 5’ column, you can see that there are TWO alleles (alleles 5a and 5b). Therefore, the allele is not fixed.

[!!!!!!] question 8 +++++++++[/!!!!!!]

[q topic = “alleles in populations”] The diagram below represents a gene pool for a population of five individuals with five genes. Is gene 2 a fixed allele?

[c*] Yes. Gene 2 is a fixed allele.

[c] No. Gene 2 is NOT a fixed allele.

[f] Correct. ‘Fixed’ allele means that there’s only one allele in the entire population for that gene. If you look at the ‘gene 2’ column, you can see that there is only one allele, making gene 2 a fixed allele.

[f] Incorrect. If you look at the ‘gene 2 column, you can see that there’s only one allele in the entire population for that gene. Having only one allele is the definition of fixed allele. Remember that when you see this question again.

[!!!!!!] question 9 +++++++++[/!!!!!!]

[q topic = “alleles in populations”]A problem threatening cheetah survival is

[c*] low fertility

[c] high sperm counts

[c] too much variability between members of the population.

[f] Correct. A problem facing cheetahs is their low fertility, meaning their intrinsic (built-in) difficulty producing young.

[f] No. Cheetahs are facing the opposite problem: low sperm counts. In terms of this question, what would low sperm counts lead to?

[f] No. Cheetahs are facing the opposite problem: there is abnormally low variability among cheetahs.

[!!!!!!] question 10 +++++++++[/!!!!!!]

[q topic = “alleles in populations”]Compared to other cat species, genetic variation among cheetahs is

[c] higher

[c] about the same

[c*] lower

[f] No. Cheetahs are facing the opposite problem. That means that their genetic variation would be…

[f] No. Cheetahs seem to be much more related to one another than other species of wild cats. That means that their genetic variation would be…

[f] Yes. Cheetahs have much lower genetic variation that other wild cat species.

[!!!!!!] question 11 +++++++++[/!!!!!!]

[q topic = “alleles in populations”] Compared to other species of wild cats, you would expect cheetahs to have

[c] fewer fixed alleles

[c*] more fixed alleles

[c] about the same number of fixed alleles

[f] No. Cheetahs have low genetic diversity. If a population has low genetic diversity, it will have few heterozygotes, and _____ fixed alleles.

[f] Yes. Cheetahs have low genetic diversity, meaning that they’ll have more fixed alleles.

[f] No. Cheetahs have low genetic diversity. If a population has low genetic diversity, it will have few heterozygotes, and _____ fixed alleles.

[x]

[restart]

[/qwiz]

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