1. Introduction: Why Harmful Alleles Persist

In the last tutorial, we saw how natural selection, by acting on favorable or unfavorable phenotypes, can shift allele frequencies in populations. It’s reasonable to think that because they produce phenotypes that make individuals less fit, harmful alleles should disappear from any gene pool. But that tends not to be the case. Because harmful alleles are often recessive alleles, they can persist in a population almost indefinitely. And, even harmful dominant alleles, despite selection against the phenotypes they produce, can also often continue to persist in gene pools. Let’s see how this happens.

2. Interactive Reading: Harmful Recessive Alleles

[qwiz qrecord_id=”sciencemusicvideosMeister1961-Pop-gen: Harmful Recessive Alleles IR (AP)”]

[h]Interactive Reading: Harmful Recessive Alleles

[q labels = “top”]

To begin with, why are most harmful alleles recessive? One way to think about this is that recessive alleles are often ___________ versions of a functioning allele. You can see this in the case of Tay-Sachs disease, a lethal disease of the brain. A baby born with Tay-Sachs disease begins life normally. But soon, he or she experiences ongoing deterioration of nerve cells and loss of mental and physical abilities. There’s no cure or treatment.

The allele that causes Tay-Sachs is recessive. If you inherit _____ of these alleles, you’re unable to break down a specific kind of lipid, which builds up in the brain, destroying nerve cells. In other words, the allele codes for a non-functioning _______. The dominant allele codes for a ________ version of the enzyme: if you inherit one version of the functioning allele, you’ll be able to produce the enzyme, and ________ the disease. For more about Tay-Sachs, follow this link to a Wikipedia article (which will open in a new tab)

[l]avoid

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

[f*] Correct!

[l]enzyme

[fx] No. Please try again.

[f*] Great!

[l]broken

[fx] No. Please try again.

[f*] Great!

[l]two

[fx] No. Please try again.

[f*] Great!

[l]working

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

[f*] Excellent!

[q]In the United States, Tay-Sachs is most common in Jews of Eastern European descent, where it occurs in 1 in every 3500 births. In the larger American population, the frequency is 1 in 320,000 births. Why wouldn’t such a harmful genetic condition simply disappear?

[c]U2hvdyB0aG UgYW5zd2Vy

[f]T25lIHJlYXNvbiBpcyB0aGF0IGxldGhhbCByZWNlc3NpdmUgYWxsZWxlcyBjYW4g4oCcaGlkZSBvdXTigJ0gaW4gaGV0ZXJvenlnb3Rlcywgd2hvIHdpbGwgcG9zc2VzcyBvbmx5IG9uZSBjb3B5IG9mIHRoZSByZWNlc3NpdmUgYWxsZWxlLiBZb3UgY2FuIHByb3ZlIHRoaXMgdG8geW91cnNlbGYgb24gdGhlIG5leHQgY2FyZCBieSB3b3JraW5nIHRocm91Z2ggYSBQdW5uZXR0IFNxdWFyZS4=

[q]The allele Tay-Sachs (t) is recessive. We’ll use “T” to represent the allele for the normal enzyme. What will happen if two people who are carriers for the alleles have children?

Step 1: determine the genotypes of the parents

Parent 1) : ___ ___
Parent 2) : ___ ___

Steps 2-4: Complete the Punnett square.

Parent 2
        __             __
Parent 1        __ ___  ___ ___  ___
       __ ___  ___ ___  ___

Step 5: Genotype ratio: ______ TT: ____Tt: ____tt

Step 6: Phenotype ratio: _____ Normal: ____Tay-sachs

[l]T

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

[f*] Correct!

[l]t

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

[f*] Great!

[l]0%

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

[f*] Correct!

[l]25%

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

[f*] Excellent!

[l]50%

[fx] No. Please try again.

[f*] Good!

[l]75%

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

[f*] Great!

[l]100%

[fx] No. Please try again.

[f*] Great!

[q]Because Tay Sachs is caused by a recessive allele, you can only have the condition if you inherit two copies (tt). Each of your parents would have to be a heterozygote (Tt). With each child they produced, there would be a one in four chance that this child would inherit two recessive alleles and have the tt genotype.

While the loss of ¼ of their offspring from the disease certainly reduces the number of alleles that get passed on, you can see why the allele doesn’t disappear. Half of the children of parents who are heterozygotes will also be heterozygotes (Tt).

In terms of the gene pool, what’s the result of this?

[c]U2hvdyB0aGUg YW5zd2VyLg==

[f]VGhlIGFsbGVsZSBwZXJzaXN0cyBpbiB0aGUgZ2VuZSBwb29sLg==

[q]Just to make sure that you’re getting this, identify the genotypes of the individuals in the pedigree on the next page, which shows inheritance of Tay-Sachs disease.

[q labels = “top”]In this pedigree for Tay-Sachs disease, carriers (heterozygotes) are shaded in half way.

 

 

[l]TT

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

[f*] Good!

[l]Tt

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

[f*] Great!

[l]tt

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

[f*] Good!

[/qwiz]

The takeaway message: this harmful allele persists in gene pools because it can hide out in heterozygotes.

3. The persistence of harmful dominant alleles

There are, of course, harmful dominant alleles. We discussed two of these, achondroplasiac dwarfism and Huntington’s disease, in a previous tutorial. Because dominant alleles always have an immediate effect upon the phenotype, they tend to be strongly selected against. However, even harmful dominant alleles can persist in a population’s gene pool. Here’s why.

Let’s look at achondroplasia first. For one thing, while it’s harmful, it’s not lethal. People with achondroplasia not only survive to adulthood: they can also reproduce. When they do, they have a 1 in 2 chance of passing on the allele to the next generation, keeping the allele in the gene pool (achondroplasiacs are never homozygous: two alleles is a fatal combination). In addition, achondroplasia can emerge as a mutation: about 80% of achondroplasiacs are born to parents who are of normal height. If you’re interested, you can read more about achondroplasia at the National Library of Medicine.

The famous American Folksinger Woodie Guthrie was diagnosed with Huntington's disease when he was 40.
The famous American Folksinger Woody Guthrie was diagnosed with Huntington’s disease when he was 40.

Huntington’s disease persists in the population for a different reason. Its key symptoms –including loss of control of movement, difficulty thinking clearly, and depression – don’t set in until middle age. Until recently, when the gene was identified, people only learned that they had Huntington’s after they had already had children (and, in fact, often not until their children were adults). Because of that, possessing the Huntington’s allele never reduced one’s chance of producing offspring, and passing on the Huntington’s allele. And as a result, this allele persists in the population.

4. Checking Understanding

This tutorial focused on why harmful alleles don’t disappear from gene pools.

If you feel that you understand the concepts in this tutorial, take this brief quiz. If not, carefully re-read the material above, and then take the quiz.

Once you’ve mastered all the concepts, you should go on to the next tutorial in the series (link is below).

[qwiz qrecord_id=”sciencemusicvideosMeister1961-Pop-gen: Why Harmful Alleles Persist (AP)”]

[h]Why harmful alleles persist in gene pools

[q topic= “why_harmful_alleles_persist”]True or false: because of natural selection, harmful alleles will inevitably disappear from a gene pool.

[c]IFRydWU=

[c]IEZh bHNl

[f]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

[f]IENvcnJlY3QuIEJlY2F1c2UgcmVjZXNzaXZlIGFsbGVsZXMgY2FuIGhpZGUgb3V0IGluIGhldGVyb3p5Z290ZXMsIHRoZXkgY2FuIHBlcnNpc3QgaW4gZ2VuZSBwb29scywgcHJhY3RpY2FsbHkgaW5kZWZpbml0ZWx5LsKgQW5kLCBhcyB3ZSYjODIxNzt2ZSByZWFkIGFib3ZlLCB0aGVyZSBhcmUgYSB2YXJpZXR5IG9mIHJlYXNvbnMgd2h5IGhhcm1mdWwgZG9taW5hbnQgYWxsZWxlcyBjYW4gcGVyc2lzdCBpbiBnZW5lIHBvb2xzIChpbmNsdWRpbmcgc2V0dGluZyBpbiBsYXRlciBpbiBsaWZlLCBhbmQgbm90IHByZXZlbnRpbmcgaW5kaXZpZHVhbHMgd2l0aCB0aGVzZSBhbGxlbGVzIGZyb20gcmVwcm9kdWNpbmcpLjwvcD4KPHAgY2xhc3M9InAxIj4=

[q topic= “why_harmful_alleles_persist”]Which type of genotype allows for recessive alleles to hide out, shielded from natural selection?

[c]IGhvbW96eWdvdXMgcmVjZXNzaXZlcw==

[c]IGhldGVyb3 p5Z290ZXM=

[c]IGhvbW96eWdvdXMgZG9taW5hbnRz

[f]IE5vLiBIb21venlnb3VzIHJlY2Vzc2l2ZXMgaGF2ZSB0d28gY29waWVzIG9mIHRoZSByZWNlc3NpdmUgYWxsZWxlLiBXaGF0ZXZlciBwaGVub3R5cGUgdGhhdCBhbGxlbGUgaXMgY29ubmVjdGVkIHdpdGggd2lsbCBiZSBmdWxseSBleHBvc2VkIHRvIG5hdHVyYWwgc2VsZWN0aW9uLiBOZXh0IHRpbWUsIHRoaW5rIGFib3V0IHdoaWNoIGtpbmQgb2YgaW5kaXZpZHVhbCBjb3VsZCBwb3NzZXNzIGEgcmVjZXNzaXZlIGFsbGVsZSwgYnV0IG5vdCBleHByZXNzIHRoYXQgYWxsZWxlIGluIGhpcyBvciBoZXIgcGhlbm90eXBlLg==

[f]IENvcnJlY3QuIEluIGhldGVyb3p5Z290ZXMsIGEgcmVjZXNzaXZlIGFsbGVsZSB3aWxsIGJlIG1hc2tlZCBieSB0aGUgZG9taW5hbnQgYWxsZWxlLiBUaGlzIGFsbG93cyB0aGUgcmVjZXNzaXZlIGFsbGVsZSB0byBoaWRlIG91dCBpbiB0aGUgaGV0ZXJvenlnb3RlLCBzaGllbGRpbmcgaXQgZnJvbSBuYXR1cmFsIHNlbGVjdGlvbi4=

[f]IE5vLiBIb21venlnb3VzIGRvbWluYW50cyBkb27igJl0IGNhcnJ5IGFueSBjb3BpZXMgb2YgdGhlIHJlY2Vzc2l2ZSBhbGxlbGUuIE5leHQgdGltZSwgdGhpbmsgYWJvdXQgd2hpY2gga2luZCBvZiBpbmRpdmlkdWFsIGNvdWxkIHBvc3Nlc3MgYSByZWNlc3NpdmUgYWxsZWxlLCBidXQgbm90IGV4cHJlc3MgdGhhdCBhbGxlbGUgaW4gaGlzIG9yIGhlciBwaGVub3R5cGUuPC9wPgo8cCBjbGFzcz0icDEiPg==

[q topic= “why_harmful_alleles_persist”]In the Punnett square below, ‘t’ represents the allele for Tay-Sachs disease, and ‘T’ the normal allele. Which square represents the genotype of an individual who is a carrier for Tay-Sachs?

[c]IFNxdWFyZSAx

[c]IFNxdW FyZSAy

[c]IFNxdWFyZSA0

[f]IE5vLiBTcXVhcmUgMSBzaG93cyBhbiBpbmRpdmlkdWFsIHdobyBoYXMgdHdvIOKAmG5vcm1hbOKAmSBhbGxlbGVzLiBBIGNhcnJpZXIgaXMgc29tZW9uZSB3aXRoIGEgbm9ybWFsIHBoZW5vdHlwZSwgYnV0IHdobyBwb3NzZXNzZXMgdGhlIGFsZWxlLg==

[f]IENvcnJlY3QuIFNxdWFyZSAyIHNob3dzIGFuIGluZGl2aWR1YWwgd2hvIGlzIGEgaGV0ZXJvenlnb3RlLiBIZSBvciBzaGUgcG9zc2Vzc2VzIHRoZSBhbGxlbGUsIGJ1dCB3b27igJl0IHNob3cgaXQgaW4gaGVyIG9yIGhpcyBwaGVub3R5cGU/

[f]IE5vLiBTcXVhcmUgNCBzaG93cyB0aGUgZ2Vub3R5cGUgb2YgYW4gaW5kaXZpZHVhbCB3aG8gaXMgaG9tb3p5Z291cyByZWNlc3NpdmUuIFdoaWxlIHRoZXkgY2VydGFpbmx5IGNhcnJ5IHRoZSBhbGxlbGUsIHRoZXnigJlyZSBub3QgY29uc2lkZXJlZCBhIGNhcnJpZXIgaW4gdGhpcyBzcGVjaWFsIGdlbmV0aWNzIHNlbnNlIG9mIHRoZSB3b3JkLiBBIGNhcnJpZXIg4oCYY2Fycmllc+KAmSB0aGUgYWxsZWxlLCBidXQgZG9lc27igJl0IGV4cHJlc3MgaXQgaW4gaGlzIG9yIGhlciBwaGVub3R5cGUuIEJlY2F1c2UgdGhpcyBpbmRpdmlkdWFsIHdpbGwgZXhwcmVzcyB0aGUgcGhlbm90eXBlLCB0aGV54oCZcmUgbm90IGNvbnNpZGVyZWQgdG8gYmUgYSBjYXJyaWVyLjwvcD4KPHAgY2xhc3M9InAxIj4=

[q topic= “why_harmful_alleles_persist”]in the Punnett square below, t represents the allele for Tay-Sachs disease, and T the normal allele. Which square shows the genotype of an individual who would have Tay-Sachs disease?

[c]IFNxdWFyZSAx

[c]IFNxdWFyZSAy

[c]IFNxdWFyZSAz

[c]IFNxdW FyZSA0

[f]IE5vLiBJbmRpdmlkdWFsIDEgaGFzIHR3byDigJhub3JtYWzigJkgYWxsZWxlcy4gWW914oCZZCBoYXZlIHRvIGhhdmUgdHdvIHJlY2Vzc2l2ZSBhbGxlbGVzIGluIG9yZGVyIHRvIGV4cHJlc3MgdGhlIHJlY2Vzc2l2ZSBwaGVub3R5cGUsIGFuZCBoYXZlIHRoZSBkaXNlYXNlLg==

[f]IE5vLiBJbmRpdmlkdWFsIDIgaXMgYSBoZXRlcm96eWdvdGUuIEhlIG9yIHNoZSBwb3NzZXNzZXMgdGhlIGFsbGVsZSwgYnV0IHdvbuKAmXQgc2hvdyBpdCBpbiBoZXIgb3IgaGlzIHBoZW5vdHlwZS4gYW5kIHdvbuKAmXQgaGF2ZSB0aGUgZGlzZWFzZS4=

[f]IE5vLiBJbmRpdmlkdWFsIDMgaXMgYSBoZXRlcm96eWdvdGUuIEhlIG9yIHNoZSBwb3NzZXNzZXMgdGhlIGFsbGVsZSwgYnV0IHdvbuKAmXQgc2hvdyBpdCBpbiBoZXIgb3IgaGlzIHBoZW5vdHlwZS4gYW5kIHdvbuKAmXQgaGF2ZSB0aGUgZGlzZWFzZS4=

[f]IFllcy4gSW5kaXZpZHVhbCA0IGlzIGhvbW96eWdvdXMgcmVjZXNzaXZlLiBIZSBvciBzaGUgaGFzIHR3byBjb3BpZXMgb2YgdGhlIHJlY2Vzc2l2ZSBhbGxlbGUsIGFuZCB3aWxsIGhhdmUgVGF5LVNhY2hzIGRpc2Vhc2U8L3A+CjxwIGNsYXNzPSJwMSI+

[q topic= “why_harmful_alleles_persist”]If two individuals who are heterozygous for a lethal (deadly) genetic condition mate and produce offspring, what is the probability that their offspring will inherit the lethal condition?

[c]IG9uZSBp biBmb3Vy

[c]IHR3byBpbiBmb3Vy

[c]IHRocmVlIGluIGZvdXI=

[c]IGZvdXIgaW4gZm91cg==

[f]IFllcy4gSWYgeW91IGNyb3NzIHR3byBoZXRlcm96eWdvdGVzLCDigJhUdOKAmSBhbmQg4oCYVHQs4oCZIHlvdSBjYW7CoHNlZSB0aGF0IG9uZSBpbiBmb3VyIG9mIHRoZSBvZmZzcHJpbmcgd2lsbCBpbmhlcml0IHRoZSBjb25kaXRpb24u

[f]IE5vLiBKdXN0IGV4YW1pbmUgdGhlIFB1bm5ldHQgc3F1YXJlIGFib3ZlLsKgV2hhdCBwcm9wb3J0aW9uIG9mIHRoZSBvZmZzcHJpbmcgd2lsbCBoYXZlIHRoZSDigJh0dOKAmSBnZW5vdHlwZT8=

[f]IE5vLsKgSnVzdCBleGFtaW5lIHRoZSBQdW5uZXR0IHNxdWFyZSBhYm92ZS7CoFdoYXQgcHJvcG9ydGlvbiBvZiB0aGUgb2Zmc3ByaW5nIHdpbGwgaGF2ZSB0aGUg4oCYdHTigJkgZ2Vub3R5cGU/

[f]IE5vLsKgSnVzdCBleGFtaW5lIHRoZSBQdW5uZXR0IHNxdWFyZSBhYm92ZS7CoFdoYXQgcHJvcG9ydGlvbiBvZiB0aGUgb2Zmc3ByaW5nIHdpbGwgaGF2ZSB0aGUg4oCYdHTigJkgZ2Vub3R5cGU/PC9wPgo8cCBjbGFzcz0icDEiPg==

[q topic= “why_harmful_alleles_persist”]The pedigree chart below shows inheritance of a lethal recessive allele. If B represents the normal allele, and ‘b’ the recessive allele, what would be the genotype of I-1?

[c]wqBCQg==

[c]wqBiYg==

[c]wqBC Yg==

[f]IE5vLiBJZiBJLTEgaGFkIHRoZSBnZW5vdHlwZSDigJhCQizigJkgdGhlbiBoZSBjb3VsZCBvbmx5IHBhc3MgYSDigJhC4oCZIGFsbGVsZSBvbiB0byBoaXMgb2Zmc3ByaW5nLiBOb25lIG9mIHRoZW0gY291bGQgaGF2ZSBpbmhlcml0ZWQgdHdvIOKAmGLigJkgYWxsZWxlcy4gU2luY2UgSUktMyBoYXMgdGhlIHJlY2Vzc2l2ZSBjb25kaXRpb24sIHNoZSBoYXMgdG8gaGF2ZSBpbmhlcml0ZWQgdHdvIOKAmGRi4oCZIGFsbGVsZXMsIGFuZCB0aGVyZWZvcmUgaGVyIGZhdGhlciBjb3VsZCBub3QgaGF2ZSBiZWVuIOKAmEJCLuKAmQ==

[f]IE5vLiBUaGUgY29uZGl0aW9uIGlzIGxldGhhbCwgd2hpY2ggbWVhbnMgdGhhdCB5b3UgY2FuIGJlIGJvcm4gd2l0aCB0aGUgY29uZGl0aW9uLCBidXQgaXQgd2lsbCBraWxsIHlvdSBiZWZvcmUgeW91IGhhdmUgdGhlIGNoYW5jZSB0byByZXByb2R1Y2UgYW5kIHBhc3MgaXQgb24uIE5vIGFkdWx0cyBjYW4gYmUg4oCYYmIu4oCZ

[f]IFllcy4gRm9yIGluZGl2aWR1YWwgSUktMyB0byBoYXZlIGluaGVyaXRlZCB0d28g4oCYYuKAmSBhbGxlbGVzLCBzaGUgaGFkIHRvIGhhdmUgaW5oZXJpdGVkIG9uZSBhbGxlbGUgZnJvbSBlYWNoIG9mIGhlciBwYXJlbnRzLiBCb3RoIHBhcmVudHMgaGFkIHRvIGJlIGNhcnJpZXJzLCBhbmQgaGFkIHRvIGhhdmUgaGFkIGEg4oCYQmLigJkgZ2VudG95cGUuPC9wPgo8cCBjbGFzcz0icDEiPg==

[q topic= “why_harmful_alleles_persist”]The pedigree chart below shows inheritance of a lethal recessive allele. If ‘B’ represents the normal allele, and ‘b’ the recessive allele, what would be the genotype of IV-2?

[c]wqBCQg==

[c]wqBi Yg==

[c]wqBCYg==

[f]IE5vLiBJbiBhIHBlZGlncmVlIGNoYXJ0LCBpZiB0aGUgc3F1YXJlIG9yIGNpcmNsZSBpcyBjb2xvcmVkIGluLCBpdCBtZWFucyB0aGF0IHRoZSBpbmRpdmlkdWFsIGhhcyB0aGUgY29uZGl0aW9uLiBBbiBpbmRpdmlkdWFsIHdpdGggZ2Vub3R5cGUgQkLCoHdpbGwgbm90IGhhdmUgdGhlIGNvbmRpdGlvbiwgc28gSVYtMiBjb3VsZCBub3QgYmUg4oCYQkIu4oCZ

[f]IFllcy4gVGhlIGZpbGxlZCBpbiBjaXJjbGUgaW5kaWNhdGVzIHRoYXQgdGhpcyBnaXJsIGhhcyB0aGUgY29uZGl0aW9uLiBTaW5jZSB0aGUgY29uZGl0aW9uIGlzIHJlY2Vzc2l2ZSwgdGhlIG9ubHkgd2F5IHRoYXQgeW91IGNhbiBoYXZlIGl0IGlzIHRvIGJlIGhvbW96eWdvdXMsIG1ha2luZyDigJhiYuKAmSB0aGUgb25seSBwb3NzaWJsZSBnZW5vdHlwZS4=

[f]IE5vLiBJbiBhIHBlZGlncmVlIGNoYXJ0LCBpZiB0aGUgc3F1YXJlIG9yIGNpcmNsZSBpcyBjb2xvcmVkIGluLCBpdCBtZWFucyB0aGF0IHRoZSBpbmRpdmlkdWFsIGhhcyB0aGUgY29uZGl0aW9uLiBBbiBpbmRpdmlkdWFsIHdpdGggZ2Vub3R5cGUgQmLCoGlzIGhldGVyb3p5Z291cyBmb3IgdGhlIGRvbWluYW50IHBoZW5vdHlwZSwgYW5kIG5vdCBoYXZlIHRoZSBjb25kaXRpb24uPC9wPgo8cCBjbGFzcz0icDEiPg==

[q topic= “why_harmful_alleles_persist”]The pedigree chart below shows inheritance of a lethal recessive allele. Which of the following individuals MUST be a carrier for the lethal allele? (Note that in the feedback for responses, we’ll have ‘B’ represent the normal allele, and ‘b’ the recessive allele)

[c]IEkt Mg==

[c]IElJLTQ=

[c]IElJSS0x

[c]IElWLTI=

[f]IFllcy4gSS0yIGhhZCBhIGNoaWxkIHdpdGggdGhlIGNvbmRpdGlvbi4gSW4gYSBsZXRoYWwgcmVjZXNzaXZlIGNvbmRpdGlvbiwgdGhlIHBhcmVudHMgYm90aCBoYXZlIHRvIGJlIGNhcnJpZXJzLCBhbmQgZWFjaCBoYXMgdG8gcGFzcyBvbmUgb2YgdGhlaXIgYWxsZWxlcyB0byB0aGUgb2Zmc3ByaW5nIHdobyBpbmhlcml0cyB0aGUgcmVjZXNzaXZlIGNvbmRpdGlvbi4=

[f]IE5vLiBJSS00IENPVUxEIGJlIGEgY2Fycmllci4gQnV0IHRoZXJl4oCZcyBhbHNvIGEgMS80IGNoYW5jZSB0aGF0IHNoZSBpcyBob21venlnb3VzIG5vcm1hbCwg4oCYQkIu4oCZIFNpbmNlIHNoZSBsZWZ0IG5vIG9mZnNwcmluZywgdGhlcmXigJlzIG5vIHdheSB0byB0ZWxsIGlmIHNoZSBpcyDigJhCQuKAmSBvciDigJhCYi7igJk=

[f]IE5vLiBBcyBvbmUgb2YgdGhlIG9mZnNwcmluZyBvZiBJSS0yLCB3aG8gbWlnaHQgYmUgYSBjYXJyaWVyLCBJSUktMSBvciBhbnkgb2YgaGVyIHNpYmxpbmdzIG1pZ2h0IGNhcnJpZXJzLiBBbmQsIGluIGZhY3QsIElJSS0zIGlzIGRlZmluaXRlbHkgYSBjYXJyaWVyLCBiZWNhdXNlIGhl4oCZcyB0aGUgcGFyZW50IG9mIGEgY2hpbGQgd2l0aCB0aGUgY29uZGl0aW9uLiBCdXQgYm90aCBJSUktMSBhbmQgSUlJLTIgY291bGQgYmUgaG9tb3p5Z291cyBkb21pbmFudCAoQkIpIG9yIGhldGVyb3p5Z290ZXMgKEJiKS4gV2l0aG91dCBrbm93aW5nIHRoZWlyIG9mZnNwcmluZywgdGhlcmXigJlzIG5vIHdheSB0byB0ZWxsLg==

[f]IE5vLiBJVi0yIGhhcyB0aGUgY29uZGl0aW9uLiBUaGV5IGhhdmUgdGhlIGRpc2Vhc2UsIGFuZCB3aWxsIGRpZSAob3IgYWxyZWFkeSBkaWVkKSBmcm9tIGl0LCBiZWNhdXNlIGl04oCZcyBsZXRoYWwgaW4gaG9tb3p5Z290ZXMuIE9ubHkgaGV0ZXJvenlnb3RlcyBhcmUgY29uc2lkZXJlZCB0byBiZSBjYXJyaWVycy48L3A+CjxwIGNsYXNzPSJwMSI+

[q topic= “why_harmful_alleles_persist”]The pedigree chart below shows inheritance of a lethal recessive allele.Which of the following individuals inherited two copies of the lethal allele? (Note that in the feedback for responses, we’ll have ‘B’ represent the normal allele, and ‘b’ the recessive allele)

[c]IEktMg==

[c]IElJLTQ=

[c]IElJSS0z

[c]IElW LTI=

[f]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

[f]IE5vLiBJbiBhIHBlZGlncmVlIGNoYXJ0LCBpbmRpdmlkdWFscyB3aG8gaGF2ZSBmaWxsZWQtaW4gY2lyY2xlcyBvciBzcXVhcmVzIGhhdmUgdGhlIGNvbmRpdGlvbi4gSWYgdGhlIGluZGl2aWR1YWzigJlzIHNxdWFyZSBvciBjaXJjbGUgaXMgbm90IGZpbGxlZCBpbiwgdGhleSBkb27igJl0IGhhdmUgdGhlIGNvbmRpdGlvbi4gQWxzbywgaW4gdGhpcyBjYXNlLCBiZWNhdXNlIHRoZSBjb25kaXRpb24gaXMgbGV0aGFsLCBubyBwYXJlbnRzIHdpbGwgaGF2ZSBpdC4gSS0yIGlzIGRlZmluaXRlbHkgYSBoZXRlcm96eWdvdGUsIHdoaWNoIG1lYW5zIHRoYXQgc2hlIGhhcyBvbmx5IG9uZSBjb3B5IG9mIHRoZSBhbGxlbGUu

[f]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

[f]IFllcy4gSnVzdCBieSB0aGUgZmFjdCB0aGF0IGhlciBjaXJjbGUgaXMgZmlsbGVkIGluLCB5b3Uga25vdyB0aGF0IElWLTIgaGFzIHRoZSBjb25kaXRpb24uIEJlY2F1c2UgdGhlIGNvbmRpdGlvbiBpcyBsZXRoYWwsIHNoZSBoYWQgdG8gaGF2ZSBpbmhlcml0ZWQgdHdvIGNvcGllcyBvZiBpdC4gSGVyIGdlbm90eXBlIHdvdWxkIGJlIOKAmGJiLuKAmTwvcD4KPHAgY2xhc3M9InAxIj4=

[q topic= “why_harmful_alleles_persist”]Achondroplasia, a genetic form of dwarfism, is caused by a dominant allele. Which of the following is NOT a reason why achondroplasia has stayed in the gene pool? In other words, from the list below, find the FALSE statement.

[c]IE9jY2FzaW9uYWxseSwgbm9ybWFsIGFsbGVsZXMgbXV0YXRlIGludG8gdGhlIGFjaG9uZHJvcGxhc2lhIGFsbGVsZS4=

[c]IEFjaG9uZHJvcGxhc2lhY3MgaGF2ZSBzb21lIGhlYWx0aCBwcm9ibGVtcywgYnV0IHRoZXkgY2FuIGJlY29tZSBhZHVsdHMgd2hvIGhhdmUgY2hpbGRyZW4gb2YgdGhlaXIgb3duLg==

[c]IERvbWluYW50IGFsbGVsZXMsIGJlY2F1c2UgdGhleeKAmXJlIGRvbWluYW 50LCB0ZW5kIHRvIGluY3JlYXNlIGluIGZyZXF1ZW5jeSBvdmVyIHRpbWUu

[f]IE5vLiBSZW1lbWJlciB0aGF0IHlvdeKAmXJlIGxvb2tpbmcgZm9yIGEgZmFsc2Ugc3RhdGVtZW50LiBUaGlzIG9uZeKAmXMgdHJ1ZS4gV2hpbGUgYWNob25kcm9wbGFzaWEgaXMgdW5jb21tb24sIGFtb25nIGFjaG9uZHJvcGxhc2lhY3MgaXTigJlzIG5vdCB1bmNvbW1vbiBmb3IgdHdvIG5vcm1hbCBzaXplZCBwYXJlbnRzIHRvIGhhdmUgYSBjaGlsZCB3aXRoIGFjaG9ucmRyb3BsYXNpYS4gVGhlIG5ldyBhbGxlbGUgaXMgYSBtdXRhdGlvbi4=

[f]IE5vLiBSZW1lbWJlciB0aGF0IHlvdeKAmXJlIGxvb2tpbmcgZm9yIGEgZmFsc2Ugc3RhdGVtZW50LjxzcGFuIGNsYXNzPSJBcHBsZS1jb252ZXJ0ZWQtc3BhY2UiPsKgPC9zcGFuPkl0IGlzIHRydWUgdGhhdCBhY2hvbmRyb3BsYXNpYWNzIGNhbiBoYXZlIGNoaWxkcmVuLiBXaGVuIHRoZXkgZG8sIHRoZXJl4oCZcyBhIDEgaW4gMiBjaGFuY2UgdGhhdCB0aGV54oCZbGwgcGFzcyBvbiB0aGVpciBhY2hvbmRyb3BsYXNpYSBhbGxlbGUgKGFjaG9uZHJvcGxhc2lhY3MgYXJlIG5ldmVyIGhvbW96eWdvdXMgZG9taW5hbnRzOiB0aGF0IGNvbWJpbmF0aW9uIGlzIGxldGhhbCku

[f]IEV4Y2VsbGVudC4gWW914oCZdmUgZm91bmQgdGhlIDxzdHJvbmc+ZmFsc2U8L3N0cm9uZz4gc3RhdGVtZW50LiBEb21pbmFudCBhbGxlbGVzIGFyZSBvbmx5IGRvbWluYW50IGJlY2F1c2Ugd2hlbiB0aGV54oCZcmUgcGFpcmVkIHdpdGggYSByZWNlc3NpdmUgYWxsZWxlLCB0aGUgZG9taW5hbnQgYWxsZWxlIGRldGVybWluZXMgdGhlIHBoZW5vdHlwZS4gPHN0cm9uZz5PbiBhIHBvcHVsYXRpb24gbGV2ZWwsIHRoZXkgc2hvdyBubyB0ZW5kZW5jeSB0byBncm93IG9yIHNwcmVhZC48L3N0cm9uZz48L3A+CjxwIGNsYXNzPSJwMSI+

[q topic= “why_harmful_alleles_persist”]Huntington’s disease is caused by a dominant allele. Why does it persist in the gene pool?

[c]IEJlY2F1c2UgZG9taW5hbnQgYWxsZWxlcywgc2luY2UgdGhleeKAmXJlIGRvbWluYW50LCB0ZW5kIHRvIGluY3JlYXNlIGluIGZyZXF1ZW5jeSBvdmVyIHRpbWUu

[c]IEh1bnRpbmd0b27igJlzIGRpc2Vhc2Ug c2V0cyBpbiBsYXRlciBpbiBsaWZlLg==

[f]IE5vLiBUaGlzIGlzIGEgbWFqb3IgbWlzY29uY2VwdGlvbiBpbiBwb3B1bGF0aW9uIGdlbmV0aWNzLCBhbmQgeW91IHNob3VsZCByZS1yZWFkIHRoZSBtaXNjb25jZXB0aW9uIGFsZXJ0IGVhcmxpZXIgaW4gdGhpcyB0dXRvcmlhbCBzZXJpZXMuIERvbWluYW50IGFsbGVsZXMgYXJlIG9ubHkgZG9taW5hbnQgd2hlbiB0aGV54oCZcmUgcGFpcmVkIHdpdGggYSByZWNlc3NpdmUgYWxsZWxlLiBJbiB0aGF0IHNpdHVhdGlvbiwgdGhlIGRvbWluYW50IGFsbGVsZSBkZXRlcm1pbmVzIHRoZSBwaGVub3R5cGUuIE9uIGEgcG9wdWxhdGlvbiBsZXZlbCwgdGhleSBzaG93IG5vIHRlbmRlbmN5IHRvIGdyb3cgb3Igc3ByZWFkLg==

[f]IEV4YWN0bHkuIEh1bnRpbmd0b27igJlzIGRpc2Vhc2Ugc2V0cyBpbiBsYXRlIGluIGxpZmUuIFRocm91Z2hvdXQgaGlzdG9yeSAoZXNwZWNpYWxseSB3aGVuIGxpZmUgc3BhbnMgd2VyZSBzaG9ydGVyIHRoYW4gdGhleSBhcmUgdG9kYXkgYW5kIHRoZXJlIHdhcyBsZXNzIGtub3dsZWRnZSBvZiBnZW5ldGljcyksIHBlb3BsZSB3aXRoIEh1bnRpbmd0b27igJlzIHJlcHJvZHVjZWQgYmVmb3JlIHRoZXkga25ldyB0aGV5IGhhZCB0aGUgZGlzZWFzZSwgb3IgdW5kZXJzdG9vZCB0aGF0IHRoZXkgY291bGQgcGFzcyBpdCBvbiB0byB0aGVpciBvZmZzcHJpbmcu

[x]

[restart]

[/qwiz]

Links