# can a recessive gene skip 2 generations?

### #1

Posted 15 May 2009 - 03:51 PM

-Albert Einstein

### #2

Posted 15 May 2009 - 04:06 PM

### #3

Posted 15 May 2009 - 04:07 PM

### #4

Posted 15 May 2009 - 04:13 PM

A recessive gene can be dormant for any number of generations, unless I'm being stupid. You can see this with Punnett squares. Why would it be limited to one?

Yup, this is basic Mendelian genetics. However, the odds that it actually will be passed on yet remain recessive for several generations is fairly low.

### #5

Posted 15 May 2009 - 07:09 PM

-Albert Einstein

### #6

Posted 15 May 2009 - 07:31 PM

However, the odds that it actually will be passed on yet remain recessive for several generations is fairly low.

That actually depends on recombination, random sampling and how many offspring are in the subsequent filial generations. The likelihood of a single recessive allele being passed on through the first generation between a homozygote and a heterozygote is 1/4.

Assuming all generations mate with a homozygote for the dominant allele and only have one offspring in each pairing the odds of it appearing in the F3 generation (great-grandchildren) is 1/4^3 = 0.015625 assuming a biallelic model with no other confounding factors.

**Edited by MedGen, 15 May 2009 - 07:32 PM.**

typo

Genetics of a Medical Persuasion

### #7

Posted 17 May 2009 - 07:47 PM

### #8

Posted 15 August 2009 - 10:09 PM

### #9

Posted 22 August 2009 - 04:32 PM

If the genotype of the parents is AA and Aa it's a 25% chance, for Aa and aa it's a 75% chance.

Now as you move along the generations you can multiply the probabilities. If you start with grandparents of an Aa genotype, there's a 50% chance for each child to carry both alleles. If they marry someone with an Aa genotype, the probabilty for a being present yet still dormant is 0.5*0.5=0.25=25%. If their partner has got a heterogenous dominant genotype, the probability is 0.5*0.25=0.125=12.5% and so on.

Haircolor is a tricky example, since it's not determined by a single gene, things get a little bit more tricky with that (http://en.wikipedia....uman_hair_color).

### #10

Posted 26 August 2009 - 12:10 AM

**First generation**. Let's say you have Mom ("M") with genes AA and Dad ("D") with genes aa. We're assuming that Dad is homozygous for the recessive "a", because otherwise we wouldn't know that he has the trait at all.

2.

**Second generation**: children C1, C2, C3, C4. Each child inherits an A from M, and an a from D. Thus, all the children are Aa, and none show the recessive allele. The trait "skips" this generation.

3.

**Third generation**: grandchildren GC1, GC2, etc. Each grandchild has a 50% chance of inheriting the a allele from its C1 parent. The chance of the grandchild inheriting another a from its other parent depends upon that other parent's genes. Suppose we take C1, who has Aa. C1 marries spouse S1. If S1 is AA, then C1's children (GC1, GC2, etc.) will inherit an A from S1, and either an A or an a from C1. Statistically, 50% of C1's children will be Aa, and the other 50% AA (statistically: of course, all of C1's children could be Aa, or all could be AA, or anything in between). If S1 is Aa, then each grandchild has a 50% chance of inheriting an a from S1, and a 50% chance of inheriting an a from C1, and thus a 25% chance of being aa and showing the recessive phenotype. If S1 shows the aa recessive phenotype, like grandpa, then the grandchildren will inherit an a from S1, and have a 50% chance of inheriting an a from C1: thus, the grandchildren would have a 50% chance of being aa, instead of the 25% chance when S1 is Aa.

If all the children in generation 3 are AA or Aa, the trait "skips" this generation as well. However, as long as some of the children are Aa, the trait is still carried.

4.

**Fourth and following generations**. Assuming that there is no selection pressure against allele a, and that each generation has enough children, the recessive allele persists. It pops up (potentially) whenever another parent bearing the recessive allele marries into the family. From the third generation on (GCs), it is possible that none of the descendents carry the a allele. However, the a allele does not die out unless all of the children of a given generation are free of it.

### #11

Posted 28 August 2009 - 10:08 AM

1.

First generation. Let's say you have Mom ("M") with genes AA and Dad ("D") with genes aa. We're assuming that Dad is homozygous for the recessive "a", because otherwise we wouldn't know that he has the trait at all.

2.Second generation: children C1, C2, C3, C4. Each child inherits an A from M, and an a from D. Thus, all the children are Aa, and none show the recessive allele. The trait "skips" this generation.

3.Third generation: grandchildren GC1, GC2, etc. Each grandchild has a 50% chance of inheriting the a allele from its C1 parent. The chance of the grandchild inheriting another a from its other parent depends upon that other parent's genes. Suppose we take C1, who has Aa. C1 marries spouse S1. If S1 is AA, then C1's children (GC1, GC2, etc.) will inherit an A from S1, and either an A or an a from C1. Statistically, 50% of C1's children will be Aa, and the other 50% AA (statistically: of course, all of C1's children could be Aa, or all could be AA, or anything in between). If S1 is Aa, then each grandchild has a 50% chance of inheriting an a from S1, and a 50% chance of inheriting an a from C1, and thus a 25% chance of being aa and showing the recessive phenotype. If S1 shows the aa recessive phenotype, like grandpa, then the grandchildren will inherit an a from S1, and have a 50% chance of inheriting an a from C1: thus, the grandchildren would have a 50% chance of being aa, instead of the 25% chance when S1 is Aa.

If all the children in generation 3 are AA or Aa, the trait "skips" this generation as well. However, as long as some of the children are Aa, the trait is still carried.

4.Fourth and following generations. Assuming that there is no selection pressure against allele a, and that each generation has enough children, the recessive allele persists. It pops up (potentially) whenever another parent bearing the recessive allele marries into the family. From the third generation on (GCs), it is possible that none of the descendents carry the a allele. However, the a allele does not die out unless all of the children of a given generation are free of it.

Interesting analysis, well it all depends on the Alleles of both parents, and finally the pair of alleles that each of the offsprings inherited. By using the mendels laws, i guess u might kinda be able to determinr if recessive alleles do skip a generation..

### #12

Posted 23 January 2010 - 01:29 AM

1.

First generation. Let's say you have Mom ("M") with genes AA and Dad ("D") with genes aa. We're assuming that Dad is homozygous for the recessive "a", because otherwise we wouldn't know that he has the trait at all.

2.Second generation: children C1, C2, C3, C4. Each child inherits an A from M, and an a from D. Thus, all the children are Aa, and none show the recessive allele. The trait "skips" this generation.

3.Third generation: grandchildren GC1, GC2, etc. Each grandchild has a 50% chance of inheriting the a allele from its C1 parent. The chance of the grandchild inheriting another a from its other parent depends upon that other parent's genes. Suppose we take C1, who has Aa. C1 marries spouse S1. If S1 is AA, then C1's children (GC1, GC2, etc.) will inherit an A from S1, and either an A or an a from C1. Statistically, 50% of C1's children will be Aa, and the other 50% AA (statistically: of course, all of C1's children could be Aa, or all could be AA, or anything in between). If S1 is Aa, then each grandchild has a 50% chance of inheriting an a from S1, and a 50% chance of inheriting an a from C1, and thus a 25% chance of being aa and showing the recessive phenotype. If S1 shows the aa recessive phenotype, like grandpa, then the grandchildren will inherit an a from S1, and have a 50% chance of inheriting an a from C1: thus, the grandchildren would have a 50% chance of being aa, instead of the 25% chance when S1 is Aa.

If all the children in generation 3 are AA or Aa, the trait "skips" this generation as well. However, as long as some of the children are Aa, the trait is still carried.

4.Fourth and following generations. Assuming that there is no selection pressure against allele a, and that each generation has enough children, the recessive allele persists. It pops up (potentially) whenever another parent bearing the recessive allele marries into the family. From the third generation on (GCs), it is possible that none of the descendents carry the a allele. However, the a allele does not die out unless all of the children of a given generation are free of it.

OK, So, I'm trying to do some math here, but I keep confusing myself. Say I have 3 generations, and all generations exhibit dominant phenotypes. What I want to know is, if the all four grandparents are of the genotype Aa, what is the percent chance the grandchild will also be Aa?

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