The Genetics thread

[txlonghorn]

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How do you guys approach genetics questions? Here are the ones I know:

1) looking for distinct gametes when given a genotype:
Use the 2^n rule, where n equals the number of heterozygous alleles.

2)looking for distinct offspring when given the genotype of each parent:
Find the number of different types of possible alleles for each gene and multiply them together. Ex. If parrents have AaBb and AaBb, you have 3 different for A and 3 different for B, so just multiply them together and you get 9 different possible distinct offspring.

3)If autosomal recessive genes are 1/50 in the population, and both parents have it, then what is the probability that the child is homozygous recessive?
For this you multiply 1/50 *1/50 * 1/4 (probability of child being homozygous recessive)

4) If colorblindness is present in 1/12 of men. How frequently does it occur in women? The solution is 1/12*1/12, as woman needs both genes - from mom and dad - to be colorblind (as colorblindness is x-linked recessive)

5) (got this from another thread) If you have two parents who are heterozygous carriers for an AR mutation and they give birth to a healthy child, what is the probability that child is homozygous normal?

Aa x Aa = AA, Aa, Aa, aa. Now since the kid is normal, it (yes, it!) can't be aa, so it is 1/3.

Ok, I kinda still suck at genetics problems, prob cause I never did any outside of bio 1. Post any other problems with solutions!

 

[LuckMC11]

great post!

except for #5, why isnt the answer 1/4? it says homozygous normal, it doesnt like "if the child is normal, what is the probability it is homozygous?" it just says homozygous normal, so wouldnt it be 1/4?

 

[nyr201]

great post!

except for #5, why isnt the answer 1/4? it says homozygous normal, it doesnt like "if the child is normal, what is the probability it is homozygous?" it just says homozygous normal, so wouldnt it be 1/4?

it's 1/3 bc it says they have already given birth to a healthy child...so the aa is negated which leaves AA= 1/3 of whats left 😀

ps i hate these lol

 

[LuckMC11]

oooh right! thanks for clearing it up!

i hate these too lol

 

[omarbasha]

Bump
We should be Genetics freaks, i heard from 4 people that took it to really focus on Genetics... so you know what to do...

 

[nyr201]

Bump
We should be Genetics freaks, i heard from 4 people that took it to really focus on Genetics... so you know what to do...

or as my neuroscience prof used to call them "gene jocks" LOL

 

[Pat Kelly]

I am not sure about question 3 - it is saying that if both parents have the gene, what are the chances that their children is homozygous recessive - the chances should be just 1/4 since it's a conditional probability right? it doesn't matter how rare (in this case 1/50) the genes are, it's already stated in the problem that both parents have it, and the question is asking what the chances are for the children to be homozygous recessive given that the parents already have the gene. If the question was what's the chance of the parents having the gene AND the child expresses the gene as well, then it would be 1/50*1/50*1/4.

edit: this is the same reasoning as your explaination for question 5.

 

[kkim85]

I really don't like genetics ....

 

[MowgliR]

How in depth should we prepare for genetics questions?



DAT is on Aug. 18th

 

[txlonghorn]

I dunno, but that is my weak spot in bio (and math). I just suck so much at probability that the probability of me getting higher than a 19 is 0. See what I mean? Lol.

 

[nyr201]

a friend of mine who took the DAT last year said that for dihybrid crosses just know 9:3:3:1 and that should be enough and i remember on topscore 1 there was a dihybrid where knowing 9:3:3:1 got me to the right answer....can anyone else confirm this for the real test tho?

 

[DoCt0rPeTe]

unfortunately each test is different so i wouldnt bet solely on 9:3:3:1 if u want a 20+

we just have to know it all =/


and even when you think youve studied all the bio you can, there will be crap even your bio professors have never heard of on there!

 

[namiie]

I love Genetics! It's like Ochem--once you have the basic concepts down pat, you're set for life, on any problem, any time. 👍

Second-ing what Doctor Pete said. Be able to recognize epistasis when you see it as well (9:7, 9:3:4, etc).

 

[LuckMC11]

I love Genetics! It's like Ochem--once you have the basic concepts down pat, you're set for life, on any problem, any time. 👍

Second-ing what Doctor Pete said. Be able to recognize epistasis when you see it as well (9:7, 9:3:4, etc).

whats a 9:7 or 9:3:4 ratio mean?

 

[namiie]

whats a 9:7 or 9:3:4 ratio mean?

So epistasis is when the alleles of one gene mask the phenotypic effects of the alleles of another gene, right?

The most common example is when one gene dictates what color something will be while another gene dictates whether or not color will be present at all. Here, a 9:7 ratio is a situation of epistasis in which being homozygous for the recessive allele of either gene results in a colorless phenotype, thereby masking the wild-type or colored phenotype. So you need at least one copy of each dominant allele to be colored. When you do this type of cross, usually the P generation are both colorless, the F1 generation is heterozygous for both alleles and is colored (since it has at least one dominant copy of each allele), and the F2 generation has 9 colored and 7 non-colored. The F1 --> F2 cross is just dihybrid self-fertilization.

The 9:3:4 ratio occurs when something happens to modify the typical 9:3:3:1 ratio so that the latter two, the 3 and 1, have the same phenotype. This usually happens because two mutant lines have the same phenotype, but were caused by mutations in different genes. If you think of it in a biochemical pathway, it's like:

Colorless precursor ----A----> Colorless intermediate -----B---> Colored pigment

Where A and B are enzymes. You can see that a mutation that knocks out either enzymes will still result in the same phenotype--colorless. Thus, the 3 and the 1 may genotypically be different, but will show the same phenotype.

I really recommend you consult a Genetics textbook if you're still fuzzy on epistasis because these are just two specific cases of it 🙂

 

[LuckMC11]

ooh ok i tihnk i got it! lol i tihnk i understood this but saw it in a diff way but i think its the same thing u r talking about. thanks for clarifying!!