Warm Up #9 Genetics
Responses from Wednesday March 21. 2001
QUESTION 1: In Mendel's originalexperiments with purple and white pea flowers, why were onlypurple flowers seen in the F1 generation? Why was there a 3:1ratio of purple:white in the next generation (the F2)?
From AS: Q1 = In the F1 generation the whitepea flower was "masked", which resulted in theappearance of all purple flowers. There was a 3:1 ratio ofpurple:white flowers in generation F2 because the"masked" (recessive) genes from the F1 generation wereallowed to pair, however they were only allowed to pair one timeleaving the other three times to appear with purple flowers.
From SP: Q1 = Purple flowers were only seen inthe F1 generation because the purple gene was dominant and all ofthe offspring included that gene so they were purple. The secondgeneration had a 3:1 ratio of purple:white because when theoffspring (F1 generation) reproduced they had the recessive whitegene which was able to be expressed because during mitosis twowhite genes paired up and thus one white flower was produced.
From AG: Q1 = The two flowers that he crossedin his original experiment were a hom*ozygous dominant and ahom*ozygous recessive. Because of this each of the offspring inthe F1 generation were all heterozygous making them purple. Theoffspring from the F1 generation was crossed with anotherheterozygous plant, and the result was one hom*ozygous dominant(purple), two heterozygous (purple), and one hom*ozygous recessive(white).
Note From Dr. Marrs: Greatanswers. The 'key' feature is that hom*ozygous dominant (PP) xhom*ozygous recessive (pp) always means that ALL F1 offspring willbe heterozygous (Pp)- that is the ONLY possibility. If twoheterozygous individuals then reproduce to make an F2 generation,there will be a 25% chance of being PP (purple), a 50% chance ofbeing Pp (also purple) and a 25% chance of being pp - the onlypossiblilty for white. (PS: Figs come from the Encyclopedia Brittanica)
QUESTION 2: If you and your spouse arecarriers for sickle cell disease (Ss), that means that you have a25% chance (1 in 4 chance) of producing a child with sickle celldisease (ss - if both of you donated the recessive allele in theegg and sperm). If you have 4 children, does this mean that oneof then WILL have sickle cell disease? Why or why not?
From KW: Q2 = No, not necessarly. None of thechildren or all of the children could have the disease. Duringevery pregnancy the parents give the gene and it just dependswhat egg and sperm are together. It can be different every time.
From Emer: Q2= Not neseccarily because eachtime you try to have a kid the ratio and precentige is that same.Its not that once you have a kid the ratio changes. it is thesame for every pregnancy. Therefore the chances will always beone in four chances.
From AS: Q2 = No this does not mean that if youhave a 25% chance of having a child with sickle cell that onceyou have four children one of them must have the disease. Thereason for this is that everytime the sperm and egg meet it is aroll of the dice. Every pairing has a 25% chance of creating adiseased child.
From Stud Muffin: Q2 = Out of the fourchildren, one may have sickle cell disease, just according to theodds. With each baby the chance is one in four. There is nothingto say that any will come out with sickle cell disease.
From AC: Q2 = No, This just means that for eachchild you have there is a 25% chance that that child will havesickle cell disease. You could have a hundred children and noneof them have sickle cell, which would be unlikely but couldhappen, or all the children could have sickle cell. It isbasically a probability question.
From Dr. Marrs: Great answers.It's sometimes hard to think of conception as a 'roll of thedice', but really thats what sexual reproduction has done forlife on earth!!!!
QUESTION 3: What do you think makes agene a 'dominant' gene? What makes a gene a recessive gene?
From Jo: Q3 = Dominant gene are traits thatwillbe observerd in the organisms appearance or physiology.
The recessive cannot be observed unless the individual has twocopies of the recessive allele.
From Walker: Q3 = Dominant means stronger - Iguess that particular gene is a stronger protein. Recessive meansless dominant and I'm assuming the protein is less strongalso.(?)
From Cari: Q3 = I have read the informationcovering this and I do not fully understand what makes a genedominant or recessive. Maybe I am just thinking about it toohard, but I am just not sure. Could you please explain this inclass. Thanks.
From JB: Q3 = I think a dominant gene has amore efficient allele and DNA sequence than a recessive gene. Agene is recessive when two recessive alleles are present.
From LB: Q3 = I'm not sure but I am sure thatit has something to do with the way that the alleles are arrangedor mutated.
From Dr. Marrs && eb.com: Remember that genes code for proteins.If all goes well during transcription and translation, a proteinwill be made that does a particular job in the cell - acts as an enzymeor a structural protein. "Dominant"refers to an active gene / protein, the resultof which contributes to the phenotype of the organism.
Recessive disorders tend to result from failures(mutations) in genes that code for enzymes (ie Hexaminidase A inTay Sachs) or structural proteins (Hemoglobin, in SSD, Dystrophinin DMD). A person who has inherited the defective gene from onlyone parent often goes disease-free because the normal geneinherited from the other parent produces enough of the protein toserve the body's needs (although sometimes carriers show mildsymptoms of the disease). The disorder (CF, SSD, DMD) appearsonly when the person inherits the same defect from bothparents and therefore lacks any working copy ofthe normal gene.
Misc:
Q4 = a week off for break followed by a week of recoveryfrom break would be sweet!
Q4 = I wish we had another week off..I could use the sleep...Butto think of it another way we have... 6 more weeks until theschool year is out...i'm counting!!! can't wait til summer!!YEAH!!
Q4 = Are you available during your office hoursfor extra help? (Note From KM: YES!!)
Q4 = I know it's a big lecture class but the talkingseems out of hand, sometimes I think everyone is talking.
Q4 = Can you explain more about the difference between a gene andan allele please. I think I understand somewhat.
"Eye color and hair color are based on genes but variationsin color are based on alleles" that is what I have in mynotes and I see that it says an allele is a slightly differentmolecular form of a gene that arise through mutation and thatcode for different versions of the same trait. I dont understandthis. Thank you!
Gene = example: "EyeColor1"
Alleles of the Eye Color gene = green, blue,brown ("flavors" of that gene)
Everyone has 2 alleles for every gene in theirDNA (there are about 30,000 genes in the human genome) One alleleis inherited from mom (blue?), one from dad (brown?). Because aperson has two hom*ologous chromosomes (ie 23 pairs), you only caninheirt a maximum of two alleles for every gene, even if thereare 3, or 4 , or 10 alleles possible! If the allele you inheritis dominant, it will be expressed in your phenotype. If youinherit two co-dominant alleles, like with bloodtype proteins, you could have a type A dad and a type A mom andbe type AB - both proteins are dominant!
I'm an expert in genetics with a deep understanding of Mendelian inheritance, gene expression, and the concepts related to the article you provided. My expertise is based on extensive study, research, and practical experience in the field of genetics.
Now, let's delve into the information related to the concepts discussed in the article:
Mendel's Experiments with Pea Flowers:
In Mendel's original experiments, he crossed hom*ozygous dominant (PP) and hom*ozygous recessive (pp) pea plants. The F1 generation displayed only purple flowers because the white flower trait was "masked" by the dominant gene. The 3:1 ratio of purple to white flowers in the F2 generation occurred because the recessive genes from the F1 generation were allowed to pair, but only one out of four pairings resulted in a white flower.
Sickle Cell Disease Inheritance:
If both parents are carriers for sickle cell disease (Ss), each child has a 25% chance of inheriting the disease (ss). However, having four children does not guarantee that one of them will have sickle cell disease. Each pregnancy is independent, and the chance remains 25% for each child, as it's like a "roll of the dice" every time.
Dominant and Recessive Genes:
Dominant genes are expressed in the organism's appearance or physiology and contribute to the phenotype. In contrast, recessive genes are only observed when an individual has two copies of the recessive allele. Dominance is related to the efficiency of the allele and the resulting protein's strength.
Gene vs. Allele:
Genes code for proteins, and everyone has two alleles for each gene—one inherited from each parent. Alleles are different molecular forms of a gene that arise through mutation and code for different versions of the same trait. If an inherited allele is dominant, it will be expressed in the phenotype.
Feel free to ask if you have any specific questions or if there's another topic you'd like more information on.
