BIO 309F Exam I Comments, thoughts, reviews, tips
Exam I is in class on Tuesday, 2/20. The exam will be designed to last less than the hour and fifteen minutes allotted, probably about 50 multiple choice questions. We’ve been told the exam can cover ~80% material from the lecture, 20% material from the textbook.
Studying consists of 4 major steps:
· Taking Careful notes during class
o This gives you an idea of what Dr. Sanders feels is and isn’t important
· Understanding all the CONCEPTS discussed in class
o There shouldn’t be a line in your notes that doesn’t make sense to you. Not only should you understand the sentence, you should understand the ideas behind it. For example, we talked in class on 2/13 about non-disjunction in meiosis and we also talked about Down’s syndrome…do you understand how non-disjunction in meiosis can lead to Down’s syndrome? Or monosomy in another chromosome?
· Memorizing all the CONCEPTS and VOCABULARY and FACTS given in class
o There’s a lot of ‘em, and you’re responsible for knowing what to memorize and committing it to memory
· Performing well on the exam
o This involves remembering what you memorized, as well as critical thinking (taking what you memorized and applying it to a new situation)
o Some questions involves remembering vocabulary or facts given in class
§ Vocabulary memorixation question:
5. An allele is ___.
A. one of the bases in DNA
B. an alternate form of a gene
C. another term for epistasis
D. present only in males and is responsible for sex
determination
E. found in mitochondria but not in nuclei
8. The fundamental Mendelian process which involves the
separation of contrasting genetic elements at the same locus would
be called ___.
A. separation of alleles
B. independent assortment
C. continuous variation
D. discontinuous variation
E. dominance or recessiveness
§ Fact memorization questions:
48. Major risk factor(s) in having a Down Syndrome child is/are:
A. delaying having children until female is older
B. chromosome 21 reciprocal translocation
C. chromosome 21 mosaic
D. A and B are correct
E. A, B, and C are correct
o Some questions involve critical thinking: taking information you were given in class and being asked about it in a way different than the way it was given
§ Example:
4. Consider a species with a diploid (2n) number of 16
chromosomes. How many chromosomes would be found in
a trisomic somatic cell?
A. 8
B. 9
C. 16
D. 17
E. 24
· In this question, we are used to thinking about humans only, which have a diploid (2n) number of 46. A trisomic human has 47 chromosomes. This questions tests to see if you understand the general concepts of diploidy and trisomy as applied to non-humans. Applying the same logic to non-humans tells you that a trisomic individual of this species has 1 extra chromosome, leading to a total of 17 chromosomes.
§ Another example:
6. In dihybrid crosses, the ratio 9:3:3:1 indicates ___.
A. Codominance
B. Independent assortment
C. Intermediate dominance
D. Three alleles for each trait
· This question is hard because we talked about 9:3:3:1 ratios, and we also talked about each of the concepts mentioned in choices A – D. The trick is, do you understand which concept a dihybrid cross illustrates? If you do, then you’re doing above average in this class.
o 1st, you have to remember vocabulary: ‘dihybrid cross’ was a cross with 2 different traits
o 2nd, you have to critical think and understand that the dihybrid phenotypic ratio is 9:3:3:1, but if you looked at each trait independently of the other, its phenotype would give a 3:1 ratio. Combining these 2 traits is what gives the 9:3:3:1. But each trait separately shows the same inheritrance patterns as when you only examine 1 trait…this means that inherited traits have nothing to do with each other, are inherited independently, which is a result of independent assortment. Reading through the book section on indepdendent assortment and understanding how the dihybrid cross illustrates this would have you more prepared for this question than simply memorizing what Dr. Sanders mentioned during lecture for this concept.
§ The critical thinking questions get even harder…look at this one:
19. A man with genophobia disease marries a normal woman.
They have 8 children (4 boys and 4 girls. All of the girls have the
disease but none of the boys have the disease. How is genophobia
inherited?
A. autosomal recessive
B. Y linked
C. autosomal dominant
D. X-linked dominant
E. X-linked recessive
· This is a VERY tough problem. How can we go about solving it? See my ‘pedigree problem solving guidelines’, but also:
· Well, I’ve never heard of genophobia, and the ultimate question I’m being asked is how this disease is inherited, so I guess I wasn’t already told in class how this disease was inherited. I suppose I’ll have to deduce the inheritance type from the given information.
· Let’s list the given information:
o Man = disease, wife = normal
o All female offspring have disease, no male offspring do
· Let’s use some process-of-elimination thinking: this disease is MOST LIKELY sex-linked, because of the emphasis placed on the gender ratios of the disease. Autosomal diseases affect both genders equally, so this female-bias of the disease indicates to me that this is most likely a sex-linked disease.
· There are 2 types of sex-linked, X and Y linked. If its Y-linked, ONLY MALES can EVER have the disease, because only males have Y chromosomes. So its not Y-linked, ‘cuz females have it.
· So is it X-linked dominant or recessive? Well, we need more process-of-elimination, so we’ll be hypothetical…what if it is X-linked dominant?
o If it is, the mom has genotype XaXa and the dad has genotype XAY. The punnett square resulting from this cross would be
o In this Punnett square, all females have the disease and all males are normal. That sounds just like what the problem said! Let’s make sure that its not X-linked recessive. If it was X-linked recessive, the mom has genotype XAX_ and the dad has genotype XaY. The only was ANY offspring can have the disease is if the mom carries it, so she MUST be heterozygous XAXa. The punnett square resulting from this cross would be:
o Half of the females show the disease as do half of the males. This COULD be what happened and there just weren’t enough kids to see the 50:50 phenotypic ratio in both genders, but that is unlikely. With 8 kids, all females with the disease and no males with the disease, its MOST LIKELY that the disease is X-linked dominant, so that’s my answer choice.
It looks like this will be a pretty tough exam, with a good scattering of critical thinking questions. How can I know exactly what all I need to study, and prepare properly for the level of difficulty of questions expected on the exam? I like to re-outline my notes, in a way that organizes ALL of the information given in class, supplements class info with info from the book, and highlights all the FACTS and VOCABULARY that I need to memorize. Here’s an example of how I would re-outline my notes from 2/1/2007.
To start this outline, I have my notes, the ‘outline’ Dr. Sanders handed out in class, and my book all open. I see that this material corresponds to Ch. 3, so I open up to that chapter. I try to follow the ‘outline’ Dr. Sanders gave in class, filling in information from my notes and from the book for each section. My final outline looks like this.
· Mendel’s experiments
o 1853
o True breeding (a plant with a trait gives rise to offspring that have the same trait, results from self-fertilization in peas) pea plants
o Mendelian laws:
§ The principle of segregation: separation of members of a gene pair during gamete formation
· During meiotic gamete formation, each gamete gets one allele (one of the possible alternative forms of a gene, usually distinguished from other alleles by its phenotypic effects) for a trait. This is what Mendel meant by the principle of segregation.
§ The principle of independent assortment: During gamete formation, alleles of one gene pair segregate in to the gametes independently of the alleles belonging to other gene pairs, resulting in the production of gametes containing all possible combinations of alleles.
· During meiosis, each gamete gets 1 copy of each chromosome, but its random which copy the gamete might get. You might get maternal chromosome 1 and paternal chromosome 2, or maternal chromosome 1 and 2. Because its random which chromosome combinations you get, its random which allele from 1 trait will be seen in a gamete with an allele from another trait. Because chromosomes assort independently, alleles for different traits do also, thus this principle.
o Mendelian inheritance: alleles for a trait are either dominant (it takes only 1 copy of the allele to see that phenotype) or recessive (it takes 2 copies of the allele to see that phenotype)
o Monohybrid cross: (cross of 1 trait, each parent is a heterozygote)
§ If T = tall and t = short, it’s a cross of tall individuals with genotypes Tt and Tt:
§
§ Results in a 3:1 phenotypic ratio (tall:short) and 1:2:1 genotypic ratio (homozygous tall:heterozygous tall:homozygous short)
· Genotype = the specific genetic constitution of an organism, made up of the alleles of a gene
· Phenotype = the observable properties of an organism
o Genotype vs phenotype: the phenotype is the characteristic of a trait that you observe, the genotype is what alleles make up the phenotype
§ Test cross: (cross of 1 true-breeding and one heterozygote)
· TT x Tt OR Tt x tt
· Results in a 1:1 phenotypic and genotypic ratio
o Dihybrid cross: (cross of 2 traits, each parent is a heterozygote for both traits)
§ We’ll use tall (T) and short (t), as well as seeds that are smooth (S) or wrinkled (s): TtSs x TtSs
· The punnett square consists of all possible allele combinations in a gamete: TS, Ts, tS, ts
· The phenotypic and genotypic ratios:
o 9:3:3:1 phenotypic
o 1:2:1:2:4:2:1:2:1 genotypic
o Lets yse R and Y instead of T and S:
RY / Ry / rY / ryRY / RRYY / RRYy / RrYY / RrYy
Ry / RRYy / RRyy / RrYy / Rryy
rY / RrYY / RrYy / rrYY / rrYy
ry / RrYy / Rryy / rrYy / rryy
· Non-Mendelian inheritance
o Incomplete dominance: (expression of a phenotype that is intermediate between those of the parents
§ Example: flower color
· RR = red, rr = white, Rr = pink
· In this instance, R codes for 50% red, r codes for lack of color (white). The combinations explain the colors.
o Codominance: (full phenotypic expression of both alleles in the heterozygote)
§ In this, neither allele is dominant or recessive
§ Example: blood types in humans
· AB heterozygotes carry both the A and B antigens on their red blood cells, blood type AB
· Introduction to pedigrees (a diagram listing the members and ancestral relationships in a famil, used in the study of human heredity)
o Afflicted Males = ■, afflicted females = ●
o Non-afflicted = □, ○
More thoughts and tips
· USE YOUR TEXTBOOK!!!!!!!!!!
o The text book has the level of detail of the material you are expected to know for the exam, whereas the lecture does not. MOST of the material we covered in lecture also appears in your text, and you need to know the material as well as the text describes. Dr. Sanders didn’t explicitly show how the dihybrid cross illustrates Mendel’s principle of independent assortment, but the book does, and this is a question that was asked on an old exam.
· USE YOUR BRAIN!!!!!!!
o Some questions on the exam involve critical thinking…only if you truly understand the concept, instead of just memorizing, will you be equipped to answer these questions on the spot. If you are having trouble understanding how to get to the point where you are prepared to answer questions like that, STUDY YOUR BOOK in depth or see me for guidance.
o For each lecture, I can come up with a list of questions/concepts that illustrate whether or not you are prepared for exm-critical thinking. Other than that, you have no way of knowing how prepared for the exam you really are. For example, for the above lecture, you’re doing great if you can answer these questions:
§ How does a punnett square illustrate the principle of segregation?
§ How does a dihybrid cross illustrate the principle of independent assortment?
§ How does meiosis help me understand what allele combinations to put in my punnett square?
§ Can you use any symbol to represent an allele?
o Here are some other quick questions that if you can answer easily, you are more prepared for critical thinking:
§ How does the timing of meiosis occurrence in females affect Down’s syndrome?
§ What are the major differences in meiosis between males and females?
§ Can you keep track of the number of chromosomes throughout meiosis?