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Exam 1: Wednesday October 10th, 2007

Exam 1  review session on Tuesday October 9th if interest and my schedule allows.  Would be at 11:00am.

Exam 1 Review Sheet

This exam review sheet is intended to help you prepare for the exam but should not be considered your only source of study.  It is possible I have inadvertently left off some items of importance, so you should use your notes to help you prepare for the exam. You are still responsible for any material we covered in class, whether on this sheet or not.  This lists serves only to highlight the main points.

Exam format: mostly short answer questions and a couple of problems

Chapter 3 Mendelian Genetics

Who was Gregor Mendel and what did he contribute to genetics?
What about Mendel's scientific approach made him successful where others were not?
What are gametes? 
Understand the terms: monohybrid cross, dihybrid cross, P generation, F1 generation and F2 generation.
Know the terms homozygous or heterozygous.
What are alleles?  How many alleles does an individual have?  How many alleles does a gamete have? How many alleles can there be in a population of organisms?
Understand the difference between dominant and recessive traits.
Know the terms genotype and phenotype.  How do they differ?  

Monohybrid cross This is a  cross involving a single pair of contrasting traits (monohybrid)

P (parental) PP X pp (crossing truebreeding parents)
F1 Genotype: Pp
Phenotype: purple
F2 ¼ PP + ½ Pp + ¼ pp Genotypes
¾ purple and ¼ white Phenotypes
A monohybrid cross results in a 1:2:1 genotypic ratio and a 3:1 phenotypic ratio

bulletThe Punnett Square: Use to determine genotypic and phenotypic ratios for monohybrid cross:
  P p
P PP Pp
P PP pp
bulletRemember, you can express your ratios however you wish, so long as it is correct!   So, for example you could express your genotype ratios as  1/4 PP, 2/4Pp, 1/4pp OR 25%PP, 50%Pp, 25%pp OR 1:2:1 PP:Pp:pp
bulletReview your handout on single Gene Inheritance which summarizes the six possible types of matings and the resulting genotypic and phenotypic ratios. 

Dihybrid cross This is a  cross involving two pairs of contrasting traits (dihybrid)

PP = purple BB = short
Pp = purple Bb = short
pp = white bb = tall

P (parental) PPBB X ppbb OR PPbb X ppBB (crossing truebreeding parents -- note that either parent shown will result in the same F1)
F1 Genotype: PpBb
Phenotype: purple, short 
F2  1/16PPBB,  2/16 PPBb, 1/16 PPbb, 2/16 PpBB, 4/16 PpBb, 2/16 Ppbb, 1/16ppBB, 2/16ppBb, 1/16 ppbb Genotypes
9/16 purple, short 3/16 purple tall 3/16 white short and 1/16 white tall Phenotypes

A dihybrid cross results in a 1:2:1:2:4:2:1:2:1 genotypic ratio and a 9:3:3:1 phenotypic ratio
bulletReview either branch line diagram or punnett square method of determining F2 phenotypic and genotypic ratios. 

**Keep in mind that the above examples of monohybrid and dihybrid crossed are by definition starting with a standard cross of truebreeding parents, and the resulting F2 ratios will always be the same as shown above no matter what trait or pair of traits you are considering.  However it is possible to look at individual matings of particular organisms which will not be truebreeding, and the resulting ratios will not be the same as above.  The nature of the cross would have to be specified in some way, and the resulting phenotypes and genotypes would be calculated in a similar manner.** 

bulletReview Mendel's 4 postulates and how they correlate to the presence of genes located on homologous chromosomes and their behavior (Figure 3-11).  Especially the concept of segregation and independent assortment. 
bulletBe familiar with what a testcross is, how it is done, and what the results tell you.  Always cross to a homozygous recessive. 

Chapter 4  Extensions of Mendelian Genetics

This whole chapter deals with situations which alter the standard monohybrid and dihybrid phenotype ratios (3:1, 9:3:3:1).   When gene expression does not adhere to a simple dominant/recessive mode, or when more than one pair of genes influences the expression of a single character, the classic ratios are modified.  Nevertheless, the fundamental principles of segregation and independent assortment still hold true in these situations.  

bulletIncomplete dominance  -- the heterozygote has an intermediate phenotype.  phenotype ratio = genotype ratio in a standard monohybrid or dihybrid cross. 
Example : four o'clocks R1R1 = red, R1R2 = pink, R2R2 = white
R1R2 x R1R2 ----> 1/4 R1R1, 2/4 R1R2, 1/4 R2R2 = 1/4 red, 2/4 pink, 1/4 white
bulletCodominance --the heterozygote shows phenotype of both alleles.  phenotype ratio = genotype ratio in a standard monohybrid or dihybrid cross.
Example: MN blood groups.
bulletMultiple alleles -- For any given gene the number of alleles within members of a population is not restricted to two.   Multiple alleles = 3 or more alleles for a given gene.  Any individual diploid organism has at most two alleles of any gene. 
Example: ABO Blood groups.  Three alleles: IA, IB, and IO.  IA, IB are dominant to IO.  IA is codominant to IB.
Table 4.1 shows the various ratios of offspring that can result from different crosses. 
Lethal alleles -- Many gene products are essential to an organism's survival.  When a mutant allele is present in may be lethal for the individual. 
bulletA recessive lethal allele is lethal only if two copies of the lethal allele are present (homozygous).  Sometimes the heterozygote has a unique phenotype (see Figure 4-4).   The other normal allele is sufficient to compensate for the mutant one.   Example: AY allele in mice, p. 85
bulletA dominant lethal allele is lethal even when only one copy of the allele is present.  So organisms homozygous or heterozygous for the lethal allele will die. Example: Huntington's disease
bulletWith lethal alleles, the phenotype ratio is altered   such that in a standard monohybrid cross, the F2 phenotype ratio would be 2:1 instead of 3:1.  The dihybrid ratio would also be affected. 
Be able to distinguish between the different types of inheritance, and be able to apply these types of inheriance to new examples.   
Combinations of two gene pairs involving two modes of inheritance modify the 9:3:3:1 ratio -- see Figure 4-5.  Be able to do this type of cross. 

 

bulletEpistasis -- an example of gene interaction; occurs when the expression of one gene pair masks or modifies the expression of another gene pair.  The involved genes control the expression of the same general phenotypic trait.  Has the effect of combining one or more of the four phenotypic categories of a standard dihybrid cross in various ways.  Three types:
bullethomozygous recessive allele at one locus --> may prevent or override the expression of other alleles at a second locus.  Example: A,B,O Blood groups affected by expression of H substance. When homozygous recessive at h allele, genotype at IA or IB allele is masked.  See class notes.   Another example involving mouse coat color  in your text. 
bulletSingle dominant allele at one locus ---> may prevent or override the expression of other alleles at a second locus.  Example: Summer squash .  The dominant allele A results in white color regardless of the the genotype at a second locus B. 
bulletTwo gene pairs may complement one another such that at least one dominant allele at each locus is required to express a particular phenotype. Example: sweet pea flowers. Flowers are only purple when one dominant allele from A locus and B locus, otherwise white flowers.  
bulletAll three types of epistasis result in altered F2 ratios.  
bulletRemember, in epistasis, even though only one characteristic is being followed (flower color for example), the phenotype ratio is expressed in 16 parts. This is a way to see that epistasis is occurring
bulletBe familar with the inheritance of sex-linked traits
bulletWhat is meant by sex-influenced?  sex-limited?

Chapter 5:  Linkage, Crossing over, and Mapping

bulletWhat is linkage? 
bulletWhat is the difference between complete linkage and linkage with recombination?
bulletBe able to distinguish between independent assortment, linkage, and linkage with recombination. 
bulletCrossover occurs between non-sister chromatids of a homologous pair of chromosomes.  When does crossover occur (specifically)? 
bulletAs distance between genes on the same chromosome increase, the degree of crossing over increases (and the number of recombinant gametes increases)
bulletThree point testcross:
bulletWhat conditions have to be met for a successful three point testcross?
bulletKnow what non crossover (parental) and single crossover, and double crossover offspring are, and be able to identify these classes of offspring if given the results of a three point testcross (which are present in the highest numbers?   The lowest?
bulletmap distance= % of all detectable exchanges occurring between 2 genes = %SCO + %DCO
bulletWhich class of offspring tells you the correct arrangement of alleles in the female parent (NCO, SCO or DCO)?
bulletWhich class of offspring tells you what is the correct sequence of the alleles (NCO, SCO or DCO)?
bulletFor the three point testcross , I will give you questions related to the things listed above, and will not ask you to solve a whole problem.  I will give you parts of a solved problem and ask you to figure out some of  the information.  For example, if I give you the SCO% and DCO% between genes, I may ask you to calculate the map distance.

 

Still need help?  email me at lrapp@stcc.edu