Presentation on theme: "Lecture for Tuesday September 23, 2003 What’s due? CH2 problem set Assignments: CH4 problems: 1-5, 8, 10, 11, 14, 16, 17, 21, 22 What’s due Thursday 9/25?"— Presentation transcript:
Lecture for Tuesday September 23, 2003 What’s due? CH2 problem set Assignments: CH4 problems: 1-5, 8, 10, 11, 14, 16, 17, 21, 22 What’s due Thursday 9/25? CH3 problem set Today’s lecture: Review material from 9/18 Human Pedigrees Begin CH4 Today’s Lab: Maize segregating ears: Dihybrid cross and chi- square analysis *Exam I is one week from today! Reading assignment: Omit sections 4.8 and 4.9
Review: Trihybrid cross- A genetic cross between two individuals involving three characters (also referred to as a three-factor cross) The Forked-Line Method (branch diagram): Recall: *The F 1 that result from a monohybrid cross (AA x aa) all have the genotype Aa and the phenotype represented by A *The F 2 that result from a cross between 2 individuals from the F 1, have a phenotypic ratio of 3:1 *Assume independent assortment of the 3 gene pairs KEY: We are examining the resulting phenotypes!
Review Chi-Square Analysis: Mendel’s monohybrid and dihybrid ratios are predictions based on the following assumptions: 1.Each allele is dominant or recessive 2.Random segregation of alleles 3.Independent assortment 4.Fertilization is random NOTE: *The outcomes of 2-4 are “chance events” and are subject to random fluctuation *As sample size increases, the average deviation from expected results decreases Establishing a null hypothesis (H 0 ): States that there is no difference between the observed and expected data An Example (for a monohybrid cross): The observed phenotypic ratio is 3:1
Review Chi-Square Analysis: The null hypothesis is analyzed statistically: *It may be rejected or *It may fail to be rejected Chi-Square (X 2 ) Analysis: *Examines deviation between observed and expected numbers *Accounts for sample size (o-e) 2 X 2 = e Interpretation: *determine df (n-1) *typically use p value of 0.05 or greater (i.e. 0.01, 0.001) *reject or fail to reject null hypothesis
Review Chi-Square Analysis: p value (probability): consider as a percentage (i.e. 0.05 = 5%) *A level of error that is acceptable to the researcher in analysis of data *5% of the time your result (or outcome) is due to chance *95% of the time your results are not due to chance *If your calculated X 2 is GREATER than that shown at p = 0.05, then you reject your null hypothesis *Therefore, we CAN NOT reject our null hypothesis! Example from Table 3.1: Calculated X 2 = 0.53
Human Pedigrees Pedigree- a family tree that shows the phenotype of a particular trait for each family member = Female = Male =Unknown *Shaded symbol=expressed phenotype *Individuals KNOWN to be heterozygous are half shaded *Horizontal lines connect parents, vertical lines lead to offspring *Proband (p)= individual in whom a genetically determined trait of interest is first determined
Chapter 4: Modification of Mendelian Ratios Allele- (short for allelomorph) alternative forms of the same gene *Wild-type allele- allele that occurs most frequently in a population (arbitrarily designated as “normal”); usually dominant *Mutant allele- allele that contains modified genetic information and often specifies an altered gene product Conventional symbols for alleles: recessive allele- initial letter of the name of the recessive trait, lowercased and italicized dominant allele- same letter in uppercase Tall = D Dwarf = d Example: BRCA1 or BRCA2- (humans) Breast Cancer susceptibility SUPERMAN- (Arabidopsis) regulates genes involved in floral development Genetic nomenclature is extremely diverse!
Incomplete or Partial Dominance Incomplete dominance- expression of a heterozygous phenotype which is distinct from, and often intermediate to, that of either parent Cross between parents with contrasting traits: Red flowers or white flowers Offspring with an intermediate phenotype: pink flowers
Incomplete or Partial Dominance con’t C R C R x C W C W CRCWCRCW C R C W x C R C W ¼ C R C R ½ C R C W ¼ C R C W
Codominance: Codominance- Condition in which the phenotypic effects of a gene’s alleles are fully and simultaneously expressed in the heterozygote Example: MN Blood group- red blood cells contain a transmembrane glycoprotein (glycophorin); two different forms of this protein exist, M and N Genotype L M L M L M L N L N L N Phenotype M MN N L M L M X L M L N ¼ L M L M ½ L M L N ¼ L M L N We can predict genotypic and phenotypic ratios
Multiple Alleles- three or more alleles of the same gene Examples: *Table 4.1: over 100 alleles at a given locus in Drosophila *ABO Blood group in humans Multiple Alleles: *Characterized by the presence of glycoprotein antigens on the surface of red blood cells *Distinct from the M and N antigens *Also exhibits codomiance Genotype I A I A I A I O I B I B I B I O I A I B I O I O Antigen A A B B A,B Neither Phenotype A A B B AB O
Lethal Alleles: Lethal Allele- recessive allele in which a homozygous genotype results in death Example: Coat color in mice *A = agouti = wild-type allele *A Y = yellow = mutant allele Dominant Lethal: Huntington’s disease (H); heterozygous individuals (Hh) have late onset
Gene interaction: Individual characteristics (discrete phenotypes) are often under the control of more than one gene Epistasis- from the greek “stoppage”, interaction between genes such that one gene interferes with or prevents the expresion of another gene Example: In Drosophila, the recessive gene eyeless (when homozygous) prevents the expression of eye color genes present in genome Novel phenotypes due to gene interaction Example: disc-shaped fruit (AABB) X long fruit (aabb) F 1 are all AaBb and disc-shaped F 2 Ratio 9/16 3/16 3/16 1/16 Genotype A-B- A-bb aaB- aabb Phenotype disc sphere sphere long Final phenotypic ratio 9/16 disc 6/16 sphere 1/16 long