Presentation on theme: "Variation, probability, and pedigree"— Presentation transcript:
1 Variation, probability, and pedigree Gamete production is source of variation and genetic diversity, an advantage of sex.As a result of segregation and independent assortment, lots of combinations possible.2n possibilities exist for diploids where n = haploid number of chromosomesIn humans, this is 8 million different gametesCrossing over during meiosis creates even more combinations of genetic informationThis diversity important in evolution, survival.
2 Product lawProduct law used to calculate odds of an outcome from independent eventsFlip a coin: heads or tails, 50:50 chance (1/2)Flip a coin 3 times, get 3 heads; the next flip, there’s still a 50:50 chance of getting a head.The chance of getting 4 heads in a row:½ x ½ x ½ x ½ = 1/16 the product law.Odds of round, yellow seeds in a cross of Ww GG x Ww gg: ¾ x 4/4 = 3/4
3 Sum Law Flip a penny and a nickel: odds of 1 heads and 1 tails? The sum law: outcomes of events are independent, but can be accomplished in more than one way.Flip a penny and a nickel: odds of 1 heads and 1 tails?There are 4 possible outcomes from this flip.1 head, 1 tail can be from the penny being heads (odds 1/4), but also from the nickel (1/4): ¼ + ¼ = ½
4 Human genetics How to determine inheritance of a trait in humans Can’t (shouldn’t) mandate breeding partnersLow numbers of offspring.PedigreesFollow inheritance of trait in familiesCompare results to other familiesDraw conclusions.
6 Pedigree sample-1 *Look for things you know must be true. Look at inheritance of trait expressed by shaded individual.You KNOW that it can’t be dominant because at least 1 of the parents would also have to show that phenotype.
7 Pedigree sample-2Beware of things that seem logical but might NOT be true.The Shaded trait is dominant.“A” dominant, “a” recessiveThe mother must be aa.The father, however, may or may not be homozygous:If the father is AA, you would expect all offspring to be Aa (AA x aa = Aa); this is what appears to be true.
8 continuedBUT, if the father is Aa, the odds for each child showing the dominant phenotype is 50:50.Just like you can flip a coin 3 times and get heads each time, you could get 3 children that are all Aa, showing the dominant phenotype.The father COULD be Aa. Likely? No. Possible? Definitely.
9 Pedigree problem from text A and a are alleles. Which is shaded? What are the genotypes?Find the sure things first.II 6 must have a recessive trait, being unlike both parents (who must be heterozygous).
10 Modification of Mendel Definitions and terms from Chap. 4Autosomes vs. sex chromosomesWild-type: “normal”, usually dominantDominant does NOT mean most commonExamples: e+/ e where e+ is wild type, slash separates alleles from homologsLower case “e” means recessiveWr+/ Wr shows mutant phenotype because Wr is a dominant mutant alleleR1 & R2; IA & IB; leu-; etc.DnaA is a protein, dnaA is the gene!!
11 Mutation and phenotype Mutations are the source of new allelesA new allele may result in a new phenotype because of changes in enzyme activityEnzyme usually has decreased or no activityEnzyme may have increased activityusually, change in a regulatory geneEnzyme may be unaltered despite change in DNAAllele only at DNA level, no other phenotype
12 Alterations to Mendel Incomplete or partial dominance Codominance Multiple allelesLethal allelesGene interactionsSex-linked, sex-limited, & sex-influencedEffect of environmentExtranuclear inheritance
13 Incomplete or partial dominance One allele only partially masks the other.Half as much enzyme makes half as much pigment.Phenotypic ratio is the same as genotypic: 1:2:1~rjh9u/snapdragon.html
14 Partial dominance-2 Partial dominance is not common A molecular phenotype showing partial dominance is more commonOne allele instead of 2 is producing enzyme, so on a gel, a protein band is half as intense.
15 Codominance M and N blood groups: LM LN Glycoprotein on blood cell surfaceIf one of each allele, both expressed.Phenotype = genotype, essentiallyHeterozygote cross: shows 1:2:1 ratio
16 Multiple allelesIn peas, Mendel following the inheritance of two contrasting traits, e.g. purple vs. white flowersOften, more than two alleles for a trait exist.Study of multiple alleles requires a population!In diploid organisms, an individual can only have a maximum of two alleles. (2 different alleles)In populations, many different alleles may be present.Classic example: the ABO blood group system
17 ABO Blood groups Series of sugars added to cell lipid creates trait. Genotypes include:AA, AO = type ABB, BO = type BOO = type OAB = type AB whereA and B are co-dominant,O is recessive, and the blood type is the phenotype.
18 Lethal allelesIn genetic crosses, information is obtained by examining the phenotype of the offspring.In some instances, the phenotype is lethalLethality may present itself late in life (Huntington Disease) or may result in no offspring.Example:Fur color in mice:Agouti on left, yellow on right.
19 Lethal alleles-2If certain genotypes are lethal, results of a cross may be quite confusing.Agouti x agouti = all agoutiYellow x yellow = 2/3 yellow, 1/3 agoutiAgouti x yellow = ½ yellow, ½ agouti2:1 ratio is tip-off that something odd happensHomozygous for yellow is lethal, so that genotype is NOT represented.For lethality, yellow allele acts as recessive.For coat color, yellow allele acts as dominantA = agouti, Ay = yellow. Heterozygote is yellow.
20 Complex inheritance and dihybrid crosses Book example: inheritance of simple trait and multiple allele trait: albinism and ABOCrossing of heterozygotes (blood group AB)Assume independent assortmentSimple trait shows 3:1 ratio, co-dominant trait shows 1:2:1 ratioPhenotypic classes in offspring no longer 9:3:3:1Actually come out 3:6:3:1:2:1Complex inheritance produces odd ratios.