2Review Chromosomes are paired (homologous) in a 2N organism. Each chromosome contains many genesHomologous chromosomes carry the same genes.Homologous pairof chromosomes
3ReviewGenes can have different versions of the same trait. These versions are called alleles.Homologous chromosomes separate into different gametes.alleles
4Alleles - Dominant and Recessive Each parent will donate one allele.Alleles can be physically apparent (dominant) or physically hidden (recessive).Mendel worked with traits that had only two alleles of each trait, one dominant and one recessive.AllelesMotherFather
5Mendel’s Laws The Law of Segregation The Law of Dominance each pair of alleles separates during meiosis (gamete formation)The Law of DominanceWhen two alleles differ,one can control the trait - Dominantthe other can be hidden - RecessiveThe Law of Independent AssortmentPairs of genes segregate into gametes randomly and independently (due to random separation of homologous pairs.
6How Can We Demonstrate these Laws? Mendel recorded phenotypes ( form of the trait that he “saw”)Based on outcomes from matings (called crosses) over several generations, he could determine the genotypes (the actual genetic makeup for that trait) of previous generationsHomozygous - both alleles for a trait are the sameHeterozygous - alleles are not identical.
7Monohybrid CrossesStudying the inheritance of a single trait while ignoring all other traitsStart with 2 “pure parents”P GenerationGG (only green)gg (only yellow)F1 generationAll green heterozygous
8Determining Genotype (1 Trait) PARENTgreenyellowF14All greenCannot see the yellowin this generationSelf- fertilizedF2Self fertilization of this generationproduces a different pattern forhomozygous vs. heterozygousYellowreturns
9Using a Punnett Square instead Grid for Organizing Genetic InformationShows Probability that a combination will occurg gParent 2GgGGg1) Determine Parent alleles2) Place alleles on edge of square3) Combine alleles inside the boxes4) Determine genotypes and phenotypesGgGgPossible offspring genotypes for F1Parent 1
10Heterozygous cross G g G g Phenotypes vs. Genotypes The recessive trait will show ONLY in the absence of the dominant (G) trait.G gGENOTYPEGgGG : Gg or gG : gg1 : :GGGgPHENOTYPEGreen Yellow3 : 1gGgg
11Test CrossesPurpose -To distinguish between a homozygous dominant and a heterozygous dominantCross the unknown with a known homozygous recessive.Create two Punnett squares.Cross a homozygous dominant with a homozygous recessiveCross a heterozygous dominant with a homozygous recessiveHow are the patterns different?
12Heterozygous + Homozygous Recessive Phenotypes vs. GenotypesThe recessive trait will show ONLY in the absence of the dominant (G) trait. What will happen?g gGENOTYPEGENOTYPEGgGG :Gg or gG : g: :PHENOTYPEGreen Yellow:
13Predicting any single outcome Given Mendel’s Laws, the probability of an outcome for any one combination can be estimated.Can Use coins to estimate probabilitySee Exercise on Probability.
14Probabilities and Proportions Should approximately match.Are only estimates of what SHOULD happen.Can be easily changed If “randomness” is compromised.
15Dihybrid crosses RY RY ry Allow the study of two traits at a time. Make a P cross (RRYY x rryy)Cross F1 plants among themselves (RrYy x RrYy)List all gameteson a 4 x 4Punnet SquareRY RYry
16Intermediate Inheritance Mendel’s studyDominant and recessiveMany genes have intermediate inheritanceHeterozygous phenotype not like either homozygous phenotype
17Types of Intermediate Inheritance Incomplete DominanceHeterozygote phenotype is “inbetween “ the two homozygous phenotypesEXAMPLE: Red + White = Pink FlowersBUT: Gametes will contain either Red or White!No dominant phenotype so only capital letters with superscripts are used.Red = FrFr White = FwFw Pink = FrFw
18Types of Intermediate Inheritance CodominanceBoth alleles express themselves fullyEXAMPLE: Blood typesA, B, AB
19Polygenic Traits Trait is controlled by two or more gene pairs Eye color, skin, hairTone amount, and distribution of pigments are all factors. Each is controlled by a different gene pair, for each attribute (color, skin, hair)
20Multiple alleles Three or more alleles in a trait Blood type IA IB and iBoth IA and IB are dominant over IIA and IA are codominant.NOT THE SAME as polygenicA single pair of genes with > 2 alleles.
21Environmental Stress on Genetics Seasonal Changes( external changes)Turning on “beard growth (internal changes)Genes can be turned on and off.