4 Law of SegregationEach individual has a pair of alleles for each traitThe alleles segregate (separate) during gamete formationEach gamete contains only one allele from each pairFertilization gives the offspring two factors for each trait
6 Modern Genetics ViewEach trait in a pea plant is controlled by two alleles (alternate forms of a gene)Dominant allele (capital letter) masks the expression of the recessive allele (lower-case)Alleles occur on a homologous pair of chromosomes at a particular gene locusHomozygous = identical allelesHeterozygous = different alleles
8 Genotype versus Phenotype Refers to the two alleles an individual has for a specific traitIf identical, genotype is homozygousIf different, genotype is heterozygousPhenotypeRefers to the physical appearance of the individual
10 Punnett SquareTable listing all possible genotypes resulting from a crossAll possible sperm genotypes are lined up on one sideAll possible egg genotypes are lined up on the other sideEvery possible zygote genotypes are placed within the squares
11 Punnett SquareCan easily calculate probability, of genotypes and phenotypes among the offspringPunnett square in next slide shows a 50% (or ½) chanceThe chance of E = ½The chance of e = ½An offspring will inherit:The chance of EE =½x½=¼The chance of Ee =½x½=¼The chance of eE =½x½=¼The chance of ee =½x½=¼
13 Monohybrid Test crossTest cross determines genotype of individual having dominant phenotypeBased on the knowledge that individuals with recessive phenotype always known homozygous recessive genotype
14 Monohybrid Test crossHowever, individuals with dominant phenotype have unknown genotypeMay be homozygous dominant, or Heterozygous
15 Two-Trait Inheritance Dihybrid cross uses true-breeding plants differing in two traitsObserved phenotypes among F2
16 Law of Independent Assortment The pair of factors for one trait segregate independently of the factors for other traitsAll possible combinations of factors can occur in the gametes
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18 Sample Dihybrid Problem Tall is dominant to short and purple is dominant to white in pea plants. Cross two plants that are heterozygous for both traits.
19 Chi-Square Chi-Square Tutorial In the garden pea, yellow cotyledon color is dominant to green, and inflated pod shape is dominant to the constricted form. Considering both of these traits jointly in self-fertilized dihybrids, the progeny appeared in the following numbers:193 green, inflated; 184 yellow constricted556 yellow, inflated; 61 green, constrictedDo these genes assort independently? Support your answer using Chi-square analysis.
20 Gene Linkage1. How are the scientists results different from Mendel’s work?2. How did they explore their results?3. How did the scientists explain their findings?Genetic linkage is very strong for genes which are located close to each other on the same chromosome. What happens in the case of two genes which are far apart on the same chromosome?
21 Are all alleles completely dominant or recessive? Incomplete DominanceThe heterozygous is in between the homozygous individuals.Phenotype reveals genotype without test cross
22 ExampleCross two pink flowers. What genotypes and phenotypes are possible?
23 Co-DominantBoth alleles are expressed in the phenotype.
24 ExampleIn humans blood type AB (IAIB) is codominant. What blood types are possible if two people with AB blood type have children?
25 Do any genes have more than two alleles? Multiple AllelesThe gene has several allelic formsBut each individual still receives two
26 PracticeMom is Type A and Dad is Type B, what are all the possible blood types for their children?
27 Is each phenotypic trait influenced by only one gene? PolygenicA trait is governed by two or more genes having different allelesEach dominant allele has a quantitative effect on the phenotypeThese effects are additiveResult in continuous variation of phenotypes
29 For genes that are on the X chromosome in humans and other mammals, what are the differences in inheritance for males vs. females?
30 X – Linked Inheritance In mammals The X and Y chromosomes determine genderFemales = XXMales = XY
31 X-linked = genes that have nothing to do with gender Carried on the X chromosome and the Y does not have these genes.Discovered in the early 1900s by a group at Columbia University, headed by Thomas Hunt Morgan.
32 Human X-Linked Disorders Several X-linked recessive disorders occur in humans:Color blindnessThe allele for the blue-sensitive protein is autosomalThe alleles for the red- and green-sensitive pigments are on the X chromosome.Menkes syndromeCaused by a defective allele on the X chromosomeDisrupts movement of the metal copper in and out of cells.Muscular dystrophyWasting away of the muscleAdrenoleukodystrophyX-linked recessive disorderFailure of a carrier protein to move either an enzyme or very long chain fatty acid into peroxisomes.HemophiliaAbsence or minimal presence of a clotting factor VIII, or clotting factor IXAffected person’s blood either does not clot or clots very slowly.
33 PracticeCross a male hemophiliac with a female whose father was a hemophiliac. What are the possible genotypes and phenotypes of their children?
34 Human Genetic Disorders/Pedigrees Autosomal – any gene not on the sex chromosomesDominant genetic disorder (brittle bone)AA and Aa have the disorderRecessive genetic disorder (cystic fibrosis)aa has the disorder
37 Autosomal Recessive Disorders Tay-Sachs DiseaseProgressive deterioration of psychomotor functionsCystic FibrosisMucus in bronchial tubes and pancreatic ducts is particularly thick and viscousPhenylketonuria (PKU)Lack enzyme for normal metabolism of phenylalanine
39 Autosomal Dominant Disorders NeurofibromatosisTan or dark spots develop on skin and darkenSmall, benign tumors may arise from fibrous nerve coveringsHuntington DiseaseNeurological disorderProgressive degeneration of brain cellsSevere muscle spasmsPersonality disorders
40 Pleioptropic EffectsPleiotropy occurs when a single mutant gene affects two or more distinct and seemingly unrelated traits.Marfan syndrome have disproportionately long arms, legs, hands, and feet; a weakened aorta; poor eyesight
44 Terminology Pleiotropy Codominance Epistasis A gene that affects more than one characteristic of an individualSickle-cell (incomplete dominance)CodominanceMore than one allele is fully expressedABO blood type (multiple allelic traits)EpistasisA gene at one locus interferes with the expression of a gene at a different locusHuman skin color (polygenic inheritance)
45 Review Blending Inheritance Monohybrid Cross Modern Genetics Law of SegregationModern GeneticsGenotype vs. PhenotypePunnett SquareDihybrid CrossLaw of Independent AssortmentHuman Genetic Disorders