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Ch. 14 and 15- Gregor Mendel genetics and the chromosomal inheritance patterns

Ch. 14 and 15- Gregor Mendel genetics and the chromosomal inheritance patterns. Terms Character-inheritable feature Trait-variant for character Gene-unit of hereditary information Allele-version of a gene Dominant allele Recessive allele Phenotype-physical characteristics

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Ch. 14 and 15- Gregor Mendel genetics and the chromosomal inheritance patterns

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  1. Ch. 14 and 15- Gregor Mendel genetics and the chromosomal inheritance patterns

  2. Terms • Character-inheritable feature • Trait-variant for character • Gene-unit of hereditary information • Allele-version of a gene • Dominant allele • Recessive allele • Phenotype-physical characteristics • Genotype-genetic makeup • Homozygous-pair of identical alleles • Heterozygous-2 different alleles

  3. Mendel’s Experiments • Hybridization* • Started with a P generation (parental) • This must be true-breeding • All offspring will be same variety as parents • Also called purebred • Are Homozygous • F1-first generation of offspring • Should be heterozygous • Also called hybrids • F2-second generation of offspring • Shows the phenotypic ratios of crossing 2 hybrids

  4. Test Cross** • Breeding of an animal with recessive trait to determine if dominant trait is homozygous or heterozygous • B?xbb • Determine second allele based on offspring • Monohybrid Cross • Crossing 2 heterozygous while comparing only one characteristic • Bb x Bb • F2 has a 3:1 ratio • Dihybrid Cross • Crossing 2 heterozygous while comparing 2 traits • BbDd x BbDd • F2 has a 9:3:3:1 ratio

  5. Mendel’s laws • Law of Segregation • The 2 alleles of a particular gene from one parent will get placed into separate gametes • Law of independent assortment • Each trait will segregate independent of one another

  6. Rules of probability • Rules of Addition • The probability of an event that can occur in two different ways is the sum of the separate probabilities. • Rule of Multiplication • Multiply the chance that 2 individual events will occur • Ex. Flipping a coin • What is the chance that the coin will land on heads twice? • ½ x ½ = ¼ • What is the chance that 2 individuals that are both Bb with each give a b allele to their offspring • ½ x ½ = ¼ • Determining pheno or genotypes of dihybrids • Determine percentage of trait inheritance through multiple generations

  7. Inheritance patterns • Complete dominance • Dominant vs. recessive • Dominant alleles do not subdue recessive alleles • Both alleles coexist in heterozygous, but do not interact • Typically a recessive allele will code for a defective gene • Incomplete dominance • F1 has an intermediate phenotype of the parental • Codominance • Both alleles are expressed in some degree • **all alleles are expressed in some fashion as dominant/ incomplete/ and codominant simultaneously**

  8. Multiple alleles • More than 2 alleles are present in population • Individuals still only contain 2 of the alleles • Ex. Blood type, IA, IB, i. • Sex-Linked traits (Ch 15) • Discovered by Thomas Hunt Morgan with fruit flies • Keyed the term wild type • Phenotype found in nature • Can be dominant or recessive (used on all inheritance patterns) • Genes located on the X sex chromosomes • F1 generation looks normal when male has recessive sex linked trait • F2, recessive only showed up in males • If female start with recessive sex linked trait, F1 will not be all hybrids • Males can pass on Y-linked genes to sons (not in book)

  9. X-inactivation • One X chromosome may be inactivated in females during embryological development • Called a Barr body • Will lead to expression of recessive X alleles in females • Pleiotrophy • Some genes can cause multiple phenotypes • Epistasis • One gene alters the expression of another gene (typically blocking the expression of) • Ex. Mice • B is black fur, b is brown fur, C allows for color to be deposited, c stops deposition of color • BbCc= black, Bbcc=white, bbCc=brown, bbcc=white

  10. Polygenic • Involve quantitative characters • Traits vary along a continuum • Skin color is controlled by at least 3 genes that are all incompletely dominant • Homozygous lethality (not in book) • The Homozygous Dominant individual does not survive • Bb x Bb = 2:1 ratio of dom/rec phenotypes • Nature vs. Nurture • How much of phenotype is genes, and how much is environment • Norm of reaction-range of phenotype due to environment • some norm or reactions are small, where others are large • Multifactorial-many factors affect phenotype

  11. Chi-Square Analysis (not in book) • Test to determine statistical significance of data • Is the data accurate enough? • See handout • Pedigrees-tracking of a particular trait through a family tree

  12. Linked Genes (ch. 15) • Terms • Parental types-offspring that look like P generation (grandparents) • Recombinants-Offspring have mixed genes of P generation • Experiment • Parental generation of a pure breeding wild type (dom) crossed with a pure breeding recessive for 2 trait • Then F1 is test crossed

  13. Analysis of test cross with F1 offspring • If 50% of offspring are recombinants then there is independent assortment of genes (genes are not linked) • 0% recombinants mean genes are completely linked (they do not sort independently) • If % are recombinants then crossing over occurs and they have incomplete linkage (or do not sort independently) • Linkage Map-Using recombinant genes to construct a genetic map (map of genes on chromosome) • Calculate recombinant frequency • # of recombinants/total • do not need to divide by 2 because you are test crossing it • 1 map unit=1% recombination

  14. ex. If gene A and B recombine 20 percent of the time, then they are 20 map units apart • If B and C recombine 10% of the time, they are 10 map units apart, but what side of B • If A and C are 10 map units apart, then the order on the chromosome is ACB, each 10 map units apart • Or A and C can be 30 map units apart, meaning the order is ABC, with 20 map units between AB, 10 between BC, Making AC 30 map units apart. • Won’t always be perfect numbers • Cytological maps-another type of genetic map • Locates genes based on other features of chromosome, not another gene • Typically will use the stained bands seen under microscopes • Comparing cytological and linkage maps, shows genes in same order, but spaced differently

  15. Karyotype alterations (ch. 15) • Nondisjunction • Homologous chromosomes do not separate properly during meiosis I Or sister chromosomes do not separate properly during meiosis II • Aneuploidy-abnormal chromosome number • Trisomic- extra chromosome • Monosomic- missing chromosome • Polyploidy • More than 2 sets of chromosomes • Triploid (3n), tetrapoid (4n) • Common in plants

  16. Chromosomal Alteration • Deletion-missing piece • Duplicated-extra piece • Inversion-section of chromosome detaches and then reattaches to original chromosome, but reversed (not as bad) • Translocation-section detaches and reattaches to non-homologous chromosome (not as bad) • Exon Shuffling (can lead to variation) • Fetal Testing (ch. 14) • Amniocentresis and Chorionic villus sampling • Test to make karyotype of fetus

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