Patterns of Inheritance Chapter 12 Copyright © McGraw-Hill Companies Permission required for reproduction or display Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Early Ideas of Heredity Mendel Gene Disorders Multiple Alleles Outline Early Ideas of Heredity Mendel Gene Disorders Multiple Alleles Pedigrees Gene Disorders Due to Protein Alteration Chromosome and Inheritance Genetic Recombination Human Chromosomes Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Early Ideas of Heredity Classical Assumption Heredity occurs within boundaries of a particular species. Traits are transmitted directly from parents to offspring. Paradox –If no variation enters a species from the outside, then all members of same species should eventually have the same appearance. Hybrids ? Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

(Early Ideas of Heredity) Early geneticists demonstrated some forms of an inherited character can: Disappear and then reappear in future generations. Segregate among offspring. Be more likely to be represented than alternative forms. Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Mendel’s use of Mathematics to study the Garden Pea Advantages of Garden Pea : Production of hybrids allowed segregation of traits. Large number of true-breeding varieties . Small and easy to grow. Short generation time. Sexual Organs enclosed in flower. Self-fertilization Cross fertilization

(Mendel’s Work) Mendel’s Experimental Design Allowed pea plants to self-fertilize for several generations. Assured constant traits. Performed crosses between varieties exhibiting alternative character forms. Permitted hybrid offspring to self-fertilize for several generations.

F1 Generation (First Filial) Mendel’s Results: F1 Generation (First Filial) Offspring of white flower and purple flower cross had flower color resembling one parent (no blending inheritance). Result: All plants exhibited purple flower color (dominant trait) and none exhibited white flower color (recessive trait). Genotype: 4PP Genotypic Ratio 4:0 Phenotype: 4 purple Phenotypic Ratio 4:0 Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

(Mendel’s Results) F2 Generation (Second Filial) Cross between seeds of F1 generation produced some plants exhibiting white flower color (recessive form reappeared). Dominant : Recessive ratio among F2 plants was always close to 3:1 (phenotypic ratio). Mendelian Ratio Discovered ¼ of recessives were always true breeding. Resulted in 1:2:1 ratio (genotypic ratio) that is disguised

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Mendel’s Model of Heredity Parents transmit discrete physiological trait information (factors) to offspring. Factors = genes=Alleles Each individual receives two factors that may code for same, or alternative, character traits. Not all copies of a factor are identical. Homozygous - Same alleles. Heterozygous - Different alleles. Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

(Mendel’s Model of Heredity) The two alleles do not influence each other in any way. Presence of a particular allele does not ensure its encoded trait will be expressed in an individual carrying that allele. Genotype - Totality of alleles (PP or Pp). Phenotype - Physical appearance (purple/white). Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Mendelian Inheritance Mendel’s First Law of Heredity: Law of Segregation Two alleles of a gene segregate during gamete formation (meiosis) and are rejoined at random, one from each parent, during fertilization Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Interpretation of Mendel’s Results Notational Convention (Punnet Square- Monohybrid Cross) P = purple (Dominant allele) p = white (Recessive allele) Gentoypes: PP - Homozygous Dominant Pp – Heterozygous pp - Homozygous Recessive Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Interpretation of Mendel’s Results P Generation (parental generation) PP x pp yielded all Pp offspring (F1) - Genotypic ratio: 4:0 Phenotypic ratio: 4:0 F1 Generation = 1st filial generation Pp X Pp yielded PP, Pp, and pp offspring (F2) Genotypic ratio: 1:2:1 Phenotypic ratio: 3:1 F2 Generation = 2nd filial generation Pp x Pp yielded: (1:2:1) ratio 1 PP 2 Pp 1 pp Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Pedigrees Used to study heredity Allow deduction of trait Inheritance patterns in families Mutations are accidental changes in genes. Rare, random, and usually result in recessive alleles. Hemophilia - Inherited condition where blood is slow to clot or does not clot at all. Only expressed when individual has no copies of the normal allele. Royal Hempohilia - Sex-linked Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Pedigrees

Dihybrid Crosses Involves 2 traits Genotypes: Phenotypes: Flower color: P= purple; p= white Height: T= tall; t= short Heterozygous purple tall x Heterozygous purple tall PpTt x PpTt Possible allelic combinations: PT, Pt, pT, pt ♂ ♀ PT Pt pT pt Genotypic ratio: Phenotypic ratio: Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Mendelian Inheritance Mendel’s Second Law of Heredity Law of Independent Assortment In a diahybrid cross, the alleles of each gene assort independently of one another; the segregation of different allele pairs is independent. Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Rules of Probability 1. Rule of Multiplication 2. Rule of Addition Cross: round yellow seeds What is the probability of round yellow (R_Y_)? What is the probability of round green (R_yy)? What is the probability of wrinkled yellow (rrY_)? Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Rules of Probability 1. Rule of Multiplication 2. Rule of Addition A trihybrid cross in which one parent has purple flowers and yellow round seeds and another parent has purple flowers and green wrinkled seeds. What is the probability of the offspring that would exhibit the recessive phenotypes for at least two of the three traits? Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

TestCross Is conducted when an individual with an unknown genotype is crossed with a homozygous recessive gentoype If you cross a plant with an unknown genotype (PP or Pp) with a homozygous recessive individual, the two possible genotypes will yield different results. pp x PP = 100% (Pp) pp x Pp = 50% (pp) : 50% (Pp) Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Mendelian Inheritance was not always easy: Some characteristics are govern by Polygenes (many genes that act sequentially or jointly on a characteristic) Vary in the population along a continuum Called Polygenic inheritance or continous Variation Continuous Variation The greater the number of genes influencing a character, the more continuous the expected distribution of character variation will be. Examples: hair and skin color; height; weight Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Pleiotrophic Effects Individual alleles often have more than one effect on the phenotype; ability of a gene to have multiple phenotypic effects Examples: cystic fibrosis- symptoms are pleiotropic effects of single defect of the gene that encodes a chloride ion transmembrane channel Sickle cell anemia – defect in the hemoglobin molecule causes anemia, heart failure, susceptibility to pneumonia, kidney failure, enlargement of the spleen Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Phenotypic Considerations Incomplete Dominance Heterozygotes are intermediate in color. Codominace Most genes in a population posses several different alleles, and often no single allele is dominant both are sometimes expressed in heterozygotes Environmental Effects Degree of allele expression may depend on the environment. Epistasis One gene interferes with the expression of another gene. Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

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Multiple Alleles: ABO Blood Group Condominant - No single allele is dominant, and each allele has its own effect. ABO Blood Groups Human gene that encodes enzyme that adds sugar molecules (cell markers) to lipids on the surface of red blood cells. IB adds galactose IA adds galactosamine i adds no sugar Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Human Genetics ≈ Mendelian Principles Gene Disorder refers to the harmful effect a detrimental allele produces when it occurs at a significant frequency in a population. Homozygous recessive Gene Disorders Most gene disorders are rare because affected individuals often die at a relatively young age, or cannot reproduce. Tay-Sachs disease Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Human Genetics ≈ Mendelian Principles Not all gene disorders are recessive. Huntington’s Disease – caused by a dominant allele Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Gene Disorders Due to Protein Alteration Sickle-Cell Anemia is a recessive inherited disorder in which afflicted individuals have defective hemoglobin, and thus are unable to properly transport oxygen to tissues. Homozygotes have Sickle-Cell. Heterozygotes usually appear normal, but are resistant to malaria. Copyright © McGraw-Hill Companies Permission required for reproduction or display Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Curable Defects in Future Cystic Fibrosis Body cells of affected individuals secrete thick mucus that clogs airways of lung. Defect in cf gene. Researchers currently working on methods of transmitting a working copy of cf gene via viruses. Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Chromosomes and Inheritance In early 20th century, it was not obvious chromosomes were vehicles of heredity information. Chromosomal Theory of Inheritance first formulated in 1902. (Walter Sutton) Evidence: 1. Diploid/haploid cells 2. chromosomes segregate during meiosis Problems : Why does the # of characters that assort independently in a given kind of organism often exceed the # of chromosome pairs the organism posses? Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Chromosomes and Inheritance Thomas Hunt Morgan (1901) A trait determined by a gene on the sex chromosome is said to be sex-linked. In Drosophila, sex is determined by the number of copies of the x chromosome. Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Genetic Recombination Crossing Over Studies in 1931 conclusively found evidence of crossing over. If two different genes are located relatively far apart on a chromosome, crossing over is more likely to occur than if they are close together. Frequency of crossings can be used to construct a genetic map. Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Human somatic cells normally have 23 pairs of chromosomes. Human Chromosomes Human somatic cells normally have 23 pairs of chromosomes. Divided into seven groups characterized by size and shape. 22 pairs of autosomes 1 pair of sex chromosomes XX = Female XY = Male Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Human Chromosomes One x chromosome in females in inactivated early in embryonic development. Visible as a darkly staining Barr body attached to the nuclear membrane. Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Abnormalities Due to Chromosome Number Failure of chromosomes to separate correctly during meiosis I or II is called primary nondisjunction. Down Syndrome caused by trisomy 21. 1 in 1700 for mothers < 20. 1 in 1400 for mothers >20<30. 1 in 750 for mothers >30<35. 1 in 16 for mothers >45. Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Nondisjunction in Sex Chromosomes XXX or XXY yields Klinefelter syndrome XO yields Turner Syndrome Y Chromosome XYY – Antisocial ? Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

High-Risk Pregnancies Couples with recessive alleles. Genetic Counseling Genetic counseling identifies parents at risk of producing children with genetic defects and assesses the state of early embryos. High-Risk Pregnancies Couples with recessive alleles. Mothers older than 35. Amniocentesis Chorionic villi sampling Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Aneuploidy or gross alterations. Proper enzyme functioning. Genetic Counseling Counselors can look for three things in cell cultures in search of genetic disorders: Aneuploidy or gross alterations. Proper enzyme functioning. Association with known genetic markers. Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Early Ideas of Heredity Mendel Gene Disorders Multiple Alleles Review Early Ideas of Heredity Mendel Gene Disorders Multiple Alleles Pedigrees Gene Disorders Due to Protein Alteration Chromosome and Inheritance Genetic Recombination Human Chromosomes Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Copyright © McGraw-Hill Companies Permission required for reproduction or display Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies