Patterns of Inheritance Chapter 8 Copyright © McGraw-Hill Companies Permission required for reproduction or display Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Mendel and the Garden Pea Advantages of Garden Pea for research: Many hybrids previously produced. Large number of true-breeding varieties. Small and easy to grow. Short generation time. Sexual Organs enclosed in flower. Self-fertilization Cross fertilization Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Mendel and the Garden Pea 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. Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Mendel and the Garden Pea F1 Generation (First Filial) Offspring of white flower and purple flower cross had flower color resembling one parent (no blending inheritance). All flowers exhibited purple flowers (dominant trait) and none exhibited white flowers (recessive trait). Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

F2 Generation (Second Filial) Mendel’s Results F2 Generation (Second Filial) Cross between seeds of F1 generation produced some plants exhibiting white flowers (recessive form reappeared). Dominant : Recessive ratio among F2 plants was always close to 3:1. Mendelian Ratio Discovered ¼ of recessives were always true breeding. Disguised 1:2:1 ratio Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

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

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

Interpretation of Mendel’s Results F1 Generation PP x pp (parental generation) yielded all Pp offspring. F2 Generation Pp x Pp yielded: (1:2:1) ratio 1 PP 2 Pp 1 pp Punnett Squares Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Mendelian Inheritance Mendel’s First Law of Heredity (Law of Segregation) Alternative alleles of a character segregate from each other in heterozygous individuals and remain distinct. Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

TestCross 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 Mendel’s Second Law of Heredity (Law of Independent Assortment) Genes that are located on different chromosomes assort independently of one another. Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Mendelian Inheritance Phenotype Considerations Continuous Variation The greater the number of genes influencing a character, the more continuous the expected distribution of character variation will be. Example: eye color, hair color, height Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Phenotypic Considerations Incomplete Dominance Heterozygotes are intermediate in color. 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

Not all defects are recessive. Huntington’s Disease Gene Disorders Gene Disorder refers to the harmful effect a detrimental allele produces when it occurs at a significant frequency in a population. Most disorders are rare because affected individuals often die at a relatively young age, or cannot reproduce. Not all defects are recessive. Huntington’s Disease Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Multiple Alleles: ABO Blood Group Codominance - No single allele is dominant, and each allele has its own effect. ABO Blood Groups Human gene that encodes enzyme that adds sugar molecules 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

Mutations are accidental changes in genes. Pedigrees Mutations are accidental changes in genes. Rare, random, and usually result in recessive alleles. Pedigrees used to study heredity. 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

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 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. Problems quickly arose in trying to track independent assortment. Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Chromosomes and Inheritance 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

Chromosomes and Inheritance 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

Can be used to determine sex of the individual Karyotype Analysis Karyotype-photograph of a person’s chromosomes during metaphase of mitosis Can be used to determine sex of the individual Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Can be used to find major chromosome errors Karyotype Analysis Can be used to find major chromosome errors Trisomy 21 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