Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies Foundations of Genetics Chapter 7 Copyright © McGraw-Hill Companies Permission required for reproduction or display

Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies Outline Gregor Mendel  Pea Experiments  Inheritance Theory Punnett Squares Epistasis Multiple Alleles Sex Linkage Human Heredity  Mutations

Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies Gregor Mendel Monk studied science and mathematics at University of Vienna.  Worked with garden peas to study heredity. - Many varieties available - Infrequent version of a trait - Small and easy to grow - Left alone, flowers do not open

Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies Mendel’s Design Let each variety self-fertilize for several generations (True breeding).  P generation Crossed two pea varieties exhibiting alternative traits such as white versus purple flowers.  Offspring resulted in F 1 generation. Allowed plants produced in crosses to self fertilize and counted numbers of each type of offspring.  F 2 Generation

Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies Copyright © McGraw-Hill Companies Permission required for reproduction or display

Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies Mendel’s Results F1 Generation  All F1 flowers were purple. - Called expressed trait dominant and unexpressed trait (white flowers) recessive. F2 Generation  Approximately 25% of individuals exhibited white flowers.

Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies Mendel’s Results F3 Generation  Ratio of purple:white flowers was 3:1. - Concluded 3:1 ratio in F2 generation was actually a 1:2:1 ratio. - (1) True-breeding dominant: (2) not-true breeding dominant: (1) true-breeding recessive.

Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies Mendel’s Proposed Theory Parents transmit information about traits to offspring (genes). Each parent contains two copies of the factor governing each trait.  Homozygous - same copies  Heterozygous - different copies

Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies Mendel’s Proposed Theory Alternative forms of a factor lead to alternative traits (alleles).  Phenotype - Appearance (Physical expression of genes).  Genotype - Alleles governing appearance. Alleles inherited from each parent do not affect each other. Presence of allele does not ensure trait expression.

Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies Punnett Squares Punnett Squares can be used to visualize crosses between two individuals.  Possible gametes of individual are listed along horizontal side. And possible gametes of the other individual are listed along the vertical side. - Genotypes of potential offspring are represented by cells within square.  Frequency expressed as probability.

Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies Punnett Squares

Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies Testcross Test unknown individual with a homozygous recessive yields two alternatives:  Unknown individual homozygous (PP) PP x pp: all offspring have purple flowers.  Unknown individual heterozygous (Pp) Pp x pp: one-half offspring will have white flowers (pp) and one-half have purple flowers (Pp).

Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies Mendel’s Laws First Law - Segregation  Only one allele specifying an alternative trait can be carried in a particular gamete, and gametes combine randomly in forming offspring. Second Law - Independent Assortment  Genes located on different chromosomes are inherited independently of one another. - Dihybrid Crosses

Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies Epistasis Epistasis - Interaction between products of two genes where one gene modifies the other gene’s phenotypic expression.  Emerson - To produce pigment, a Zea mays plant must possess at least one functional copy of each enzyme gene.

Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies Epistasis

Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies Multiple Alleles Often each allele has its own effect and the alleles are considered codominant.  Human ABO Blood type - Type A individuals only galactosamine - Type B individuals add only galactose - Type AB individuals add both sugars - Type O individuals add neither sugar  Rh Blood Group - Rh cell surface marker

Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies ABO Blood Groups Copyright © McGraw-Hill Companies Permission required for reproduction or display

Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies Genotypic Modifications Pleiotrophic alleles have more than one effect on the phenotype.  May have different effects on different phenotypic consequences.  Characteristic of many inherited disorders. Incomplete Dominance  Some alleles produce a heterozygote phenotype that is intermediate between those of the parents.

Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies Sex Linkage 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.

Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies Human Chromosomes An individual’s particular array of chromosomes is known as a karyotype.  Failure of chromosomes to separate correctly during meiosis I or II is called nondisjunction, which leads to aneuploidy (abnormal number of chromosomes). - Down Syndrome caused by trisomy 21.  Overall odds are 1 in 750.  Much higher in women over 45.

Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies Nondisjunction in Sex Chromosomes X Chromosome  XXX or XXY yields Klinefelter Syndrome  XO yields Turner Syndrome Y Chromosome  XYY - Antisocial ?

Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies Mutations in Human Heredity 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

Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies Sickle-Cell Anemia 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. - Heterozygotes usually appear normal. - Homozygotes have Sickle-Cell, but are resistant to malaria. Copyright © McGraw-Hill Companies Permission required for reproduction or display

Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies Other Disorders Tay-Sachs  Incurable heredity disorder which causes brain to deteriorates. - Nonfunctional form of hexosaminidas A enzyme. Huntington’s Disease  Inherited condition caused by dominant allele causing progressive brain deterioration. - Symptoms usually develop late in life.

Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies Genetic Counseling and Therapy Process of identifying parents at risk of producing children with genetic defects and assessing genetic state of early embryos.  Amniocentesis  Ultrasound  Chorionic Villi Sampling

Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies Review Gregor Mendel  Pea Experiments  Inheritance Theory Punnett Squares Epistasis Multiple Alleles Sex Linkage Human Heredity  Mutations

Johnson - The Living World: 3rd Ed. - All Rights Reserved - McGraw Hill Companies Copyright © McGraw-Hill Companies Permission required for reproduction or display