Lecture 7 white squash x white squash WwYy x WwYy Chapter 5: Extensions and Modifications of Basic Principles, Part 2 Problem Set 1B due on Monday Genotype W_Y_ 9/16 W_yy 3/16 wwy_ 3/16 wwyy 1/16 Phenotype white white yellow green 12:3:1 Read First Bacteriophage evolution lab write-up and bring to lab next week. Complementation Test Summary Points 1. Genes are discrete units that control the phenotype of organisms. 2. Inheritance follows the rules of segregation and independent assortment. 3. Dominance is a function of interaction between alleles. 4. Genes control the production of proteins and thus the function of biochemical pathways. 5. Epistasis occurs when one gene masks the effect of another gene. It is thus a function of interaction between genes. Determines whether two independently isolated mutations are at the same locus or different loci. One type of complementation test: Cross two parents that are each homozygous for different mutations. mutation a mutation b If all the offspring are wild-type, each mutation was at a different locus. If all the offspring are mutant, each mutation was at the same locus. Example: white eyed Drosophila mutants and apricot eye color mutants. All offspring have light eyes. 1
Sex-linked characterisitics Sex-influenced characteristics Sex-limited characteristices Sex-influenced characteristics: determined by autosomal genes, but expressed differently in males and females. Sex-limited characteristics: determined by autosomal genes, but expressed only in one sex. Beards in goats: B b B b bearded in males or females B b B + bearded in males, beardless in females B + B + beardless in either sex Pattern baldness in humans is also sex influenced in the same way. AND other factors, such as testosterone, can affect the extent of baldness. What pattern of inheritance is this? Sex influenced. 2
Male-limited precocious puberty: An autosomal dominant allele, P. PP or Pp male: puberty before age of 4. pp male: normal Sex-limited characterisitics PP, Pp, or pp female: normal Cytoplasmic inheritance Due to extranuclear genes that are found in chloroplasts or mitochondria. Usually transferred by the mother (eggs larger, containing these organelles). Random segregation of differing mitochondria or chloroplasts can give widely differing phenotypes. Males in a pedigree would pass this trait AND express it in an autosomal dominant fashion. Females? pass it on? Yes, with expression in their sons in an autosomal dominant fashion. Express it? No pollen plant (male) Leaf variegation in the four-o clock plant Genetic maternal effect The genotype of the mother controls the phenotype of her direct offspring. Often due to some factor stored in the cytoplasm of the mother s egg. Seed plant (female) Enheritance comes completely from the female. Due to a defective chloroplast gene for production of chlorophyll. 3
Shell coiling in the snail, Limnea peregra s + (dextral allele) dominant over s (sinistral allele) male female s + s genotype encodes material for dextral shells Sinistral, because mother is ss: Sinistral, because mother is ss: Dextral because mother is s+s: The genotype of the mother controls the phenotype of her immediate progeny Genomic imprinting Phenotype of offspring depends on whether the altered allele comes from the maternal side or the paternal side. (only the allele inherited from one parent is expressed.) Mechanism: Not completely understood, but involves DNA methylation of either the maternal allele or the paternal allele during development. If gene is methylated in appropriate region, its inactivated. General example: A = functional allele, and a = defective allele (at a locus showing imprinting), where the allele coming from maternal side gets methylated and thus inactivated. mat A/pat a mat ACH3/pat a: defective phenotype mat a/pat A mat ach3/pat A: functional phenotype For some imprinted genes the maternal allele gets imprinted, for other genes the paternal allele gets imprinted. A side note: In X chromosome inactivation, coating of the X by Xist RNA leads to DNA methylation, which is essential for inactivation of the genes on that X. 4
Specific example of genomic imprinting : Prader-Willi syndrome and Angerman syndrome Both due to the same deletion of 15q11-15q13. Paternally inherited deletion: Prader-Willi syndrome: mental retardation, lack of muscle tone, insatiable hunger obesity, tiny hands and gonads. Maternally inherited deletion: Angelman syndrome: jerky, repetitive, lurching body movements; seizures; incoherent speech; loud bursts of laughter. The deleted region has at least two imprinted genes. Anticipation Environmental Effects on Gene Expression The severity of a trait increases or age of onset gets earlier as it is passed from generation to generation. Example: Fragile X syndrome Huntington Disease Grandfather s age of onset: 50 (CAG) 25 Mother s age of onset: 35 (CAG) 50 Child s age of onset: 5 (CAG) 120 Traits showing anticipation: mutant alleles are unstable and may change with each generation. Many are caused by trinucleotide repeats. As the number of repeats goes up, the chance of getting changes in that number increases, and the severity and age of onset gets worse. Temperature sensitive alleles There are lots of gene x environment interations! 5
Concentration here: on discontinuous characteristics Many phenotypes show continuous characteristics These must be described quantitatively, thus often called quantitative characteristics. Coming later this semester to a class near you! 6