Chapter 14

Genetic Variation Heritable variations observed daily in populations Transmitted from parents to offspring ‘Blending hypothesis’ Genetic material mixes and eventually produces uniformity Breeding experiments and everyday observances disprove ‘Particulate hypothesis’ Discrete units called ‘genes’ passed from parents to offspring Retain separate identities generation to generation

Gregor Mendel Monk considered father of genetics Used peas to demonstrate ‘particulate’ mechanism Characters (hair color) with different traits (blonde or brown) Controllable breeding Hermaphrodites that commonly self-fertilize; could cross- fertilize Lots of offspring with short generation time Chose ‘either or’ not ‘more or less’ characters Cross-pollinate (hybridize) contrasting true-breeding lines Parents are P generations and offspring are F1 generation F2 generation to deduce 2 fundamental principles Law of Segregation and Law of Independent Assortment

Mendel’s Work

Useful Genetics Terminology

Alleles Different versions of a gene Dominant mask other when both are present (P) Recessive are easily masked by others (p) Dominance implies it determines phenotype, not superiority or increased prevalence Recessive traits often more common Actual combination determines genotype Resulting physical expression determines phenotype

Genotype versus Phenotype Homozygous dominant (PP) 2 dominant alleles Express dominant Heterozygous (Pp) 1 dominant and 1 recessive allele Express dominant Homozygous recessive (pp) 2 recessive alleles Express recessive

Punnett Squares Predicts the allelic composition of offspring from a cross between individuals of known genotype Testcross between an unknown dominant phenotype and a recessive homozygote Monohybrid cross deals with only 1 character at a time Dihybrid cross deals with 2 characters at the same time

Testcross

Monohybrid Cross Each parent contributes only 1 type of allele to gamete F1 heterozygotes contribute 50% of each type of allele F2 offspring have a 3:1 ratio

Dihybrid Cross F2 offspring with 9:3:3:1 phenotypic ratio Same as monohybrid for each of 2 characters Only for nonhomologs

Principles of Heredity Law of Segregation Two alleles for a heritable character separate during gamete formation and end up in different gametes Law of Independent Assortment Each pair of alleles segregate independently of other pairs of alleles during gamete formation

Reviewing Concepts What is a true-breeding individual and how does this relate to allelic composition? Assume brown is the dominant character for eye color, what case letter would represent the allele? What are the possible genotype(s) for a brown eyed individual? If a person’s phenotype is blue eyes, what is their genotype and what term most accurately applies to them?

Extending Mendelian Genetics Mendel used characters exhibiting complete dominance, offspring like one of two parents Not applicable to all characters Genotype and phenotype relationship not all so simple Single genes can have alleles that aren’t completely dominant or recessive Characters can have 1+ genes Basic principles of segregation and independent assortment persist

Incomplete Dominance Offspring phenotype is between both parental types Doesn’t support blending Each genotype has own phenotype

Codominance Both alleles contribute to the phenotype Blood type demonstrates multiple alleles too What blood type is the universal donor? What blood type is the universal acceptor?

Other Patterns of Inheritance Epistasis: gene at one locus alters the phenotypic expression of another gene at a second locus Polygenic Inheritance: an additive effect of 2+ genes on one phenotypic trait; opposite of pleiotropy (1 gene = multiple phenotypes)

Pedigree Analysis Can’t control human matings, so look at existing Trait information from phenotypes of a family Arrange into a pedigree Can deduce genotypes of most from phenotypes Mendelian genetics and logic Is this a dominant or recessive trait?

Genetic Disorders Recessive only affect homozygous recessives Heterozygotes are carriers Prevents complete removal of allele from a population Dominant affect all but homozygous recessive Lethal forms less common When rare unlikely 2 carriers will mate Inbreeding increases rate of occurrence More likely both carrying recessive allele Albinism Achondroplasia

Autosomal Recessive Cystic fibrosis Most common lethal genetic disease in US 2 recessive alleles have defective or lack chloride transport channels Cause mucus build up in multiple organs, has pleiotropic effects Sickle cell anemia Most common among people of African descent Only homozygous recessive have full blown disease Heterozygous only when O2 exposure down Protection against malaria increases incidence of

Autosomal Dominant Can persist if death occurs only after advanced age Genes passed on to offspring already Huntington’s disease Degeneration of nervous system No phenotypic effect till years old Can now test for genetically before symptoms occur

Genetic Counseling Allows people with family histories of genetic disorders to make informed decisions Many types of genetic disorders can now be tested for Potential problems inherent in medical records Fetal testing Amniocentesis extracts amniotic fluid to be tested Chronic villi sampling extracts cells from placenta, but can be done sooner Karyotyping to view chromosomal abnormalities Ultrasounds produce anatomical images, no risks Biochemical testing for certain genetic markers