Genetics
Early Ideas about Heredity People knew that sperm and eggs transmitted information about traits Blending theory mother and father s traits blended together Problem: Would expect variation to disappear Variation in traits persists (Example: freckles show up in unfreckled parents offspring!) + =
Mendel s Experimental Approach Mendel was a monk with training in plant breeding and mathematics considered to be the founder of modern genetics! (1822 1884) He studied the garden pea (Pisum sativum), which breeds true for a number of traits
The Garden Pea Plant Self-pollinating True breeding (different alleles not normally introduced) Can be experimentally crosspollinated excellent for genetic manipulation!!! Small, easy to grow, can self-fertilize, easily distinguishable traits!
Terms Used in Modern Genetics Genes Heritable units of information about traits Parents transmit genes to offspring Each gene has a specific locus on a chromosome Diploid cells (chromosome number 2n) have pairs of genes on homologous chromosomes
Terms Used in Modern Genetics A mutation is a permanent change in a gene May cause a trait to change Alleles are different molecular forms of a gene A hybrid has nonidentical alleles for a trait Offspring of a cross between two individuals that breed true for different forms of a trait are hybrids
Terms Used in Modern Genetics An individual with nonidentical alleles of a gene is heterozygous for that gene An individual with identical alleles of a gene is homozygous for that gene
Terms Used in Modern Genetics An allele is dominant if its effect masks the effect of a recessive allele paired with it Capital letters (A) signify dominant alleles; lowercase letters (a) signify recessive alleles Homozygous dominant (AA) Homozygous recessive (aa) Heterozygous (Aa)
Terms Used in Modern Genetics Gene expression The process by which information in a gene is converted to a structural or functional part of a cell or body Expressed genes determine traits
Terms Used in Modern Genetics Genotype The particular alleles an individual carries Phenotype An individual s observable traits
Terms Used in Modern Genetics P stands for parents, F for filial (offspring) AA x aa F1: First generation offspring of parents Aa x Aa F2: Second generation offspring of parents AA, Aa, Aa, aa
Key Concepts Where Modern Genetics Started Gregor Mendel gathered the first experimental evidence of the genetic basis of inheritance His meticulous work gave him clues that heritable traits are specified in units The units, which are distributed into gametes in predictable patterns, were later identified as genes
Mendel s Law of Segregation Garden pea plants inherit two units of information for a trait, one from each parent
Monohybrid Experiments Monohybrid experiments Testcrosses that check for a dominance relationship between two alleles at a single locus May be crosses between true breeding (homozygous) individuals (AA x aa), or between identical heterozygotes (Aa x Aa)
Mendel s Monohybrid Experiments Mendel used monohybrid experiments to find dominance relationships among pea plant traits When he crossed plants that bred true for white flowers with plants that bred true for purple flowers, all F1 plants had purple flowers When he crossed two F1 plants, ¾ of the F2 plants had purple flowers, ¼ had white flowers
Segregation of Alleles at a Gene Locus
Mendel s Monohybrid Experiments
Calculating Probabilities Probability A measure of the chance that a particular outcome will occur Punnett square A grid used to calculate the probability of genotypes and phenotypes in offspring
Construction of a Punnett Square
Phenotype Ratios in a Monohybrid Experiment
Phenotype Ratios in a Monohybrid Experiment
Mendel s Law of Segregation Mendel observed a phenotype ratio of 3:1 in the F2 offspring of his monohybrid crosses Consistent with the probability of the aa genotype in the offspring of a heterozygous cross (Aa x Aa) This is the basis of Mendel s law of segregation Diploid cells have pairs of genes on pairs of homologous chromosomes The two genes of each pair separate during meiosis, and end up in different gametes
Key Concepts Insights from Monohybrid Experiments Some experiments yielded evidence of gene segregation: When one chromosome separates from its homologous partner during meiosis, the alleles on those chromosomes also separate and end up in different gametes http://www.youtube.com/v/f3akldl6jzg - practice with Punnett Squares!!! To be done on your own or with me in office hours!!!
Mendel s Law of Independent Assortment Mendel s law of independent assortment Many genes are sorted into gametes independently of other genes
Dihybrid Experiments Dihybrid experiments Tests for dominance relationships between alleles at two loci Individuals that breed true for two different traits are crossed (AABB x aabb) F2 phenotype ratio is 9:3:3:1 (four phenotypes) Individually, each dominant trait has an F2 ratio of 3:1 inheritance of one trait does not affect inheritance of the other
Independent Assortment at Meiosis
Mendel s Dihybrid Experiments
Mendel s Law of Independent Assortment Mendel s dihybrid experiments showed that units specifying one trait segregated into gametes separately from units for other traits Exception: Genes that have loci very close to one another on a chromosome tend to stay together during meiosis
Key Concepts Insights from Dihybrid Experiments Some experiments yielded evidence of independent assortment: Genes are typically distributed into gametes independently of other genes
Beyond Simple Dominance Mendel focused on traits based on clearly dominant and recessive alleles; however, the expression patterns of genes for some traits are not as straightforward
Continuous Variation Continuous variation Traits with a range of small differences The more factors that influence a trait, the more continuous the distribution of phenotype
Incomplete Dominance Incomplete dominance One allele is not fully dominant over its partner The heterozygote s phenotype is somewhere between the two homozygotes, resulting in a 1:2:1 phenotype ratio in F2 offspring Example: Snapdragon color RR is red Rr is pink rr is white
Incomplete Dominance in Snapdragons
Genes and the Environment Expression of some genes is affected by environmental factors such as temperature, altitude, or chemical exposure The result may be variation in traits
Effects of Temperature on Gene Expression Enzyme tyrosinase, works at low temperatures
Codominance in ABO Blood Types Codominance Two nonidentical alleles of a gene are both fully expressed in heterozygotes, so neither is dominant or recessive May occur in multiple allele systems Multiple allele systems Genes with three or more alleles in a population Example: ABO blood types
Codominance in ABO Blood Types
ABO Blood Type: Glycolipids on Red Cells Type A - Glycolipid A on cell surface Type B - Glycolipid B on cell surface Type AB - Both glyocolipids A & B Type O - Neither glyocolipid A nor B
Regarding the Unexpected Phenotype Phenotype results from complex interactions among gene products and the environment identical twins!!! Enzymes and other gene products control steps of most metabolic pathways Mutations, interactions among genes, and environmental conditions may affect one or more steps
Key Concepts Variations on Mendel s Theme Not all traits appear in Mendelian inheritance patterns An allele may be partly dominant over a nonidentical partner, or codominant with it The environments also influences gene expression
Chromosomal Abnormalities some examples Errors sometimes occur during meiosis Down Syndrome extra copy of Chromosome #21 Klinefelter Syndrome (XXY male) sterile male with female characteristics and diminished mental capacity (extra X). Normally XX and XY Turner Syndrome (XO female) sterile female with webbed neck and diminished stature. Missing all or part of 2nd X chromosome