Observing Patterns In Inherited Traits Ø Where Modern Genetics Started/ Gregor Mendel Ø Law of Segregation Ø Law of Independent Assortment Ø Non-Mendelian Inheritance Ø Complex Variations in Traits
Genetics: The Scientific Study of Heredity and Hereditary Variation
19 th century biologists: Blending hypothesis Hereditary material from both parents mix in forming offspring Failed to explain how traits that are not present in parents appear in the children (freckles)
Gregor Mendel: Father of Modern Genetics
Pisum sativum (garden peas Developed two laws of genetics based on his crossing experiments: Law of segregation Law of independent assortment
Why Garden Peas? Short generation time Available in many varieties Self pollinated Could control crossing Could start with true breeding varieties
Mendel s Experiments Crossing experiments using true breeding varieties of peas
Inheritance in Modern Terms Ø Individuals share certain traits because their chromosomes carry the same genes (same or different alleles) Ø The DNA sequence of each gene occurs at a specific location: locus Ø An individual carrying identical alleles for a gene is homozygous Ø An individual carrying two different alleles of a gene is heterozygous
Ø Hybrids are heterozygous offspring resulting from a cross between homozygous parents Ø The particular set of alleles that an individual carries is their genotype Ø The observable traits, such as flower color, make up an individual s phenotype Ø An allele is dominant (A) when its effect masks that of a recessive (a) allele paired with it
Punnett square How Are Alleles Distributed Into Gametes? Ø A grid used to predict the genetic and phenotypic outcome of a cross
Monohybrid Cross A cross where parents are heterozygous for the same trait The starting cross is between two true breeding varieties (homozygous)
Question: What is the outcome of a cross between a pea plant with purple flowers with one that has white flowers? Possible outcomes: 1) If blending hypothesis is true, then all plants would be of an intermediate color: light purple 2) Some plants will be purple, others will be white 3) All plants will be purple or all plants will be white
Outcomes of Mendel s Monohybrid Crosses In the first generation Only one trait appeared: the purple flower color Purple: dominant trait White: recessive trait
In the second generation/ Monohybrid cross White flowers were present The F1 plants with purple flowers must be carrying the genetic information to produce white flowers
Mendel always observed the 3:1 phenotypic inheritance pattern among the F 2 offspring for the various crosses he made Hereditary information passes from one generation to another in discrete units (alleles)
First law of Genetics: The Law of Segregation Separation of alleles during the formation of gametes Meiosis I (separation of homologous chromosomes) After fertilization, the paired condition is restored
A cross where the parents are heterozygous for two traits Dihybrid Cross
Outcomes of Mendel s Dihybrid Crosses Mendel always observed the 9:3:3:1 phenotypic inheritance pattern among the F 2 offspring for the various crosses dihybrid crosses he made
Second Law of Genetics Law of Independent Assortment During meiosis, members of a pair of genes (alleles) on homologous chromosomes get distributed into gametes independently of other gene pairs Independent assortment of chromosomes at Metaphase I of meiosis
An example of using Mendelian genetics to determine unknown genotypes Cross an individual of unknown genotype with the homozygous recessive Testcross Phenotypes of offspring indicate the genotype of the parent
Please make sure you know these terms Chromosome Homologous chromosomes Locus Gene Alleles Dominant Recessive Homozygous Heterozygous Phenotype Genotype F1 and F2 generations Punnett square
Are all genes inherited in a Mendelian pattern? Ø Codominance Ø Incomplete dominance Ø Epistasis Ø Pleiotropy Ø Multiple alleles for a single gene
Codominance
Incomplete Dominance Incomplete dominance results in intermediate phenotypes Phenotypic ratio 1:2:1
Epistasis When a trait is influenced by the products of other genes
Pleiotropy One gene influencing many characters Sickle cell disease
Do All Traits Occur in Distinct Forms? Many traits show a continuous range of variation The additive effects of two or more genes on a single phenotypic character (polygenic inheritance) Human skin color Human height Human eye color
Figure 13-15 p216
Genes with multiple alleles may also result in continuous variations Short tandem repeats Ø Some genes have regions of DNA in which a series of two to six nucleotides is repeated hundreds or thousands of times in a row Ø Example: 12 alleles of a gene that influences face length in dogs
Nature and Nurture But remember that only genetic influences are inherited Examples of characters: Risk of heart disease Risk of cancer Flower color in Hydrangeas Skin color Examples of environmental factors: Nutrition, exercise level Pollutants Soil ph Sun exposure Acid Neutral/Alkaline