Gregor Mendel (1822-1884) • Austrian Monk, “Father of Genetics” • Bred Garden Peas (Pisum sativum) • Developed a simple set of rules to accurately predict patterns of heredity which form the basics of genetics • Years later we found that traits are determined by genes encoded in DNA
Heredity History • Heredity – transmission of traits from parents to offspring… before DNA was discovered it was one of the great mysteries of science! • Modeled experiments after British farmer T.A. Knight who bred garden peas and concluded purple flowers show a stronger tendency to appear than white flowers • Mendel used a mathematical approach and counted the number of each kind of offspring
Why did Mendel choose peas? • Many easily distinguishable characteristics • 2 possible traits (forms) of each characteristic • Quantitative – he could count plants with or with out trait • P. sativum were small, easy to grow, mature quickly, and produce lots of offspring • Pea plants can self-pollinate • Male (pollen) and Female (pistil) parts are enclosed in the same flower and it can fertilize itself • Pea plants can cross-pollinate • Transfer pollen from one plant to the pistil of another plant
Mendel’s Experimental Design • Parental Generation (P generation): ensure that ea/plant was true breeding – all offspring display only one form of the characteristics for subsequent generations • First Filial Generation (F1 generation): Mendel cross pollinated 2 plants from P generation w/ contrasting traits, offspring called F1 generation • Second Filial Generation (F2 generation): Mendel allowed the F1 generation to self-pollinate, offspring called the F2 generation • Mendel then counted his results…
Mendel’s Results • F1 • The recessive traits disappears • The expressed trait is said to be dominant • F2 • The recessive trait reappears!! • Mendel obtained a 3:1 ratio of dominant to recessive for each trait of the F2 generation!
Mendel proposed a Theory of Heredity • Parents pass on “units of information” that operate in the offspring to produce a trait (today we know these to be genes!) • For each characteristic there are 2 factors or alleles(1 from mom and 1 from dad) at ea/locus • Homozygous - if 2 of the same alleles are inherited (true-breeding) • Heterozygous – if 2 different alleles are inherited (hybrid) • Genotype – combination of alleles an individual has • Phenotype – physical appearance as a result of the alleles inherited
Mendel’s Theory Became Laws of Heredity • Law of Segregation • The members of each pair of alleles separate when gametes are formed • Law of Independent Assortment • Pairs of alleles separate independently of one another during gamete formation (only applies to genes far apart on the same chromosome or separate chromosomes) • Mendel published paper in 1866 – no interest, rediscovered in early 1900’s
Analyzing Heredity • Use letters to represent alleles • Capital letters represent dominant alleles • Lowercase letters represent recessive alleles • Same letter designates 2 forms of the same trait (letter of dominant trait) • Ex. Tallness in pea plants • T = tall dominant allele • t = short recessive allele
Genotype vs. Phenotype • 2 alleles for each trait make up genotype
Probability • Probability – likelihood that a specific event will occur • Probability = # of specific outcome total # of all possible outcomes • Use this formula to predict the outcome of a genetic cross
Monohybrid Cross • Monohybrid Cross - provides data about 1 pair of contrasting traits • Ex. Homozygous tall x homozygous short • Punnett Square – diagram used to predict the probable outcome of a cross • Write parental cross (genotypes) • Draw box, genotype of 1 parent goes on one side, other parents genotype on the other side • Fill in the boxes with 1 allele from each parent to indicate possible offspring genotypes • Determine probability of traits • Genotypic Ratio: homozygous dominant : heterozygous : homozygousrecessive • Phenotypic Ratio: dominant: recessive
Test Cross • Test cross is used to determine unknown genotypes • Cross unknown with a homozygous recessive individual for that trait • If ALL offspring show dominant trait, then the unknown is homozygous dominant • If any (about 1/2 ) offspring show recessive trait, then the unknown is heterozygous
Do Now: • Leslie has a long palmar muscle. Leslie has a brother, who does not have a long palmar muscle. Leslie’s parents also lack the muscle. Leslie is married to Lamont, who does have the long palmar muscle. Their first two children are identical twin boys (Larry and Lance), who both have a long palmar muscle. Use the letters M and m to represent the alleles for this trait. • What are the genotypes of everyone in this problem? • Leslie, Louis, Lamont, Larry, Lance, Leslie’s Parents • What is the most probable method of inheritance (dominant or recessive) for this trait? Explain.
Dihybrid Cross • Dihybrid Cross involves 2 pairs of contrasting traits • Ex. Homozygous round yellow seeds (RRYY) x homozygous green wrinkled seeds (rryy) • Punnett Square has 16 boxes • Determine possible allele combinations for each parent and put on sides of Punnett square • Fill in boxes with possible allele combinations for offspring
Extra Credit – Trihybrid Cross • Round is dominant to wrinkled seeds • Yellow seeds are dominant to green seeds • Purple flower color is dominant to white flower color • Show a trihybrid cross, and use a Punnett square to determine the phenotypic ratio for possible offspring from parents that are each heterozygous for all traits
Complex Patterns of Heredity • Do not follow Mendelian Genetics • Incomplete Dominance • Codominance • Multiple Alleles • Sex linked traits • Polygenic Inheritance
Incomplete Dominance • Incomplete dominance occurs when an intermediate form of the trait is displayed in heterozygous individuals • Ex. Snapdragons Red x White = 100% Pink!
Codominance • Codominance – 2 dominant alleles are both expressed at the same time • Ex. Roan horses Red x White horse = 100% Roan horse (has both red and white hair)
Do Now: • Thomas has sickle cell but his wife, Susie, does not have sickle cell. Their daughter, Kelly has both regular cells and sickle cells. • What pattern of inheritance does sickle cell follow? How do you know? • What is the probability that Kelly and her husband Regis (who does not have sickle cell) will have a child with all normal red blood cells?
Multiple Alleles • Traits with more than 2 possible alleles • Ex. Blood Type (A,B, and O) • 3 possible alleles • IA,IB (dominant), • i (recessive)
Linked Genes • Discovered by Thomas Hunt Morgan • Studied Drosophila melanogastar • Crossed wildtype red-eyed female x mutant white-eyed male • Concluded white-eye mutation linked to sex chromosome (X)
Sex-linked Traits • Sex-linked traits – genes are found on the X chromosome but not on the Y chromosome • Females have 2 X chromosomes, therefore 2 alleles for each trait and a heterozygous female would exhibit the dominant trait • Males have only 1 X chromosome, therefore only 1 allele to determine traits found on the x chromosome and will always exhibit that trait even if it is recessive • Ex. Sex-linked traits: Hemophilia, Red-Green color blindness, Male-Pattern baldness, Duchenne Muscular Dystrophy
Genetic Recombination • Unlinked Genes - typically see 50% freq of recombination for any 2 genes located on different chromosomes due to independent assortment of metaphase I • Linked Genes – freq of recombination varies depending on distance between linked genes due to crossing over during prophase I • Using the freq of recombination can construct a genetic map (ordered list of loci along chromosome)
Polygenic (Multi-gene Inheritance) Polygenic Inheritance – several genes influence 1 trait, therefore we see a variety of phenotypes and a continuum from one extreme to another
Linked Genes • Linked genes – genes located on the same chromosome that tend to be inherited together • Linked genes do not always follow Mendel’s Law of Independent Assortment (he used genes on different chromosomes)
X Inactivation in Female Mammals • Although female mammals inherit 2 copies of the X chromosome, one X chromosome becomes inactivated during embryonic development and is called a Barr Body • The inactivation of an X chromosome occurs randomly in each embryonic cell, therefore females consist of a mosaic of 2 types of cells (active x from mom or active x from dad) • Ex. Tortoise shell cats • Some cells express black fur and others express orange fur
Pedigree Analysis • Pedigree - diagram of family history of a trait or disease used to study heredity • By studying a pedigree, it is possible to infer the pattern of heredity
Analyzing a Pedigree • Determine if trait is sex-linked or autosomal • Sex-linked usually seen in males • Autosomal appears in both sexes equally • Determine if trait is dominant or recessive • If every individual w/trait has a parent w/trait then it is dominant • If individual has parents w/o trait then it is recessive • Determine if the trait is determined by a single gene or several • If determined by a single recessive gene, than normal parents should produce affected children with a 3:1 ratio • If determined by several genes the proportion would be much lower
Ex. Pedigree 1 Pedigree 2 Example Pedigree