BIO 211 Introductory Genetics I/Genetics I Lecture Notes PDF

BIO 211/BIO201 INTRODUCTORY GENETICS I/ GENETICS I  MENDELIAN LAWS OF INHERITANCE Mendel's First Law of inheritance (Law of Segregation): Law of monohybrid inheritance Second Law of inheritance (Law of independent assortment): Law of Di-hybrid inheri...

BIO 211/BIO201 INTRODUCTORY GENETICS I/ GENETICS I  MENDELIAN LAWS OF INHERITANCE Mendel's First Law of inheritance (Law of Segregation): Law of monohybrid inheritance Second Law of inheritance (Law of independent assortment): Law of Di-hybrid inheritance Genetic analysis predates Gregor Mendel, but Mendel's laws form the theoretical basis of our understanding of the genetics of inheritance.  Mendel made two innovations to the Science of genetics: 1. developed pure lines 2. counted his results and kept statistical notes Pure Line - a population that breeds true for a particular trait [this was an important innovation because any non- pure (segregating) generation would and did confuse the results of genetic experiments]  Results from Mendel's Experiments F1 Phenoty Parental Cross F2 Phenotypic Ratio F2 Ratio pe Round x 5474 Round:1850 Round 2.96:1 Wrinkled Seed Wrinkled Yellow x Green 6022 Yellow:2001 Yellow 3.01:1 Seeds Green Red x White Red 705 Red:224 White 3.15:1 Flowers Tall x Dwarf Tall l787 Tall:227 Dwarf 2.84:1 Plants  Terms and Results Found in the Table Phenotype - literally means "the form that is shown"; it is the outward, physical appearance of a particular trait. Mendel's pea plants exhibited the following phenotypes: - round or wrinkled seed phenotype - yellow or green seed phenotype - red or white flower phenotype - tall or dwarf plant phenotype  Seed Colour: Green and yellow seeds.  Seed Shape: Wrinkled and Round seeds.  Dominant - the allele that expresses itself at the expense of an alternate allele; the phenotype that is expressed in the F1 generation from the cross of two pure lines Recessive - an allele whose expression is suppressed in the presence of a dominant allele; the phenotype that disappears in the F1 generation from the cross of two pure lines and reappears in the F2 generation Mendel's Conclusions The hereditary determinants are of a particulate nature. These determinants are called genes. Each parent has a gene pair in each cell for each trait studied. The F1 from a cross of two pure lines contains one allele for the dominant phenotype and one for the recessive phenotype. These two alleles comprise the gene pair. One member of the gene pair segregates into a gamete, thus each gamete only carries one member of the gene pair. Gametes unite at random and irrespective of the other gene pairs involved.  Mendelian Genetics Definitions Allele - one alternative form of a given allelic pair; tall and dwarf are the alleles for the height of a pea plant; more than two alleles can exist for any specific gene, but only two of them will be found within any individual Allelic pair - the combination of two alleles which comprise the gene pair Homozygote - an individual which contains only one allele at the allelic pair; for example DD is homozygous dominant and dd is homozygous recessive; pure lines are homozygous for the gene of interest Heterozygote - an individual which contains one of each member of the gene pair; for example the Dd heterozygote Genotype - the specific allelic combination for a certain gene or set of genes  Using symbols we can depict the cross of tall and short pea plants in the following manner:  The F2 generation was created by selfing the F1 plants. This can be depicted graphically in a Punnett square. From these results Mendel coined several other terms and formulated his first law. First the Punnett Square is shown. D d Union of DD Dd Punnet D Gametes (Tall) (Tall) t At Random Square Dd dd d (Tall) (Short)  The Punnett Square allows us to determine specific genetic ratios Genotypic ratio of F2: 1 DD: 2 Dd: 1 dd (1: 2: 1) Phenotypic ratio of F2: 3 tall: 1 dwarf (or 3 D_ : 1 dd) Mendel's First Law - the law of segregation; during gamete formation each member of the allelic pair separates from the other member to form the genetic constitution of the gamete  Confirmation of Mendel's First Law Hypothesis With these observations, Mendel could form a hypothesis about segregation. To test this hypothesis, Mendel selfed the F2 plants. If his law was correct he could predict what the results would be. And indeed, the results occurred has he expected.  From these results we can now confirm the genotype of the F2 individuals. Phenotypes Genotypes Genetic Description F2 Tall Plants 1/3 DD Pure line homozygote 2/3 Dd dominant Heterozygotes Pure line homozygote F2 Dwarf Plants all dd recessive Thus the F2 is genotypically 1/4 DD : 1/2 Dd : 1/4 dd  Mendel performed one other cross to confirm the hypothesis of segregation --- the backcross. Remember, the first cross is between two pure line parents to produce an F1 heterozygote. At this point instead of selfing the F1, Mendel crossed it to a pure line, homozygote dwarf plant. Backcross: Dd x dd  Male Gamete s d Femal DD D e (Tall) Gamet dd d es (Short) Backcross One or (BC1) Phenotypes: 1 Tall : 1 Dwarf BC1 Genotypes: 1 Dd : 1 dd Backcross - the cross of an F1 hybrid to one of the homozygous parents; for pea plant height the cross would be Dd x DD or Dd x dd; most often, though a backcross is a cross to a fully recessive parent  Testcross - the cross of any individual to a homozygous recessive parent; used to determine if the individual is homozygous dominant or heterozygous Monohybrid cross - a cross between parents that differ at a single gene pair (usually AA x aa) Monohybrid - the offspring of two parents that are homozygous for alternate alleles of a gene pair Remember --- a monohybrid cross is not the cross of two monohybrids. Monohybrids are good for describing the relationship between alleles. When an allele is homozygous it will show its phenotype. It is the phenotype of the heterozygote which permits us to determine the relationship of the alleles. Dominance - the ability of one allele to express its phenotype at the expense of an alternate allele; the major form of interaction between alleles; generally the dominant allele will make a gene product that the recessive cannot; therefore the dominant allele will express itself whenever it is present  Variations to Mendel's First Law of Genetics Co-dominance - a relationship among alleles where both alleles contribute to the phenotype of the heterozygote Example: Co-dominance Species: Four o'clock plants Trait: Flower colour Pure line phenotypes: red or white flower Parental cross: Red x White F1: We would expect red or white flowers in this generation, depending upon which allele is dominant. But, the F 1 plants produced pink flowers. As with any experiment of this sort, the F1 plants are selfed. The results that were obtained were: F2 phenotypic ratio: 1/4 Red : 1/2 Pink : 1/4 White Snapdragon Flower Colour It appears as if the red and white alleles are interacting in the heterozygote to generate the pink flowers.  Scientists have devised another approach, called pedigree analysis, to study the inheritance of genes in humans. Pedigree analysis is also useful when studying any population when progeny data from several generations is limited. The analysis is also useful when studying species with a long generation time.  A series of symbols are used to represent different aspects of a pedigree. Below are the principal symbols used when drawing a pedigree.  Once phenotypic data is collected from several generations and the pedigree is drawn, careful analysis will allow you to determine whether the trait is dominant or recessive. Here are some rules to follow. For those traits exhibiting dominant gene action: affected individuals have at least one affected parent the phenotype generally appears every generation two unaffected parents only have unaffected offspring  Pedigree of a trait controlled by dominant gene action.  Traits exhibiting recessive gene action: unaffected parents can have affected offspring affected progeny are both male and female The following is the pedigree of a trait controlled by recessive gene action.  Mendel's Law of Independent Assortment Mendel also performed crosses in which he followed the segregation of two genes. These experiments formed the basis of his discovery of his second law, the law of independent assortment. A few terms are presented: Dihybrid cross - a cross between two parents that differ by two pairs of alleles (AABB x aabb) Dihybrid- an individual heterozygous for two pairs of alleles (AaBb) Again a dihybrid cross is not a cross between two dihybrids. Now, let's look at a dihybrid cross that Mendel performed. Parental Cross: Yellow, Round Seed x Green, Wrinkled Seed F1 Generation: All yellow, round F2 Generation: 9 Yellow, Round, 3 Yellow, Wrinkled, 3 Green, Round, 1 Green, Wrinkled  Using diagram for the cross using specific gene symbols. Choose Symbol Seed Colour: Yellow = G; Green = g Seed Shape: Round = W; Wrinkled = w The dominance relationship between alleles for each trait was already known to Mendel when he made this cross. The purpose of the dihybrid cross was to determine if any relationship existed between different allelic pairs. Let's now look at the cross using our gene symbols.  Female Gametes GW Gw gW gw GGWW GGWw GgWW GgWw GW (Yellow, (Yellow, (Yellow, (Yellow, round) round) round) round) GGWw GGww GgWw Ggww Gw (Yellow, (Yellow, (Yellow, (Yellow, round) wrinkled) round) wrinkled) Male Gametes GgWW GgWw ggWW ggWw gW (Yellow, (Yellow, (Green, (Green,