Mendel s Work (cont d)... His observations made him believe that information passed from parents to their young as packages he called units or factors...the factors for one trait are inherited as a unit...an idea Mendel called the Unit Theory of Inheritance. Led him to formulate 3 principles of heredity based on his pea plant experimentation... The Principle of Dominance The Principle of Segregation The Principle of Independent Assortment - His early experimentation (crossing homozygous dominant with homozygous recessive) showed that when two different variations of a trait are crossed, only the dominant form is expressed in the F1 generation. = his Law of Dominance...for traits having two alleles, one is dominant, and must be shown in any offspring carrying it. Only homozygous recessive creatures actually show the recessive form of a trait. - he also concluded...a Law of Segregation... = Each parent has two factors for a trait, which separate into different sex cells, one is inherited by each offspring. Backed up by his experimentation : Crossing F1 plants to make the F2 generation... he got 75% dominant phenotype, and 25 % recessive phenotype (a 3:1 ratio every time), shown using Punnett squares this is only possible if parents have 2 factors that separate, and each offspring gets one from each parent to make their own pair of factors.
- his Law of Independent Assortment... = The factors inherited for one trait have no influence on what factor you d inherit for another trait. Backed up by his experimentation : it is possible to produce plants having the dominant form of some traits, and also having the recessive form of other traits. Ex. A white flowered tall plant with green wrinkled peas, with axial flower position. In more modern terms...
Completing Punnett Squares Correctly Punnett squares are diagrams used to A. Predict the results of a particular cross...what are the genotypes of the offspring, and their phenotypes? B. Figure out the ratio of pure dominants to hybrids to pure recessives You must know how to do two kinds of Punnett squares...simpler, 4 block ones, used to look at ONE trait at a time (called a monohybrid cross)...and more complicated 16 block ones used to look at TWO traits at a time (called a dihybrid cross) We use letters to represent the factors/alleles for a trait, capitals representing dominant alleles, lower case represents recessive : Ex. TT = homozygous dominant genotype Tt = heterozygous ( hybrid ) tt = homozygous recessive Questions will often tell you what form of a trait is dominant, which form is recessive, or even which letter to use. Monohybrid cross example : 1. Choose a letter, or use the one mentioned in a question. 2. Write yourself a key... what uppercase and lower case stand for. 3. Determine the factor pair (genotype) of each parent from a description given in a question.
4. Write one parents factors along one side of the P. Square, the other parents factors along another side. 5. Fill in each block with factors, always writing any dominant one first, recessive second. 6. Summarize the results in 2 ways...a genotypic ratio and a phenotypic ratio Genotypic ratio : 2 AA : 2 Aa Phenotypic ratio : 4 dominant : 0 recessive Monohybrid punnett squares are set up slightly differently when other patterns of inheritance are involved ( incomplete dominance, co-dominance, and multiple allele traits). incomplete dominance - there is no recessive form of a trait, so NO lowercase letters are used at all! Use a symbol to show how one capital letter is different from the other. Ex. B for black andalusian feathers B for white BB for gray co-dominance - again, there is no recessive, so don t use any lowercase letters. Use two different capital letters. Ex. R for red cattle W for white cattle RW for roan cattle
multiple allele traits - blood type questions. I is used to represent both the dominant alleles (A and B). A capital letter is used as an exponent. Lowercase i represents the recessive O allele. A A A Ex. Blood type A : I I (homozygous ) or I i (hybrid) B B B Type B : I I (homozygous ) or I i (hybrid) Type O : ii sex linked traits - the alleles of these traits are on the sex chromosomes ( X and Y) that determine gender. The X and Y letters are used to represent the sex chromosomes of each parent, and the alleles are written as exponents. Two kinds... X-linked and Y-linked. If a trait is X-linked, the exponents are only written by X chromosomes. Ex. Normal visioned (N) female hiding recessive N n nearsightedness (n) would be written as X X Ex. A nearsighted man would be written as X n Y Predicting results using the Product Rule If you know the chances (or probability) of something happening, you can mathematically predict the odds of a series of events happening by just multiplying the odds of one event by the odds of the other event. If the probability of inheriting any one allele is ½, or 50 %. The probability of inheriting any particular two letter set is figured out by multiplying the odds of each event... ½ x ½ = 1/4 or 25 % = the product rule... each independent probability is multiplied to get an answer ex. the chances of having 3 daughters in a row is ½ x ½ x ½, or 1/8