GEN 3000 Notes Set 3 01 13 2016 Dr Tsai Clemson University Chapter 2 Continued Beginning of Chapter 3 Meiosis Continued Meiosis is a longer process in females It starts in the embryonic ovary and freezes in meiosis I Then upon ovulation in a young adult female it restarts and freezes in Meiosis II and only completes if fertilization occurs Keep in mind that plants also have this cycle They go between diploid and haploid also called the sporophyte and megaspore female side of plants respectively Chapter 3 Mendelian Genetics About Johann Gregor Mendel Mendel s work is the basis of what we know about genetics He was carrying out his work in the 1850s 1860 s He may have read some of Darwin s work during this time but it isn t certain He was a monk and after he carried out his work and published his findings he was called to other duties He did not return to genetics At the time of publishing his work wasn t really recognized In the early 1900s as other scientists began doing similar experiments and achieve similar results Mendel s work begins to get recognized and built off of Mendel s scientific and experimental approach were very detailed and allowed little room for error Mendel used the garden pea plant which was a perfect model for studying genetics and heredity due to the following characteristics o Grow rapidly o Produce many offspring o Had genetically pure strains to start with o Stem characteristics color seed shape and seed coat color allowed him to study genetic variation and heredity o Mendel avoided base plants with a lot of variation Vocab Gene Allele A genetic factor that is responsible for determining characteristics Johannsen One of the possible forms of a gene In Mendel s experiments the same gene established the term gene from the original term of pangenesis with a different allele was what produced the two different colors of pods or wrinkled versus round as well as the other varied characteristics A location on a chromosome where an allele is located The set of alleles than an individual possesses Can be heterozygous or homozygous This is the ONLY thing that is inherited Phenotype is not see below An Individual with two different alleles at a locus An individual with two of the same allele at a locus Heterozygote Homozygote Locus Loci Genotype Phenotype or Trait Character or characteristic The appearance or manifestation of a character this is equal to the genotype combined with the environmental factors that contribute Homologous chromosomes can have alleles that occupy the same locus but may have different characteristic exhibition This is true across species An attribute or feature Blue or green eye color etc Mendel s work Started with a monohybrid cross or two pure parents that had one characteristic opposite of each other such as two tall plants with green pods and seed coats but one plant had a smooth seed coat the other wrinkled seed coats He did this to look at one characteristic at a time Pea plants prefer to self fertilize so Mendel cut off the anthers pollen provider of the flower so the plants could not fertilize themselves This way he could control phenotypic variables He did crosses in both directions such as where first the female plant would have the wrinkled seeds then the second time the male plant would have the wrinkled seeds This was called a reciprocal cross By doing this he showed that all inheritance was independent of the sex of the parent 1 The Experiment began with the Parent generation abbreviated P that were homozygous for round seeds or wrinkled seeds He cross pollinated the plants to produce the F1 generation filial 1 2 Within the F1 generation he noticed that one of those phenotypes wrinkled seeds disappeared from the offspring characteristics He then allowed the plants to self fertilize to produce the F2 filial 2 generation 3 This second set of offspring would exhibit the wrinkled seeds again but in a 3 1 ratio of round to wrinkled seeds He did this for all of the characteristics Mendel s conclusions Through this process performed hundreds of times Mendel discovered that the unit factors came in pairs what we now call alleles so that each trait has 2 different unit factors that result in different traits and gives 3 possible combinations This experiment also showed that Dominant and Recessive traits existed and that those that disappeared were recessive and required 2 of them to show in the phenotype That s why it returns in F2 He determined that the alleles had equal genetic opportunity in hybrid or heterozygous individuals and are separated into the gametes each parent only contributes o This led to the Principle of Segregation Mendel s first law He concluded that plants that produced purely round seeds contained only dominant alleles RR and wrinkled seeds alone were recessive only seeds rr When those alleles were separated and combined by fertilization they would separate equally and produce only Heterozygous individuals with round seeds Rr Then in the F2 generation there are even greater possibilities of combinations RR Rr rR rr Mendel did continue onto the F3 generation and it further supported his theories Punnett Square Developed by R C Punnett in 1917 Allows us to quickly determine the possible genotypic and phenotypic ratios based on parental allele contribution For the above example a simple Punnett Square would look as follows Both parents heterozygous phenotypically round seeds R round r wrinkled R round RR homozygous dominant round rR heterozygous round r wrinkled Rr heterozygous round rr homozygous recessive wrinkled Types of Genetic Crosses Mendel Used Backcross Test cross Experimental action where an offspring is fertilized back to a parent individual Can be done with either parent Cross of a dominant expressing individual of unknown genotypic makeup R to a heterozygous recessive individual rr to see what offspring are produced This allows us to determine the second allele in the individual with a dominant trait A test cross MUST be done with a homozygous recessive individual o If there is a 50 50 split of offspring with dominant and recessive traits a test cross shows there is a recessive allele with the dominant in the test plant Rr o If there are only dominant traits in the offspring the test plant is homozygous dominant RR Genotypic and Phenotypic Ratios of Single Characteristics from Specific Parent Crosses 1 2 1 Genotypic ratio 1RR 2Rr Two heterozygous parents Rr x Rr Heterozygous parent crossed to a