Genetics Lecture 3, 9/3: Mendelian Genetics

Today's audio. NOTE: I have included the apx. times of when he goes over particular topic in the notes. Ex. He goes over Mendels monohybrid cross at (19:10))

• Practice problems (due before next fridays exam)
• 2,9,17,23,33,35
• Gregor J. Mendel -1866-
• Laid the foundation for the science of Genetics. Thus this form of genetics is called Mendelian Genetics or transmission Genetics. This is study of how traits are passed from generation to generation. Genotype to phenotype.
• Brief bio
• born in 1822 in central europe @Heinzendorf
• 1843 admitted to Augustinian Monastary - St. Thomos Brno.
• spends time as a pastor but in 1849 is relieved
• begins teaching and then went to school from 1851-1853. He studied physics and botany at the U of Vienna
• 1854 he returns to the monastary and teaches physics and natural science for 16 years
• 1856 he performs first experiment. Continues this research until 1868.
• In 1868 he becomes the abbott (top guy)
• 1884 he dies.
• Why was Mendel successful? (slide 2 chap 3)
• chose a good organism - common pea plant
• easy to grow
• can produce lots of data in one growing season
• control of its mating - which parents are for which offspring
• chose a series of traits to study with relatively straightfoward inheritance patterns
• all of the traits came in two varieties and in all of his relationships you have one trait that is completely dominant over a second trait.
• mendel had access to a variety of seed types.
• true breeding strain --> a strain that, when fertilized, you produce offspring that are all identical to the parent.
• Mendel took GREAT records. Allowed him to have a large amount of data. Enough so he could do quantitative analysis on. This gave creedence to Mendel's conclusions.
• Mendels monohybrid cross. Monohybrid cross and follow one trait at a time. (19:10)
• His experiment. Take tow pairs (true -breeding) with opposing characteristics.
• Tall cross that with a short plant. P generation --> parental generation
• produce and examine the offspring, called the F1 generation or First filial generation.
• F2 generation is the second filial generation and the F2 generation is a result of self-crossing a member of the F1 generation. Or crossing two identical members of the F2 generation.
• Results
• F1 generation all of the offspring are tall. In all of the crosses --> one trait appears and masks the second trait
• He then self fertilizes a member of the F1 generation and produces F2's. He gets 787 tall plants and 277 dwarf plants, 3:1 ratio of one trait to the 2nd trait. This phenomenon was not sex dependent. He did the cross two ways: tall sperm and dwarf on the egg and he got 3/4 tall and 1/4 dwarf. He switched and the numbers were the same.
• From his work he developed a series of postulates on the nature of inheritance.
• Genetic characteristics are controlled by unit factors (genes) which exist in pairs in individuals. Two copies of each gene one from mom and one from dad.
• When you have different forms of the same unit factor (alleles) in an individual one of the traits dominates over the other. Referred to as the dominant trait. In Mendels experiments it appeared solely in the F1 generation and 3/4 in the F2 generation. Recessive trait which appears as 1/4 of the F2.
• this is true for a lot of situations but it is not universally true.
• During gamete formation the unit factors (genes) segregate independent of each other. Roughly an equal likelihood of a gamete receiving a particular trait. Basically if an individually has both genes (an allele for tall and one for short) 50% of gametes will be tall and the other half will be dwarf.
• Genotype - refers to the nature of the genes for a particular trait. Are you "tall tall" "tall dwarf" or "dwarf dwarf" (36:25)
• Phenotype - physical appearance. Different genotypes lead to different phenotypes which means they can either be homozygous or heterozygous.
• homozygous - two identical copies of the same gene. You can be homozygous dominant (two alleles for tall) or you can be homozygous recessive.
• Heterozygous - two different alleles for a particular trait (heterozygous dominant). Can't be heterozygous recessive because by the nature of the relationship between the dominant and recessive allele the heterozygous individual will end up with the dominant phenotype. To figure this out you do a mono-hybrid cross on a punnet square.
• A true breeding plant is homozygous by nature. A heterozygous plant will produce offspring with both characteristics. SIDE NOTE: capital letter=dominant cell . . . lower case letter=recessive allele
• Homozygous tall x homozygous dwarf. (start as a diploid organism and your gametes are haploids with one allele each, shown here)

• 	D	D
  d	Dd	Dd
  d	Dd	Dd

• In the F1 generation all of the offspring are Dd-->Heterozygous (all tall)
• F2 generation production (46:45)
• Self fertilize tall x tall
• 1/4 offspring are homozygous
• 1/2 are heterozygous
• 1/4 are homozygous recessive
• This is how you end up 3/4 tall (homozygous dominant plants and heterozygous dominant plants will show the tall phenotype) and 1/4 (homozygous recessive) dwarf.
• SIDE NOTE: Gene vs. allele.
• gene: unit factor of inheritance. Passed down from generation to generation
• allele: you have alleles of a gene (different forms of a gene). Ex. Your gene is a pea plant and your allele is whether you are tall or short.
• You have a dominant plant in the F2. How do you know if its homozygous or heterozygous? (slide 4 chap 3)
• Do a test cross.
• Cross your dominant individual with a homozygous recessive individual. Two possibilities homozygous dominant or heterozygous dominant.
• If your F2 plant is homozygous dominant --> all Dd --> all tall
• If it is heterozygous dominant then --> 1/2 tall Dd and 1/2 homozygous recessive dwarf dd
• In a test cross if your individual is heterozygous that individual will produce the recessive phenotype.