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Biology 1201A
Richard Gardiner

Gregor Mendel During the 1700s Anton Van Leeukuehoek saw “animolecules” in sperm of humans and animals Some scientists speculated they saw little men in each sperm Pangenesis- males and females formed “pangenes” in every organ These pangenes then moved through their blood to the genitals and then to children Concept originated with Greeks and influenced biology up to around 100 years ago The term “blood relative”, “full blooded” and “royal blooded” Francis Galton (Darwin’s cousin) disproved the theory in the 1870s. Why peas? Mendel also worked with other species  Peas worked the best  Peas had 7 traits that gave results Mendel could understand  Other traits could not Rule #1 of a good scientist  Work with things that work Heredity: For much of human history people were unaware of how heredity worked Greek Philosophy:  Male flowers caused females flowers to open  Seeds produced by various body parts and transmitted to offspring at time of conception Greg Mendel: Grew up on a form in what is now Czech Republic (Austria) 1842 entered monastery University of Vienna Became a teacher Interest in breading plants at monastery Mendel’s Method: In typical breeding experiment, Mendel would cross-pollinate two contrasting, true breeding variables.  The true breeding parents are the P generation and their hybrid offspring are the F1 generation Mendel would then allow the F1 hybrids to self-pollinate an F2 generation It was mainly Mendel; quantitative analysis of F2 plants that revealed the two fundamental principles of heredity; the law of segregation and the law of independent assortment Terms: Character- a heritable feature Trait- a variant of character True breeding- organism that they breed produces the same character Hybridization- mating or crossing of two varieties Monohybrid cross- cross that tracks the inheritance of a single character P generation- parental F1 generation- first filia- generation following the parental F2 generation- second filia- F1XF1 Alleles- alternative version of gene Dominant Allele- fully expressed allele Recessive Allele- allele showing no noticeable affects Summary of Mendel’s Model: Adult (2N) traits involve 2 alleles Allele segregate in gametes (N) (no blending of traits)  Each gamete contains only one allele o Law of Segregation Dominant alleles make the expression of recessive alleles in heterozygotes True breeding adults have identical alleles they are homozygous How does this relate to meiosis? Homologous chromosomes may have different alleles for seeds colour Terms: Dominant Allele- capital letter “A” Recessive Allele- lower case letter “a” Homozygous- has 2 identical alleles Heterozygous- has 2 different alleles Phenotypes- an organism expressed visible traits Genotype- an organism genetic builds up For flowers colour in peas, both PP and Pp plants have the same phenotype (purple) but different genotypes (homozygous vs. heterozygous) The only way to produce a white phenotype is to be homozygous recessive (pp) for the flower colour gene It is not possible to predict the genotype of an organism with a dominant phenotype  The organism must have dominant allele, but it could be homozygous dominant or heterozygous By the law of independent assortment, each pair of alleles segregates into gametes independently. Mendel’s experiments that followed the inheritance of flowers colour or other characters focused on only a single character via monohybrid crosses. He continued other experiments in which he followed the inheritance of 2 different characters, a dihybrid across. In one hybrid cross experiment, Mendel studied the inheritance of seed colour and seed shape  The alleles for yellow seeds (Y) is dominant to the allele for green seeds (y)  The allele for round seeds (R) is dominant to the allele for wrinkled seeds (r) Mendel crossed true breeding plants that had yellow, round seed (YYRR) with true breeding plants that had green, wrinkled seeds (yyrr) A hypothesis is that the two pairs of alleles segregate independently of each other  The presence of one specific allele for one trait has no impact on the presence of a specific allele for the second trait In our example, the F1 offspring would still produce yellow, round seeds. However, when the F1’s produced gametes, genes would be packaged into gametes with all possible alleles combinations  4 classed of gamete (YR, Yr, yR, yr) would be produced in equal amounts One possibility is that the 2 characters are transmitted from parents to offspring as a package  The Y and R alleles and the y and r alleles stay together If this were the case, the F1 offspring would produce yellow, round seeds The F2 offspring would produce two phenotypes in a 3:1 ratio, just like a monohybrid cross This was not consistent with Mendel’s result Mendel repeated the dihybrid cross experiment for other pairs of characters are always observed a 9:3:3:1 phenotypic ratio in the F2 generation Each character appeared to be inherited independently The independently assortment of each pair of alleles during gamete formation is now called Mendel’s Law of indep
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