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Basic principles of heredity

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Dalhousie University
BIOL 2030

January 10th (lecture 3) Basic principles of heredity Mendel's work Laid foundation of the science of genetics by discovering principles of transmission of hereditary traits from generations to generations In 1854, he began a series of cross- breeding experiments with the garden pea to learn about the mechanisms of heredity Published his findings in 1866 Rediscovery of Mendel's principles of heredity by 3 scientists in 1900's Mankind knew by this time that they was some sort of hereditary nature of traits Tried to understand how heredity worked Set up to do systematic series of cross-breeding experiments with the garden pea Mendel used crosses of true-breeding varieties homozygous for the gene(s) controlling the traits The traits that he studied had 2 forms (= 2 phenotypes) and they happen to have a simple genetic basis Used true breeding variety (if you propagate this breed all the offsprings look like the parents - thy don't change) of peas - looked at 7 traits (had only 2 phenotypes) and they all happened to have simple genetic basis - excellent model species cross breeding - know definition Alot of plants have both female and male organs and they're self fertilized --- he didn't want this he wanted cross- breeding - cut out all pollen parts and took the pollen parts from another plant and looked at the pea he was getting Monohybrid crosses - Mendel's first law (principle of segregation) Crosses between 2 individuals that differ by 1 trait - derived 1st rule - principle of segregation F1 - first felio generation P - parental generation F1 - self fertilize to get an F2 -- 3/4 round and 1/4 wrinkled that wasn't present in the F1 generation - it re-appeared Concluded that this phenotype was controlled by 2 factors 1 coming from male plant and another from the female 2 forms (alleles) 1 form coding for round and 1 form coding for wrinkle phenotype round form was dominant over the other one Gene: an inherited factor that determine a trait/characteristic Genes exist in different forms called alleles, found at the same specific place on the chromosome (locus). Different alleles can code for different forms of the traits (different phenotypes) The genotype is the set of alleles of an individual (2 alleles at one locus for diploid organisms).An individual possessing 2 identical alleles is homozygous at that locus; it is heterozygous if it possesses 2 different alleles at that locus Genotype - 2 one from mother and 1 from after (generic composition) true breeding round seed in homozygous zygote with carry either a big R or a big R F1 - all individuals are heterozygous (all look round) Now this individual can produce 2 types of gametes (R or r) Self fertilization = four combinations RR, rr, Rr, Rr F2 - 1/4 RR etc Mendel's first law (principle of segregation) The two members of the gene pair (alleles) segregate from each other in the formation of gametes - half the gametes carry one allele and the other half carry the other allele Link with previous lecture. Remember that genes are on chromosomes. The specific location of a gene on the chromosome is called its locus. Gene segregation (separation) parallels the separation of homologous pairs of chromosomes at anaphase I of meiosis (He also proposed the concept of dominance) All do to meiosis Crossing over - how do we still end up with the same combination 2 R and 2 r? The locus is the only thing we're interested in and that doesn't change and therefore crossing over doesn't change the results - we have 50/50 chance of getting each allele Confirming the principle of segregation Mendel noted the same results (3:1 ratio in the F2) for crosses with all seven characters He performed reciprocal crosses and obtained the same results Mendel also did some crosses to confirming his postulated principle of segregation Selfcrosses of F2 --- 1/4 RR; 1/2 Rr /// 1/4 rr = 3/4 round and 1/4 wrinkled) F2 wrinkled should be true breeding - should only give wrinkled Testcross A testcross is the cross of an individual of unknown genotype (usually with the dominant phenotype), with a k
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