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Lecture 10, Mendelian_Genetics_1_NOTES

Lecture 10, Mendelian_Genetics_1_NOTES - Lecture 10...

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Unformatted text preview: Lecture 10: Mendelian Genetics, Part 1 Mendelian What we’ll cover in the next few lectures HEREDITY is the study of how the genetic composition of an organism (its _GENOTYPE_) and the environment _GENOTYPE_ and influence its the physical appearance (its ____PHENOTYPE_) ____PHENOTYPE_ • What are the Mendelian laws of inheritance? • What is the relationship between genes and What chromosomes? chromosomes? • How do alleles interact? • How do genes interact? • What about traits that are controlled by multiple genes? I. Pre-Mendelian thinking about inheritance In 1694, Nicolas Hartsoeker discovered "animalcules" iin the n discovered semen of humans and other animals animals This was the beginning of This _spermists__ theory, who held _spermists__ the belief that the sperm was in fact a "little man" (_homunculus_) that was that placed inside a woman for growth into a child growth ( see Wikipedia: “Homunculus”) homunculi I. Pre-Mendelian thinking about inheritance Long before about 1700 AD, most regular people knew that there was an association between sex & reproduction... was But it was not until ~1700, that Camerarius showed that plant sex But required the union of pollen & ovules_(recent discoveries) required And it wasn’t until 1830 that Hertwig showed that sexual reproduction And in animals also required the union of ___sperm and egg_ in • So, whatever it was that caused the So, resemblance between parents & offspring had to fit inside a sperm & egg offspring • But, no one had the slightest idea what But, the stuff was inside sperm & eggs, much less what happened during fertilization fertilization I. Pre-Mendelian thinking about inheritance By the mid-19th century, plant & animal breeders worked under a reasonable (but incorrect) assumption about inheritance: reasonable When you breed 2 different parental lines that differ for some When trait(s), each of which is _true-breeding_, to produce hybrid _true-breeding_ to offspring, these offspring all look pretty much alike, and (often) look like a mix of the 2 parental types. Thus there is _blending inheritance inheritance Looks like blending inheritance inheritance I. Pre-Mendelian thinking about inheritance This was a problem for Darwin’s theory of natural selection: This With Blending, populations should eventually become phenotypically With uniform, with no variation upon which selection can act uniform, Father x Mother P1 Offspring Offspring Offspring F1 Mendel to the rescue! II. Mendelian Genetics A. Gregor Mendel Gregor Mendel (1822-1884) (1822-1884) He headed a monastery in He Moravia, Austria (now the Czech Republic), bred sweet peas, and solved Darwin’s problem Darwin’s II. Mendelian Genetics A. Gregor Mendel • Mendel was a contemporary of Darwin • He was unusually well-trained in the physical He sciences (& it showed in his entire statistical/probabilistic approach to a biological problem) problem) • In 1866, he sent Darwin his pea paper, but Darwin In either didn’t understand the implications, or never read it Darwin never mentions the paper in his Darwin notebooks, though there is a copy of the paper in Darwin’s collection paper Darwin performed similar crosses on flowers, Darwin but did not grasp the significance! but II. Mendelian Genetics B. What Mendel did • He repeatedly self-fertilized plants so that they were TRUE BREEDING He for each of 7 BINARY, discrete characters in sweet peas for • He crossed these plants in various combinations in careful experiments • He analyzed the results from these experiments using simple He mathematics and statistics, and deduced the nature of genes and the laws governing heredity ... Without any knowledge of chromosomes or laws .. DNA! (smart guy...) DNA! II. Mendelian Genetics B. What Mendel did, 1. General Methods Mendel’s controlled crosses with true-breeding lines Mendel’s stimga anthers ovary Plant the seeds F1 plant Transfer pollen from one parent to the stigma of the other parent _Parental Generation =P_ _Parental Resulting offspring =__first filial Resulting generation of F1 generation Self-pollinate or cross F1 plants to produce second filial generation or F2 second II. Mendelian Genetics II. B. What Mendel did, 2. Test of blending: monohybrid crosses Hypothesis: Hypothesis: Traits from the parents are irreversibly blended in the offspring Traits Predictions?: II. Mendelian Genetics B. What Mendel did, 2. Test of blending: monohybrid crosses Methods (STEP 1): • Make true-breeding lines for each trait • Cross parental plants (P) differing in just Cross one character one • F1 generation are thus _homozygous_____________ _homozygous_____________ Results (STEP 1): • One trait of each pair disappeared in One the F1 generation the – these traits are these ____heterozygous__________ ____heterozygous__________ • The trait that appears in the F1 is the __dominant_______________ trait __dominant_______________ trait Parental plants F1 generation 100% 100% 100% 100% II. Mendelian Genetics B. What Mendel did, 2. Test of blending: monohybrid crosses Methods (STEP 2): • Cross 2 of these resulting F1 plants • Count the numbers of each trait type in Count the F2 generation the Results (STEP 2): • See next slide... Cross two F1 plants II. Mendelian Genetics B. What Mendel did, 2. Test of blending: monohybrid crosses Results (STEP 2): Results The recessive trait always re-appeared in the F2’s The ratio of dominant to recessive phenotypes in the F2 was ______3:1_______________ Reciprocal crosses yielded the same results (didn’t matter which parent gave pollen or ovule) He repeated this with 7 traits and got the same results... II. Mendelian Genetics B. What Mendel did, 3. Law of Segregation Mendel’s inference from the results of the monohybrid crosses: Mendel’s • Each gamete (pollen/sperm or egg) contains only one “factor”, but the Each gamete zygote contains two—because it is produced from the fusion of two gametes gametes – We now call these “factors” __genes________________ We __genes________________ This lead to _____the law of segregation____ This _____the • When any individual produces gametes, the two copies of a gene When separate/segregate, so that each gamete receives only one copy separate/segregate, 1 • Thus from every parent of the P generation, every individual of the F generation receives one gene copy chosen randomly chosen II. Mendelian Genetics B. What Mendel did, 3. Law of Segregation Terms you should make yourself very comfortable with: very • • • • The location of a gene on a chromosome is called a _locus_______(plural = The _locus_______(plural _loci____) _loci____ Variants of a gene are called alleles e.g. purple vs. white flowers) Variants e.g. When an individual has two of the same alleles, they are homozygous When homozygous (homozygotes); when different alleles, heterozygous (heterozygotes) (homozygotes) when heterozygous The combination of each pair of alleles for a given gene is an organism’s The _genotype____________, iit’s outward characteristics are the _genotype____________ t’s __phenotype______________ __phenotype______________ Allele for an organism The totality of all genes inpurple flowers is the __________________ The __________________ • genotype Locus for flower Locus color gene color Homologous Homologous pair of chromosomes chromosomes phenotype SS Ss ss Allele for white flowers II. Mendelian Genetics B. What Mendel did, 3. Law of Segregation When and where does segregation occur? When What is this process? Where does it occur? Is the parent homozygous or Is heterozygous? heterozygous? II. Mendelian Genetics B. What Mendel did, 3. Law of Segregation Back to Mendel’s original experiment... Now we’ll examine Back the monohybrid cross with a ____Punnet Square_ ____Punnet The Punnett square is a tool to help visualize a cross and predict The the genotypes & phenotypes of the offspring the 1. Heterozygous F1 plant produces haploid gametes gametes 2. Male & female gametes produced by F1 are arrayed outside the Punnett Square. are 3. Different combinations of alleles from Different each parent produce 2 different seed phenotypes in the F2 generation. phenotypes 4. Seed PHENOTYPES appear in _3:1 ratio 4. II. Mendelian Genetics II. B. What Mendel did, 3. Law of Segregation The R allele is dominant and codes for red flowers, r is recessive The and codes for orange. If an Rr individual is crossed with an Rr _RR____ individual, what will be the resulting ratio of phenotypes? phenotypes ...
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