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Date Shared: 19 February 2021
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Review Video: https://www.youtube.com/watch?v=btud-uew9co Introduction to Genetics
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A. An example of Mendel's experiments demonstrates his careful research methods. Mendel worked with plants that had been true-breeding for many _generations_ for either of two forms of a trait, tall and short pea plants. He cross-pollinated the two plants by _transferring_ pollen from one plant to another plant with the different form of the trait by _carefully_ avoiding self-pollination of the flowers. All the first generation plants were tall. He allowed the first generation plants to _self-pollinate._ He analyzed the data mathematically and found that he obtained a 3:1 ratio of tall to short plants in the second generation. He concluded that each of his true-breeding plants has two factors _(alleles)_ for each of the traits. These factors separate when _gametes_ are formed. Mendel did similar _monohybrid_ crosses with seven pairs of traits in pea plants. In every case he found similar results. B. The two alleles for round and wrinkled seeds separate when gametes are formed in the F1 generation and are passed at _random_ to the F2 generation. Plants that inherit either one or two alleles for round seeds have round seeds, and plants that have two alleles for wrinkled seeds have wrinkled seeds. The _probability_ for a plant to have at least one allele for round seeds is three fourths. C. The frequency of _crossing-over_ between genes during meiosis provides a clue to the genes' locations. If two genes are close together, then crossovers between them should be _rare._ If two genes are far apart, then crossovers between them should be more _common._ By this reasoning, one can use the frequency of crossing-over between genes to determine their distances from each other.
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A. The frequency of crossing-over between genes during meiosis provides a clue to the genes' locations. If two genes are _close_ together, then crossovers between them should be rare. If two genes are _far_ apart, then crossovers between them should be more common. By this reasoning, one can use the frequency of crossing-over between genes to determine their _distances_ from each other. B. Homologous chromosomes form _tetrads_ during prophase I of meiosis. Sometimes, the chromosomes may exchange segments in an event called crossing over. C. During meiosis, the nucleus of the cell divides _two_ times. The first division separates homologous chromosomes, and the daughter cells are _haploid._ When haploid gametes combine, the result is a new cell that has different genes than either of the parent cells. D. To find all the possible gametes, list all of the different allele _combinations_ of the three genes: TYPr; TYpr; Typr; TyPr; tYPr; tYpr; typr; tyPr E. In meiosis, DNA replication is followed by two rounds of cell division to produce _four_ daughter cells. Each cell with _half_ the number of chromosomes as the original parent cell. F. The difference between meiosis I and mitosis is that _homologous_ chromosomes form tetrads and separate during meiosis I, forming daughter cells with the haploid number of chromosomes. In mitosis, sister _chromatids_ separate, preserving the _diploid_ number of chromosomes.
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A. Polygenic traits are controlled by _more_ than one gene. They generally show a wide range of _phenotypes._ Human height and skin color are other examples of polygenic traits. B. Because IA is dominant to i and produces antigen _A_ on the surface of red blood cells, a person with blood type A has either a genotype of IAIA or IAi. C. The gene I for blood type codes for a _protein_ found on the surface of red blood cells. This is a single gene with three alleles IA, IB, i. The IA and IB alleles are _codominant_ to each other. This means if a person inherits the IA allele from one parent and the IB from the other parent, the red blood cells will carry _both_ A and B surface proteins and the person will have type AB blood. D. Human mitochondria are inherited from the mother's egg cell _cytoplasm._ As a result, genetic disorders in human mitochondrial DNA follow a pattern of _maternal_ inheritance. E. Environmental conditions, such as temperature, can affect gene _expression_ and influence genetically-determined traits F. Coat color in cattle is an example of codominance, in which both the red and white alleles are _expressed_ in the heterozygous genotype. Note that codominance differs from incomplete dominance, in which one allele _partially_ dominates the other.
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A. The F2 generation of a _dihybrid_ cross resulting from self-pollination of the F1 plants produces _four_ different types of plants in the F2 generation following Mendel's law of independent assortment. When the tomato plant produces gametes, the alleles Tt segregate from each other (law of _segregation)_ and separate from the Rr alleles (law of _independent_ assortment). These alleles can then recombine in four different ways: 9 tall vine/red fruit; 3 tall vine/yellow fruit; 3 dwarf vine/red fruit; 1 dwarf vine/yellow fruit. B. The parent P generation involved true-breeding plants, either tall with red fruit (TTRR) or dwarf with yellow fruit (ttrr). As a result, the offspring in the F1 generation all have the same _genotype_ (TtRr), which means they all exhibit the dominant traits of tall height and red fruit. Also, note that a Punnett square describes a cross between heterozygous individuals for both traits (TtRr x TtRr) would be a cross between the F1 generation offspring, the resulting offspring is the _F2_ generation. C. Remember that Punnett squares and probability calculations show the _expected_ or predicted outcomes of a cross. They do not show _actual_ results, especially from a small sample size, such as only three offspring. In this example, the results of the first litter have no effect on the probable outcomes for the _second_ litter.
Draw a Punnett square on scratch paper. Cross a heterozygous parent (Dd) with a homozygous recessive parent (dd). What percentage of their offspring could be heterozygous?
Draw a Punnett Square on scratch paper. A person with blood type IAIB marries a person with blood type IAi. What is the percentage that their children will be IBi?
Draw a Punnett Square on scratch paper. 16 boxes this time...its a dihybrid cross. Cross a true-breeding tall red tomato plant (TTRR) with a true-breeding short yellow tomato plant (ttrr) What is the phenotypic ratio for Tall red: Tall yellow: Short red: short yellow plants
19 February 2021
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