Chap 14 Mendel and the Gene Idea

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For each observable trait (phenotype), an organism inherits 2 alleles, one from each parent. These alleles make up its genotype. If the 2 alleles at a locus (the region on a chromosome where a gene is found) differ, the organism is heterozygous, otherwise it is homozygous.

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Mendel's law of independent assortment. The P plants are true-breeding: one with yellow-round seeds and the other with green-wrinkled seeds. The F1 dihybrids are heterozygous for both characters. Self-pollination of the F1 yields a phenotypic ratio of 9:3:3:1 in the F2, NOT the 3:1 typical of a monohybrid cross. Thus the 2 traits assort independently of each other. One way to ensure you have all the combination of gametes for a dihybrid cross is the FOIL method.

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GenotypeAntigens
on RBCs
Antibodies
in Serum
Blood Group
IAIA or IAiAAnti-BA
IBIB or IBiBAnti-AB
IAIBA and BneitherAB
iineitherA and BO


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The A, B, AB, or O phenotypes are affected by 3 different alleles. IA and IB alleles produce different antigens on the surface of red blood cells, thus are dominant to the i allele which produces no antigen. IA and IB are codominant to each other because the RBCs bear both antigens.

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A gene at one locus may affect phenotypic expression of a gene at another locus by epistasis. The C/c gene, which is epistatic to the B/b gene, controls whether or not pigment of any color will be deposited in the hair. A homozygous recessive cc mouse has no hair pigment and will be albino regardless of its B genotype.

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An observble character, or trait (phenotype), such as flower color, is inherited in units called genes. Alternative versions of a gene are called alleles. An organism's total genes is its genome.

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Phenotype versus genotype. A monohybrid cross yields a 3:1 phenotypic ratio in the F2, assuming purple flower color is dominant and white flower color is recessive. The genotypic ratio is 1:2:1, since there are 2 types of purple–flowered plants, PP (homozygous) and Pp (heterozygous). The true-breeding P generation must have identical alleles for that gene and are homozygous.

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Incomplete dominance in snapdragon color. When red snapdragons are crossed with white ones, the F1 hybrids have pink flowers. Superscripts indicate alleles for flower color: CR for red and CW for white. The F2 generation produces a 1:2:1 ratio for both genotype and phenotype.

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Human skin pigmentation is influenced by multiple genes which produce different melanin pigment molecules and shows quantitative variation. This polygenic inheritance also exhibits incomplete dominance.

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 EXPERIMENT   True-breeding purple-flowered plants and white-flowered plants were crossed. The F1 hybrids were allowed to self-pollinate.
 RESULTS   Both purple-flowered plants and white-flowered plants appeared in the F2 generation, in a ratio of about 3 purple : 1 white. Similar ratios were observed many other pea characters. Mendel called the purple flower trait dominant, and the white flower color trait recessive.

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The garden pea has closed flowers and can self-fertilize, but also allow for manual cross-pollination.  APPLICATION   By crossing (mating) two true-breeding varieties of the pea, Mendel studied patterns of inheritance of flower color.

 RESULTS   When pollen from a white flower fertilizes eggs of a purple flower, the first-generation hybrids (F1) all have purple flowers. The result is the same for the reciprocal cross, the transfer of pollen from purple flowers to white flowers.

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Pedigree analysis. In these family trees, squares represent males and circles represent females. Shaded squares and circles represent individuals who exhibit the trait. A dominant trait such as widow's peak cannot skip a generation.

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Pedigree analysis. A recessive trait such as attached earlobe may skip a generation. A dot may be placed within a symbol to represent known heterozygotes (carriers who do not exhibit the recessive phenotype). Identify the carriers. ( Hint )

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Segregation of alleles and fertilization as chance events. When a heterozygote (Rr) forms gametes, segregation of alleles is like the toss of a coin. We can determine the probability for any genotype among the offspring of 2 heterozygotes by multiplying the individual probabilities of a gamete having a particular allele (R or r).

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Mendel's law of segregation. Each plant inherits 1 allele for flower color from each parent. The 2 alleles segregate (separate) and end up in different gametes during meiosis. To construct a Punnett square, list all the possible female gametes along one side and all the possible male gametes along the other side. Random fertilization between gametes yield predictable ratios in the offspring.

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An organism that exhibits a dominant trait, such as purple flowers, can be either homozygous for the dominant allele or heterozygous. To determine the organism’s genotype, a testcross is done: the individual with the dominant phenotype is crossed with a recessive phenotype (white flowers), since we know the latter's genotype is homozygous recessive.
By observing the phenotypes of the offspring, we can deduce the genotype of the purple-flowered parent.