CHAPTER 13 Beyond Mendel

Transcription

1 CHAPTER 13 Beyond Mendel

2 In the 20th century, geneticists have extended Mendelian principles not only to diverse organisms, but also to patterns of inheritance more complex than Mendel described. In fact, Mendel had the good fortune to choose a system that was relatively simple genetically. Each gene has only two alleles, one of which is completely dominant to the other. Each character (but one) is controlled by a single gene.

3 In Mendel s pea plants each gene has only two alleles, one of which is completely dominant to the other. However, in early twenty century, geneticists trying to determine of Mendel s principles apply to all traits in all organisms were faced with the following dilemma. In Anthirrinum (Snapdragons), when two pure lines, red and white, are crossed, it would be expected that the F1 shows one of the two parental phenotypes (the dominant one). (Following Mendel s principles).

4 However, a cross between a white-flowered plant and a red-flowered plant will produce all pink F 1 offspring. Self-pollination of the F 1 offspring produces 25% white, 25% red, and 50% pink offspring. This is a textbook example of incomplete dominance.

5 Incomplete dominance is an allelic relationship where dominance is only partial. In a heterozygote, the recessive allele is not expressed. The one dominant allele is unable to produce the full phenotype seen in a homozygous dominant individual. The result is a new, intermediate phenotype. A similar system is found in carnations.

6 Another example is plumage color in chickens: a. Crossing a true-breeding black chicken (C( B C B ) with a true-breeding white one (C( W C W ) produces an Andalusian blue F1 (C( B C W ). b. When the F1 interbreed, the F2 include black (C( B C B ), Andalusian blue (C( B C W ), and white (C( W C W ) birds, in a ratio of 1;2;1. c. At the molecular level, two copies of C B produce black, while one copy is sufficient to produce only the grey Andalusian blue phenotype.

7 b. When the F1 interbreed, the F2 include black (CBCB), Andalusian blue (CBCW), and white (CWCW) birds, in a ratio of 1;2;1. c. At the molecular level, two copies of CB produce black, while one copy is sufficient to produce only the grey Andalusian blue phenotype.

8 Another example of incomplete dominance is found in cats. In cats, the gene S codifies for white spots. The gene s is for solid color or spot-less. The gene is incomplete dominant. A cat that is SS cat has white spots in more than 2/3 of their body independently of their base color. For example a black cat with white spots or a white cat with white spots. If the spot is in the eye region, it may cause that the eyes are also blue. Booger aka Boo Baby kitty

9 A Ss cat has also white spots but since S is incomplete dominant a Ss cat has 1/3 to ½ spots. KinnerD, 1995 A ss cat does not have white spots expressing a solid color phenotype. Kooguakoogua

10 Another textbook example of incomplete dominance in found in horses. When two true breeding horses with the C gene for Chesnut is mated with a homozygous C cr for cremello, they produce an F1 of intermediate color known as palomino. Write the genotypes for these three phenotypes.

11 Multiple Alleles Not all genes have only two alleles; many have multiple alleles. No matter how many alleles for the gene exist in the multiple allelic series, a diploid individual will have only two alleles, one on each homologous chromosome

12 A well known multiple allele system is the one expressing the fur color of rabbits. The gene C (color) has four alleles. C+, C ch, C h and C a. In this case + is the wild type or normally gray, ch is chinchilla phenotype, h is himalayan phenotype and a, albino phenotype. These genes are completely dominant. Write all the genotypes that you could have in this allele system.

13 Multiple alleles in cats. There are two chemically different kinds of melanin: eumelanin and phaeomelanin. Eumelanin granules are thought to be spherical in shape and absorb almost all light, giving black pigmentation. Phaeomelanin granules are thought to be elongated "footballs" in shape, and reflect light in the red-orange-yellow range.

14 The multiple allele system is comprised of three genes in order of dominance: B, b and b. The alleles are completely dominant. A cat B is black a b is brown (chocolate) and a b is light brown or cinnamon. B b b b

15 Codominance Most genes have more than two alleles in a population (multiple alleles). The ABO blood groups in humans are determined by three alleles, IA, IB, and I. Both the IA and IB alleles are dominant to the i allele The IA and IB alleles are codominant to each other. Because each individual carries two alleles, there are six possible genotypes and four possible blood types.

16 Both IA and IB are dominant to i, while IA and IB are codominant to each other. The resulting phenotypes are: a. People with genotype i/i are blood type O. b. People with genotype IA/IA or IA/i are blood type A. c. People with genotype IB/IB or IB/i are blood type B. d. People with genotype IA/IB are blood type AB.

17 Karl Landsteiner discovered human ABO blood groups in the early 1900s, and received the 1930 Nobel Prize in Physiology or Medicine for this work. Landsteiner's Rule: If an individual has the antigen, he/she will not have the antibody. This is a universal law with few exceptions. ABO antigens are glycolipids (on the surface of the RBC) or glycoproteins (in secretions). ABO antigens are found on RBC's, lymphs, platelets, tissue cells, bone marrow, and organs. These antigens can be secreted by tissue cells if the appropriate genes are present. These antigens must be matched for ransfusions and wrong combinations can lead to pregnancy/newborn problems

18 Biochemistry of the RBC antigens. The A & B antigens are derived from a common precursor known as the H antigen. Activity of the I A gene product, a-n-acetylgalactosamyl transferase, converts the H antigen to the A antigen. Activity of the I B gene product, a-d-galactosyltransferase, converts the H antigen to the B antigen. Both enzymes are present in an IAIB individual, and some H antigens will be modified to the A antigen while others are modified to the B antigen. Neither enzyme is present in an ii individual, and so the H antigen remains unmodified.

19 Production of the H antigen is controlled by a different genetic locus from the ABO enzymes. Rarely, an individual lacks the dominant allele H needed for H antigen production. This hh genotype results in the Bombay blood type, which is similar to type O except that Bombay blood type individuals produce anti-o antibodies that are not seen in true type O individuals.

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21 Figure 14.10x ABO blood types

22 ABO antigens are developed in the uterus at 5-6 weeks of gestation. And full expression of ABO antigens occurs between 2-4 years of age. Another group of cell antigens are the M-N red blood cell antigens that are less important in transfusions but also show codominance. There are three types: Type M, with genotype LM/LM. Type MN, with genotype LM/LN. Type N, with genotype LN/LN. Type Frequency O 45% A 40% B 11% AB 4%

23 In Summary: In codominance, since both alleles are dominant, the heterozygote s phenotype includes the phenotypes of both homozygotes. Examples include: The ABO blood series, in which a heterozygous IA/IB individual will express both antigens, resulting in blood type AB. And the human M-N blood group where individuals with genotype LM/LN will express both antigens.

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