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Chapter 4

Chapter 4. MULTIPLE ALLELES. When a given gene has several alleles, not just two A diploid individual still has a maximum of 2 alleles, one on each homologous chromosome. MULTIPLE ALLELES. ABO Blood Groups Discovered in early 1900s Important when considering tranfusions

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Chapter 4

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  1. Chapter 4

  2. MULTIPLE ALLELES • When a given gene has several alleles, not just two • A diploid individual still has a maximum of 2 alleles, one on each homologous chromosome

  3. MULTIPLE ALLELES • ABO Blood Groups • Discovered in early 1900s • Important when considering tranfusions • 4 types; 3 alleles

  4. MULTIPLE ALLELES • ABO Blood Groups • A: IAIA; IAi • B: IBIB; IBi • AB: IAIB • O: ii

  5. MULTIPLE ALLELES • ABO Blood Groups • Antibody – protein molecule that recognizes and binds to foreign material • Antigen – molecule that is recognized as foreign and stimulates antibody production • Don’t stimulate antibody formation in organism expressing them (exception: Autoimmune diseases)

  6. MULTIPLE ALLELES • ABO Blood Groups • IA : specifies the ‘A’ antigen; antibodies against ‘B’ and will clump onto the IB • IB : specifies the ‘B’ antigen; antibodies against ‘A’ and will clump onto the IA • AB : have both antigens, but no “anti-” antibodies • O (ii) : have no antigens and no “anti-” antibodies

  7. MULTIPLE ALLELES • ABO Blood Groups • Safe Transfusions: • A (IAIA /IAi) – can receive A or O • Can give to A or AB • B (IBIB /IBi) – can receive B or O • Can give to B or AB • AB (IAIB) – can receive AB or O • Can give to only AB • O (ii) – can receive only O • Can give to any blood group, A, B or O

  8. MULTIPLE ALLELES • What does this have to do with molecular genetics? • The base pair sequence of a gene specifies amino acid sequence of a protein…this protein function depends on the sequence of amino acids • So, a simple change in the base sequence can drastically change the protein function

  9. Modifications of Dominance • Complete Dominance • One allele is dominant to another, so the heterozygous individual shows the dominant phenotype • Incomplete Dominance • One allele is not completely dominant to another (partial dominance), heterozygous individual shows a new intermediate phenotype

  10. Modifications of Dominance • Incomplete Dominance • Ex. Plumage color in chickens • Cross a true-breeing black (CBCB) with a true-breeding white (CWCW) and the heterozygous F1 offspring (CBCW) exhibits a bluish-grey plumage • C : color • B : black • W : white • Can’t be true-breeders…why?

  11. Modifications of Dominance • Incomplete Dominance • Explanation: believed to occur for this reason… • CB : produces color gene expression • CW : produces no gene expression • So a heterozygous individual produces “half” a dose of gene expression • Insufficient • Heterozygotes that produce a “normal” dominant appearance are described as haplosufficient

  12. Modifications of Dominance • Codominance • one allele is not dominant to another, instead the phenotype produced exhibits both dominant phenotypes • Ex. ABO blood grouping (AB) • Explanation…believed to occur because • Both alleles for competing phenotypes are expressed

  13. In Review • Complete dominance • A/A & A/a produce the same phenotype and can be written as A/- because the second allele does not change the expression of the gene • Incomplete dominance & Codominance • A/A & A/a do not produce the same phenotype so they must be written out as they appear

  14. Modified Mendelian Ratios • Production of NEW Phenotypes • Ex. Comb Shape in Chickens (may be true-breeders) • a) R/- p/p • b) R/- P/- • c) r/r P/- • d) r/r p/p • Assuming recessives do not take any action, we can assume the single comb is a product of other genes while the others are due to the activity of the R and P alleles

  15. Modified Mendelian Ratios • Production of NEW Phenotypes • Ex. Fruit Shape in Summer Squash – available in long, sphere, and disk-shaped • Sphere – A dominant allele of either gene and homozygous recessive of the other • Disk-shaped – A dominant allele of both genes • Long – double homozygous recessive

  16. Modified Mendelian Ratios Produces NO new phenotype • Epistasis • Involves a gene masking or modifying the phenotypic expression of another gene • Interaction between 2 or more genes to control a single phenotype • Confined to dihybrid crosses where two pairs of alleles assort independently • Does not produce a new phenotype, only masks • Epistatic gene – the gene that masks another • Hypostatic gene – the gene that is masked

  17. Modified Mendelian Ratios • Epistasis • Recessive – must be homozygous • Ex: coat color in rodents – natural coat color in wild rodents is a greyish color (produced by alternating bands of black and yellow – agouti pattern) • Aids in camouflage • Found in mice, squirrels, etc • Other colorations exist, but are recessive to agouti • (A/– agouti; a/a nonagouti) • (C/– pigment; c/c albino) • (B/– black; b/b brown) • c is epistatic when homozygous (recessive gene) • A is hypostatic

  18. Modified Mendelian Ratios • Epistasis • Recessive - must be homozygous • Ex: coat color in labrador retrievers – available in black, yellow, & chocolate • One gene specifies black pigment (B/-) or brown (b/b) • An independent gene either allows (E/-) or hides (e/e) the expression of the Black / brown gene • Black: B/- E/- • Chocolate: b/b E/- • Yellow: -/- e/e (B/- black noses; b/b brown noses) • e is epistatic when homozygous (recessive gene) • B is hypostatic

  19. Modified Mendelian Ratios • Epistasis • Dominant • Ex: fruit color in summer squash – available in white, yellow, or green • W/-, -/- white • w/w, Y/- yellow • w/w, y/y green • W – epistatic (homozygous or heterozygous) • y - hypostatic

  20. Modified Mendelian Ratios • Epistasis • Dominant • Ex: Greying in horses • It doesn’t matter what color the horse’s base is (sorrel, black, bay, etc) over time the Grey gene will mask that phenotype • It is a progressive process • Grey does not affect skin or eye color, only hair

  21. Modified Mendelian Ratios • Epistasis • Duplicate Genes • When a gene at one locus produces a phenotype identical to that produced at another locus • Ex: sweet peas flower colors • C: colored • c: no color • P: purple • p: white • Purple flowers: C/- P/- • White flowers: c/c -/- OR C/- p/p (duplicate recessive epistasis OR complementary gene action) when 1 or both loci are homozygous recessive

  22. Modified Mendelian Ratios • Essential & Lethal Genes • Mutations not only change phenotypes, they can also cause death (which I guess technically does change the phenotype) • Alleles resulting in death are lethal alleles, caused by essential genes (essential to the normal functioning of the organism) • When caused by a dominant lethal allele both the heterozygous and homozygous individuals will show the lethal phenotype • When caused by a recessive lethal allele, only the homozygous individual will show the lethal phenotype

  23. Modified Mendelian Ratios • Essential & Lethal Genes • Lethal alleles • Ex: Yellow body color in mice • Acts dominant in determining body color, but acts recessive in determining lethality (only heterozygotes survive to birth) • Ex: Huntington’s disease in humans • Autosomal dominant (can’t be studied until reproductive age) • Onset doesn’t appear until early-thirties, and death in forites • Ex: Hemophilia in humans • X-linked recessive

  24. Modified Mendelian Ratios • Gene Expression • Penetrance – frequency with which a gene manifests itself in individuals in the population • Depends on genotype and environment • Expressivity – degree to which a gene or phenotype are expressed in an individual • Environment – • Age of onset: creates internal environmental changes • Genes are not “on” all the time; genes can be activated or deactivated over time • Pattern baldness • Muscular Dystrophy

  25. Modified Mendelian Ratios • Gene Expression • Environment – • Sex – expression of genes are influenced by gender • Sex-limited traits –autosomal genes that affect only 1 gender and not the other • Ex: milk production • Ex: appearance of horns in some species • Ex: facial hair • Temperature – Reactions are catalyzed by enzymes, which function in a certain range. • Ex: fur color in Himalayan rabbits • >30 C – all white • <25 C – typical coloration (black paws, ears, nose tail) and anywhere it is artificially cooled

  26. Modified Mendelian Ratios • Gene Expression • Environment – • Chemical – can have significant effect on an organism • EX: Phenylketonuria (PKU): autosomal recessive, trouble metabolizing amino acid phenylalanine, diet determines severity (proteins) • Nature vs Nurture • What are the relative contributions of genes and the environment to the phenotype? • Ex: Height – influenced by genes (potential) and environment (diet, overall health, hormones) • Ex: Alcoholism – influenced by genes (susceptibility) and environment (choice) • Ex: Intelligence – influenced by genes (potential) and environment (learning, challenges)

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