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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.

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Presentation on theme: "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."— Presentation transcript:

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: I A I A ; I A i  B: I B I B ; I B i  AB: I A I B  O: ii

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6 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)

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

8 MULTIPLE ALLELES ABO Blood Groups – Safe Transfusions: A (I A I A /I A i) – can receive A or O – Can give to A or AB B (I B I B /I B i) – can receive B or O – Can give to B or AB AB (I A I B ) – 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

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10 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

11 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

12 Modifications of Dominance Incomplete Dominance – Ex. Plumage color in chickens Cross a true-breeing black (C B C B ) with a true-breeding white (C W C W ) and the heterozygous F 1 offspring (C B C W ) exhibits a bluish-grey plumage – C : color – B : black – W : white Can’t be true-breeders…why?

13 Modifications of Dominance Incomplete Dominance – Explanation: believed to occur for this reason… C B : produces color gene expression C W : 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

14 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

15 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

16 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

17 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

18 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

19 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

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21 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

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23 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/ygreen » W – epistatic (homozygous or heterozygous) » y - hypostatic

24 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

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29 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

30 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

31 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

32 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

33 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

34 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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