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 4 types; 3 alleles

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

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)

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

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

Figure 4.4 Antigenic reactions that characterize the human ABO blood types. Blood serum from each of the four blood types was mixed with blood cells from the four types in all possible combinations. In some cases, such as a mix of B serum with A cells, the cells become clumped.

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

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

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?

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

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

In Review Complete dominance Incomplete dominance & Codominance 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

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

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

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

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

Figure 4.11 Recessive epistasis: generation of an F2 9 agouti : 3 black : 4 white ratio for coat color in rodents.

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

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

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

Figure 4.15 Dominant epistasis: Dominant greying allele in the horse causes the coat to turn grey as the horse matures. A horse (my Lipizzaner) is shown at age 4 (top) and age 7 (bottom).

Figure 4.15 Dominant epistasis: Dominant greying allele in the horse causes the coat to turn grey as the horse matures. A horse (my Lipizzaner) is shown at age 4 (top) and age 7 (bottom).

Figure 4.15 Dominant epistasis: Dominant greying allele in the horse causes the coat to turn grey as the horse matures. A horse (my Lipizzaner) is shown at age 4 (top) and age 7 (bottom).

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

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

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

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

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

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)