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Lab. 8 Deviation of Mendel’s first law “Monohybrid” part 1 Main topics: Deviation of Dominance a. Co-dominance b. Incomplete Dominance Deviation of lethal alleles a. Dominance lethal alleles b. Recessive lethal alleles Deviation from Mendel’s law Sometimes during the crosses the ratio obtained does not obey Mendelian’s law. Deviation from Mendel’s law I: 1. Dominance: a. Co-dominance b. Incomplete dominance 2. Lethal alleles c. Dominance lethal alleles d. Recessive lethal alleles 3. Mutation: a. Multiple alleles 4. Sex : a. Sex- linkage b. Sex- limited c. Sex- influences Principles of Genetics 1 1. Deviation of Dominance a. Co-dominance: Co-dominance: is basically means existing together. This means when there are two alleles that share a co-dominant relationship, progeny will express both alleles. For example: (1) In roan horses, the white horse (2) In roan cows, these cows have possibilities below is the father of the foal. The red of several colors of hairs that aren't a blend of horse on the right is the mother and the the roan foal (baby horse) is on the because the traits for each phenotype are far right. dominant. RR = red cow hairs, Rr = red and dominant and recessive phenotypes white cow hairs, rr = white cow hairs Ex. Color of Shorthorn cattle Genetics Analysis: P: Gp: F1: Gf1: F2: Principles of Genetics 2 b. Incomplete Dominance It is the expression of alleles that are neither dominant nor recessive. These alleles mix together and result in expression of a physical trait that is a mixture of the two alleles. For example: In snapdragon flower, the phenotypic ratio and genotypic ratio for F2 generation is the same which is 1:2:1. This is due to incomplete dominance where recombinant progeny is produced. A red flower and white flower cross would produce 1 red flower, 2 pink flowers and 1 white flower. This is due to the intensity of the pigment cannot express itself in heterozygous state and produce a combination of the pigment red and white which is pink. Ex. Color of Snapdragon flower Genetics Analysis: P: Gp: F1: Gf1: F2: 2. Lethal Alleles In 1905, Lucien Cue not observed unusual patterns when studying inheritance of a coat color gene in mice. After mating 2 yellow mice, observed that the offspring never showed a normal 3:1 phenotype but observed 2:1, with 2 yellow mice and one non-yellow mice. Principles of Genetics 3 Essential genes are those that are absolutely required for survival and the absence of their protein product leads to a lethal phenotype • It is estimated that about 1/3 of all genes are essential for survival Nonessential genes are those not absolutely required for survival This is lethal genes cause the death of the organisms that carry them . Sometimes, death is not immediate it may take yeas depending on the gene. Kind of Lethal Genes : a. Postnatal lethal alleles cause abnormalities in the progeny that cause them to die early on in development. An example is parrot jaw, which is observed in cattle. The phenotype results in a deformed mouth so the calf is unable to eat correctly and eventually starves to death. b. Embryonic lethal cause the death of the fetus, and fertility studies are often required in order to positively determine that an embryonic lethal exists. An example of an embryonic lethal is the AY allele in mice (seen in the diagram on the right). This allele causes a yellow phenotype and is dominant to the a+ allele, which causes the grayish agouti color seen in many wild mice. However, embryos that are homozygous for the AY allele die before they are born. Thus, the live progeny from an AYa+ x AYa+ mating show a 2 yellow: 1 agouti phenotypic ratio. Principles of Genetics 4 1. Dominant Lethal Alleles • The ratio 2:1. EX: Huntington disease , coat color gene in mice and Manx cat (Modifies 3:1 ratio to 2:1) • Organisms die before they can produce the progeny. • So, the mutant dominant lethal allele is removed from the population in the same generation it arose Ex. Color fur mice Genetics Analysis: P: Gp: F1: Gf1: F2: Other example: Manx cat Example of Manx Inheritance patterns Mendel's ratio has now changed to 2:1. Principles of Genetics 5 2. Recessive Lethal Allele The recessive lethal allele kills the carrier individual only in homozygous condition. They maybe of two kinds (i) One which has no obvious phenotypic effect in heterozygotes. (ii) One which exhibits a distinctive phenotype when in heterozygous condition. • In many cases, lethal genes become operative at the onset of sexual maturity • Conditional lethal alleles may kill an organism only when certain environmental conditions prevail. For example, temperature-sensitive (ts) lethal A developing Drosophila larva may be killed at 30 C But it will survive if grown at 22 C. Principles of Genetics 6 Ex. Maize seedlings Genetics Analysis: P: Gp: F1: Gf1: F2: Principles of Genetics 7 Exercise8 1- A red carnation is crossed with a white carnation and the resulting offspring are all pink (indicating incomplete dominance). What would be the result of a cross between a pinkand a white carnation? Pink carnation genotype: RW White carnation genotype: WW 2- A curly haired person and a straight haired person mate and all their offspring have wavy hair (note that wavy hair is a phenotype in between that of the curly and straight haired individuals). What would be the result of a cross between two wavy haired individuals? (Give phenotype percent). Principles of Genetics 8 3- In shorthorn cattle, when a red bull (RR) is crossed with a white cow (WW), all the offspring are roan—a spotted, red and white or milky red color. 1. What offspring are expected from mating a roan bull and a roan cow? 2. What phenotypes would you expect from a cross between a red bull and a white cow? 4- What is the F1 genotypic and phenotypic ratio from crossing two yellow individuals? Yy produces yellow fur YY lethal yy Agouti Principles of Genetics 9