Mendelian Genetics Some of the Basics.

Alleles Alleles are alternate forms of the same gene. A homologous pair of chromosomes contain two alleles, one in the chromosome derived from the father, and the other in the chromosome derived from the mother.

Allele types Dominant alleles Trait shown will mask the recessive allele carried. Represented by a capital letter (B) Example: black fur is dominant in bunnies. Recessive alleles Trait will only show if two of these alleles are present. Represented by a lowercase letter. (b) Example: white fur is recessive in bunnies.

Genotype and Phenotype Genotype The actual alleles present in the cells. ex:Bb Phenotype The trait that is shown. ‘What we see.” ex: pink flowers, blue eyes or sickle-cell anemia.

Complete dominance traits Complete dominance traits come in three possible gentoypes, but only show two possible phenotypes.

Homozygous dominant Two dominant alleles are present in the genotype: AA True-breeder. Shows dominant trait in the phenotype. Human example: brown hair.

Homozygous recessive Two recessive alleles are present in the genotype: aa True-breeder. Shows recessive trait in the phenotype. Human example: blonde hair.

Heterozygous dominant One dominant allele and one recessive allele are present in the genotype: Aa Hybrid, carrier. Shows dominant trait in the phenotype. Human example: brown hair.

Punnett squares and Monohybrid crosses Punnett squares are a record keeping tool. We can determine genotypic ratio and phenotypic ratio in the offspring using them. Parent cross is shown as Cc x Cc. We call it monohybrid because we are only using one trait at a time.

Genotypic ratio For complete dominance there are three possible genotypes. AA:Aa:aa The punnett square at the left shows a ratio of 1:2:1

Phenotypic ratios There are only two possible phenotypes for complete dominance Dominant:recessive The punnett square at the left shows a ratio of 3:1

Ratios The cross would be written as RR x Rr. The genotypic ratio is 2:2:0 The phenotypic ratio is 4:0

What goes inside? What is placed into the following punnett square? What is the genotypic ratio? What is the phenotypic ratio? Rrrr Rrrr 0:2:22:2

Probability What is the probability that the kids of a heterozygous dominant male and a homozygous recessive female will have brown hair? It will be 50%

Why they are called carriers

Using a Test Cross. We will assume that green is dominant and that clear is a recessive trait in fish. Since a green fish can be either GG or Gg, we could label it “ G?.” What should “ G?” be crossed with to determine which alleles it has? A test cross should be done, using a “homozygous recessive” clear fish. G? x gg

Diagram the possible crosses If G? is GG, then GG x gg If G? is Gg, then Gg x gg All offspring G green bodies All offspring (baby fish) will be Gg and have green bodies. Half the offspring Ggreen bodies Half the offspring will be Gg - green bodies, half half will be gg – clear. GGgGGg GGgGGg G g gg G G gg GGgGGg GGgGGg GGgGGg GGgGGg g g

Conclusion of test cross If G? is GG, then all of the offspring (baby fish) will be Gg and have green bodies; -OR- If G? is Gg, then half of the offspring will be Gg and have green bodies, half will be gg and have clear bodies.