Punnett Squares

Dominant Vs. Recessive A trait can be either dominant or recessive The trait that WILL be expressed in the offspring if the parent contributes it Represented by a CAPITAL letter Recessive The trait that must be contributed by both parents to be expressed in the offspring Represented by a lowercase letter

Dominant vs. Recessive Each parents will have two letters which represent one allele inherited from of their parents. They will pass one of those alleles to their offspring. An organism with two copies of the same allele, either two dominant or two recessive alleles, is said to be HOMOZYGOUS or purebred (AA,aa) An organism with one dominant and one recessive allele is said to be HETEROZYGOUS or hybrid (Aa)

Punnett Squares Because trying to think of all the possible outcomes of a combination of genes for an offspring can be difficult, we use a tool called a Punnett Square A Punnett Square is used to organize all the possible combinations of alleles that a set of specific parents could pass onto an offspring The letters representing the genes of the parents go on the outside of the square with one letter going on the outside of each box The letters are then carried down and across the squares The result is all of the possible combinations of alleles that the offspring could have

Examples g g 100% Gg (Heterozygous) G G g G g G G g G g A green pea plant (GG) is crossed with a yellow pea plant (gg). Genotype: Phenotype: g g 100% Gg (Heterozygous) G G g G g G G g G g 100% Green pea plants

The Results The results of each Punnett square give us a genotype and a phenotype The phenotype is an organism’s appearance or other detectable characteristics Genotype is the genetic makeup of the organism The results are always expressed as a ratio or percentage

Examples B B 50% BB (homozygous dominant) B B B B B b B b B b A brown mouse (Bb) is crossed with another brown mouse (BB). Tan is the recessive color. Genotype: Phenotype: B B 50% BB (homozygous dominant) 50% Bb (heterozygous) B B B B B b B b B b 100% Brown mice

Examples H h H H H H h h H h h h 75% Tall Plants 25% Short Plants A heterozygous tall plant is crossed with another heterozygous tall plant. Genotype: Phenotype: H h 25% HH(homozygous dominant) 50% Hh (heterozygous) 25% hh (homozygous recessive) H H H H h h H h h h 75% Tall Plants 25% Short Plants

50% Ee (heterozygous) Examples 50% ee (homozygous recessive) e e E E e A heterozygous floppy eared rabbit is crossed with a recessive homozygous straight eared rabbit Genotype: Phenotype: e e 50% Ee (heterozygous) 50% ee (homozygous recessive) E E e E e e e e e e 50% floppy eared 50% straight eared