Genotype and Phenotype

1. Genotype and Phenotype • The genotypeis the genetic makeup of an organism. • If an individual has two identical alleles of a certain gene, the individual is homozygous for the related character. • If an individual has two different alleles of a certain gene, the individual is heterozygous for the related character. • The phenotype is the appearance of an organism. • Thus, genotype determines phenotype.

2. Probability • Probability is the likelihood that a specific event will occur. number of one kind of possible outcome • Probability = ------------------------------------------------------ total number of all possible outcomes • A probability may be expressed as a fraction or a percentage

3. Predicting Results of Monohybrid Crosses • A cross in which one characteristic is tracked is a monohybrid cross. • A Punnett square can be used to predict the outcome of genetic crosses. • A Punnett square shows all of the genotypes that could result from a given cross. • The simplest Punnett square consists of a square divided into four boxes. • The combination of letters in each box represents one possible genotype in the offspring.

4. Monohybrid Crosses • Homozygous x Homozygous • In tomatoes, red fruit (R) is dominant over yellow fruit (r). A plant that is homozygous for red fruit is crossed with a plant that has yellow fruit. Use a Punnett square to illustrate the offspring produced.

5. Homozygous x Heterozygous • In guinea pigs, short hair (S) is dominant to long hair (s). What are the genotypic and phenotypic ratios of a cross between a heterozygous short haired guinea pig with a long haired guinea pig?

6. Heterozygous x Heterozygous • For humans, brown eyes (B) are dominant over blue eyes (b). A heterozygous brown-eyed man marries a heterozygous brown-eyed female. Give the genotypic and phenotypic ratios of the offspring.

7. Testcross • How can we tell the genotype of an individual with the dominant phenotype? • Such an individual must have one dominant allele, but the individual could be either homozygous dominant or heterozygous • The answer is to carry out a testcross: breeding themystery individual with a homozygous recessive individual • If any offspring display the recessive phenotype, the mystery parent must be heterozygous

8. In dogs, there is an hereditary deafness caused by a recessive gene, “d.” A kennel owner has a male dog that she wants to use for breeding purposes if possible. The dog can hear, so the owner knows his genotype is either DD or Dd. If the dog’s genotype is Dd, the owner does not wish to use him for breeding so that the deafness gene will not be passed on. Explain how the owner can determine if the dog’s genotype is DD or Dd. Use Punnett squares to support your point