Mendel s Second Set of Experiments Dihybrid Crosses

Transcription

1 Mendel s Second Set of Experiments Dihybrid Crosses Mendel s first set of experiments proved the Law of Segregation; an individual contains factors which segregate from one another during meiosis. Each parent contributes one member of a pair of alleles to an offspring. For his second set of experiments he crossed pea plants to consider the outcomes for seed shape and pod colour at the same time: Seed Shape: R round Pod Colour: Y- yellow r - wrinkled y green Round and yellow X Wrinkled and green F1 All were round and yellow F2 9 round yellow, 3 round green, 3 wrinkled yellow, 1 wrinkled green The F2 generation proved that alleles for different characteristics assort independently. For example: Round shape was not always found with yellow colour in the offspring of F2. This is known as the Law of Independent Assortment. Proving Mendel s Dihybrid Crosses: Cross #1 (F1): RRYY x rryy Cross #2 (F2): RrYy x RrYy

2 Example: Black hair is dominant to red. Dark eyes are dominant to light eyes. What are the phenotypic ratios of the offspring produced when a heterozygous black haired, light eyed female has children with a red haired, male heterozygous for dark eyes? Hints: Each gamete (sex cell) has to get one allele from each characteristic. The gamete options can be simplified to make the punnett square smaller. Keep letters for each characteristic together and in the same order. Always write a dominant allele first.

3 Dihybrid Cross Practice 1. In humans, right-handedness is dominant to left-handedness and green eyes are dominant over grey. a. In a family, the husband is homozygous dominant for right-handedness and for green eyes. His wife is left-handed and has grey eyes. Determine the possible genotypes and phenotypes of their children. b. If one of the children (from part a) marries a person who is homozygous dominant for both traits, what are the possible genotypes and phenotypes of their children? 2. In rabbits, spotted coat is dominant to solid coat and black colour is dominant to brown. Pure bred black solid rabbits are mated with brown pure bred spotted. a. What are the genotypes of the F1? b. If two of the F1 rabbits mate, what would be the appearance of the F2? 3. What are the expected phenotypes of offspring produced by a cross between a heterozygous black, short-haired guinea pig and a homozygous white, long-haired guinea pig? Assume black colour and short hair are dominant characteristics. 4. In watermelons, the genes for green colour and for short shape are dominant over the alleles for striped colour and for long shape. Suppose a plant with long striped fruit is crossed with a plant that is heterozygous for both of these characteristics. What phenotypes would this cross produce and in what ratio? 5. In humans the ability to taste the chemical called PTC is dominant to not being able to taste it. If a person with type O blood that can t taste PTC has children with a person who is heterozygous for type A blood and homozygous for being able to taste the paper: a. What is the percent chance they will have a type O, PTC tasting child? b. What are the phenotypic ratios for the offspring combinations?

4 Mendel s Second Set of Experiments Dihybrid Crosses Mendel s first set of experiments proved the Law of Segregation; an individual contains factors which segregate from one another during meiosis. Each parent contributes one member of a pair of alleles to an offspring. For his second set of experiments he crossed pea plants to consider the outcomes for seed shape and pod colour at the same time: Seed Shape: R round Pod Colour: Y- yellow r - wrinkled y green Round and yellow X Wrinkled and green F1 All were round and yellow F2 9 round yellow, 3 round green, 3 wrinkled yellow, 1 wrinkled green The F2 generation proved that alleles for different characteristics assort independently. For example: Round shape was not always found with yellow colour in the offspring of F2. This is known as the Law of Independent Assortment. Proving Mendel s Dihybrid Crosses: Cross #1 (F1): RRYY x rryy Cross #2 (F2): RrYy x RrYy

5 Example: Black hair is dominant to red. Dark eyes are dominant to light eyes. What are the phenotypic ratios of the offspring produced when a heterozygous black haired, light eyed female has children with a red haired, male heterozygous for dark eyes? Hints: Each gamete (sex cell) has to get one allele from each characteristic. The gamete options can be simplified to make the punnett square smaller. Keep letters for each characteristic together and in the same order. Always write a dominant allele first.

6 Dihybrid Cross Practice 1. In humans, right-handedness is dominant to left-handedness and green eyes are dominant over grey. a. In a family, the husband is homozygous dominant for right-handedness and for green eyes. His wife is left-handed and has grey eyes. Determine the possible genotypes and phenotypes of their children. b. If one of the children (from part a) marries a person who is homozygous dominant for both traits, what are the possible genotypes and phenotypes of their children? 2. In rabbits, spotted coat is dominant to solid coat and black colour is dominant to brown. Pure bred black solid rabbits are mated with brown pure bred spotted. a. What are the genotypes of the F1? b. If two of the F1 rabbits mate, what would be the appearance of the F2? 3. What are the expected phenotypes of offspring produced by a cross between a heterozygous black, short-haired guinea pig and a homozygous white, long-haired guinea pig? Assume black colour and short hair are dominant characteristics. 4. In watermelons, the genes for green colour and for short shape are dominant over the alleles for striped colour and for long shape. Suppose a plant with long striped fruit is crossed with a plant that is heterozygous for both of these characteristics. What phenotypes would this cross produce and in what ratio? 5. In humans the ability to taste the chemical called PTC is dominant to not being able to taste it. If a person with type O blood that can t taste PTC has children with a person who is heterozygous for type A blood and homozygous for being able to taste the paper: a. What is the percent chance they will have a type O, PTC tasting child? b. What are the phenotypic ratios for the offspring combinations?