In guinea pigs, the allele for black hair (B) is dominant to the allele for white hair (b) and the allele for long hair (L) is dominant to the allele for short hair (l). A double homozygous guinea pig with long, black hair was bred with another double homozygous guinea pig with long, white hair.
State the genotypes of the two parent guinea pigs.
In 1905 a group of scientists were investigating Mendelian inheritance by carrying out experiments on sweet pea plants. They were looking at the inheritance of two traits, flower colour and pollen grain shape. Flower colour is determined by two alleles, the dominant P for purple and recessive p for red. Pollen grain shape is determined by two alleles, the dominant L for long and recessive l for round.
They started by breeding together a double homozygous dominant plant (PPLL) with a double homozygous recessive plant (ppll) to produce a generation of plants that were all double heterozygous (PpLl).
They then bred together several double heterozygous plants and ended up with 381 offspring.
The outcome of the crosses are as follows:
Phenotype and genotype
Observed
Expected from 9:3:3:1 ratio
Purple flower, long pollen
284
Purple flower, round pollen
21
Red flower, long pollen
21
Red flower, round pollen
55
Complete the third column of the table above with the expected number of individuals of each phenotype if the expected 9:3:3:1 ratio of offspring traits had occurred.
The work of these scientists helped to establish the idea that not all dihybrid crosses produce Mendelian ratios and was later expanded on by another scientist through his work with Drosophila.
Another characteristic that was surveyed was hair colour.
It was found that most individuals had black, brown, blonde, or ginger hair, but a small number of individuals had hair colours like pink, blue and green.
Describe the factors that can cause variation in hair colour.
Some characteristics that are more likely to be examples of continuous variation are those that are coded for by several genes that work in combination to produce the phenotype.
State the scientific term used for this type of characteristic.
In tomato plants, the genes for height and for the type of leaf are autosomally linked.
The allele T, for a tall plant, is dominant to the allele t, for a dwarf plant. The allele M, for normal leaves, is dominant to the allele m, for mottled leaves.
A tomato plant is heterozygous, with both dominant alleles located on one chromosome and both recessive alleles located on the other.
Draw the correct notation to represent the genotype of this plant.
