Practise genes-and-phenotype questions by building genetic diagrams, testing expected ratios and tracing how alleles change proteins, traits or disease outcomes.
Wing pattern in the butterfly species Heliconius melpomene is controlled by genes on autosomal chromosomes.
The gene for banding pattern on the upper wing has two alleles:
- a dominant allele coding for a full band
- a recessive allele coding for a broken band.
The gene for ray pattern on the lower wing has two alleles:
- a dominant allele coding for rays
- a recessive allele coding for no rays.
Scientists crossed a butterfly that was homozygous dominant for both genes with a butterfly that was homozygous recessive for both genes. The scientists wanted to check whether the phenotypic ratio for offspring in the F2 generation agreed with the expected phenotypic ratio of 9:3:3:1.
The results of this genetic cross are shown in Fig. 4.2.
Fig. 4.2
Explain the term F1 generation.
Suggest two reasons why phenotypic ratios in the F2 generation do not always match the expected ratios.
A mutation causing coats of mice to be woolly in appearance is in a gene located on chromosome 11. The mutation causes a very shortened polypeptide product. Mice with the woolly coat phenotype have longer fur than mice with normal coats.
The inheritance of the woolly coat characteristic was investigated.
Draw a genetic diagram to show a cross between two heterozygous parents with normal coats.
Use the symbols A and a for the alleles.
parental genotypes
gametes
offspring genotypes
offspring phenotypes
Some plants are grown commercially for their flowers. Many of these plants are varieties that have short stems.
Two factors that affect the height of stems are:
- gibberellin
- the Le/le gene.
The Le/le gene has two alleles, Le and le.
Explain how the Le/le gene and gibberellin are involved in affecting the height of plant stems.