Inheritance: Answers

Which type of base always pairs with guanine and which always pairs with adenine?

Guanine always pairs with cytosine and adenine always pairs with thymine. This happens because these pairs of bases have matching (complementary) shapes.

What are the two differences between DNA and RNA?

RNA contains the base uracil instead of thymine and RNA is single-stranded whereas DNA is double-stranded.

Describe the process of translation, by which the RNA molecule is used to build a protein.

The RNA molecule leaves the nucleus and enters the cytoplasm where it attaches itself to a ribosome. The ribosome moves along the RNA stand, ‘reading’ the different codons. A codon is a sequence of three bases (e.g. AUG) which correspond to a particular amino acid. A tRNA molecule with the complementary anti-codon sequence binds temporarily to the RNA, bringing an amino acid with it. As the ribosome moves along the RNA, more amino acids are dropped off. Bonds form between each of the amino acids to produce a polypeptide chain which eventually folds into a protein.

What is meant by the terms ‘dominant’ and ‘recessive’ alleles?

An allele is a version of a gene. We have two alleles for each gene, one on each chromosome. A dominant allele will always be expressed, even if there is only one of them. A recessive allele will only expressed in the absence of a dominant allele (i.e. two recessive alleles are present).

Using a Punnet square, predict the genotypes and phenotypes of the offspring formed from a man which has a homozygous dominant genotype and a woman who has a homozygous recessive genotype for eye colour (brown is dominant and blue is recessive).

Remember that homozygous means that both alleles are the same. When we have two different alleles, we call that heterozygous.

The homozygous dominant genotype is BB and the homozygous recessive genotype is bb. If we write these on the top and sides of a Punnet square, then fill in the genotypes of the offspring in the middle, we can see that all of them have a heterozygous phenotype (Bb). Since the dominant allele coding for brown eyes (B) is present, it will be expressed and all of the offspring will have brown eyes.