Question

5. How many recessive alleles for a trait must an organism inherit in order to have that trait?

6. Gregor Mendel crossed true-breeding purple-flowered plants with true-breeding white-flowered plants. Explain why all the offspring were purple.

7. Name and explain the categories of gene mutations.

8. What is the role of tRNA in translation?

9. Explain the makeup of the DNA molecule.

10. Define codon.

11. What do the filled-in squares of a Punnett square represent?

Answer 1

5. If a dominant allele is present it masks the presence of any recessive allele in the organisms. Since each gene has two alleles. The two alleles must be recessive allele to have the trait associated with that allele. Therefore, an organism must inherit two recessive alleles in order to have the trait associated with the allele

6. When true breeding purple and white flowered plants are crossed it results in an heterozygous individual carrying one allele coding for purple coloured flower and one allele coding for white coloured flower. As dominant allele masks the presence of recessive allele all the offsprings will express dominant phenotype. Since, purple coloured flower is a dominant characteristics, all plants have purple coloured flower .

7. Mutations are undesirable change in the nucleotide sequence of a gene. There are various types of it which are listed in the table below

S.No	Mutation	Characteristics
1	Substitution	A nucleotide is substituted by another nucleotide
2	Deletion	Single or set of nucleotides removed from the DNA
3	Insertion	Single or set of nucleotides are inserted in the original DNA
4	Translocation	In which a part of DNA sequence is translocated to a new position where it is originally not present

Point mutation - Change in a single nucleotide of a gene. It is of three types

S.No	Mutation	Characteristics
1	Silence	Change in the nucleotide that codes for the same or similar amino acid as the original nucleotide codes for
2	Missense	Change in the nucleotide that codes for different amino acid than that is coded by the original nucleotide
3	Non-sense	Change in the nucleotide that converts a codon specifying an amino acid into a stop codon

Transition mutation - where a purine nucleotide is replaced by another purine nucleotide and a pyrimidine nucleotide is replaced by pyrimidine nucleotide

Transversion mutation - where a purine nucleotide is replaced by a pyrimidine nucleotide and vice versa

8. tRNA - transfer RNA which is in a clover leaf like structure is a type of RNA that involves in transfer of amino acids to the ribosomes involved in synthesizing proteins

9. DNA molecule is made up of four deoxyribonucleotides namely dATP, dGTP, dCTP and dTTP attached to each other in the same strand by phosphodiester bond and in the opposite strands by hydrogen bonding. DNA molecule is packed in a condensed manner by a set of proteins called as Histones. Histone proteins are of five types H1, H2a, H2b, H3 and H4 where the H1 binds to linker DNA and other four forms a core called nucleosomes.

10. A codon is a tri-nucleotide in the messenger RNA that codes for an amino acid during the translation process

11. Punnet square - It is a square diagram used to predict the genotypes of the offsprings resulting from a cross between two individuals.

The filled in squares of the Punnet square represents the genotypes of the offsprings that will result when the two individuals under study are crossed. It can be used to predict the ratios of different genotypes that result from a cross.