IB 3.1 Genes

What is a difference between two alleles of a gene?

• A.  Their positions on homologous chromosomes

• B.  Their amino acid sequence

• C.  The characteristic they influence

• D.  Their base sequence

A short sequence of amino acids (represented by letters) in the protein cytochrome c is shown for six vertebrates. Letters in bold indicate identical amino acids for all species. Which statement refers to this section of cytochrome c?

• A.  The DNA base sequences for human and rhesus monkey are different.

• B.  The most DNA base changes are between humans and snapping turtles.

• C.  The largest number of amino acid differences is two.

• D.  The amino acid at position 32 is the most variable.

The table shows the estimated total number of genes in several organisms.

What can be deduced from the information in this table?

• A. Throughout evolution, the number of genes increases.

• B. The domestic dog is more closely genetically related to the fruit fly than to the human.

• C. The number of genes does not determine evolutionary success.

• D. Humans produce about half as many proteins as rice.

Answer explanation

The number of genes does not translate 1:1 to the number of proteins made by an organism. (some proteins need polypeptide chains from more than one gene, some genes can code for more than one protein, some genes code for RNA). All of the organisms in the table are successful, because they still exist today.

Which statement defines alleles?

• A. They are the different forms of a gene that have the same effect on the phenotype.

• B. They are the similar forms of a gene in different positions of a chromosome.

• C. They are the various forms of a gene with slight differences in their base sequences.

• D. They are the different forms of a gene coding for identical polypeptide chains.

The electron micrograph shows a section through a plant cell.

In which structure(s) is the genome of the cell contained?

• A. Z only

• B. X, Y and Z only

• C. W and X only

• D. X and Y only

Answer explanation

Section 3.1 (Genes) in the guide defines the genome as "the whole of the genetic information of an organism". Section 1.5 (The origin of cells) requires the candidates to have reviewed the evidence for the endosymbiotic theory so knowledge that some organelles also carry genetic material would be expected.

What is a feature of the human genome?

• A. Plasmids

• B. Messenger RNA

• C. Transfer RNA

• D. Mitochondrial DNA

A mutation is a change in the ........ (select all that apply)

• genetic makeup of an organism

• DNA of an organism

• nucleotide/base sequence of a gene

• phenotype of an organism

• Amino acid sequence of a protein

Answer explanation

Mutations occur in the DNA. They MAY affect the amino acid sequence and phenotype, but many mutations are 'silent' and have no effect on either of these things!

Some yeast genes can be replaced by human genes that then continue to produce the same human proteins in the yeast cells. Which statement helps to explain this evidence?

• A. The DNA of yeast and humans is identical.

• B. Yeast and humans have the same number of chromosomes.

• C. The genetic code is universal.

• D. Yeast and humans are both eukaryotes.

What determines the genomic size of a species?

• A. The total amount of DNA

• B. The total number of genes

• C. The total number of alleles

• D. The total number of chromosomes

What distinguishes an allele from a gene?

• A. An allele is made of RNA.

• B. An allele is shorter.

• C. An allele is a variety of a gene.

• D. An allele cannot be transferred during genetic modification.