25: Basic Genetics

Question 1

State the definition of heredity and variations.

Answer 1

Heredity is the passing on of characteristics from parents to offspring. Variations are the differences among the individuals of a species.

Question 2

State the definition of monohybrid inheritance.

Answer 2

A type of inheritance involving one pair of contrasting characters.

Question 3

State the definition of a gene.

Answer 3

A gene is the basic unit of heredity. It is a segment of a DNA molecule on a chromosome that controls a particular characteristic.

Question 4

State the definition of alleles.

Answer 4

Alleles are different forms of a gene, which occur at the same position on a pair of homologous chromosomes.

Question 5

State the definition of gene locus.

Answer 5

A gene locus is the position of a gene on a chromosome.

Question 6

Explain the difference between heterozygous and homozygous individuals.

Answer 6

Homozygous individuals have the same two alleles at the same gene locus on a pair of homologous chromosomes, while heterozygous individuals have different alleles at the same gene locus on the pair of homologous chromosomes chromosomes.

Question 7

State the difference between genotype and phenotype.

Answer 7

Genotype is the genetic make-up of an individual while the phenotype is the observable character of an individual.

Question 8

Explain the difference between dominant and recessive alleles.

Answer 8

Dominant alleles can express itself in heterozygous condition, while recessive alleles are masked in heterozygous condition.

Question 9

Name two graphical representations of monohybrid cross.

Answer 9

Genetic diagrams, Punnett square

Question 10

Explain the difference in the actual outcomes of genetic crosses compared to those predicted by genetic diagrams or Punnett squares.

Answer 10

If the sample size is large, the proportions of offspring having a certain genotype or phenotype can be predicted. However, if the sample size is small, the observed results will differ because fertilisation of gametes is random. The outcome of any cross is also unrelated to that of any other.

Question 11

Explain the meaning and application of a test cross.

Answer 11

A test cross is a cross of an organism with a dominant character but of an unknown genotype with a homozygous recessive organism. This can identify the genotype of the organism.

Question 12

State the expected results of a test cross.

Answer 12

If only organisms showing the dominant characteristic appear in the offspring, the organism being tested is probably homozygous dominant.
If organisms showing the recessive character appear in the offspring, the organism being tested is probably heterozygous.