2 Diploid organisms contain two copies of each gene in each of their cells. In sexual
reproduction, gametes are formed containing one copy of each gene. Each off spring
receives two copies of each gene, one from each of its parents.
3 The cells produced by meiosis are genetically different from each other and from their
cell. This results from independent assortment of the chromosomes as the bivalents line
up on the
equator during metaphase I, and also from crossing over between the chromatids of
chromosomes during prophase I.
4 Genetic variation also results from random fertilisation, as gametes containing diff
of genes fuse together to form a zygote.
5 An organism‟s genetic constitution is its genotype. The observable expression of its
genes is its phenotype.
6 Different varieties of a gene are called alleles. Alleles may show dominance,
codominance or recessiveness. An organism possessing two identical alleles of a gene
is homozygous; an organism possessing two different alleles of a gene is heterozygous.
If a gene has several diff erent alleles, such as the gene for human blood groups, these
are known as multiple alleles.
7 The position of a gene on a particular chromosome is its locus.
8 A gene found on the X chromosome but not on the Y chromosome is known as a sexlinked gene.
9 The genotype of an organism showing dominant characteristics can be determined by
looking at the off spring produced when it is crossed with an organism showing
recessive characteristics. This is called a test cross.
10 Monohybrid crosses consider the inheritance of one gene. Dihybrid crosses consider
the inheritance of two diff erent genes.
11 The χ2 test can be used to find out whether any diff erences between expected
results and observed results of a genetic cross are due to chance, or whether the
difference is significant.
12 The genotype of an organism gives it the potential to show a particular characteristic.
In many cases, the degree to which this characteristic is shown is also affected by the
13 Mutation can be defined as an unpredictable change in the base sequence in a DNA
molecule (gene mutation) or in the structure or number of chromosomes (chromosome
mutation). New alleles arise by gene mutation. Gene mutations include base
substitutions, deletions or additions. The HbS (sickle cell) allele arose by base
substitution. Such mutations may affect the organism‟s phenotype.
1. End-of-chapter questions
1. A cell in the process of meiosis was seen to have a spindle with sister chromatids
being drawn towards opposite poles of the cell. In what stage of meiosis was the
A anaphase I
B anaphase II
C metaphase I
D metaphase II
2 All the offspring of a cross between pure-bred red-flowered and pure-bred whiteflowered snapdragons were pink.
Two of these pink-flowered plants were interbred. What proportion of the offspring were
3 A man has haemophilia. Which statement correctly describes the inheritance of the
gene causing his condition?
A He inherited the recessive allele from his mother.
B He inherited the dominant allele from his father.
C He can pass the recessive allele to a son.
D He can pass the dominant allele to a daughter.