MINISTRY OF EDUCATION
SECONDARY ENGAGEMENT PROGRAM
GRADE 11
BIOLOGY
Week 13
Lesson 1
Topic:
Continuity and variation
Sub-topic:
Inheritance
Objectives:
After reading handout, students will:

Correctly define genetic terms.

Accurately explain the inheritance of traits.

Correctly predict the results of crosses involving one pair of alleles in the
heterozygous, homozygous dominant and recessive conditions.
Content
All people are similar because they have some characteristics in common, for example, two eyes,
a nose, a mouth, etc. Yet every human is unique because we inherit varied characteristics from our
parents.
Genetic terms

Dominant: if an allele is dominant, it will always express itself in the phenotype

Recessive: if an allele is recessive, it is only expressed when the dominant allele is not
present

Genotype: describes and individual’s genetic makeup for one characteristic

Heterozygous: refers to an individual in whom a single characteristic is controlled by two
non-identical alleles of the same gene, for example, A a

Homozygous (pure breeding): refers to an individual in whom a single characteristic is
controlled by two identical alleles of the same gene, for example, A A

Phenotype: describes an individual’s physical appearance, for example, brown eye, or
biochemical makeup, for example, blood group A.
Dominant and recessive genes
Each chromosome in a pair carries one allele for each particular characteristic. In a pair of
chromosomes, you can either have two identical alleles (R R) or (r r) or you can have two different
alleles (R r) or (r R). Usually one of the alleles is stronger, this allele is said to be dominant and
it controls what happens when the character is passed on. For example, R is dominant, so a righthanded person would have either the (R R) alleles or the (R r) alleles. Only people with the (r r)
combination of alleles would be left-handed. we say that the allele for the left-handedness is
recessive. In characteristics such as eye and hair colour, the darker colour genes are usually
dominant over recessive, lighter colour genes.
Punnett square
A geneticist R.C. Punnett developed a system known as a Punnet square to show possible
combinations of alleles. The alleles of the gametes of one parent are written along the top. The
alleles of the gametes of the other parent are written down the side. The possible combinations of
alleles are written in the blocks.
Figure showing the use of the Punnett’s square to determine height of offspring
(image taken from Pinterest.com)
Homework
1. Provide an example to illustrate what is meant by incomplete dominance.
2. In some plants, a cross between a dark purple flowering plant and a white flowering plant
results in pale purple coloured offspring.
a. Is this an example of incomplete dominance or co-dominance? Explain why.
b. Draw a diagram to show the offspring and the proportions of each that you could
expect if you crossed two pale purple flowering plants.
References
Exploringnature.org. (n.d.). Punnett's Square Activity - Shows Mendel's Genetics. Retrieved September
16, 2020, from Pinterest.com: https://www.pinterest.com/pin/149674387594360746/
Kirby, P.-G., Madhosingh, L., & Morrison, K. (2014). Biology for CSEC (2nd ed.). Cheltenham, UK: Nelson
Thornes Ltd.