This document provides information about genetics and inheritance from a grade 12 lesson. It discusses: 1. What genetics is and how it relates to heredity and similarities/differences between parents and offspring. 2. Gregory Mendel's pioneering genetics experiments in the 1800s using pea plants which discovered the basic principles of heredity and inheritance through generations. He demonstrated dominant and recessive traits and that genes exist in different alleles. 3. How to solve genetic cross problems by determining the genotypes and phenotypes of parents and predicting offspring based on dominant and recessive traits. It provides examples of monohybrid and dihybrid cross problems involving traits like plant height, eye color in rabbits, and seed shape

1. GENETICS AND INHERITANCE
GRADE -12
Ms O MANTYI SLIDES

2. 1. WHAT IS GENETICS
• Genetics: The study of heredity.
• Heredity is the relations between
successive generations.
• Why do children look a little bit like
their parents but also different?
• What is responsible for these
similarities and differences?

3. 2. MENDEL’S GENETICS
• Gregory Mendel is the father of
Genetics.
• Mendel discovered the basic
principles of heredity by breeding
garden peas in carefully planned
experiments.
• Advantages of pea plants for genetic
study: Cross-pollination (fertilization
between different plants) can be
achieved by dusting one plant with

5. • He also used varieties that were true-
breeding (organisms with only one variety
of a type e.g. red flowers can only produce
red flowers)
• In a typical experiment, Mendel mated two
contrasting, true-breeding varieties, a
process called hybridization
• The true-breeding parents are the P
generation.
• The hybrid offspring of the P generation
are called the F1 generation
• When F1 individuals self-pollinate, the F2

6. • When Mendel crossed contrasting, true-
breeding white and purple flowered pea
plants, all of the F1 hybrids were purple

7. • When Mendel crossed the F1 hybrids,
many of the F2 plants had purple
flowers, but some had white
• Mendel discovered a ratio of about
three to one, purple to white flowers,
in the F2 generation.

9. • Mendel reasoned that only the purple
flower factor was affecting flower color
the F1 hybrids.
• Mendel called the purple flower color a
dominant trait and the white flower
recessive trait
• What Mendel called a “heritable factor”
is what we now call a gene
• He did 7 other crosses using different traits
and found the same phenomenon.

11. • Mendel noted that the gene for flower color
for example exists in two versions, one for
purple flowers and the other for white
• These alternative versions of a gene are now
called alleles
• Each gene is found at a specific locus
(position) on a specific chromosome.

12. • The two alleles at a locus on a
homologous chromosome pair may be
identical, as in the true-breeding plants
they are then said to be homozygous for
that trait/gene.
• Alternatively, the two alleles at a locus
may differ – they are said to be
heterozygous for that gene/trait.
• If the two alleles at a locus differ, then
one (the dominant allele) determines the
organism’s appearance (we refer to it as
its phenotype), and the other (the

14. • Mendel then formulated the law of
segregation, states that the two alleles
a heritable character separate
during gamete formation and end up in
different gametes
• Thus, an egg or a sperm gets only one of
the two alleles that are present in the
somatic cells of an organism.

15. • An organism traits are indicated via its
genotype and phenotype.
 Genotype: The genetic composition of
the gene, indicated by letters e.g. GG,
Gg, gg. (A capital letter represents a
dominant allele, and a lowercase letter
represents a recessive allele)
 Phenotype: The external appearance of
the gene e.g. Brown hair, white hair.

16. 3. GENETIC CROSSES
• HOW CAN WE NOW MORE OF LESS
DETERMINE WHAT WILL BE THE
OUTCOME IF 2 ORGANISMS HAVE A
BABY?

17. TWO TYPES OF
GENETIC CROSSES
• MONOHYBRID CROSSES: A cross
between 2 organisms where we only
look an one pair of contrasting traits.

18. MONOHYBRID CROSS -EXAMPLE
• Determine the outcome/ F1
generation of a cross between a
homozygous tall plant and a
homozygous short plant. Tall plants
are dominant over short plants.

19. STEPS TO SOLVE A CROSS PROBLEM
1. What trait are we looking at?
2. Choose a letter to represent the trait.
3. See if you can identify which trait is dominant –
allocate the capital letter to that trait.
4. Identify the recessive trait and allocate a lower
case letter to that trait.
5. Determine the genotypes of the parents. –
Homozygous dominant – Two capital letters e.g.
GG
Homozygous recessive – Two lower case letter.
E.g. gg
Heterozygous – One capital letter and one lower
case letter e.g. Gg

20. SOLUTION
1. Trait – Size of plant.
2. Letter chosen to represent size of plant =
T/t
3. Tall plants are dominant. (Given in
problem) – Given the – “T” (capital T)
4. Short plants are recessive – given the “t”
(lower case t)
5. One parent is homozygous tall – TT
other parent is homozygous short - tt

21. CROSS SHOWN AS A GENETIC DIAGRAM
Why?
Tall is dominant over short plants – Babies have
both alleles: tall and short

22. Cross shown as a punnet square

23. CROSS BETWEEN F1 GENERATION
INDIVIDUALS (INTERBREED F1 GENERATION)

24. MONOHYBRID CROSS –EXAMPLE 2
A heterozygous blue eyed rabbit is
crossed with a rabbit with pink eyes.
What is the possibility of the babies
being born with pink eyes?

25. SOLUTION
1. Trait: eye colour of rabbit.
2. Letter used: E/e
3. Dominant trait: Blue eyes (Why? The first
is heterozygous – both alleles – but blue is
being expressed in rabbit eyes.) = E
4. Recessive trait: pink eyes = e
5. Rabbit one – heterozygous: Ee
Rabbit two – homozygous: ee (why?)
The only way that a rabbit can have pink eyes
expressed externally is if both alleles code
pink eyes.

27. EXAMPLE OF A DIHYBRID CROSS
Determine the F2 generation of a cross
between yellow round seeded peas
and wrinkled green seeded peas.
Yellow and round seeds are