Download Mendel`s Experiments

Noadswood Science, 2012
Mendel’s Experiments

Tuesday, May 23, 2017
To be able to explain Mendel’s experiments and why he is deemed
the ‘father of genetics’
Inherited

Children usually look a little like their father, and a little like their
mother, but they will not be identical to either of their parents

Why is this?

Offspring get half of their inherited features from each parent

During fertilisation, the nucleus from the sperm cell joins with the
nucleus in the egg cell, and a new nucleus is formed with all the
genetic information needed
Inherited

Some variations are inherited, whilst other variations are due to
environmental factors

Inherited variation is a characteristic you have got from your parents
- what can you inherit?
 Gender
 Eye colour
 Hair colour
 Skin colour
 Lobed or lobeless ears
Lobed (left) & lobeless (right) ears
Alleles

Some characteristics, such as eye colour and the shape of the
earlobe, are controlled by a single gene – these genes may have
different forms which are called alleles (one form of a gene)

The gene for eye colour has an allele for blue eye colour and an
allele for brown eye colour

Alleles can be dominant
(expressed) or recessive
(masked or suppressed when
in the presence of a dominant
allele)
Alleles

The characteristic controlled by a dominant allele develops if the
allele is present on one or both chromosomes in a pair

The characteristic controlled by a recessive allele develops only if
the allele is present on both chromosomes in a pair

The allele for brown eyes is
dominant, while the allele for
blue eyes is recessive

An individual who inherits one or
two alleles for brown eyes will
have brown eyes

An individual will only have blue
eyes if they inherit two copies of
the allele for blue eyes
Individuals A and B have brown eyes - only
individual C has blue eyes
Mendel

Gregor Mendel (1822-1884) studied the inheritance of different
characteristics in pea plants

He found that when he bred red-flowered plants with white-flowered
plants, all the offspring produced red flowers

If he bred these plants with each other, most of the offspring had red
flowers, but some had white – this was because the allele for red
flowers is dominant, and the allele for white flowers is recessive…
Genetic Diagrams

Genetic diagrams show all of the possible alleles for a particular
characteristic

There will be two alleles from one parent, and two from the other
parent, making four altogether

Lines show all the possible ways that these alleles could be paired
in the offspring

There will be four possible ways, but some or all of them could be
repeated

In genetic diagrams, the dominant allele is shown as a capital
letter, while the recessive allele is shown as a lower-case letter
Genotype & Phenotype

The allele pair for each characteristic is called the genotype

The physical expression of an allele pair is the phenotype

What are the phenotypes of these genotypes?
RR
rr
Rr
Mendel’s Experiment

Mendel took 2 plants – one which is pure-bred for tallness and one
pure-bred for shortness

He then crossed them: -
Mendel’s Experiment
All the plants
produced were tall
Two of these plants were then crossed…
3 out of every 4
plants were tall
Mendel hypothesised that for every characteristic there must be two determiners
Genetic Diagrams

Mendel’s first cross – all the offspring have red flowers, even though
they carry the recessive allele for white flowers
Genetic Diagrams

Mendel’s second cross – three-quarters of the offspring have red
flowers and a quarter have white flowers
Mendel

Unfortunately, nobody knew about chromosomes or genes
when Mendel published his findings so no one believed him
until many years after his death (when more powerful
microscopes were available)