Noadswood Science, 2012

Mendel’s Experiments To be able to explain Mendel’s experiments and why he is deemed

the ‘father of genetics’

Wednesday, May 3, 2023

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?

rrRR 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

Two of these plants were then crossed…

All the plants produced were tall

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)