Transcript Genetics - onlinebiosurgery

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Monohybrid inheritance 4/10
Objectives:
*Define monohybrid inheritance
**Explain examples of monohybrid crosses
Starter:
What is monohybrid inheritance?
Inheritance of a single characteristic i.e inheritance of alleles of one
gene
Gregor Mendel was a school teacher in Austria who
had a passion for research.
He is known as the father of genetics.
Mendel did many experiments with peas.
We are only going to focus on 3 experiments.
Experiment 1: He crossed pure bred tall pea plants with pure bred small pea plants.
Experiment 2: He crossed the plants resulting from the first experiment together.
Experiment 3: He crossed the plants resulting from the first experiment with pure bred
small pea plants.
Draw punnet square or stick diagram to show the phenotype and genotype of
each experiment
Let’s look at the how the idea works in detail.
Parents
T
T
Experiment 1
t
t
Sex cells
T
T
t
t
Offspring
Tt
Tt
Tt
Tt
Tall
Tall
Tall
Tall
F1 generation
All of the offspring are tall and have inherited the gene for smallness.
Experiment 2
Parents
T
t
T
t
Sex cells
T
t
T
t
Offspring
TT
Tt
tT
tt
Tall
Tall
Tall
Small
25% of the offspring should be small i.e 1 in 4.
F2 generation
Test cross
Experiment 3
Parents
T
t
t
t
Sex cells
T
t
t
t
Offspring
Tt
Tt
tt
tt
Tall
Tall
1:1 tall : small.
small
small
Let’s summarize the results of the experiments
Tall (pure) X Small (pure)
ALL Tall (?)
F1 Generation
Phenotype
Gametes
What has happened to the characteristic for small ?
3:1
Tall (?) X Tall (?)
75% Tall (?) and 25% Small (?)
F1 genotype
F2 Generation Phenotype
The characteristic for small has resurfaced !
Therefore the characteristic for small must have been passed on.
It also suggests that ‘Tallness’ is dominant over ‘Smallness’.
Test cross
You can see in above 2 crosses in F1 recessive allele is not expressed but
expressed in F2. So it is not possible to know the genotype of tall from its
phenotype when you do a test cross to determine genotype
Tall (?) X Small (pure)
Tall (?) X Small (pure)
50% Tall (?) and 50% Small (?)
100% Tall (?)
Mendel came up with the idea of ‘hereditary units’ (later called genes).
These units (genes) control characteristics and are inherited from
generation to generation.
How did he use this idea to explain the results of his experiments ?
How can the ratios be explained ?
Mendel further proposed that a characteristic is controlled by 2 factors
which are inherited.
(These factors later become known as genes.)
Let’s call the gene for Tallness ‘big’ T
T
Let’s call the gene for Smallness ‘little’ t
t
Mendel also proposed that the 2 factors which control a characteristic are
separated when sex cells are made.
Eg if a plant has the genes T and t for height, then when
the sex cells are made:
50% of the sex cells will have the T gene
The other 50% of the sex cells will have the t gene
A plant which has the genes T and t will be tall because ‘Tallness’ is dominant.
Let’s use the idea of genes to try and explain the results of the experiments.
Tall (TT) X Small (tt)
ALL Tall (Tt)
The characteristic for small has been inherited but not expressed.
Tall (Tt) X Tall (Tt)
75% Tall (?) and 25% Small (t)
What have all the Tall plants inherited ?
Why are 1 in 4 plants small ?
Tall (Tt) X Small (t)
50% Tall (Tt) and 50% Small (t)
The Tall plants must have inherited the characteristic for small.
More evidence that ‘Tallness’ is dominant over ‘Smallness’.
The theory, using the idea of pairs of genes controlling characteristics can
account for the observations.
Each pair of genes is separated when sex cells are made.
When the sex cells unite (fertilisation), the pairs of genes are restored BUT
now the combination may be different.
Some genes are dominant and some are recessive.
A genes which control a characteristic are called an alleles.
Determination of gender:
female
Parents
X
male
X
X Y
Sex cells
X
X
X
Y
Offspring
XX
XY
XX
XY
female
male
female
male
50% of the offspring should be male and 50%
should be female, eg a ratio of 1:1.
The End