The
Genetics
Slide Show
Gregor Mendel
GHB 2004
Gene:
The unit of inheritance
Genes
A
Chromosome:
B
A bundle of DNA
containing many
genes
C
Chromosome
B
Alleles:
Different versions of
the same gene
b
A cassette is a
large bundle of
tape containing
many tracks.
It is like a… chromosome
Tape:
-it is used to
record
information
- it is like… DNA
One song or
track on the
tape is like a… gene
Different versions of the
same song are like… alleles
B
b
Phenotype:
Physical appearance
Red flowers
Brown eyes
White flowers
Blue eyes
Genotype:
The combination of alleles
which you carry for a
particular gene
B
B
Genotype: BB
B
b
Genotype: Bb
b
b
Genotype: bb
Dominant allele:
An allele which is
always expressed
when it is present…
… it cannot be
masked by another
allele.
B
b
Recessive allele:
An allele which can be
masked by another
allele…
… it is only expressed
when two copies are
present.
b
b
If an individual has
two copies of same
allele of a gene on a
pair of chromosomes
the individual is…
…homozygous
B
B
b
b
The prefix homomeans… ‘same’
If an individual has
two different alleles
of a gene on a pair
of chromosomes the
individual is…
…heterozygous
B
b
The prefix heteromeans… ‘other’
A cell which contains
two copies of each
chromosome is… diploid
Normal body cells
are diploid
A cell which contains
one copy of each
chromosome is… haploid
Gametes (sex cells) are
haploid
Sperm
Plant egg
cell
Pollen
Ovum
Sperm cells are produced by … meiosis
Diploid parent
cell
Four haploid
daughter cells
are produced
The
chromosomes in
the nucleus are
copied
Each haploid
daughter cell
contains only one
of the two alleles
from the diploid
parent cell
Diploid
parent cell
Ova are also produced by
meiosis
Haploid
Fertilization:
An ova and a sperm fuse to form a diploid zygote
Ova
Zygote
Sperm
The zygote contains one
allele from each parent
This process is random
Inheritance of eye color
b
b
Genotype is: bb
Phenotype is:
Blue
B
B
Genotype is: BB
Phenotype is:
Brown
B
b
Genotype is: Bb
Phenotype is:
Brown
b
B
Genotype is: Bb
Phenotype is:
Brown
The same rules apply to inheritance in plants as apply to
inheritance in animals.
The male gamete
or sex cell is …
… pollen
The female
gamete is…
… the egg cell
Plant genes can have dominant and recessive alleles
Example:
R
The dominant allele
R produces …
… a red flower
r
The recessive
allele r produces …
… a white flower
A genetic cross in plants: Rr x Rr
There is only one way to make a homozygous plant with
two dominant genes…
R
R
RR
… and only one way to make a homozygous plant with two
recessive genes
r
r
rr
In the genetic cross Rr x Rr there are two ways of
making a heterozygous plant…
R
OR
r
r
R
Rr
Rr
… so, heterozygous offspring are twice as likely as either
homozygous dominant or homozygous recessive offspring
We can show the likeliness or probability of different
genotypes in different ways:
1. As a percentage -
Probability of RR is… 25%
Probability of Rr is… 50%
Probability of rr is… 25%
2. As a fraction -
Probability of RR is…
Probability of Rr is…
Probability of rr is…
3. As a ratio -
¼
½
¼
RR : Rr : rr = 1 : 2 : 1
There are three ways to make a plant with red flowers…
… one produces a homozygous plant…
R
R
RR
… and two produce heterozygous plants…
R
r
Rr
r
R
Rr
…but, there is only one way to produce a plant with white
flowers…
r
r
rr
So, in the genetic cross, Rr x Rr…
…red flowers are three
times more likely than
white flowers!!!
We can show the likeliness or probability of different
phenotypes in different ways:
1. As a percentage -
Probability of red is…
75%
Probability of white is… 25%
2. As a fraction -
Probability of red is…
Probability of white is…
3. As a ratio -
red : white =
3:1
¾
¼
The Punnett Square
We can use the Punnett Square to work out the probability of
the different genotypes and phenotypes in a genetic cross
Step 1: Write down the genotypes of the parents.
x
= Rr x Rr
Step 2: Write down the genotypes of the gametes that
each parent produces.
pollen
Egg
cells
= R+r
= R+r
Egg cells
Step 4: Write the genotype
of the different gametes
into your Punnett Square
R
Step 5: Write in the
different ways in which the
gametes can be combined…
…this shows you the
possible genotypes of the
offspring
Pollen
Step 3: Draw your Punnett Square
r
R
RR R r
r
rR
Genotypes of
offspring
rr
Probability of Genotypes
Egg cells
Count up how many times
each genotype appears in
the Punnet Square – this
gives you its probability:
Probability of RR is…
¼
Probability of Rr is…
2
4
Probability of rr is…
¼
Pollen
There are four possible ways to
combine egg cells and pollen
R
r
R
RR R r
r
rR
rr
= 25 %
= ½ = 50 %
= 25 %
The ratio of
RR : Rr : rr is…
1:2:1
Probability of Phenotypes
Egg cells
…decide which phenotype
each one should produce.
Count up how many times
each phenotype appears in
the Punnet Square – this
gives you its probability:
Pollen
Look at the genotypes of the
offspring in your Punett Square…
Probability of red is…
¾
= 75 %
Probability of white is…
¼
= 25 %
R
r
R
RR R r
r
rR
rr
The ratio of
red : white is…
3:1