Genetics Gregor Mendel

GHB 2004

The unit of inheritance

Gene:

Chromosome

Genes

Chromosome:A bundle of DNA

containing many genes

B

A

C

Different versions of the same gene

Alleles:

B b

Physical appearance

Phenotype:

Brown eyes

Blue eyes

Red flowers

White flowers

The combination of alleles which you carry for a

particular gene

Genotype:

B B bB bb

Genotype: BB Genotype: Bb Genotype: bb

An allele which is always expressed

when it is present…

Dominant allele:

… it cannot be masked by another

allele.

B b

An allele which can be masked by

another allele…

Recessive allele:

… it is only expressed when two copies are

present.

bb

…homozygous …heterozygous

If an individual has two copies of same allele of a gene on a pair of chromosomes the individual is…

If an individual has two different alleles of a gene on a pair of chromosomes the individual is…

bBB B bb

The prefix homo- means…‘sam

e’

The prefix hetero- means…‘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

Ovum

Pollen

Plant egg cell

Sperm cells are produced by …

meiosis

The chromosomes in the nucleus

are copied

Four haploid daughter cells are produced 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

Diploid parent cell

Fertilisation:

An ova and a sperm fuse to form a diploid zygote

Ova

Sperm

Zygote

The zygote contains one allele from each

parentThis process is random

Inheritance of eye colour

bb

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

… a red flower

r The recessive allele r produces

…

… a white flower

R

The dominant allele R produces

…

Example:

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

rrr r

In the genetic cross Rr x Rr there are two ways of making a heterozygous plant…

R r

Rr

Rr

RrOR

… 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…

¼½

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… ¼

3. As a ratio - RR : Rr : rr =

1 : 2 : 1

There are three ways to make a plant with red flowers…

R R RR

R r

r R

Rr

Rr

… one produces a homozygous plant…

… and two produce heterozygous plants…

…but, there is only one way to produce a plant with white flowers…

rrr r

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…

¾¼

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

The Punnett Square

We can use the Punnett Square to work out the probability of the different genotypes and phenotypes in a

genetic crossStep 1: Write down the genotypes of the parents.

= Rr x Rrx

Step 2: Write down the genotypes of the gametes that each parent produces.

pollen Egg cells

= R + r = R + r

Step 3: Draw your Punnett Square

Step 4: Write the genotype of the different gametes into your Punnett SquareStep 5: Write in the different ways in which the gametes can be combined…

Egg cells

Polle

n

R r

R

r

RR R r

rR r r

…this shows you the possible genotypes of the offspring Genotypes of

offspring

Egg cells

Polle

n

R r

R

r

RR R r

rR r r

Probability of Genotypes

There are four possible ways to combine egg cells and pollen

Count up how many times each genotype appears in the Punnet Square – this gives you its probability:

¼Probability of RR is…Probability of Rr is…Probability of rr is… ¼

= 25 %=

½

24

= 25 %

= 50 %

The ratio of RR : Rr : rr is…

1 : 2 : 1

Egg cells

Polle

n

R r

R

r

RR R r

rR r r

Probability of Phenotypes

Look at the genotypes of the offspring in your Punett Square… …decide which phenotype each one should produce.Count up how many times each phenotype appears in the Punnet Square – this gives you its probability:Probability of red is…

¾ = 75 %

Probability of white is…

¼ = 25 %

The ratio of red : white is…

3 : 1

Test Cross

• Which is the genotype of an organism showing the dominant characteristic in its phenotype?

Inheritance of blood groups