GENETICSGENETICS

• The nucleus of a cell carries the genetic information which allows it to control all of the activities of the cell and determines the overall characteristics of the organism

nucleus

The nucleus of a living cell contains

threadlike structures called chromosomes.

A chromosome is a threadlike structure

which carries genetic information.

All the nuclei of the body cells of a living

organism contain identical copies of the

chromosomes. nucleus

chromosomes

Chromosome complement

The number of chromosomes present in the

cells of a living organism is called the

chromosome complement and depends on

the species.

Humans have 46 chromosomes

in every body cell.

Species Chromosome complement

Horse 64

Sheep 54

Human 46

Mouse 40

Maize 20

Pea plant 14

Drosophila fruit fly 8

Chromosomes and Genes

Chromosomes are made from tightly coiled

molecules of DNA.

DNA is a long chain made up of a backbone

with bases attached.

DNA = deoxyribonucleic acid!!

DNA backbone

bases

Centromere

DNA of 1 gene uncoiling

= Adenine (A)

= Guanine (G)

= Thymine (T)

= Cytosine (C)

Positions of individual genes

DNA coiled into a chromosome

Label the following diagram

• There are 4 different types of base within a strand of DNA.

- Adenine (A)

- Thymine (T)

- Guanine (G)

- Cytosine (C)

The DNA carries pieces of coded genetic information.

An individual section of DNA with a single piece of genetic

information is a gene.

Chromosomes are thought of as lots of genes in a chain

The Function of DNA

DNA molecules carry genetic instructions which allow the cell to make specific protein molecules.

Proteins are made from amino acid units linked together to form long chains.

The order of DNA bases encodes the information for the sequence of amino acids in proteins

AMINO

ACID T

AMINO

ACID S

AMINO

ACID RAMINO

ACID S

Sets of 3 BASES on a DNA strand carry the codes for………….

….a chain of amino acids which join up to make a protein molecule.

Protein molecule forming

DNA backbone

An amino acid is a unit of protein structure.

A base is a part of DNA structure

DNA base sequence

Amino acid coded for

P

Q

R

S

T

This table shows the information about the base sequences for some amino acids

It is your job to decode the next few diagrams of pieces of DNA and to draw the chain of amino acids they encode.

1.

2.

3.

4.

5.

1.

2.

3.

4.

5.

P Q R

R S T

T

P

QQ

PS

P

RT

Protein Structure & Function

• The chains of amino acids are folded and twisted to give the molecules 3-D shapes.

• The sequence of amino acids is determined by the sequence of the DNA bases.

• The sequence of the amino acids dictates the structure and function of the protein produced.

e.g. Enzymes

Enzymes are made of protein.

The folding of the chains of amino acids

allows the formation of the active sites which

makes the enzymes specific to their

substrate.

Relationship between proteins present in a Relationship between proteins present in a cell and the organisms characteristicscell and the organisms characteristics

• Inherited characteristics are the result of many biochemical

processes controlled by enzymes (which are made of protein!)

• In humans, enzymes control the reactions that lead to the formation of hair or a certain texture, eye irises of a certain colour etc..

• The protein haemoglobin gives red blood

cells their red colour.

• The body also possesses many hormones which are also made of protein. Hormones are chemical messengers around our body.

MEIOSISMEIOSIS

• Meiosis is the name given to the process which produce gametes (sex cells).

Chromosome revisionChromosome revision

centromere one chromatid

Double chromosomesingle chromosome

In cells that are not about to divide, chromosomes are found as single chromosomes. In cells that are about to divide, the DNA makes an extra copy of itself (shown above as the dark strand) and the chromosomes become double chromosomes. Each strand is called a chromatid and the chromatids are held together by a centromere.

Sperm mother cell

Egg mother cell

meiosis

meiosis

Egg cell

sperm cell

zygote

Zygote ready to divide

fertilisation

The Process of Meiosis

1) Gamete mother cell containing 4 double chromosomes

2) Matching chromosome pair and line up across the middle

3) The pairs separate to either end of the cell and the cell divides into 2

4) The chromosomes turn and line up. Each cell then divides again.

5) The centromeres split and the chromatids are pulled apart. After the chromatids are separated from each other they are known as single chromosomes

6) 4 gametes are produced, each with only 1 set of single chromosomes.

Meiosis reduces the total number of sets of double chromosomes from 2 matching sets in the gamete mother cell to 1 set of single chromosomes in each gamete.

SO:Gamete mother cell = 2 sets chromosomesGametes = 1 set chromosomes

The 2 sets of chromosomes are restored at fertilisation.

Chromosome shuffling

• The different ways that the matching chromosomes can pair increases the total number of gamete varieties.

• Any process which increases the number of different gametes must also increase the variety of offspring.

• The random assortment of chromosomes during meiosis leads to variation in offspring.

So instead of this arrangement

You get this instead

Which results in 4 gametes like this

Because…..

Confused??!!!??!!

Sex Determination

In humans, each male gamete has an X or a Y chromosome.So males are XY.

Each female gamete has an X chromosome.So females are XX.

The sex chromosomes of an individual determine their sex.

Genetic Symbols

= male symbol

= female symbol

He’s a man, man!

Stick in the ‘Sex chromosomes’ cut out

• All egg cells will contain an X chromosome.

• Half of sperm cells will be X and half will be Y.

• It is the sperm cells that determine the sex of the baby – the egg will always be X but the sperm will either be X or Y.

male female

XY XX

X Y X X

Baby girl XX XY Baby boy

• Collect a sex determination grid and complete the blank squares to show the 4 possible combinations in the offspring.

• Use a crayon lightly to shade the boxes to show the male and female offspring.

• Complete the ratio information below the grid

• The ratio of males to females is 1:1

• But the process of fertilisation is random so it is a matter of chance which sperm will fertilise the egg – X or Y.

• It is for this reason that there will be roughly a 1:1 ratio of males to females.

Genetics for Beginners!

• Genes are parts of chromosomes

• Alleles are the different forms of a gene.

• Each gamete will carry one allele of the gene.

• E.g. Gene for height in pea plants.

Pea Plants can be tall or dwarf.

Each plant will carry 2 copies of a gene, onefrom each parent.

The alleles are represented by letters and will be T for tall and t for dwarf.

A tall plant will either be TT or TtA dwarf plant will be tt.

• If a tall plant and a small plant cross, the offspring are all tall.

• This means that ‘Tall’ is dominant.• ‘Dwarf’ is recessive.

• The dominant form of the gene always gets a capital letter e.g. T = tall.

• The recessive form of the gene always gets the same letter but lower case e.g. t = dwarf

X

tall dwarf

All tall

Complete the Symbols for Alleles Table

Symbols for Alleles

Organism Gene Dominant allele Recessive allele

Word Symbol Word Symbol

Pea plant Height Tall T Dwarf t

Human Eye colour Brown B Blue b

Drosophila Wing length Long L Short l

Maize Grain colour Purple P Yellow p

Guinea pig Coat colour Black B White b

• An individual with 2 of the same allele is said to be HOMOZYGOUS. (e.g. tt or TT)

• An individual with 2 different alleles of a gene it is said to be HETEROZYGOUS. (e.g. Tt)

• The genetic symbols an individual has is its GENOTYPE, e.g. Tt

• The physical appearance an individual has is its PHENOTYPE e.g. Tall

E.g. flower colour in pea plants• A pea plant with lilac flowers was crossed with a white flowered plant.

• All offspring were lilac.

x

All lilac

• Which is the dominant characteristic?

• What letter would we give the dominant allele?

• What letter would we give the recessive allele?

• What is the recessive characteristic?

Lilac

L

l

white

• If a pea plant had the alleles ll – would the individual be homozygous or heterozygous?

• What colour would it be?

• If a plant had the alleles Ll – would the individual be homozygous or heterozygous?

• What colour would it be?

• If a plant had the alleles LL – would the individual be homozygous or heterozygous?

• What colour would it be?

homozygousWhite

heterozygous

Lilac

homozygous

Lilac

Genotype and PhenotypeAn organism can have the same phenotypebut have a different genotype.

Example: Organism: Pea plants

Gene height flower colourTT = tall LL = lilac

Tt = tall Ll = lilac

True Breeding

Parents (P) X

1st generation (F1)

X

2nd generation (F2)

X

XMembers of F1 cross

True breeding lilac strain True breeding white strain

All lilac All white

• When 2 lilac parent plants cross, the offspring are all lilac. When the lilac offspring cross, all their offspring are lilac.

• When 2 white parent plants cross, the offspring are all _______. When the ______ offspring cross, all their offspring are _______.

• So, when the flower colour of the offspring is identical to the parent flower colour, the members of the strain are true breeding. (they are always homozygous, e.g. LL or ll)

Terms for Monohybrid Crosses

P = parents

F1 = 1st filial generation.

F2 = 2nd filial generation

Monohybrid Crosses

A monohybrid cross is a cross that involves

only one difference between the original

parents, e.g. flower colour or height.

Parents in monohybrid crosses are usually

true breeding and show different phenotypes

How to do Monohybrid Crosses

Question: A true breeding black mouse

was crossed with a true breeding white

mouse. All of the offspring were black.

Show this as a monohybrid cross using

appropriate symbols right through to the F2

generationX

P Black mouse White mouse

phenotype Black white

genotype BB bbX

F1phenotypegenotype

All BlackBb

F2Black Blackphenotype

genotypeX

Bb Bb

gametes B b B b

Punnet Square

Sperm

B b

Eggs B BB Bb

b Bb bb

F2 genotypic ratio 1BB:2Bb:1bb

F2 phenotypic ratio 3 black: 1 white

Try for yourself….

A true breeding pea plant with round seeds was crossed with a true breeding pea plant with wrinkled seeds. All the F1 generation had round seeds. Show this as a monohybrid cross using appropriate symbols right through to the F2 generation.

X

P

phenotype

genotypeX

F1phenotypegenotype

All ________

F2phenotypegenotype

X

gametes

Punnet Square

Sperm

Eggs

F2 genotypic ratio : :

F2 phenotypic ratio :

P Round seeds Wrinkled seeds

phenotype Round wrinkled

genotype RR rrX

F1phenotypegenotype

All RoundRr

F2Round Roundphenotype

genotypeX

Rr Rr

gametes R r R r

Punnet Square

Sperm

R r

Eggs R RR Rr

r Rr rr

F2 genotypic ratio 1RR:2Rr:1rr

F2 phenotypic ratio 3 round: 1 wrinkled

• Now do the Possible monohybrid crosses in mice sheet.

Observed vs Predicted RatiosObserved vs Predicted RatiosMonohybrid crosses that we have seen so far, always produce a 3:1 ratio in the F2 generation.

However, there is often a difference between the observed and predicted numbers of the different types of offspring as an exact 3:1 ratio rarely happens in nature as you can see from the table below…(stick in table)

This is due to the fact that fertilisation is a random process involving an element of chance.

We can show this by experiment…..

Bead Experiment….flower colour in pea plants

Pick a female and a male gamete at random and record your results in the table, then work out the ratio – is it 3:1?

PP

Pink

Pp

Pink

pp

yellow

Co-dominanceWhen 2 alleles of a gene are codominant this means that neither allele is dominant to the other. Both alleles are expressed equally in the phenotype of an organism with the heterozygous genotype.

e.g. Coat colour in horses and cattle, feather colour in domestic fowl, flower colour in carnations.

Black stallion White Marex

All offspring grey roan

Black coat is codominant to white coat. They are expressed equally and so offspring have coats with black and white hairs, these are called grey roans.

Genotypes in Co-Dominance

• Neither allele in co-dominance is recessive so neither symbol has a small letter. Both are capital letters since both alleles are equally dominant.

Phenotype GenotypeBlack coat BBWhite coat WWGrey roan BW

 Red coat RRWhite coat WW

Red roan RW

• Time to do some co-dominance problems….

Remember co-dominant alleles are expressed equally. They are

equally dominant. Neither is recessive so both alleles have a

capital letter.

Polygenic Inheritance

• This is when characteristics are controlled by the alleles of more than one gene.

• E.g. Skin colour in humans, seed mass in plants.

• The characteristics arise due to the interaction of the alleles of several genes.

• Remember back to continuous and discontinuous variation?…….

Discontinuous variation is controlled by a single

gene and is an example of single-gene

inheritance.

Continuous variation is controlled by the alleles

of more than one gene and is an example of

polygenic inheritance.

Example of Polygenic Inheritance

When a characteristic is controlled by 2

genes there may be 4 alleles working

together.

In maize, kernel colour is controlled by

several genes, we will say 2 genes for this

example. (Each gene will have 2 alleles)

• Each gene has a dominant allele giving a red colour to the kernel and a recessive allele giving a white colour to the kernel.

• R1 = red R2 = red• r1 = white r2 = white

• If a maize inherits all dominant alleles, (R1 R1 R2 R2), it will have very dark red kernels

• If a maize inherits all recessive alleles

(r1 r1 r2 r2) then it will have white kernels

genotype R1 R1 R2 R2 x r1 r1 r2 r2

Gametes R1 R2 r1 r2

R1 r1 R2 r2

F1 Phenotype Medium red

F1 Genotype

Parent Phenotypes Very dark red white

If 2 of the F1 generation are crossed….

R1 r1 R2 r2 R1 r1 R2 r2xMedium red Medium red

See your diagram of polygenic inheritance in maize and

complete the missing genotypes of the offspring.

??

So…..

The more genes there are for a particular

characteristic, the more different phenotypes

there are.

FAMILY TREESFAMILY TREES• Family tree diagrams are set out in a standard way.

• Male =

• Female =

• The squares and symbols can be shaded in or left, depending on the phenotype.

• Parents are joined by a horizontal line

• Offspring are connected by a branched line

• Parents are joined to offspring by a vertical line.

Brown-eyed male

Blue-eyed male

Brown-eyed female

Blue-eyed female

1 2

3 4 5 6 7

8 9 10 11 12 13

B = brown eyes b = blue eyes

COMPLETE THESE STATEMENTSa) The phenotype of person 2 is:-

b) The phenotype of person 3 is:-

c) The genotype of person 1 is:-

d) The genotype of person 4 is:-

e) Person 7 is likely to be homozygous dominant because….

f) The genotype of person 8 is….

g) The genotype of person 9 cannot be stated with certainty because…

Blue-eyed female

Brown-eyed male

Bb

Bb

All offspring are brown eyed.

bb

Could be Bb or BB

Environmental Impact on PhenotypeEnvironmental Impact on Phenotype

• The final appearance of an organism is the result of its genotype and the effects of the environment.

• If organisms of identical genotype are subject to different environmental conditions they show considerable variation (differences).

• These changes are not genetic so they are not passed on from one generation to the next.

Twins reared together

Both twins fed same healthy diet – both reach full potential height.

Twins separated at birth

1 twin fed healthy diet – reached full potential height

1 twin fed poor diet – did not reach full potential height

So: The environment has an effect on our overall phenotype…

Genotype + Environment = Phenotype