Post on 26-Dec-2015
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.
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.
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)
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 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
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