Demonstrate understanding of biological ideas relating

to genetic variationScience 1.9 (AS90948)

Specific Learning Outcomes

Role of DNA in carrying instructions◦Explain DNA structure and

label a diagram Include:

◦Nucleotide, Triplet, Sugar, Phosphate, Base, Base pair,

◦Explain why DNA is important in making each organism unique Include

◦Gene, Allele, Cell, Organism, Characteristic, Genetic code

◦Explain DNA replication◦Explain mitosis

Specific Learning Outcome

Role of DNA in determining phenotype◦Explain how sex is determined◦Explain meiosis◦Explain how chromosomes and

genes are inherited◦Explain how the genetic code

determines our characteristics ◦Use a punnett square/pedigree

tree to predict genotype and phenotype

◦Use a punnett square/pedigree tree to help explain how you test for a pure bred organism

Specific Learning Outcomes

Link between phenotype, variation and species survivalExplain the ways

inheritable and non-inheritable variation occurs

Discuss how phenotypes related to survival value

Discuss advantages and disadvantages of sexual and asexual reproduction

DNA

DNA (deoxyribonucleic acid) has 2 functions◦To control cell activities◦To pass genetic information from generation to

generationDNA is made up of small units called

nucleotides

DNA Structure

All living things have the same basic DNA structure◦2 chains made up of

nucleotides that looks like a ladder

◦Each chain is joined at the bases (A,T,G,C)

◦The bases, paired A-T and C-G, are called base pairs

◦This ladder exists in a helix shape to save room

Importance of DNA

DNA provides the information for all cellsThe sequence of bases on each strand

forms the genetic codeEach organism has its own unique code

because its order of bases is uniqueSections of the code are called genes and

code for certain characteristics (traits)The particular trait we display is called our

phenotype

DNA, Alleles, Genes, Chromosomes and Characteristics

Our DNA is wound up into chromosomes

We have 2 copies of each chromosome (1 from our Mum and 1 from our Dad)

Each of the chromosomes could have a different version of the same trait (alleles) The particular combination of alleles is called our genotype

Gene 4

Tour

Alleles

A genotype can be either:◦Homozygous (2 alleles that are the same)◦Heterozygous (2 alleles that are different)

As well as◦Dominant (always shows up)◦Recessive (only shows up if homozygous)

BB = black coat because dominant B allele is present

Bb = black coat because dominant B allele is present

bb = white coat because dominant B allele is not present

DNA replication

When an organism grows, repairs itself, reproduces or replaces old cells, new cells need to be made

Every new cell needs its own copy of the DNA so it is replicated

The double

strands of DNA

coil up into a

helix

Each of the two

newly formed DNA

double helix

molecules will

become a chromatid

Cell Division - Mitosis

Mitosis is the process of cell division for growth and repair

It makes an exact copy of the original cell to produce 2 identical daughter cells

2 daughter cells identical to original

Chromosomes line up to so that one copy can go to each cell

Chromosomes now split

Parent cell

Cell Division - Meiosis

Meiosis is the process to produce sex cells

It produces 4 daughter cells each with half the number of chromosomes

Due to crossing over and independent assortment, these cells produce inheritable variation

Replicated chromosomes separate

Homologous pairs separate into 2 cells

4 non-identical daughter cells produced

Sexual v Asexual Reproduction

Sexual reproduction involves 2 parents and produces offspring that are genetically different to the parent

Asexual reproduction involves 1 parent and produces offspring that are identical to that parent (clones)

 Asexual Reproduction

Sexual Reproduction

Name of an example    

Are gametes involved    Is fertilisation involved    

Is mitosis involved    

Is meiosis involved    Are the offspring genetically identical    Can it be done by only one parent    

Strawberry runner Puppy

No

No

Yes

No

Yes

Yes

Yes

Yes

Yes

Yes

No

No

Sex determination

Our sex is determined by either an X or a Y chromosome

Females are homozygous males are heterozygous

During sexual reproduction an egg and a sperm come together. Every egg has an X but the sperm has a 50/50 chance of being an X or a Y due to its formation in meiosis

XY XX

All genes are inherited through sexual reproduction

We can use a punnett square to determine what the chance of offspring having certain traits is going to be

Inheritance and Punnett Squares

X

X

X

Y

Sex determination

b

B

b b

A man who is heterozygous for eye colour and a woman who has blue eyes (bb)

Bb

Bb bb

bbXX XY

XYXX

Pure Breeding

A pure bred organism is one that is homozygous for the trait concerned (AA or aa)

A pure bred organism always breeds pure and has no hidden genetic characteriestic

Test Cross

A test cross is used to determine whether an individual with the dominant phenotype pure bred or not

You cross your individual with one that is homozygous recessive When you complete the cross, if any offspring have the

recessive trait the individual must have been heterozygous Eg In mice, grey coat is dominant (G) over white. A breeder

wants to know if his grey male is homozygous for colour

G G

g

g

Homozygous – no white offspring

Gg

Gg

Gg

Gg

Gg gg

ggGgg

g

gG

Heterozygous ½ grey offspring, ½ white

Pedigrees

Pedigree trees show inheritance patternsThey show the phenotypes over various

generations and can be used to determine most genotypes

Pedigree Example

Variation

Variation is the differences in a population that can come from inherited genes and the influence of the environment

Variation in inherited genes can come during sexual reproduction (crossing over and independent assortment) or from mutations

Variation is important for the survival of a species because if the environment changes, some of the population should be able to survive and continue to reproduce if the traits that they have are favourable

Evolution

Evolution is a result of this variationNatural selection is the fact that the traits

that are favourable are “selected for” in an environment

Eventually these traits will become the norm, this is how evolution occurs