9 mutations, mendelian nonmendelian geneticsmembers.optusnet.com.au/~romainedb/notes/Lecture 9 grey.pdf2004 Biology Olympiad Preparation Program 13 Mutations, in summary • Mutagens

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2004 Biology Olympiad Preparation Program 2

MUTATIONS


Mutagens Mutagens are physical or chemical agents that

give rise to mutations.

High energy radiation – UV, X, gamma.

Base analogues, intercalators, chemical change inducers.

Affect DNA structure, base pairing, etc.

Spontaneous mutations arise from errors in DNA replication, repair,

recombination.


Mutations Mutations are changes in the genetic material of a cell.

They are important for evolution – the ultimate source of new genes.

Mutations in germ line cells can be passed onto offspring. Mutations in other somatic cells are

not hereditary.

2 broad types

Point mutations Chromosomal mutations


Point mutations – substitutions Replacement of one base pair with

another pair of nucleotides.

Silent mutation – no AA change due to redundancy of genetic code.

Missense mutation – AA change.

Nonsense mutation – stop codon creation from nucleotide substitution.


Point mutations – insertions & deletions Additions or losses of base pairs

in a gene. Often disastrous frameshift mutations.

Frameshift causing extensive missense.

Frameshift causing immediate nonsense.

No extensive frameshift.


Sickle-cell disease

Point mutation – substitution, causing missense.

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Chromosomal mutations

Altered chromosome number

Aneuploidy Polyploidy

Altered chromosome structure


Alterations of chromosome number

Nondisjunction occurs where chromosomes or chromatids do not

separate properly during telophase of meiosis.

Daughter cells receive abnormal numbers of chromosomes.


Aneuploidy Aneuploidy arises when an

abnormal gamete with n+1 or n-1 chromosomes is fertilised

with a normal gamete.

Trisomic cells have 2n+1 chromosomes.

Monosomic cells have 2n-1 chromosomes.

Down syndrome – trisomy 21.


Polyploidy Polyploidy is where

more than 2 complete sets of chromosomes are

present in a cell.

3 sets is triploid, 4 sets is tetraploid, etc.

Normal is 2 sets, diploid.

Common in plants – wheat is hexaploid!


Alterations of chromosome structure

Deletions remove chromosomal segments.

Duplications repeat chromosomal segments.

Inversions invert chromosomal segments.

Translocations move segments between non-homologous chromosomes. Often reciprocal.


Mutations, in summary •  Mutagens are physical or chemical agents that cause mutations. •  Spontaneous mutations arise from errors in DNA replication,

repair and recombination. •  Mutations are changes in the genetic makeup of cells. •  They can be inherited if found in germ line cells. •  Mutations are the ultimate source of genetic variation on which

evolution acts. •  Point mutations are changes in one base pair of a gene and

include substitutions, insertions and deletions. •  Frameshift mutations may result from insertions and deletions. •  Aneuploidy and polyploidy are alterations in chromosome

number and can be caused by nondisjunction during meiosis.

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MENDELIAN GENETICS


Gregor Mendel’s peas

Mendel transferred pollen from one flower of one plant (eg white) to carpel of another (eg purple).

Mendel always knew the parentage of new seeds.

Thousands of replicates; chose the right characters to study.


Characters, traits and generations Characters – heritable features. Traits – variations of characters.

All offspring of true-breeding parents are of the same variety as

each other and the parents.

P generation is hybridized.

Hybrid offspring of P generation is F1 generation.

F2 generation arises from self-pollination of F1 generation.


Alleles Alleles are alternative versions of genes found at the specific locus

(position) of that gene.

Organisms inherit two alleles – one from each parent.

Homozygous individuals have a pair of identical alleles for a character.

E.g. PP or pp

Heterozygous individuals have two different alleles for a character.

E.g. Pp


Genotype and phenotype

The dominant allele is expressed fully, whilst the

recessive allele has no noticeable effect.

E.g. Allele P overshadows the effect of allele p.

Only recessive homozygotes have the recessive phenotype.

Genotype – organism’s genetic makeup, e.g. Pp. Phenotype – organism’s traits, e.g. purple.


The law of segregation The law of segregation states that the two alleles for each character segregate during

gamete production.

True-breeding parental gametes are P from purple parent and p from

white parent.

F1 gametes are either P or p.

F2 generation: 25% PP, 50% Pp, 25% pp.

Punnet square.

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*Example

Gametes: B b

P:

BB bb

F1: Bb Bb

Gametes: B b B b

F2:

BB Bb bb

1 2 1


Independent segregation of alleles

A A A A

B B B B

a a a a

b b b b

A A

B B

a a

b b

A A

B B

a a

b b

A A

B B a a

b b

A A

b b

B B

a a


The testcross

Useful for determining genotype of individuals with known

phenotype.

A testcross involves the breeding of a recessive

homozygote (white, pp) with an organism of dominant phenotype (purple) but

unknown genotype (PP or Pp).


The law of independent assortment

Two pairs of alleles segregate independently of each other.

Dihybrid cross.


*Example P:

BBGG bbgg

BG Gametes: bg

F1: BbGg

Gametes: BG Bg bG bg

F2:

BBGG BbGg

BBgg Bbgg

bbGG bbGg

bbgg

9 3 3 1

BbGg

BG Bg bG bg


Mendelian genetics and probability If a gene has 2 alleles, one gamete has a 50% chance

of having a particular allele for that gene.

Chance of PP is ½ × ½ = ¼

Chance of Pp is ½ × ½ = ¼ Chance of pP is ½ × ½ = ¼

Therefore chance of heterozygote is ¼ + ¼ = ½

Chance of pp is ½ × ½ = ¼

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Example P: Ppyyrr PpYyRr ×

F1:

Chance of getting at least two recessive traits:

ppyyRr

ppYyrr

Ppyyrr

PPyyrr

ppyyrr

Prb(pp) and Prb(yy) and Prb(Rr) = ½×½ × ½ × ½ = 1/16

½ × ½ × ½ = 2/16

¼ × ½ × ½ = 1/16

¼ × ½ × ½ = 1/16

¼ × ½ × ½ = 1/16

“Pp” means Pp or pP hence

½×½ + ½×½ = 2/4 = ½

6/16 2004 Biology Olympiad Preparation Program 27

Mendelian genetics, in summary •  Mendel performed his experiments so that he could determine

patterns of inheritance by knowing the parentage of new seeds. •  Characters are heritable features and traits are variations of

characters. •  True-breeding individuals are homozygous for the particular

gene under study. •  The P generation is the parental generation, and F1 and F2 are

the first and second filial generations respectively. •  Alleles are different variations of a gene found at a particular

locus on a chromosome. •  Dominant alleles are expressed fully while recessive alleles

have no noticeable effect on appearance.


Mendelian genetics, in summary •  Homozygous individuals (homozygotes) have two of the same

allele for a gene. •  Heterozygous individuals (heterozygotes) have two different

alleles for a gene. •  An organism’s genetic makeup is its genotype. •  Its phenotype is an expression of its genotype. •  Recessive homozygotes express the recessive phenotype, while

heterozygotes and dominant homozygotes express the dominant phenotype.

•  The law of segregation states that two alleles for each character segregate during gamete formation.


Mendelian genetics, in summary •  The testcross is a useful tool where an unknown genotype

dominant phenotype individual is crossed with an individual with recessive phenotype.

•  The law of independent assortment states that two pairs of alleles segregate independently of each other.

•  Results of Mendelian crosses can be quickly and easily calculated using the laws of probability.


NON-MENDELIAN GENETICS


Incomplete dominance & codominance Mendel’s laws assume complete

dominance – i.e. one allele is dominant, another recessive.

Incomplete dominance – heterozygotes have phenotype

intermediate between homozygous phenotypes. E.g.

snapdragon colour (left).

Codominance – both alleles affect phenotype. E.g. MN blood groups.

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Multiple alleles & pleiotropy

Multiple alleles – more than 2 alleles

for a gene. E.g. ABO blood

types.

Important in determining blood

compatibility.

Pleiotropy – one allele has many phenotypic effects. E.g. sickle cell disease.


Epistasis & polygenicity

Epistasis – one gene affects the phenotypic expression of another.

E.g. coat colour in mice.

Polygenic inheritance – two or more genes affecting a single phenotype. E.g. skin colour.


Phenotype and the environment Phenotype depends on environment as well as genotype.

Multifactorial characters are affected by many factors.

E.g. hydrangea colour.

Flowers turn different colour in different soil pHs. Red in alkaline,

blue in acid.

P = G + E


Pedigrees

Ff Ff Ff ff

FF/ Ff ff ff Ff Ff ff

ff FF/ Ff

I

II

III

1 2 3 4

Nomenclature:

. .

Normal male Normal female

Carrier female Carrier male

Affected female Affected male


Chromosomes and Mendel’s laws Alternate arrangements of

homologous chromosomes on the metaphase plate during

metaphase I account for the law of independent

assortment.

The separation of homologues and sister

chromatids in anaphase I and II, respectively, account for the law of segregation.


Sex determination and chromosomes In most animals, there is a chromosomal

basis behind sex determination.

XY: mammals. Sperm is either X or Y. XX female, XY male.

X0: some insects. Sperm either carries or doesn’t carry the X (sex)

chromosome. XX female, X0 male.

ZW: birds, fish, some insects. Ovum is either Z or W. ZW female, ZZ male.

Haplo-diploid: bees and ants. Females diploid, males haploid.

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Sex linkage Sex-linked genes are carried on sex

chromosomes (either X or Y).

E.g. eye colour in Drosophila, the fruit fly studied by Morgan.

Wild type alleles (normal phenotype) of a character written with a superscript +.

Mutant alleles written without it.

F2 generation showed typical 3:1 ratio, but only males were white eyed – it

was linked to sex.


Linked genes distance between two

genes, likelihood of being inherited together – these are

linked genes.

E.g. Fruit fly body colour and wing type (left).

Independent assortment does not occur all the time

with linked genes.

Recombinant phenotypes have new combinations of traits. Parental phenotypes have the parental combinations.


Recombination

Recombinants produced by crossing over during meiosis I.

Recombination frequency is a measure of how closely linked

two genes are.


Linkage maps Recombination frequency is

representative of the relative distance between two genes on the same

chromosome.

We can build linkage maps out of recombination frequency data to

determine the relative positions of genes on chromosomes.

Good for Drosophila as they breed quickly. Not so good for humans. So

how do we do it?


Non-Mendelian genetics, in summary •  Incomplete dominance involves heterozygotes having a

phenotype intermediate between those of the two types of homozygotes.

•  In codominance, a heterozygote exhibits phenotypes for both its alleles.

•  Many genes exist in multiple alleles in a population. •  Pleiotropy is the ability of a single gene to affect multiple

phenotypic traits. •  In epistasis, one gene affects the expression of another gene. •  Characters that are quantitative, varying continuously, indicate

polygenic inheritance, an additive effect of two or more genes on a single phenotypic character.


Non-Mendelian genetics, in summary •  Environment plays a role in determining an organism’s

phenotype. •  Family pedigrees can be used to deduce the possible

genotypes of individuals and make predictions about future offspring.

•  Mendelian inheritance has its physical basis in the behaviour of chromosomes during meiosis and fertilisation.

•  Sex is an inherited phenotypic character usually determined by the presence or absence of a special chromosome.

•  Mammals use the XY system of sex determination. •  Sex-linked genes have unique patterns of inheritance. •  Linked genes are situated close to each other on the

chromosome and are likely to be inherited together.

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Non-Mendelian genetics, in summary •  The recombination frequency is a measure of how close two

genes are on the same chromosome. •  Recombinant phenotypes are due to crossing over during

meiosis. •  Linkage maps can be constructed from recombination

frequencies.


Next time…

•  Eukaryotic genetics •  Bacterial and viral genetics

When?

Documents

9 mutations, mendelian nonmendelian geneticsmembers.optusnet.com.au/~romainedb/notes/Lecture 9 grey.pdf2004 Biology Olympiad Preparation Program 13 Mutations, in summary • Mutagens