Inherited ChangeInherited Change

MeiosisMeiosis

Used in sexual reproduction to allow for Used in sexual reproduction to allow for variation.variation.Homologous chromosome – chromosomes that Homologous chromosome – chromosomes that have the same genes but not the same alleles. have the same genes but not the same alleles. One from mum and one from dad.One from mum and one from dad.Bivalent – when the homologous chromosomes Bivalent – when the homologous chromosomes have replicated they join together by a chiasma have replicated they join together by a chiasma and this forms a bivalent made of 4 chromatids.and this forms a bivalent made of 4 chromatids.Crossing over – the non-sister chromatids that Crossing over – the non-sister chromatids that lie next to each other in the bivalent may join lie next to each other in the bivalent may join temporarily and then break off swopping some of temporarily and then break off swopping some of their genetic information. their genetic information.

VariationVariation

Due to:Due to:– Crossing over of non-sister chromatids in the Crossing over of non-sister chromatids in the

bivalentbivalent– Random assortment of bivalents and sister Random assortment of bivalents and sister

chromotids during metaphase I and IIchromotids during metaphase I and II– Random fertilisationRandom fertilisation

DefinitionsDefinitions

Gene – part of a chromosome that codes Gene – part of a chromosome that codes for a particular proteinfor a particular protein

Locus – position of a gene on a Locus – position of a gene on a chromosomechromosome

Allele – different forms of a gene. Eg. The Allele – different forms of a gene. Eg. The gene is eye colour but the allele is blue or gene is eye colour but the allele is blue or brownbrown

Only one allele can be expressed most of Only one allele can be expressed most of the time. Eg. Eye colour does not the time. Eg. Eye colour does not constantly change even if you inherit two constantly change even if you inherit two different alleles from your parent.different alleles from your parent.

This means that one is normally dominant This means that one is normally dominant over the other and is shown in the physical over the other and is shown in the physical appearance of a person.appearance of a person.

Recessive is the allele that does not show Recessive is the allele that does not show in the physical appearance but it may in the physical appearance but it may show in the next generation.show in the next generation.

Codominant – when both alleles show in Codominant – when both alleles show in the physical appearance. Eg. Red flowers the physical appearance. Eg. Red flowers crossed with white flowers give pink crossed with white flowers give pink flowers.flowers.

Homozygous – alleles are the same on Homozygous – alleles are the same on both chromosomes.both chromosomes.

Heterozygous – alleles are different on the Heterozygous – alleles are different on the homologous chromosomeshomologous chromosomes

Phenotype – physical appearancePhenotype – physical appearance

Genotype – genetic make-upGenotype – genetic make-up

Monohybrid crossesMonohybrid crosses

Genetic crosses that involve just one Genetic crosses that involve just one gene.gene.

Eg. Eye colourEg. Eye colour

(Make sure the outline of the cross is written (Make sure the outline of the cross is written out exactly as this gains the marks in an out exactly as this gains the marks in an exam)exam)

Let B represent brown eyes. (parent is homozygous) Let B represent brown eyes. (parent is homozygous) Let b represent blue eyes (parent is homozygous)Let b represent blue eyes (parent is homozygous)

Parent phenotypes?Parent phenotypes?Parent genotypes?Parent genotypes?

Gametes?Gametes?

F1 genotype?F1 genotype?F1 phenotype?F1 phenotype?Ratio?Ratio?

Dihybrid CrossesDihybrid Crosses

MendelMendel

Looked at pea plantsLooked at pea plants

Colour of seeds and round or wrinkledColour of seeds and round or wrinkled

Colour of flowers and tall or shortColour of flowers and tall or short

Dihybrid means showing two Dihybrid means showing two characteristics in one genetic cross.characteristics in one genetic cross.

Round seeds are dominant over wrinkled Round seeds are dominant over wrinkled seedsseeds

Yellow seeds are dominant over green Yellow seeds are dominant over green seedsseeds

Let the dominent allele for shape be?Let the dominent allele for shape be?

Let the recessive allele for shape be? Let the recessive allele for shape be?

Let the dominant allele for colour be?Let the dominant allele for colour be?

Let the recessive allele for colour be?Let the recessive allele for colour be?

Parent Phenotypes?Parent Phenotypes?

Parent Genotype (pure breeding)?Parent Genotype (pure breeding)?

Gametes?Gametes?

(there are four(there are four

Possible)Possible)

F1 genotype (see punnett square)F1 genotype (see punnett square)

F1 phenotypeF1 phenotype

Try June 2006 paper 4 Try June 2006 paper 4

Monohybrid phenotype outcomesMonohybrid phenotype outcomes

4 : 0 homozygous dominant crossed with 4 : 0 homozygous dominant crossed with homozygous recessivehomozygous recessive

or homozygous dominant crossed with or homozygous dominant crossed with heterozygousheterozygous

3 : 1 heterozygous crossed with heterozygous3 : 1 heterozygous crossed with heterozygous

1 : 1 homozygous recessive crossed with 1 : 1 homozygous recessive crossed with heterozygousheterozygous

Codominance – heterozygous cross Codominance – heterozygous cross heterozygous 1 : 2 : 1heterozygous 1 : 2 : 1

CodominanceCodominance

If both alleles show in the phenotype in the If both alleles show in the phenotype in the heterozygous condition this is called heterozygous condition this is called codominance.codominance.

Eg. Red and white alleles make pink.Eg. Red and white alleles make pink.

Still use one letter though.Still use one letter though.

RR is red, rr is white, Rr is pinkRR is red, rr is white, Rr is pink

Main example is blood groupsMain example is blood groups

Sex LinkageSex Linkage

When genes are located on the either the X When genes are located on the either the X chromosome or Y chromosome they are said to chromosome or Y chromosome they are said to be sex linked.be sex linked.

Eg. Ability to see particular colours and blood Eg. Ability to see particular colours and blood clotting.clotting.

These are both found on the X chromosome.These are both found on the X chromosome.

Therefore a recessive allele will be more likely to Therefore a recessive allele will be more likely to show in a male than in a female. As there is no show in a male than in a female. As there is no other X chromosome to mask it.other X chromosome to mask it.

HaemophiliaHaemophilia

When blood does not clot normally.When blood does not clot normally.

Sex-linked character caused by a recessive Sex-linked character caused by a recessive allele carried on the X chromosome.allele carried on the X chromosome.

If the male has the recessive allele then he has If the male has the recessive allele then he has the disease.the disease.

The female only gets the disease if she inherits The female only gets the disease if she inherits two recessive alleles. two recessive alleles.

If the female is heterozygous she is known as a If the female is heterozygous she is known as a carrier.carrier.

Let XLet Xhh represent haemophiliac represent haemophiliacLet XLet XHH represent normal clotting represent normal clotting

Parent PhenotypeParent Phenotype

Parent GenotypeParent Genotype

GametesGametes

Test crossTest cross

If we don’t know the genotype of an If we don’t know the genotype of an individual but we can find out by doing a individual but we can find out by doing a test cross.test cross.

This is a cross with a homozygous This is a cross with a homozygous recessive individual.recessive individual.

Let T represent tallLet T represent tallLet t represent dwarfLet t represent dwarf

Parent phenotype Tall Dwarf Parent phenotype Tall Dwarf

Parent genotypeParent genotype TT tt TT tt

GametesGametes

Chi – squared TestChi – squared Test

To see if there is a difference between To see if there is a difference between numbers observed in an experiment and numbers observed in an experiment and the numbers given in an hypothesis.the numbers given in an hypothesis.

And if that difference is real or if it is due to And if that difference is real or if it is due to chance or sampling error.chance or sampling error.

Chi-squared = Chi-squared =

Null hypothesis – where we assume there Null hypothesis – where we assume there is no difference between the expected and is no difference between the expected and the observed.the observed.

Which means our hypothesis (genetic Which means our hypothesis (genetic diagram) will be backed up by actual diagram) will be backed up by actual results if we did the experiment.results if we did the experiment.

Eg. If we said there should be a 3:1 ratio Eg. If we said there should be a 3:1 ratio then the actual result will be 9 white then the actual result will be 9 white rabbits and 3 pink ones.rabbits and 3 pink ones.

To calculate the chi-squared valueTo calculate the chi-squared value

first determine the number first determine the number expected expected in in each category. each category.

If the ratio is 3:1 and the total number of If the ratio is 3:1 and the total number of observed individuals is 880, then observed individuals is 880, then the the expected numerical values expected numerical values should be should be 660 white and 220 pink. 660 white and 220 pink. 

Say that the actual cross between two Say that the actual cross between two rabbits yields a population of 880 rabbitss, rabbits yields a population of 880 rabbitss, 639 with white fur and 241 with pink fur.639 with white fur and 241 with pink fur.

Now plug it into the equation:Now plug it into the equation:

The chi-squared value will be changed into The chi-squared value will be changed into a probability value using a chi-squared a probability value using a chi-squared table.table.The table contains degrees of freedom.The table contains degrees of freedom.Degrees of freedom ≡ mathematical term Degrees of freedom ≡ mathematical term relating to the number of free variables in relating to the number of free variables in the system. It is always (n – 1).the system. It is always (n – 1).n is the number of categories or n is the number of categories or phenotypes in a given example. In our phenotypes in a given example. In our example 2 (white and pink) therefore example 2 (white and pink) therefore degrees of freedom would be 2 – 1 = 1 degrees of freedom would be 2 – 1 = 1

Use the chi-square distribution table to Use the chi-square distribution table to determine significance of the value.determine significance of the value.

a) Determine degrees of freedom and a) Determine degrees of freedom and locate the value in the appropriate column.locate the value in the appropriate column.

b) Locate the value closest to your b) Locate the value closest to your calculated calculated chi-squared valuechi-squared value on that  on that degrees of freedom row.degrees of freedom row.

Move up the column to determine the p Move up the column to determine the p value.value.

Probability value means the probability that our Probability value means the probability that our observed will match up to our expected every observed will match up to our expected every time we do the practical.time we do the practical.State your conclusion in terms of your State your conclusion in terms of your hypothesis.hypothesis.If the If the pp value is  value is p > p > 0.05, accept your 0.05, accept your hypothesis. hypothesis. – 'The deviation is small enough that chance alone 'The deviation is small enough that chance alone

accounts for it. A accounts for it. A pp value of 0.6, for example, means  value of 0.6, for example, means that there is a 60% probability that any deviation from that there is a 60% probability that any deviation from expected is due to chance only. This is within the expected is due to chance only. This is within the range of acceptable deviation.range of acceptable deviation.

If the p value is If the p value is p < p < 0.05, reject your hypothesis, 0.05, reject your hypothesis, – conclude that some factor other than chance is conclude that some factor other than chance is

operating for the deviation to be so great. For operating for the deviation to be so great. For example, a p value of 0.01 means that there is only a example, a p value of 0.01 means that there is only a 1% chance that this deviation is due to chance alone. 1% chance that this deviation is due to chance alone. Therefore, other factors must be involved. Therefore, other factors must be involved.

So bigger than 0.05 backs the null So bigger than 0.05 backs the null hypothesis.hypothesis.

Smaller than 0.05 means there is a Smaller than 0.05 means there is a significant difference between the significant difference between the hypothesis and the actual results so chuck hypothesis and the actual results so chuck your hypothesis out and start again.your hypothesis out and start again.

Eg. Pg 233 and 235 Eg. Pg 233 and 235

VariationVariation

Phenotype results from the interaction between Phenotype results from the interaction between the genotype and the environment.the genotype and the environment.

The genotype determines the potential of an The genotype determines the potential of an individual but the environment determines to individual but the environment determines to what extent that potential is fulfilled.what extent that potential is fulfilled.

Divided into Divided into – Discontinuous – usually genetic only with an either or Discontinuous – usually genetic only with an either or

outcome. Distinct groups.outcome. Distinct groups.– Continuous – environmental effects come in to play. Continuous – environmental effects come in to play.

Groups merge in to one another.Groups merge in to one another.

Mutation and phenotypeMutation and phenotypeGene mutationGene mutation is a change in the base is a change in the base sequence of DNAsequence of DNA

Alters primary structure of proteinsAlters primary structure of proteins

– SubstitutionSubstitution - one base is swopped for - one base is swopped for anotheranother

– DeletionDeletion – one base is removed – one base is removed– InsertionInsertion – one base is added – one base is added– DuplicationDuplication – one or more bases are repeated – one or more bases are repeated– InversionInversion – a sequence of bases is reversed – a sequence of bases is reversed

examplesexamples

Sickle-cell anaemiaSickle-cell anaemia – base substitution – base substitutionNormal haemoglobin is:Normal haemoglobin is:– Val – His – Leu – Thr – Pro – Glu – Glu – LysVal – His – Leu – Thr – Pro – Glu – Glu – Lys

Sickle-cell is:Sickle-cell is:– Val – His - Leu – Thr – Pro – Val – Glu – LysVal – His - Leu – Thr – Pro – Val – Glu – Lys

Valine is non-polar and hydrophobicValine is non-polar and hydrophobicIn low oxygen concentrations haemoglobin becomes In low oxygen concentrations haemoglobin becomes

less soluble and crystallisesless soluble and crystallisesSticky fibres then distort the red blood cell shape.Sticky fibres then distort the red blood cell shape.

Cystic fibrosisCystic fibrosis– Recessive mutation on chromosome 7Recessive mutation on chromosome 7

Normal gene codes for a chloride channel Normal gene codes for a chloride channel protein called CFTRprotein called CFTRIn cystic fibrosis sufferers channel defective.In cystic fibrosis sufferers channel defective.

Result –Result –– as chloride ions do not flow outas chloride ions do not flow out– sodium ions rush in to balance charge and this sodium ions rush in to balance charge and this

prevents water leaving the cellprevents water leaving the cell– Mucus becomes thick and sticky.Mucus becomes thick and sticky.– Affects lungs, pancreas, liverAffects lungs, pancreas, liver– Mucus clogs ducts and passagesMucus clogs ducts and passages

Down’s syndromeDown’s syndrome– Chromosome mutation on 21Chromosome mutation on 21

Sufferers have 47 chromosomes instead Sufferers have 47 chromosomes instead of 46 as there is an extra copy of 21.of 46 as there is an extra copy of 21.Called trisomyCalled trisomy

SymptomsSymptoms– Mental retardationMental retardation– Short statureShort stature– heart defectsheart defects– Coarse hairCoarse hair

Enviromental effect on phenotypeEnviromental effect on phenotype

HeightHeight

WeightWeight

Dark tips on rabbit ears and tails. Areas Dark tips on rabbit ears and tails. Areas exposed to cold temperatures cause a exposed to cold temperatures cause a pigmentation to be formed in those areas.pigmentation to be formed in those areas.