Inheritance

Genotype

genetic makeup of an individual/organism

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Phenotype

visual characteristics of an individual (not just external physical attributes - for example, being affected by Diabetes Mellitus is a phenotype)

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An individual’s phenotype is a result of its...

genotype and its environment. This is the basis of the nature vs nurture debate. It explains why genetically identical twins may have diverging phenotypes, especially as they get older and are exposed to more environmental stimuli.

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A single gene may have different allele. Since chromosomes come in pairs, there are always two copies of a gene in an individual’s DNA - and each gene can have a different alleles. What is an allele?

An allele is a version of a gene which gives it a certain characteristic

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What are the blood types?

The gene for blood group type can be A, B, or O

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Dominant Alleles

Expressed regardless of allele on the other chromosome/gene at the same locus. Whether homozygous or heterozygous ‘Expressed’ means that it is coded for and transcribed and translated into a protein E.g. Bb and BB express B (same phenotype).

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Recessive Alleles

Recessive alleles are only expressed if homozygous Present at both chromosomes Will only be expressed if bb - not if BB or Bb.

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Co-dominance

When both alleles present in the genotype contribute to the phenotype. WW (white) and BB (black)= Speckled (BW).

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Name an example in human of co-dominance with multiple alleles.

human blood groups. An individual possessing the A allele - with or without the O allele - expresses A. An individual possessing the B allele - with or without the O allele - expresses B. But AB both expressed. A + B proteins synthesised.

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Colour coat in cattle- E.g. of co-dominance.

Red allele AND white allele = roan coat, speckled white and red coat Both alleles are expressed

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Genetic Diagrams

Genetic diagrams are used to illustrate the possible genotypes of offspring from two parents, and the probability of occurrence of each.

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If you are asked to draw a genetic diagram in the exam, your answer should include a punnett square. You need to be able to draw monohybrid and dihybrid crosses. What are they?

Monohybrid = investigations that examine the inheritance of a single characteristic: e.g. wrinkly or smooth peas.Dihybrid = investigations that examine the inheritance of two characteristics: e.g. cut or potato-leaved + dwarf or tall leaves.

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Your genetic inheritance diagram should contain: (4)

Parent phenotypes Parent genotypes Parent gamete genotypes Punnett square

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Linkage

Linked genes more likely to be passed on together since their loci are close together on the chromosome. Linkage = when two or more genes are located on the same chromosome

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Define autosomal linkage and sex linkage

Autosomal linkage = linked genes which are on non-sex chromosomes Sex linkage = linked genes are on sex chromosomes - therefore specific characteristic is more likely to be inherited in either male or female offspring

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Sex linkage

Genes are more likely to be X-linked-found on X chromosome. So, female offspring only show recessive genes if homozygous. Male offspring show recessive X-linked genes even if only present on X chromosome E.g. colour blindness more common in males.

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Autosomal linkage

Linkage of genes which are found on autosomal (non-sex) chromosomes

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EXAM TIP

After drawing genetic diagrams to predict the phenotypes of F1 individuals - if the expected vs observed phenotypes are significantly different. Use chi-squared to determine if significant.

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Epistasis

interaction of non-linked genes where one masks the expression of the other.

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Recessive epistasis

Homozygous recessive alleles mask expression of another allele at different locus

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E.g. of recessive epistasis is flower colour in Salvia.

Two gene loci: A/a, B/b.B = purple; b = pink; a = white Plant with genotype AABB is purple Plant with genotype AAbb is pink But any plant with genotype aa-- will be white - even if alleles at B/b are homozygous dominant

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Homozygous aa is epistatic to both alleles of the B/b gene

Neither B nor b is expressed unless at least one dominant A is present

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The Chi-Squared (χ2) Test

statistical test to find out whether the difference between observed vs expected data is due to chance or ‘goodness of fit’ of a hypothetical model. Genetic diagrams give us an indication of probable offspring genotypes.

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However,

Due to the random nature of gamete fusion, these are rarely 100% accurate predictions

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When to use the chi-squared test: (4)

The data are in categories (i.e. discrete variation) The sample size is large enough to be representative The data indicate absolute numbers: not percentages No data values are equal to null

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Step 1 of Chi-Squared Test

Define null hypothesis States that there is no significant difference between observed/expected data I.e. states that the difference is due to chance

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Step 2- Applying the test

x2 =the sum of [each observed number (o) - each expected number (E)]each expected number (E)

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Once x2 is found using equation what do you do?

Determine the number of degrees of freedom (number of categories - 1). E.g. studying 10 cat traits, 10-1=9.

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Now using the degree of freedom, determine value of p from distribution table and decide whether the result is significant. They are significant if/

If calculated value is more than critical value there is significant differences.

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In general, small P value cause you to reject null hypothesis, shows significant difference.

Very large P value (e.g.95% due to chance) cause you to accept the null hypothesis, suggesting not a significant difference.

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Define genotype and phenotype

Genotype = genetic makeup of an individual Phenotype = visual characteristics of an individual (not just external physical attributes - for example, being affected by Diabetes Mellitus is a phenotype)

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An individual’s phenotype is a result of its genotype and its environment. This is the basis of the nature vs nurture debate. It explains why genetically identical twins may have diverging phenotypes...

especially as they get older and are exposed to more environmental stimuli.

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Genetic factors

Genetic variation caused by mutations Those which are expressed in the phenotype contribute to phenotypic variation

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Caused by mutagenic agents eg…

X-rays Benzopyrene found in tobacco smoke Viruses Gamma rays

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Caused by chromosomal mutations...

What types could this include? (of mutations)

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Deletion

Removal of base pair, resulting in change in protein structure and ultimately it's functioning. May be advantageous or disadvantageous within it's habitat.

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Invertion

section of chromosome breaks off, then is re-inserted in the opposite direction

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Translocation:

section of chromosome breaks off then is re-inserted on a different chromosome

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Duplication:

part of a chromosome occurs twice

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Non-disjunction:

one pair of chromosomes fails to separate, so the gamete and zygote has an extra chromosome (e.g. Down's syndrome)

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Aneuploidy = chromosome number is not a multiple of the haploid number for that organism Polyploidy = diploid gamete fertilised by a haploid gamete

Resulting zygote is triploid (3n chromosomes)

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Genetic variation could be cause of normal sexual reproduction. Suggest why?

Meiosis produces genetically different gametes Alleles shuffle around Independent assortment of chromosomes Contribute to genetic diversity Random fusion of gametes at fertilisation

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Environmental factors

Variation caused by the environment alone E.g. losing a limb in an accident

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Variation caused by the environment interacting with genes

nvironmental conditions can affect the expression of some genes- This is called epigenetics

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Epigenetics

Humans are learning how to control the expression of genes by altering the epigenome, and how to alter genomes and proteomes of organisms. This has many medical and technological applications.

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Epigenetic control of gene expression in eukaryotes.

Epigenetics involves heritable changes in gene function, without changes to the base sequence of DNA. These changes are caused by changes in the environment that inhibit transcription by:

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increased methylation of the DNA or decreased acetylation of associated histones.

The relevance of epigenetics on the development and treatment of disease, especially cancer. In eukaryotes and some prokaryotes, translation of the mRNA produced from target genes can be inhibited by RNA interference (RNAi).

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Genes are put in certain ‘modes’ where they might behave in a certain way

E.g. plants reacting to light It is thought that epigenetics can be passed vertically (from parent to offspring)

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Discontinuous variation

Phenotypes are in distinct categories E.g. blood types or sex Cannot be ‘between’ blood type A and B Determined by a single allele Monogenic Qualitative

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Continuous variation

Phenotypes fall in a range Smooth gradient between phenotypes, which are often distributed in a bell curve Eg tail length in mice; birth weight; height; skin colour; heart rate Controlled by more than one gene Polygenic Quantitative

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EXAM TIP Don’t get confused about what polygenic means. If a characteristic is polygenic, it is determined by the interactions of several genes and the alleles at those loci - in that one individual.

Name some examples of continuous and discontinuous variation within the human population.

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Height is an example of continuous variation - individuals can have a complete range of heights, for example, 1.6, 1.61, 1.62, 1.625 etc metres high.

Other examples of continuous variation include: • Weight; Hand span; Shoe size; Milk yield in cows

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Factors affecting specie variation

Evolution is driven by phenotypic variation. Some individuals will have phenotypic traits that make them better adapted to their environment. They will live long and reproduce, passing on the advantageous alleles.

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What will happen over time to these alleles?

will become more and They will become more frequent in the population. Allele frequency will change.

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Directional selection

Environment favours individuals at one extreme of the bell curve E.g. faster cheetahs are able to survive better The mean changes Mean sprint speed of cheetahs becomes faster and faster over time

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Stabilising selection

Environment favours individuals close to a specific value which doesn’t change Individuals with extreme phenotypes are less likely to survive E.g. birth mass Too heavy = birth problems Too small = developmental problems

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So what does this mean for the mean of the babies birth weight?

The mean stays the same Standard deviation becomes smaller over time

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Genetic drift

Occurs when population is small to begin with Small gene pool Chance mutations that are not beneficial or harmful might cause changes in allele frequency

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Population isolation

can ‘drift’ and become very different to the parent population. Think the Galapagos Islands

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Genetic bottleneck

When a population shrinks and then increases again

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Name an example of a genetic bottleneck

E.g. disease spreads through a population and few survive. New population has reduced genetic diversity as genes derive from a few individuals. However in the case of disease, they will have acquired resistance to the disease which allow survival.

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Founder effect

If a new population is established from very few ‘founding’ individuals, there will be little genetic diversity Small gene pool Specific type of genetic drift

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Be clear that genetic bottleneck and genetic drift occur as a result of random mutations - not the other way round.

Random mutations occur irrespective of the size of the population, but if the population is notably small, they affect a larger proportion of it, and therefore have a greater effect and are more likely to be carried into future generations.

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Hardy-Weinberg Principle predicts the ongoing frequencies of alleles and genotypes within a closed, freely and randomly breeding population.

p+q=1. P and Q are the probability of 2 different alleles.

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No change in the allele frequencies within a population Assuming:

Large population No migration or immigration No mutation to new alleles Random mating, with respect to the genotype of the organism

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EXAM TIP

No need to memorise the Hardy-Weinberg equation as it will be provided in the exam. However, try out a few practice questions to make sure you know how to apply it.

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Speciation

the formation of a new species.

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Parent species must be split into 2 groups

Requires the isolation of one group which will go on to become new species Different selection pressures on each group Speciation occurs when individuals from the 2 groups have significant genetic differences Can no longer interbreed

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Isolating mechanisms

Name the types

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Geographical isolation- pre-zygotic

2 groups separated geographically Eg. ocean/mountains/rivers 2 groups do not meet and cannot interbreed ‘Allopatric speciation’ = speciation in different countries

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Behavioural isolation- pre-zygotic

Occurs when populations are separated by different courtship rituals and other behaviours involved in reproduction

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Temporal isolation- pre-zygotic

Occurs when populations reproduce at different times.

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Mechanical isolation- pre-zygotic

Copulation is attempted, but transfer of sperm does not take place.

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Reduced hybrid fertility

Hybrids fail to produce functional gametes, cannot reproduce/ sterile.

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Habitat isolation

Populations inhabit similar regions but occupy different habitats, e.g. one species of snake lives on land the other lives in water.

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Gametic isolation

The gametes do not fuse so no zygote will be produced.

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Hybrid breakdown

F1 generation are fertile but the F2 generation fail to develop or are infertile. After a few generations sterile offspring are produced!

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Reduced hybrid viability

Hybrids are produced by fail to develop to reproductive maturity

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Artificial selection

The agricultural revolution marked the beginning of man’s implementation of artificial selection. This is where farmers and breeders select individuals from a population with desirable traits

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Name an example of these desired traits

e.g. cereal with resistance to drought, or cows with high milk yields.

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Define natural and artificial selection

Natural selection = selection of genes which adapt a species to survival Artificial selection = selection of genes for optimal economic yield

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Hybrid vigour

Excessive selective cross-breeding can cause inbreeding depression Breeding of related individuals Gene pool diversity reduced Chances of expression of recessive harmful gene increased

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Hybrid vigour

To avoid this problem, breeders must maintain a resource of genetic material They outcross individuals from different varieties Often with wild types The resulting F1 are heterozygous at many loci

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Hybrid vigour leads to...

Decreased genetic variety = increased widespread susceptibility to disease

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Domesticated animals are less able to defend themselves against predators They are also often less able to hunt prey E.g. dogs who became scavengers and were no longer under the selection pressure to be able to hunt as a pack

Livestock animals are biomorphically different and this may not support a happy life E.g. chickens which grow too rapidly - bones unable to support weight Pigs vulnerable to low temperatures during the winter because of reduced fat percentage

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Pedigree dogs subject to narrow gene pool and therefore often have higher susceptibility to certain diseases E.g. Boxer: cancer, heart disease Labrador: abnormal hip and shoulder joints, lameness West Highland Terrier: dry eye, skin irritation

This is cruel and unethical. Make sure you know the ethics of artificial selection

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Inheritance

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