Patterns Of Inheritance

Genetic Variation 

Genetic variation - a measure of the genetic differences that exist within a population. Genotype: the genetic makeup of an organism. Phenotype: the visible characteristics of an organism.  Causes of variation: => Mutation => Sexual reproduction => Environmental variation  Gene mutations: there are many causes. Mutagens are biological, chemical and physical agents which speed up the rate of mutation. Physical = x-ray and gamma ray. Chemical = nitrous acid and mustard gas. Biological = some viruses and food contaminants. Chromosome mutation: => Deletion => Insertion => Inversion => Duplication => Non-disjunction  

Aneuploidy: The chromosome number is not an exact multiple of the haploid number for that chromosome. This is caused by non-disjunction. Polyploidy: If a diploid gamete is fertilised by a haploid gamete the resulting zygote is triploid. Sexual Reproduction: Crossing over - prophase 1 Independent assortment - metaphase 1 and 2 Random fertilisation - any egg can be fertilised by any sperm. Mutations can also occur during the replication of DNA before meiosis.  Environmental factors: => Area you live for dialect. => Scaring => Chlorosis is an example which is caused by lack of minerals, virus and lack of light.  Animal body mass is an example of variation caused by genetics and the environment. Body mass can be caused by the amount of food an organism eats or the presence of a disease (environment). But it could also be cause by genetic factors. For example obesity in mice can be caused by a mutation in chromosome 7.

Meiosis Stages

Meiosis 1 Prophase 1:  Chromosomes join together to form a bivalent Chromatids cross over. Metaphase 1: Bivalents line up on the equator. Independent assortment occurs. Anaphase 1: Chromosomes separate. Telophase 1: Nuclei form Cell divides. Cells have two chromosomes not 4 chromatids.

Meiosis 2 Prophase 2: Nuclear envelope breaks down Spindles form Metaphase 2: Chromosomes line up on the equator.  Independent assortment occurs. Prophase 2: Centromeres split Sister chromatids separate. Telophase 2: 4 haploid cells each with 2 chromatids.

Continuous and Discontinuous Variation

Continuous variation A characteristic that can take any value within a range. Caused by the environment and genetics. Controlled by a number of genes: polygene. E.g. skin colour. Scatter graph Discontinuous variation: A characteristic that can only appear in specific (discrete) values. Mostly caused by genetics. Controlled by 1 or 2 genes. E.g. blood group. Bar chart

Monogenetic inheritance

Monogenetic: caused by a single gene, such as colour of flower. Allele: version of a gene. Heterozygous: having different alleles at a particular gene locus on a pair go homologous chromosomes. Homozygous: having identical alleles. To draw a genetic cross diagram: State the phenotypes of both parents. State the genotype for both parents. State the gamete of each parent, place a circle around each. Use a punnet square. State the proportion of each genotype. State the corresponding phenotype.

Co-dominance = when a gene is coded for by two alleles which are equally dominant. Multiple alleles = genes which have more than two versions.

Sex Linkage

Characteristics which are determined by genes carried on the sex chromosomes are known as sex-linked. Sex is determined by one of the 23 pairs of chromosomes called the sex chromosomes. The other 22 pairs are called autosomes. Each of the autosomal pairs are fully homologous - they match for length and contain the same gene at the same loci. The X and Y (sex chromosomes) are not fully homologous. A small part of one matches a small part of the other. This is so these chromosomes can pair up before meiosis.  If a female has one abnormal allele on one of her X chromosomes she will probably have a functioning allele of the same gene on her other X chromosome. However, if a male inherits from his mother an abnormal allele for a particular gene he will inherit the disease. Males are functionally haploid/hemizygous for X-linked genes.

Examples include: haemophilia, colour blindness and sex linkage in cats for the colour of their fur.  As the Y chromosome is much smaller than the X, there are a number of chromosomes on the X which a male will only have one copy of.

Autosomal Linkage

Two or more genes are located on the same chromosome, they are said to be linked. It is autosomal linkage if these genes are found on chromosomes 1-22. This is significant because: Linked allele combinations will be inherited together. Only crossing over during meiosis can separate linked allele combinations. Two alleles are less likely to be separated during crossing over when they are closer together on a chromosome.  Recombinants: they are new combinations of genes not found in the parents. They are produced by crossing over in prophase 1.    

Recombination frequency 50% recombination frequency = no linkage. <50% recombinant frequency = gene linkage. The greater the distance between the genes, the greater the frequency of recombinant chromatids. 

Chi-squared Test

It measures the size of difference between the observed and expected results. It helps us determine whether the differences are significant or not. When is Chi-squared test used? The data is in categories and is not continuous. We have a strong biological theory to use to predict expected values. The sample size is large. There are no 0 scores in the raw data count. The procedure: Calculate the value of chi-squared. Determine the degrees of freedom (n = number of categories - 1). Determine the value of p from the distribution table. The value of 0.05 identifies the level that could occur by chance just 5 times in 100.

Determining whether it is significant or not? If the calculated value of chi squared is less than the critical value at p = 0.05, the difference is not significant and we accept the null hypothesis. If the calculated value of chi squared is greater than the critical value at p = 0.05, the difference is significant and we reject the null hypothesis. 

Epistasis

Epistasis is where a gene masks or suppresses the expression of another. It is the interaction of genes at different loci. Gene regulation is an example of epistasis with regulatory genes controlling the expression of structural genes, such as lac operon. Dominant epistasis: if dominant, then having one copy of the gene will mask the expression of another gene. You would expect the 12:3:1 ratio. Recessive epistasis: if recessive then having both copies of the gene will mask the expression of another You would expect 9:3:4.

Evolution

Natural selection: Mutations and migration introduce new alleles into a population.  Some individuals in the population will be better adapted to survive in the environment, due to their differences in genotype and phenotype.  These individuals are more likely to survive and reproduce, passing on their advantageous alleles to the next generation.  Over time the allele frequencies in the population will change. This is natural selection.  There are three main types of selection: stabilising, directional and disruptive.  Stabilising selection occurs when an organisms environment remains unchanged and it favours the intermediate phenotypes. The extremes are favoured against.

Directional selection: It is caused by a change in environment. The normal (intermediate) is no longer favoured. Organisms which are less common and have more extreme phenotypes are positively selected. This shifts the allele frequency. This type of selection is used by plant and animal breeders to produce desirable traits. Disruptive selection: Here the extremes are selected for and the norm is selected against. It is the opposite of stabilising. An example would be the finches in the Galapagos Islands. Those with a bright beak are too threatening, and those with the dull brown beaks are not threatening. Therefore these are left alone and the intermediate are attacked.

Evolution

Genetic drift: variation in the relative frequency of different genotypes in a small population, which can lead to the disappearance of particular genes. This occurs due to chance and it is random. The effects of genetic drift are strongest in a small population. If can have a major effect when a population is sharply reduced in size (genetic bottleneck) or when a small group splits off and forms a new colony (founder effect). It leads to a reduction in the gene pool.

Founder effect: It is where a few isolated species form a new colony. They have small gene pools so less genetic variation. The frequency of alleles that were rare will have a much higher frequency in the new smaller population. These will have a much greater impact during natural selection.  Genetic bottleneck: large reductions in a population size which last for at least one population. Humans have experienced at least one bottleneck. The gene pool will be greatly reduced. A positive of this would be that a beneficial mutation will have a much greater impact and lead to the quicker development of a new species.

Problems with a small population

Small populations with limited genetic diversity can't adapt as easily leading to extinction. This is a problem if a disease arises. The pathogen could easily wipe out the population.  The size of a population can be affected by many factors, there are two types of limiting factor: Density dependent: such as disease, predation and competition Density independent: such as earthquakes and climate change. 

Speciation

It is the evolution of a new species which cannot interbreed with the original population following some kind of isolation.  There are two types of speciation: allopatric and sympatric. Allopatric: Speciation taking place due to geographical isolation preventing gene flow. Sympatric: Speciation taking place in the same geographic area. This is most likely caused by reproductive isolation.  Reproductive isolation Hybrid sterility: odd number of chromosomes in a hybrid meaning they can't produce gametes.  Gametic: gametes released in the same place but chemical incompatibility means fertilisation is impossible.  Mechanical: Reproductive structures are incompatible. Behavioural: Different mating calls not recognised by different species.  Temporal: organisms come in to season at different times so can't reproduce. Pre-zygotic barrier: prevents fertilisation from happening. Post-zygotic barrier: mating and fertilisation do occur but the offspring are not fertile.   

Artificial Selection

The selective breeding of organisms involves humans choosing the desired phenotypes and interbreeding those. Selective breeding programmes are a common method of artificially selecting organisms. It reduces the gene pool. If related individuals are crossed, inbreeding depression can occur. Inbreeding depression is the reduced biological fitness as a result of inbreeding.  Gene banks: store biological samples. They are used to increase genetic diversity. Often used to over come problems with inbreeding.  Examples of gene banks: Rare breed farms Wild populations Crops in cultivation Botanic gardens and zoos. Seed banks.

Problems with inbreeding

It reduces the gene pool and reduces the chances of a population of inbred organisms evolving. Many genetic disorders are caused by recessive alleles. Organisms that are closely related are genetically similar so they are likely to have the same recessive alleles. Inbreeding mean offspring have a greater chance of being homozygous for these recessive traits. Therefore they are more susceptible to genetic disorders. This reduces there chances of surviving and reproducing. They are less biologically fit.

Ethics fo artificial selection

Live stock selected to have more lean meat but less fat meaning they are at risk of extreme temperatures. Some coat colours are selected because humans like them but they may be disadvantaged in their habitat.  Traits considered desirable by humans may put an organism at a disadvantage. 

Hardy-Weinburg Principle

Factors affecting a gene pool: Population size Mutation rate Migration Natural selection Gene flow Genetic Drift  Non-random mating  

The Hardy-Weinburg principle describes and predicts a balanced equilibrium in the frequencies of alleles and genotypes within a breeding population. It can be used to determine the frequency of those carrying a genetic disorder.  The principle assumes: the population is large enough to make sampling errors negligible. Mating within a population is random no selective advantage for any genotype no selection no mutation no migration no genetic drift p^2 + 2pq + q^2 = 1 (genotypes) p + q = 1 (allele) Always find q first.