7A: Genetics

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  • Created by: DBaruch
  • Created on: 05-01-17 18:09

Genes and alleles

  • A gene is a sequence of bases on a DNA molecule that codes for a protein which results in a characteristic.
  • You can have 1 or more versions of the same gene. These different versions are called alleles. There can be many different alleles of a single gene, but most plants and animals, including humans, only carry two alleles of each gene, one from each parent. The order of bases in each allele is slightly different, that is because each allele codes for different verisons of the same characteristic. 
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Loci, Genotype and Phenotype

  • Humans are diploid organisms. This means we have 2 copies of each chromosome, one from each parent. Its why we have 2 alleles of each gene. The allele of each gene is found at fixed position called a locus, on each chromosome in a pair.
  • The genotype of an organism is its genetic make up or the different alleles an organism has. This could be a list of all its allele but usually its just the alleles for one characteristic at a time
  • The phenotype of an organism is the expression of the genes and its interaction with the environment. This is just what characteristics an organism has a result of both its genes and the effect the environment has on its gene
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Dominant and recessive allele, codominant, homozyg

  • A dominant allele is always expressed in the phenotype, even when theres only one copy it. Dominant alleles are shown by a capital letter. Recessive alleles are those with characteristics that only appear in the phenotype if two copies are present. They're shown by lower case letters.
  • Some alleles are both expressed in the phenotype because neither 1 is recessive. They are said to be codominant alleles.
  • At each locus in a diploid organism, the genotype can be homozygous or heterozygous. If an organism carries two copies of the same allele, its said to be homozygous at that locus. If an organism carries two different alleles for a gene, then its heterozygous
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Genetic diagrams

  • Diploid organisms have 2 alleles for each gene. Gametes contain only 1 allele for each gene, they are haploid. When 2 haploid gametes from 2 parents fuse together, the alleles they contain form the genotype of the diploid offspring that is produced. 
  • Genetic diagrams can be used to predict the genotypes and phenotypes of the offspring produced if 2 parents are crossed. 
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Monohybrid inheritance

  • Monohybrid inheritance is the inheritance of a characteristic controlled by a single gene. Monohybrid corsses show the likelihood of the different alleles of that gene being inherited by offspring of certain parents.
  • Image result for monohybrid inheritance (http://www.s-cool.co.uk/assets/learn_its/gcse/biology/genetic-crosses/monohybrid-crosses/g-bio-cross-dia02.gif)
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Phenotypic ratios

  • The phenotypic ratio is the ratio of different phenotypes in the offspring. Genetic diagrams allow you to predict the phenotypic ratios in F^1 and F^2 offspring.
  • Usually whenever you do a monohybrid cross with 2 heterozygous parents you get a 3:1 ratio of dominant:recessive characteristics in the offspring. However, sometimes you won't get the expected phenotypic ratio. For example, codominant alleles and sex linkage can both alter phenotypic ratios in the offspring of monohybrid crosses
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Punnett squares

  • Image result for punnett squares
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Monohybrid inheritance of codominant alleles

  • Alleles somethimes show codominance meaning that both alleles are expressed in the phenotype and neither one is recessive. An example of this is the allele for sickle-cell anaemia.
  • Image result for monohybrid cross codominance (http://sctritonscience.com/Wilson/biolog4psa.gif)
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Multiple allele crosses

  • Inhereitance is more complicated when there are more than 2 alleles of the same gene.
  • Image result for multiple allele crosses (http://sctritonscience.com/Wilson/biolog12psa.gif)
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Multiple allele crosses

  • Inhereitance is more complicated when there are more than 2 alleles of the same gene.
  • Image result for multiple allele crosses (http://sctritonscience.com/Wilson/biolog12psa.gif)
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Dihybrid crosses

  • Dihybrid inheritance is the inheritance of 2 characteristics, which are controlled by different genes. Each of the 2 genes will have different alleles. Dihybrid crosses can be used to show the likelihood of offspring inheriting certain combinations of the 2 characteristics from particular parents.
  • Image result for dihybrid cross (http://www.biology.arizona.edu/Mendelian_Genetics/problem_sets/dihybrid_Cross/graphics/03tg.gif)
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Inheritance of sex-linked characteristics

  • The genetic information for gender is carried on two sex chromosomes. In mamals, females have 2 X chromosomes and males have 1 X and 1 Y. The chance of a baby being male or female is 50/50
  • Some characteristics are sex-linked. Meaning that the alleles that code for them are located on a sex chromosome. The Y chromosome is smaller than the X so carried less genes. So most genes carried on sex chromosomes are carried on the X chromosome.
  • As males only have 1 X, they often only have 1 allele for sex linked genes. So because of this they express the characteristic of the allele even if its recessive. This makes males more likely than females to show recessive phenotypes for genes that are sex-linked.
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Examples of sex-linked characteristics

  • Genetic disorders caused by faulty alleles are located on sex chromosomes include both colour blindness and haemophillia. They are both carried on the X chromosome however Y-linked disorders to exist but are less common.
  • Image result for sex linked inheritance
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Linkage of autosomal genes

  • Autosome is another way of saying any chromosome that isn't sex related. Autosomal genes are the genes located on the autosomes. Genes on the same autosome are linked as they stay together during independent segregation in meiosis 1 and their alleles will be passed on to the offspring together
  • The only reason this won't happen is if crossing over splits them up first. The closer together two genes are on the autosome, the more closely they are said to be linked.
  • If 2 genes are autosomally linked, you wont get the phenotypic ratio you expect in the offspring of a cross
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What is epistasis

  • Many different genes can control the same characteristic, they interact to form the phenotype. This can be because the allele of one gene masks the expression of the alleles of other genes.
  • Image result for epistasis (http://learn.genetics.utah.edu/content/pigeons/epistasis/images/epistasis.jpg)
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Phenotypic ratios for epistatic genes

  • Recessive- if the epistatic allele is recessive then 2 copies of it will mask the expression of the other gene. If you cross a homozygous recessive parent with a homozygous dominant parent will produce a 9:3:4 ratio of dominant both:dominant epistatic, recessive other:recessive epistatic in the F^2 generation
  • Dominant- if the epistatic allele is dominant, then having at least 1 copy of it will mask the expression of the other gene. Crossing a homozygous recessive parent with a homozygous dominant parent will produce a 12:3:1 ratio of dominant epistatic:recessive epistatic, dominant other :recessive both in the F^2 generation
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Chi-squared test

  • The chi-squared test is a statistical test thats used to see if the results of an experiment support a theory. First, the theory is used to predict a result- this is called the expected result. Then the actual experiment and the result recorded this is called the observed result.
  • To see if the results support the theory you have to make a null hypothesis which states that there is no significant difference between the observed and expected results.
  • Your experimental result will usually be different from what you expect, but you need to know if the difference is just due to chnace, or because your theory is wrong. The outcome either supports or rejects the null hypothesis.
  • Image result for chi squared formula (http://www.statisticshowto.com/wp-content/uploads/2013/09/chi-square-formula.jpg)
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