Cellular Control, Bio (Pt3)

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master.director
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Cellular Control, Bio (Pt3)
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2013-04-22 18:27:34
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cellular control biology a2 genes
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Some terms you need to know, using genetic diagrams
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  1. What does the term "genotype" mean?
    Genetic makeup of an organism. It describes the alleles present within cells of an individual, for a particular trait/characteristic.
  2. An organism with two identical alleles for a particular gene is called what? And what about those who have two different alleles of the same gene.
    • Homozygous: organism with two identical alleles for a particular gene
    • Heterozygous: organism with two different alleles of the same gene.
  3. Briefly explain how domant and recessive genes work, using cystic fibrosis as an example.
    • Heterozygotes have one normale allele (CF) and one abnormal allele (cf), but have no symptoms. This is because they have some abnormal chloride ion channels (coded for by abnormal allele), but enough normal channels (coded for by dominant allele), for lungs to function.
    • Only when both alleles are abnormal, in the case of cystic fibrosis, would one get the symptom.
  4. Explain what is meant by phenotype.
    Refers to the observable characteristics that are expressed in the organism. Determined by its genotype and the environment in which it has developed.
  5. Explain the term dominant.
    • An allele is said to be dominant if it is always expressed in the phenotype, even if a different allele for the same gene is present in a heterozygous genotype.
    • Can also describe the characteristic and its inheritance pattern as dominant too.
  6. Explain the term recessive.
    • An allele is said to be recessive if it is only expressed, in the phenotype, in the presence of another identical allele, or in the absence of a dominant allele, for the same gene.
    • Again, can be used to describe the characteristic or inheritance pattern. (eg. cystic fibrosis is a recessive characteristic and has a recessive inheritance pattern).
  7. Explain the term codominant.
    • A characteristic where both alleles contribute to the phenotype.
    • Two alleles are described as codominant if they both contribute to the phenotype of a heterozygote.
  8. Explain what is meant by the term linkage.
    Genes for different characteristics that are present at different loci on the same chromosome are linked.
  9. Explain what is meant by sex linkage.
    • A characteristic is sex-linked if the gene that codes for it is found on one of the sex (X and Y) chromosomes. In most animals, small Y chromosome has few genes so most sex-linked genes are likely to be found on X.
    • eg. haemophilia (is a sex-linked characteristic)
    • When it is linkage but not sex linkage, then it is called autosomal linkage.
  10. Give two examples of sex-linked disorders.
    • Haemophilia A
    • Duchenne muscular dystrophy (DMD)
  11. A few conventions when drawing genetic diagrams.
    • Start by showing parental phenotypes.
    • Work out the different gametes to use in the diagram.
    • Gene represented by single letter, with upper case for dominant allele and lower case for recessive.
    • Where gene in question has more than two alleles (in a population, that is), then gene has upper case letter and the alleles in superscript (eg. CCw) Also provide a key for the symbols.
    • In sex-linked questions, make the base letter X and Y for the two chromosomes.
  12. Imagine a genetic diagram showing (punnet square) the different genotypes offsprings could be of a carrier female and normal male for a sex-linked characteristic such as haemophilia.
  13. REMEMBER
    Practice questions using genetic diagrams involving both sex-linked and codominance problems.
  14. Why is haemophilia much rarer in girls than in boys?
    Because it is a sex-linked disease, and a recessive characteristic, only homozygous girls will get haemophilia, whereas in boys, if they have the recessive allele in only the X chromosome, that is enough to get it, because the Y is too small to have that gene.
  15. Give the different genotypes of sickle-cell anaemia. Is it a recessive or codominant condition?
    • Genotype of people with normal haemoglobin can be denoted HAHA
    • Genotype of people with sickle-cell anaemia is HSHS
    • Genotype of symptomless heterozygote is HAHS
    • Heterozygotes are symptomless carriers and at whole-organism level condition can be considered to be recessive. However, at molecular level, because both alleles contribute to phenotype as observed in RBC, it is codominant.
  16. Sometimes we want to look at the inheritance of two genes at the same time. This is known as ___ ___.
    Dihybrid inheritance
  17. In a normal dihybrid cross, what is the normal ratio? (So interbreeding of RrYy for example).
    • 9:3:3:1
    • (Dominant dominant; dominant recessive; recessive dominant; recessive recessive).
  18. Often (I think) a dihybrid question will start with two crosses (AABB and aabb) which produces F1 of RrYy. They then interbreed it. How do you tackle this question?
    • Start by identifying all the different games there could be. (ie AB, Ab, aB, ab) for each.
    • Use punnet square against the male and female.
    • Identify all the different genotypes 
    • Work out ratio of F2 phenotype (I think it should be 9:3:3:1?)
  19. What is epistasis?
    The interaction of different gene loci so that one gene locus masks or suppresses the expression of another gene locus.
  20. Describe the three types of epistasis in the textbook.
    • Recessive epistasis: having two recessive epistatic allele masks the expression of the other gene.
    • Dominant epistasis: having at least one dominant epistatic allele masks the expression of the other gene.
    • Working in complementary fashion: homozygous recessive condition at either locus masks the expression of the dominant allele at other locus. (Because it works with creating an intermediate compound - need 2 enzymes to work).
  21. Considering these different types of epistasis... Once we breed AABB and aabb to get all AaBb, the epistatic ratios are variations on the normal ratio from a dihybrid inheritance of what? What are the different ratios for the different types of epistasis?
    • Normal: 9:3:3:1
    • Recessive epistasis: 9:3:4 (dominant both, dominant epistatic recessive other, recessive epistatic)
    • Dominant epistasis: 12:3:1 (dominant epistatic, recessive epistatic dominant other, recessive both) (or, 13:3)
    • Complementary fashion: 9:7
    • [You can prove this with the genetic diagram and knowing the type of epistasis]
  22. What do you use the chi-squared (x2) test for?
    To find out if the difference between observed categorical data (data in categories) and expected data is small enough to be due to chance.
  23. What are the criteria for the chi-squared test?
    • For categorical data
    • Where there is a strong biological theory that we can use to predict expected values.
    • Sample size must be relatively large.
    • Only raw counts, not percentage or ratios
    • There are no zero scores
  24. Briefly explain how you would tackle a chi-squared test question.
    • Create a table with the different categories and create columns for the different values needed for the formula for each (eg. Oberved value, expected value, O-E etc).
    • Work out x2
    • If you have 4 categories, the degree of freedom is 3. Degree of freedom is number of categories - 1.
    • Use the x2 table to find out the critical value for the degree of freedom (usually for p=0.05)
    • If the x2 value is smaller than critical value then we say the difference is due to change and is not statistically significant (ie accept null hypothesis).
  25. What is discontinuous variation? Give examples.
    • This describes qualitative differences between phenotypes. Qualitative differences fall into clearly distinguishable categories. There are no intermediate categories.
    • eg. Male or female; blood group O,A,B or AB.
  26. What is continuous variation? Give examples.
    • This described quantitative differences between phenotypes. These are phenotypic differences where there is a wide range of variation within the population, with no distinct categories.
    • eg. Height and mass in humans, grain colour in wheat
    • [Remember, the spec is to describe difference between continuous and discontinuous variation]
  27. Explain the basis of discontinuous variation with reference to the number of genes that influence the variation.
    • Different alleles at a single gene locus have large effects on the phenotype.
    • Different gene loci have quite different effects on the phenotype.
    • Examples include codominance, dominance nad recessive patterns of inheritance.
    • In many cases there may only be one gene involved, and these are called monogenic.
    • (Both types of variation may be result of more than one gene, but in discontinuous, if there is more than one gene involved, usually the genes interact in an epistatic way. )
  28. Explain the basis of discontinuous variation with reference to the number of genes that influence the variation.
    • Traits exhibiting continuous variation are controlled by 2 or more genes.
    • Each gene provides an additive component to the phenotype.
    • Different alleles at each gene locus have a small effect on the phenotype.
    • A large number of different genes may have a combined effect on phenotype. These are known as polygenes and the characteristic they control described as polygenic. (these genes are unlinked - on different chromosomes).
    • Eg. Dominant allele of a gene may add 2cm to cob length, whereas recessive may add 1cm, and if 3 genes contribute to length, then AABBCC will be 12cm, aabbcc will be 6cm etc..
  29. In many examples of discontinuous variation there may only be one gene involved. Such characteristics are called what?
    Monogenic
  30. Both __ and __ contribute to phenotype variation. For example, a child may have the __ ___ to become intelligent, but not having a stimulating learning environment may __ the __ of the genes.
    • genotype 
    • environment
    • genetic potential
    • limit
    • expression
  31. The expression of ___ traits is influenced more by the environment than is the expression of ___ traits.
    • polygenic
    • monogenic
  32. What are the two different types of selection?
    • Natural
    • Artificial
  33. Explain why variation is essential in selection.
    • When environment changes, those individuals that are well adapted will survive and reproduce, passing on their advantageous alleles to their offspring. This is the basis for evolution.
    • Because of variation, some members will be better adapted to survive and reproduce than others. Variation is therefore essential for selection.
  34. What is heritability?
    The proportion of phenotypic variation in a population that is attributable to genetic variation among individuals.

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