Pack 12 - Inheritance

Pack 12 - Inheritance

1. Genotype:

   • Genetic makeup of an organism, all alleles an organism has.
   • Phenotype:
     • Observable or biochemical characteristics

2. What is a gene

   • A sequence of nucleotide bases coding for a polypeptide or functional RNA
   • An allele is a different version of the same gene
     • Each gene of diploid cells should have 2 alleles, unless sex chromosones

3. The locus is the position of a gene on a chromosome

4. Types of chromosomes:

   • Homozygous
     • Alleles on each chromosome are the same
   • Heterozygous
     • 2 alleles on the chromosome are different

5. Types of alleles:

   • Dominant
     • When present in genotype it's always expressed in phenotype
   • Recessive
     • Only expressed when homozygous recessive

6. Codominance

   • This is when two alleles both contribute to the phenotype

7. Monohybrid Inheritance

   • This is the inheritance of a single gene

8. Pedigree Diagrams

   • Diagrams show ancestral relationships and transmission of genetic traits
   • Exam hint:
     • If asked whether an allele is dominant or recessive, look for two parents that do not have the characteristic but have children that do have it
       • Shows it is recessive

9. Dihybrid Inheritance

   • Two different characters, determine by different genes located on two different chromosomes are inherited

10. Mendel's Laws

    • Law of segregation:
      • Characteristics of an organism are controlled by genes that occur in pairs, only one of each pair can be carried in a gamete
    • Law of independent assortment
      • Each member of a pair of alleles may combine randomly with either of another pair
    • In terms of meiosis:
      • Homologous chromosomes pair up in prophase 1
      • In metaphase 1:
        • Homologous chromosomes line up on equator in pairs
        • Random segregation of c'somes either side of equator leads to different combinations and so different alleles in gametes
      • Homologous chromosomes separated in Anaphase 1

11. Autosomal linkage

    • An autosome is a chromosome that is not a sex chromosome
    • Any two genes occuring on the same chromosome are said to be linked
      • Autosomal linkage is when two or more genes are carried on the same autosome

12. Sickle cell anaemia

    • Due to single base substitution.
      • Results in one amino acid being altered in the 146 long Beta polypeptide chain

13. Multiple alleles

    • Some genes have more than two alleles
      • Only two of these alleles can be present in an individual at one time
      • e.g. human blood groups

14. Epistasis

    • This is where an allele of one gene affects or masks the expression of another in the phenotype
    • Example: Coat colour in mice
      • Gene A controls melanin distribution in hair
        • Dominant = black bands
        • Recessive when homozygous = uniformly black
      • Gene B determines the expression of gene A:
        • Dominant allele = production of melanin
        • Recessive = when homozygous no melanin produced.

15. Sex inheritance in humans

    • Humans have 22 pairs of autosomes and a pair of sex chromosomes
    • Females have two X's
      • Males have one X one Y
        • Produce gametes with X and Y
      • Produce gametes of only X
    • Sex Linkage
      • All genes carried on the sex chromosomes are transmitted along with those during sex
      • Because Y is smaller than X, there is no equivalent part of X to half of Y
        • This means that in males, recessive alleles on non-homologous part of X will be expressed
          • This is because there is no equivalent dominant allele
      • Expressed as: X^N X^a and X^N Y

16. Haemophilia

    • This is the failure of the blood to clot and is potentially fatal if not treated.
    • One cause is the recessive allele of a gene that codes for a protein needed in the clotting process

17. Stats Tests

    • The chi-squared test
      • Used to test whether deviations between observed and expected numbers are significant or not
      • Null Hypothesis:
        • No statistically significant difference between observed and expected frequency, any difference is due to chance
      • Cut off point where p=0.05 is whether you accept or reject null hypothesis
        • If X^2>critical value or equal too, reject null hypothesis
          • The probability that the differences occured by chance is less than or equal to 0.05
        • If X^2<critical value, accept null hypothesis
          • No significant difference, probability that difference is due to chance is greater and 0.05
      • Then, calculate degrees of freedom (number of categories minus one)

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