Inherited Change - chapter 17

Inherited Change - chapter 17

1. Studying Inheritance

   • Genotype
     • the genotype is is the genetic make-up of an organism. It describes all the alleles an organism has.
     • The genotype determines the limits within which the characteristics will vary
   • Phenotype
     • The observable or biochemical characteristics of an organism. It is the result of the interaction between the expression of the genotype and the environment
     • The environment can alter a phenotype
   • Genes
     • A length of DNA (a sequence of nucleotide bases) that normally code for a particular polypeptide
     • Genes exist in two, or more, different forms called alleles
     • The position of a gene on a DNA molecule is known as a locus.
   • Alleles
     • An allele is one of the different forms of a gene.
   • Dominant and Recessive
     • An allele can be dominant or recessive and expressed by homozygous (the same) or hetrozygous (different) chromosomes
     • Dominant - an allele that is always expressed in the phenotype of an organism
     • Recessive - the effect of an allele is apparent in the phenotype of a diploid organism only in the presence of another identical allele

2. Monohybrid Inheritance

   • The inheritance of a single gene
   • The law of genetics - In diploid organisms, characteristics are determined by alleles that occur in pairs. Only one of each pair of alleles can be present in a single gene.
   • When the hetrozygous organismof the F1 generation are crossed with another, the offspring are always an approx. ratio of 3:1

3. Dihybrid Inheritance

   • When two characteristics, determined by two different genes located on different chromosomes are inherited
   • *when doing a dihybrid cross, the gentoypes are crossed within each other e.g. RrGg = RG Rg rG rg*
   • Theoretical Ratio = 9:3:3:1
   • Mendel's law of independent assortment = each member of a pair of alleles may combine randomly with either of another pair

4. Codominance and Multiple Alleles

   • Codominance
     • both alleles are expressed in the phenotype
     • Codominance occurs where instead of one allele being dominant and the other recessive, both alleles are equally dominant. This means both alleles are expressed in the phenotype
       • e.g. snapdragon plants = one allele codes for an enzyme that catalyses the formation of a red pigment in flowers. The other allele codes for another enzyme that doesn't produce a pigment. The flowers are codominant so they can be red, pink or whits
   • Multiple Alleles
     • there are more than two alleles, of which only two may be present at the loci of an individual's homologous chromosomes
     • An example of multiple alleles is blood type
       • there are 3 alleles associated with gene I. They lead to the presence of different antigens on the cell surface membrane of red blood cells

5. Sex-linkage

   • Sex-linkage in humans
     • The sex-chromosomes in humans are X and Y
     • Females have two X chromosomes so all the gametes are the same in that they contain a single X chromosome
     • Males have one X and one Y chromosome so produce half X gametes and half Y gametes
   • Any gene that is carried on the X or Y chromosome is said to be sex linked
     • The X chromosome is much longer than the Y chromosome meaning that for most of the length of the X chromosome there is no equivalent homologous portion of the Y chromosome
     • Those characteristics which are recessive will appear more frequently in men as there is no portion on the Y chromosome which may contain the dominant allele
   • Pedigree charts
     • Male = square
     • Female = circle
     • shading indicates presence of a character

6. Autosomal Linkage

   • Any chromosome which is not a sex chromosome is an autosome
   • The name given to the situation where two or more genes are carried on the same autosome is called autosomal linkage
   • Assuming there is no crossing over, all the linked genes remain together during meiosis and so pass into gametes
     • They do not segregate in accordance with Mendel's Law of Independent Assortment
     • ratio = 9:3:3:1

7. Epistasis

   • When the allele of one gene affects or masks the expression of another in the phenotype
   • Recessive epistasis = 9:4:3
     • Dominant epistasis = 12:3:1
   • *exam questions = normally one enzyme which is not present leads to the inhibition of another molecule*