Inherited Change - chapter 17

Studying Inheritance
1. Genotype
  1. the genotype is is the genetic make-up of an organism. It describes all the alleles an organism has.
  2. The genotype determines the limits within which the characteristics will vary
2. Phenotype
  1. The observable or biochemical characteristics of an organism. It is the result of the interaction between the expression of the genotype and the environment
  2. The environment can alter a phenotype
3. Genes
  1. A length of DNA (a sequence of nucleotide bases) that normally code for a particular polypeptide
  2. Genes exist in two, or more, different forms called alleles
  3. The position of a gene on a DNA molecule is known as a locus.
4. Alleles
  1. An allele is one of the different forms of a gene.
5. Dominant and Recessive
  1. An allele can be dominant or recessive and expressed by homozygous (the same) or hetrozygous (different) chromosomes
  2. Dominant - an allele that is always expressed in the phenotype of an organism
  3. Recessive - the effect of an allele is apparent in the phenotype of a diploid organism only in the presence of another identical allele
Monohybrid Inheritance
1. The inheritance of a single gene
2. 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.
3. When the hetrozygous organismof the F1 generation are crossed with another, the offspring are always an approx. ratio of 3:1
Dihybrid Inheritance
1. When two characteristics, determined by two different genes located on different chromosomes are inherited
2. *when doing a dihybrid cross, the gentoypes are crossed within each other e.g. RrGg = RG Rg rG rg*
3. Theoretical Ratio = 9:3:3:1
4. Mendel's law of independent assortment = each member of a pair of alleles may combine randomly with either of another pair
Codominance and Multiple Alleles
1. Codominance
  1. both alleles are expressed in the phenotype
  2. 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
    1. 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
2. Multiple Alleles
  1. there are more than two alleles, of which only two may be present at the loci of an individual's homologous chromosomes
  2. An example of multiple alleles is blood type
    1. 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
Sex-linkage
1. Sex-linkage in humans
  1. The sex-chromosomes in humans are X and Y
  2. Females have two X chromosomes so all the gametes are the same in that they contain a single X chromosome
  3. Males have one X and one Y chromosome so produce half X gametes and half Y gametes
2. Any gene that is carried on the X or Y chromosome is said to be sex linked
  1. 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
  2. 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
3. Pedigree charts
  1. Male = square
  2. Female = circle
  3. shading indicates presence of a character
Autosomal Linkage
1. Any chromosome which is not a sex chromosome is an autosome
2. The name given to the situation where two or more genes are carried on the same autosome is called autosomal linkage
3. Assuming there is no crossing over, all the linked genes remain together during meiosis and so pass into gametes
  1. They do not segregate in accordance with Mendel's Law of Independent Assortment
  2. ratio = 9:3:3:1
Epistasis
1. When the allele of one gene affects or masks the expression of another in the phenotype
2. Recessive epistasis = 9:4:3
  1. Dominant epistasis = 12:3:1
3. *exam questions = normally one enzyme which is not present leads to the inhibition of another molecule*

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