not mock (genetic diagrams)

• what do monogenic crosses show?

  the probability of the different alleles of a gene being inherited by offspring
• why might the actual ratio of offspring having a phenotype not be as predicted?

  random fusion of gametes during fertilisation

  differential mortality (some more likely to die than others so can't reproduce)
• what are codominant alleles?

  both alleles are expressed in the phenotype if present
• what does it mean if some genes have multiple alleles?

  a gene has more than 2 possible alleles at a single locus e.g. blood groups can be A, B or O --> A and B are codominant
• what do dihybrid crosses show?

  the inheritance of two characteristics which are controlled by different genes
• what is the first step for making a dihybrid cross?

  identify the gametes by finding the combinations of alleles from the parents genotype
• what is the classic phenotypic ratio for a monogenic cross of all heterozygous?

  3:1
• what is the classic phenotypic ratio for a dihybrid cross of all heterozygous?

  9 : 3 : 3 : 1
• why would the classic phenotypes not be observed?

  random fusion of gametes and random mating

  • sex linkage
  • autosomal linkage
  • epistasis
• how can you tell if a characteristic is sex-linked?

  if one sex is more affected than the other
• which sex chromosomes do females have?

  XX
• which sex chromosomes do males have?

  XY
• what is a sex-linked characteristic?

  the allele that codes for it is located on a sex chromosome
• why are males more likely to be affected by a recessive genetic condition?if it's sex-linked

  males only have one X chromosome so they often only have one allele so they only need one recessive allele to be affected unlike females which need two recessive alleles to be affected
• what evidence can be seen from a family pedigree diagram that a condition is sex-linked?

  if the condition is more common in males the harmful recessive allele must be linked to the X chromosome as males only inherit one X chromosome
• what are autosomes?

  Non-sex chromosomes
• what is autosomal linkage?

  where 2 genes are located close together on the same chromosome
• what is the effect of autosomal linkage?

  the two genes are close together on the same chromosome so the alleles are often inherited together therefore, each offspring is more likely to share both characteristics with a particular parent so a higher proportion of the offspring will have their parent's genotype and phenotype
• why are autosomally linked alleles more likely to stick together?

  they'll stay together during the independent assortment of chromosomes in meiosis 1, the only reason this won't happen is if crossing over splits them up first
• why are the offspring more likely to have the parental phenotypes with autosomal linkage?

  the alleles are inherited together, this means that the heterozygous parent can produce only two possible gametes, not four, this limits the number of offspring phenotypes, however, occasionally recombinant offspring are produced due to crossing over
• what is epistasis?

  the allele of one gene masks the expression of the alleles of other genes e.g. if you have the gene for baldness, it doesn't matter if you gave the allele for a curved hairline as you have no hair
• how can you identify a codominant allele from phenotypic ratios?

  there will be three phenotypes for one characteristic and the third is a mix of the other two e.g. red, white and pink flower colour