genetic crosses

• what is meant by monogenic inheritance?

  the inheritance of a single gene
• what is meant by codominance?

  • two alleles occurring for a gene, which are both dominant
  • therefore both of the alleles of the gene are expressed within the phenotype equally
• what is the term given to characteristics determined by sex chromosomes?

  Sex-linked traits
• why are sex-linked characteristics more common in males than females?

  • females have XX and males have XY chromosomes
  • the Y chromosome is much smaller than the X chromosome
  • so Y chromosome will only have a gene to form male, while the X chromosome contains most other genes
  • if sex-linked characteristic present on an X chromosome of a female the other X chromosome could have the dominant version of the normal gene
  • but if X chromosome of a male has the recessive allele, the Y chromosome can't have a corresponding dominant allele so the condition will develop
• what is haemophillia?

  • example of a sex-linked genetic disorder
  • blood clots extremely slow
  • can result in prolonged bleeding, even fatal in some cases
  • most common in males as it's sex-linked
• What is meant by multiple alleles?

  When a gene has three or more alleles instead of the usual two
• What is a monohybrid?

  The inheritance of one gene
• what is meant by a dihybrid cross?

  The inheritance of two genes
• what is meant by linkage?

  where genes are located on the same chromosome
• explain why linked genes don't produce the expected phenotypic ratio

  • linked genes are inherited together as they are on the same chromosome
  • so independent assortment doesn't separate these alleles during meiosis. Linked alleles can only be separated if they experience crossing over at chiasmata
  • eg- Bb and Aa genes are linked (represents two different characteristics)
  • B and A are on the same chromosome
  • b and a are on the same chromosome
  • Ba and bA would be the result of crossing over- which is the only way BA and ba can be separated, but this doesn't happen if the alleles are too closely linked

  
• what are recombinant offspring?

  offspring with a different combination of alleles to their parents
• how can recombinant frequency be calculated?

  recombination frequency = number of recombinant offspring / total number of offspring
• what does a recombination frequency of 50% represent?

  no linkage and the genes are on separate chromosomes
• what does a recombination frequency of less than 50% represent?

  • indicates gene linkage
  • independent assortment is hindered
• what is autosomal linkage?

  when the genes that are linked are found on one chromosome in each homologous pair

  • in the diagram we can see that AB are linked and found on one chromosome in one homologous pair, and that ab are linked and found on one chromosome in the OTHER homologous pair

  
• what is epistasis?

  a form of gene interaction when one gene masks the effect of another
• what are the three types of epistasis?

  • recessive- where two recessive alleles prevent a characteristic
  • dominant- where one OR two dominant alleles prevent a characteristic
  • complimentary action- where two recessive alleles of one characteristic have an affect on the other characteristic, and this other characteristic can have two recessive alleles that affect the first characteristic
• what is the ratio for recessive epistasis?

  9:3:4
• what is the ratio for dominant epistasis?

  12:3:1
• what is the ratio for complementary action?

  9:7