17 - Inherited Change

• What is the Genotype?

  The genetic constitution of an organism (the alleles it has for a gene)
• What is the Phenotype?

  The expression of the genes and its interaction with the environment
• What is meant by Homozygous?

  A pair of homologous chromosomes carrying the same alleles for a single gene
• What is meant by Heterozygous?

  A pair of homologous chromosomes carrying two different alleles for a single gene
• What is a Recessive Allele?

  An allele that is only expressed in the phenotype in the absence of a dominant allele
• What is a Dominant Allele?

  An allele that is always expressed in the phenotype
• What is meant by Codominant?

  When both alleles are equally dominant so are both expressed in the phenotype
• What is meant by Multiple Alleles?

  More than two alleles for a single gene
• What is meant by sex-linkage?

  A gene whose locus is on the X chromosome
• What is meant by Autosomal Linkage?

  Genes that are located on the same chromosome (not the sex-chromosomes)
• What is meant by Epistasis?

  Where one gene modifies or masks the expression of a different gene at a different locus
• What is meant by Monohybrid?

  Genetic inheritance cross of a characteristics determined by one gene
• What is meant by Dihybrid?

  Genetic inheritance cross of a characteristics determined by two gene
• What must you include in all genetic crosses?

  1. The parents' genotypes
  2. The gametes
  3. The offspring genotype
  4. The offspring phenotypes
  5. The probability of the offspring having the condition/feature
• Cystic fibrosis is caused by a recessive allele. If two carriers reproduce, what is the probability they will have a daughter with cystic fibrosis?

  25% of cystic fibrosis so 12.5% of it being a girl

  
• Cows can be red, white or roan in colour. Red and white are both dominant. If two roan cows reproduce, what is the probability they will produce red offspring?

  25% as when red and white are both expressed, it creates roan

  
• Parents with blood group AB and blood group O reproduce. I^A and I^B are codominant and I^O is recessive. What is the probability that they will have an offspring with blood group A?

  50%

  
• Colour blindness is caused by a recessive allele found only on the X chromosome. If a non colour-blind male reproduces with a female carrier of the allele, what is the probability their children will be colour-blind?

  25%

  
• Labrador coat colour is controlled by two genes:

  Gene 1: Controls whether pigment will be expressed Allele E – dominant and codes for pigment production Allele e – recessive and codes for no pigment production (dogs will always be yellow)

  Gene 2: Controls which pigment is expressed Allele B – dominant and codes for black fur Allele b – recessive and codes for brown fur What would be the ratio of the phenotypes of the offspring if two black dogs who were heterozygous for both genes reproduced?

  9:4:3 ---- Black:Yellow:Brown

  
• In a dihybrid cross, what ratio would you expect to see in the offspring if both parents were heterozygous for both genes?

  9:3:3:1
• What are two reasons why you may not observe the above ratio you would expect?

  1. Autosomal linkage
  2. Crossing over in meiosis
• Which statistic could you use to see if there is a difference between the expected phenotype frequencies and the frequencies of the phenotypes actually observed?

  Chi-squared