Topic 1 Heredity: Genetic Inheritance

Question 1

• Extranuclear genes (genes present in organelles other than the nucleus) are found in mitochondria and chloroplasts
  i. Defects in mitochondrial DNA can reduce a cell’s ATP production, and because mitochondrial DNA is inherited only from the mother, all related mitochondrial defects/ diseases are also inherited. Note that mitochondria have their own 70S ribosomes that make mitochondrial proteins within the mitochondrial matrix.
  ii. Mitochondrial DNA is also an exception to the universality of the genetic code

Answer 1

Extranuclear Inheritance

Question 2

• As mentioned throughout this chapter, homozygous refers to when two copies of the same allele (AA or aa) are inherited. Heterozygous inheritance is when different alleles of the same gene (Aa) are inherited. Hemizygous is when one single copy of a gene is inherited instead of two.
  i. One example of a hemizygous genotype is males, who have XY sex chromosomes, aka only a single copy of both X and Y genes inherited
  ii. Important note - don’t confuse XY with homologous pairs
  iii. Phenotype and genotype ratios are not necessarily the same - if there are multiple sets of alleles, treat each of the crosses individually and then multiply the individual probabilities to get the overall probability of one specific genotype outcome
  a. Refer to the biology question bank for sample questions that use this method!

Answer 2

2. Patterns of Inherence

Question 3

• Imagine we cross Aa x Aa, and get 1 AA, 2 Aa, and 1 aa. If “aa” is lethal, our genotypic ratio would be AA and Aa present in a 1:2 ratio.
• If a phenotype skips generations, be suspicious of an autosomal recessive disorder. If no skip, it is most likely an autosomal dominant disorder. Be suspicious for X-linked recessive if a father doesn’t have the phenotypes and none of his daughters display it.

Answer 3

3. Lethal Gene

Question 4

• Codominance is represented as both alleles being dominant (e.g., AB) while incomplete dominance is represented as A and A’ since A’ is not completely recessive. Other than these cases, assume there is standard complete dominance/recessiveness

Answer 4

Note