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MCDB 1041 Class 13 Complex patterns of inheritance, cont. Problems in meiosis Learning Goals: • Compare and be able to identify the different examples of inheritance patterns. • Draw the ways in which non-disjunction can impact the chromosomal content of gametes • Compare the results of non-disjunction in meiosis I with non-disjunction in meiosis II • Determine from the genotype of a set of gametes whether nondisjunction happened in meiosis I or meiosis II Complex pa*erns con/nued Another example of different allelic relationships: One breed of dog, two very different phenotypes “Powder Puffs” Genotype is hh “Chinese Crested” Genotype is Hh People love the hairless Chinese Crested dogs. However, breeders have seen that in litters of pups from two Chinese Crested dogs, there are frequently dead puppies. This can happen when a particular combination of alleles is lethal. Which allele causes lethality when homozygous? How would dog breeders get the most Chinese crested hairless pups per mating, with the fewest dead puppies? a. b. c. d. e. mate two hairless dogs mate two powderpuff dogs mate a hairless dog with a powderpuff dog a or c b or c Final example of complex inheritance patterns: Mitochondrial Inheritance The Mitochondria Mitochondria are organelles in the cytoplasm They have their own DNA (they were likely once independent organisms!) • small circle of DNA with 37 genes. – These genes func/on in energy produc/on and protein assembly • Mitochondria are only passed along from the mother, because they are present in the egg at fer/liza/on, while the sperm contributes only its nucleus to the egg (no cytoplasm, and thus no organelles). Mitochondral disorders Typically cause fatigue Mitochondrial myopathies: muscular weaknesses (defective mitochondria can t produce energy) Can also cause degeneration of muscles or nerves Example: In genera0on 3, which individuals have the chance of passing on the mitochondrial disorder to their offspring? a. Anyone with an affected parent b. Anyone who is affected (Individuals 4,5,6,10,11,12,13) c. Individuals 4,10,13 d. Individuals 5,6,11,12 Chromosomal abnormalities Chromosomes contain many genes, each with a unique function The number of copies of each gene is important: in general, we need two functional copies of every gene to have a normal phenotype. When there are too few or too many copies of a chromosome, problems ensue. Examples: Phenotypes caused by abnormal chromosomal number Turner Syndrome : Only one X Chromosome Female Short and have a low hairline Broad chest and folds of skin on neck Normal intelligence Sterility Trisomy 21 (Down’s Syndrome): 3 copies of ch. 21 Developmental disabilities Characteristic facial features Decreased longevity Heart defects Chromosomal number abnormalities result from mistakes in meiosis • EITHER a homologous pair of chromosomes or sister chromatids can fail to be pulled apart in meiosis I or meiosis II • When this happens, the gamete ends up with too few or too many of a particular chromosome This is called “non-disjunction” , which means “failure to separate” This is a relatively rare event, not something that happens in the production of all gametes #1 A #1 a DNA replication A F #2 #2 f #1 #1 F Spermatogonium Metaphase of meiosis I A A a a F F f f A #1 #1 a a #2 #2 F #2 #2 f f Errors during meiosis I a a A A F F A A a a F F f f f f a a A F A f f F Too many chromosomes! Too few chromosomes! Errors during meiosis II A A A F F A A a a F F f f a f a f a a A f F Too many chromosomes! F Too few chromosomes! f Normal Work on handout Differences between meiosis I and II errors Errors in meiosis I a a A A f f F F Too many chromosomes! Too few chromosomes! Errors in meiosis II A a a A f F Too many chromosomes! F Too few chromosomes! f Normal When nondisjunc0on happens in meiosis II rather than in meiosis I, the outcome is: a. The same as when it happens in meiosis I b. In meiosis II, all gametes are abnormal, while in meiosis I only ½ the gametes are abnormal c. In meiosis II, only ½ the gametes are abnormal, while in meiosis I, all gametes are abnormal Prac0ce. You are presented with a pa0ent who has Down syndrome (3 copies of chromosome 21). The mother was Bb for a gene of interest on chromosome 21 The father was BB. You know the nondisjunc0on event occurred in the mother. If the resul0ng genotype of the pa0ent is BBb, when during meiosis did the nondisjunc0on event occur (assuming no recombina0on)? #21 B a. During meiosis I b. During meiosis II #21 #21 c. It could have happened during meiosis I or II b B