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ANNOUNCEMENTS

Learn about pedigree analysis, including the inheritance patterns and characteristics of autosomal recessive, autosomal dominant, and X-linked traits. Discover complications such as incomplete penetrance and sex-limited expression. Practice reading pedigrees and determining genotype probabilities.

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ANNOUNCEMENTS

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  1. ANNOUNCEMENTS • Homework #2 is due on Monday in lecture. • Change to 1b. Do not calculate a c2 value. Just calculate the expected phenotypic ratios if the genes for brown-ness and disease-resistance are actually the same gene (or completely linked). Then, say whether complete linkage is possible given the observed data. • Don’t forget about my office hours (MWF 9-11) and the Thursday night Q&A sessions.

  2. From last time... Extensions of Mendelian analysis • Genes follow Mendel’s law of inheritance, but differences in gene action can generate more complex inheritance patterns for phenotypes • Single genes - dominance, codominance, incomplete dominance, overdominance, allelic series, pleiotropy, lethals • Multiple genes - epistasis, polygenic traits • Genes & the environment - sex-influenced traits, environment-dependent gene expression, incomplete penetrance

  3. Today... Pedigree analysis • In humans, pedigree analysis is an important tool for studying inherited diseases • Pedigree analysis uses family trees and information about affected individuals to: • figure out the genetic basis of a disease or trait from its inheritance pattern • predict the risk of disease in future offspring in a family (genetic counseling)

  4. Today... Pedigree analysis • How to read pedigrees • Basic patterns of inheritance • autosomal, recessive • autosomal, dominant • X-linked, recessive • X-linked, dominant (very rare) • Applying pedigree analysis - practice

  5. female male affected individuals Sample pedigree - cystic fibrosis

  6. Autosomal recessive traits • Trait is rare in pedigree • Trait often skips generations (hidden in heterozygous carriers) • Trait affects males and females equally

  7. Autosomal recessive diseases in humans • Most common ones • Cystic fibrosis • Sickle cell anemia • Phenylketonuria (PKU) • Tay-Sachs disease • For each of these, overdominance (heterozygote superiority) has been suggested as a factor in maintaining the disease alleles at high frequency in some populations

  8. Autosomal dominant pedigrees • Trait is common in the pedigree • Trait is found in every generation • Affected individuals transmit the trait to ~1/2 of their children (regardless of sex)

  9. Autosomal dominant traits • There are few autosomal dominant human diseases (why?), but some rare traits have this inheritance pattern ex. achondroplasia (a sketelal disorder causing dwarfism)

  10. X-linked recessive pedigrees • Trait is rare in pedigree • Trait skips generations • Affected fathers DO NOT pass to their sons, • Males are more often affected than females

  11. X-linked recessive traits ex. Hemophilia in European royalty

  12. X-linked recessive traits ex. Glucose-6-Phosphate Dehydrogenase deficiency • hemolytic disorder causes jaundice in infants and (often fatal) sensitivity to fava beans in adults • the most common enzyme disorder worldwide, especially in those of Mediterranean ancestry • may confer malaria resistance

  13. X-linked recessive traits ex. Glucose-6-Phosphate-Dehydrogenase deficiency

  14. X-linked dominant pedigrees • Trait is common in pedigree • Affected fathers pass to ALL of their daughters • Males and females are equally likely to be affected

  15. X-linked dominant diseases • X-linked dominant diseases are extremely unusual • Often, they are lethal (before birth) in males and only seen in females ex. incontinentia pigmenti (skin lesions) ex. X-linked rickets (bone lesions)

  16. Pedigree Analysis in real life: complications Incomplete Penetrance of autosomal dominant traits => not everyone with genotype expresses trait at all Ex. Breast cancer genes BRCA-1 and BRCA-2 & many “genetic tendencies” for human diseases

  17. Pedigree Analysis in real life: complications Sex-limited expression => trait only found in males OR females

  18. Pedigree Analysis in real life • Remember: • dominant traits may be rare in population • recessive traits may be common in population • alleles may come into the pedigree from 2 sources • mutation happens • often traits are more complex • affected by environment & other genes

  19. What is the pattern of inheritance? What are IV-2’s odds of being a carrier?

  20. Sample pedigree - cystic fibrosis What can we say about I-1 and I-2? What can we say about II-4 and II-5? What are the odds that III-5 is a carrier? What can we say about gene frequency?

  21. What is the inheritance pattern? What is the genotype of III-1, III-2, and II-3? What are the odds that IV-5 would have an affected son?

  22. III-1 has 12 kids with an unaffected wife 8 sons - 1 affected 4 daughters - 2 affected Does he have reason to be concerned about paternity?

  23. Breeding the perfect Black Lab How do we get a true-breeding line for both traits?? black individuals = fetch well grey individuals = don’t drool

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