Lecture 2 - Heredity (PDF)

Lecture 2: Basic principles of heredity 08-22-2024  Pea plants... Easy to grow. Easy to cross. Can self. Lots of offspring. Variation.  The monohybrid cross Mendel reasoned that the F1 plants MUST contain “factors” to make the...

Lecture 2: Basic principles of heredity 08-22-2024  Pea plants... Easy to grow. Easy to cross. Can self. Lots of offspring. Variation.  The monohybrid cross Mendel reasoned that the F1 plants MUST contain “factors” to make the wrinkled seed. Each plant contains two genetic factors (alleles). Equal probability of alleles separating into gametes. Reciprocal cross.  Mendel’s ”laws” 1) Principle of segregation. Alleles separate with equal frequency. 2) Dominance.  Multiplication rule The probability of two or more independent events occurring together is calculated by multiplying their independent probabilities. Roll 1 has NO effect on roll 2.  Addition rule The probability of any one of two or more mutually exclusive events is calculated by adding the probability of each event. Key words when using addition rule are EITHER and OR.  The dihybrid cross F1 followed same pattern as monohybrid cross. F2 saw a 9:3:3:1 phenotypic ratio. Dominance determines the ratio. Mendel’s third law: Principle of independent assortment  Statistics and genetics We can make a prediction about the inheritance of a trait. Can use a chi-square goodness of fit test to determine if actual numbers differ from expectations.  Using the chi-squared test: H0 and H1 Is coat color in cats inherited under Mendelian expectations? X Bb Bb Observed n= 30 n=20 Expected¾ X 50 = 37.5¼ X 50 = 12.5  Now, plug these numbers into chi-square formula. Observed – expected black kittens = 30 – 37.5 = - 7.5. Square this number = 56.25. Divide this number by expected = 56.25/37.5 = 1.5. Repeat with grey kittens (20-12.5) 2 /12.5= 4.5. Add both values = 6.0. This is the chi-squared value.  Significance table. Does it appear that coat color in cats is inherited via Mendelian expectations? Looking past pea plants: Why you should care about Mendelian inheritance… Many diseases are Mendelian in their inheritance. Disease may be dominant or recessive. Pedigrees/family trees are a good way to track diseases through generations.  Autosomal recessive diseases Most genetic diseases are recessive. Examples include Tay- Sachs and Cystic Fibrosis. Caused by mutations in genes that produce a defective protein.  Autosomal dominant diseases Different pattern of inheritance. Examples include Huntington’s chorea. Doesn’t begin to show symptoms until late in life.  Mendelian Inheritance in Man Staff at John Hopkins University curate a database of traits inherited via Mendelian principles. Jan 2020: 5,512 diseases known genetic basis: 3,824 genes. www.ncbi.nlm.nih.gov/omim  Genetic heterogeneity Most genetic disease can be caused by different mutations. CF508 = deletion of Phe at amino acid 508 in CFTR protein 70% of CF patients. Over 1000 over mutations known in CFTR. The identification of Mendelian traits in the 2020s Mendelian diseases are rare. Need multiple cases to identify and validate mutation that causes the defective disorder. However, new discoveries are often being made e.g., familiar autism. Although their patterns of inheritance are simple, still much to learn!  Genetic counseling Again, personalized genomics means many people can obtain genetic information. If I have an allele with a link to Alzheimer's, what does that mean?  Summary Understand the principles of Mendelian genetics, and how that informs patterns of inheritance and link Mendel’s laws to meiosis. Be able to carry out predictions/use probability for dihybrid and trihybrid crosses. Be able to use chi-square to determine if character is inherited under Mendelian principles or not.