University:
Amherst College
Course:
CHEM 330 | Biochemical Principles of Life at the Molecular Level
Academic year:

2009
Views:

210

Pages:

15
Author:

Vasientep9m

Chemical Biology 03 Dec 9, 2010 Inheritance II abstract concept and behavior of genes Mitosis: 2n  2n Regular cell division (mitosis) maternal and paternal chromosomes do not interact Meiosis: 2n  1n Generation of germ cells (meiosis) maternal and paternal chromosomes DO interact homologous chromosomes (maternal and paternal) 1850s Gregor Mendel came up with the idea that discrete genetic factors (“genes”) exist and are passed unchanged from one generation to another. He followed the behavior of these abstract units of heredity by observing inheritance of pea plant traits: 1. Each gene can exist in multiple forms (“alleles”) a A IA IB IC 2. Each individual carries a pair of alleles for each gene -two alleles can be the same (“homozygous”) aa AA -or different (“heterozygous”) aA the alleles may be co-dominant AIB I OR, in other cases, there may be one dominant and one recessive allele Aa Mendel’s First Law of Segregation: •Each individual carries a pair of alleles for each gene aa Aa •The two alleles segregate from one another during gametogenesis •Alleles unite at random (one from each parent) during fertilization A 1/2 a a a AA A gametes A 1/2 Aa x a A a A parent Aa a A a A fertilization = random combination Using math to predict the outcome of a cross: Aa parent 1 Aa parent 2 Aa x Aa 1/2 A; 1/2 a gametes What is the probability of getting aa progeny? 1/2 A; 1/2 a gametes Product Rule: Probability that two independent events will occur simultaneously = product of the probability of each event occurring on its own. Use Product Rule: 1/2 x 1/2 = 1/4 (and) probability that the egg carries a probability that the sperm carries a Using math to predict the outcome of a cross: Aa parent 1 Aa parent 2 Aa x Aa 1/2 A; 1/2 a gametes What is the probability of getting Aa progeny? 1/2 A; 1/2 a gametes Sum Rule: If an event can occur more than one way, the probability of that event is the sum of the independent probabilities. Use Sum Rule: (1/2 x 1/2) + (1/2 x 1/2) = 1/2 (or) probability of egg carrying A and sperm carrying a probability of egg carrying a and sperm carrying A Visualizing probabilities of potential progeny using the Punnett Square 2. List parent 2’s possible gametes along the left (one row for each possible genotype) 3. Fill in products of fertilization x Aa A A gametes 1. List parent 1’s possible gametes across the top (one column for each possible genotype) Aa a AA Aa a gametes Aa aa progeny 1/4 homozygous 1/2 heterozygous Aa x Aa 1/4 homozygous Mendelian Genotype Ratio: A a A AA Aa a Aa aa 1/4 : 2/4 : 1/4 What about Phenotype Ratio? Depends on relationship of alleles: 1. Pheno. Ratio dominant and recessive alleles: Aa = AA (___phenotypes) 2. codominant alleles aa = Aa = AA 3. incomplete dominance: aa < Aa < AA 3/4 : 1/4 (___phenotypes) 1/4 : 2/4 : 1/4 (___phenotypes) 1/4 : 2/4 : 1/4 Most human traits are caused by multiple genes rather than a single gene However there are 1000s of known human traits (most are rare diseases) that are simple: one gene (= “Mendelian Trait”) Sickle Cell Anemia Albinism Cystic Fibrosis Hemophelia Polydactyl Tay Sachs Disease Achondroplasia Huntington’s Disease Phenylketonuria (PKU) Adenosine deaminase deficiency (ADA) Consider an example: Sickle Cell Anemia: Glu7->Val caused by one recessive allele of the β globin gene aa = sc anemia Aa = healthy (but called “carrier”) AA = healthy Aa x Aa What is the probability of this couple having a child with sc? What is the probability of having a healthy child? **What is the probability that their healthy child is a carrier? Aa x AA answer same questions as above Mendel came up with the idea that discrete genetic factors (“genes”) exist and are passed unchanged from one generation to another. 1. Each gene can exist in multiple forms (“alleles”) a A IA IB IC 2. Each individual carries a pair of alleles for each gene -two alleles can be the same (“homozygous”) aa AA -or different (“heterozygous”) aA the alleles may be co-dominant OR, in other cases, IAIB there may be one dominant and one recessive allele Aa 3. During gametogenesis: alleles are segregated from each other: gametes carry only one allele 4. Alleles unite at random during fertilization A Aa 5. The alleles of different genes behave independently (independent assortment) a 5. The alleles of different genes behave independently (independent assortment) If we follow two genes in a cross, we see that they behave independently of each other; so we simply combine their individual probabilities: parent Aa Bb gametes 1/2 A; 1/2 a 1/2 B; 1/2 b considering a parent of genotype AaBb , how many different types of gametes are produced and in what proportions? 1/2 A x 1/2 B = 1/4 AB 1/2 A x 1/2 b = 1/4 Ab 1/2 a x 1/2 B = 1/4 aB 1/2 a x 1/2 b = 1/4 ab Aa Bb x Aa Bb Aa Bb gametes: 1/4 1/4 1/4 1/4 AB Ab aB ab Aa Bb Branch Diagram Aa Bb x Aa Bb •Use for multiple genes •Consider one gene (or one phenotype) at a time •Combine individual probabilities ? ¼ AA ½ Aa ¼ aa combined B genotype A genotype BB ½ Bb ¼ bb ¼ BB ½ Bb ¼ bb ¼ BB ½ Bb ¼ bb ¼ = = = ¼ x ¼ = 1/16 AABB ¼ x ½ = 1/8 AABb = = = ½ x ¼ = 1/8 AaBB ½ x ½ = 1/4 AaBb = = = ¼ x ¼ = 1/16 AAbb ½ x ¼ = 1/8 Aabb ¼ x ¼ = 1/16 aaBB ¼ x ½ = 1/8 aaBb ¼ x ¼ = 1/16 aabb Branch Diagram Aa Bb x Aa Bb Last example was done for Genotype This one is worked out for Phenotype ? A dom. phenotype ¾ A phenotype B dom. phenotype ¾B ¼ bb ¼ aa phenotype ¾B ¼ bb phenotype phenotype phenotype phenotype combined = 3/4 x 3/4 = = 3/4 x 1/4 = 3/16 A_ bb = 1/4 x 3/4 = 3/16 aa B_ = 1/4 x 1/4 = 1/16 aa bb 9/16 A_B_

Chemical Biology 03 Dec 2, 2009 Inheritance I

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