Inheritance: Mendelian Genetics I. Gregor Mendel (1865) A. Before Mendel B. Mendel’s experimental approach II. Genetic terms 1. genes 2. gene pair 3. alleles 4. homozygous/heterozygous 5. dominant/recessive 6. homozygous dominant and recessive/heterozygous 7. genotype/phenotype III. Genetic crosses A. Monohybrid crosses B. Dihybrid crosses IV. Mendel's discoveries A. Principle of segregation B. Principle of independent assortment C. Genes are particles
Darwin and Mendel A. Before Mendel The blending theory (paradigm) Inheritance of acquired characteristics • statistics • amateur • inductive leap
B. Mendel’s experimental approach Hermaphrodite Perfect flowers Genetic barriers pollen egg
Pure breeding lines Breed true
Why was Mendel’s experimental findings ignored during • his lifetime? • He was an amateur • There was a problem with inductive leaps • The dominant paradigm was opposed to his findings • He used statistics • All of the above
II. Genetic terms 1. genes/ loci 2. gene pair = homologues 3. alleles 4. homozygous/heterozygous 5. dominant/recessive 6. homozygous dominant and recessive/heterozygous 7. genotype/phenotype
Human traits Hand folding
III. Genetic crosses Punnett Square
Red coat in foxes is a dominant trait; white is the recessive trait. If a red fox whose mother had a white coat is bred to a white fox, what will be the probable percentage of red kits (baby foxes)? a. 25% b. 50% c. 75% d. 100%
Tay-Sachs is a lethal disorder resulting in death by the • age of 4. A couple who are normal have a child with Tay- • Sachs. Which of the following is true of the parents? • One parent is homozygous dominant and the other is • homozygous recessive • b. both parents are heterozygous • c. both parents are homozygous dominant • d. both parents are homozygous recessive
Sickle cell trait: recessive disorder Homozygous dominant = normal, not malaria resistant Heterozygous = malaria resistant Homozygous recessive = sickle cell anemia
Huntington’s disease: dominant disorder CAGCAGCAG Normal, 26 times HD, 40 to >100 times Manifests after age 40 Why is dominant lethality less common than recessive?
Dwarfism: dominant trait Homozygous dominant = lethal Heterozygous = dwarf Homozygous recessive = normal height Hardy-Weinburg rule
If two achondroplasic dwarfs have children, what fraction of the children would be expected to be dwarfs like their parents? a. 1/4 b. 1/2 c. 2/3 d. 3/4 • What determines how common a trait is in a population? • it’s frequency • Dominance • Selection • Chance
B. Dihybrid crosses: Human traits Dark hair dominant to light hair Curly hair incompletely dominant to straight hair Brown eyes dominant to blue Dimples dominant to no dimples
In humans, a widow's peak is dominant and a straight hairline is recessive. Dimples are dominant and no dimples are recessive. A male who is heterozygous for both widow's peak and dimples has a child with a woman who has a straight hairline and no dimples. What is the phenotype ratio of children can they produce? a. 3: 1 b. 2:2 c. 1:1:1:1 d. 4:0
IV. Mendel’s discoveries A. Principle of segregation Sexually reproducing diploid organisms have 2 alleles of each gene. These 2 alleles segregate from each other to form gametes that contain only 1 allele of each gene.
B. Principle of independent assortment Different genes on different chromosomes segregate into gametes independently of each other.
Mendel’s Principle of Segregation is based on which • event in meiosis? • Anaphase I • Anaphase II • Metaphase I • Metaphase II Mendel’s principle of Independent Assortment is based on which event in meiosis? a. Anaphase I b. Anaphase II c. Metaphase I d. Metaphase II
V. Eugenics Social Darwinism Francis Galton Positive Eugenics
Eugenics today? IVF/ PGD Gene “therapy”
Non-Mendelian Inheritance I. Dominance relations A. Incomplete dominance B. Codominance II. Interactions between different gene pairs: epistasis III.Multiple effects of single genes: pleiotropy IV. Environmental effects on phenotype V. Polygenic inheritance VI. Sex chromosomes VII.Linkage and crossing overVIII. Genes and behavior A. Incomplete dominance
B. Co-dominance Bombay phenotype ABO blood typing system Three alleles IA, IB, i Rh factor is completely dominant trait (R, r)
Population O A B AB Armenians .289 .499 .132 .080 Austrians .427 .391 .115 .066 Bolivian Indians .931 .053 .016 .001 Chinese .439 .270 .233 .058 Danes .423 .434 .101 .042 Eskimos .472 .452 .059 .017 French .417 .453 .091 .039 Irish .542 .323 .106 .029 Nigerians .515 .214 .232 .039 U.S. whites (St. Louis) .453 .413 .099 .035 U.S. blacks (Iowa) .491 .265 .201 .043 Source: Mourant, Kopec, and Domaniewska-Sobczak, 1976, The Distribution of the Human Blood Groups, 2nd Ed., London, UK: Oxford University Press Blood type by population
Jack has B+ blood and Jill has A- blood. Their daughter, • Jenna has B- blood. What is Jack’s genotype? • Heterozygous for B and heterozygous for Rh • Homozygous dominant for B and heterozygous for Rh • Heterozygous for B and homozygous for Rh • Cannot tell from this information
II. Epistasis One gene pair masks the expression of another gene pair B and E genes Black: B_ E_ Chocolate: bb E_ Yellow: _ _ ee
A black lab female whose mother was yellow and father was chocolate has a litter of puppies sired by a chocolate male whose father was a yellow lab. What is the probability that one of the puppies will be a yellow lab? a. 1/8 b. ¼ c. ½ d. ¾
III. Pleiotropy One gene has multiple effects: pigmentation and eye convergence
IV. Environmental effects on phenotype Height of American school-age children Siamese cats Cortisol and stress
V. Polygenic Inheritance: eye color Eye color: two master genes Brown/blue and Green/blue Three modifier genes Density of pigment Distribution of pigment Tone of pigment
V. Polygenic Inheritance The result of polygenic inheritance is continuous variation.
VI. Sex chromosomes Y genes • Y • 78 genes • 95% junk • SRY • anti-mullerian • sperm production • housekeeping genes • inversion region
Disease genes X • Brain genes • Gay genes? Xq28 • 1,098 genes
X-linked traits Male Pattern Baldness Red-green color deficiency
Jon has male pattern baldness; his partner Michelle is not bald but her father has male pattern baldness. What is the probability that, if they have a son, he will be bald? a. 1/8 b. ¼ c. ½ d. ¾
Single Active X At 500 to 1000 cell stage in female mammals, one of X’s in each cell is turned off Remaining X is called the single active X Being a random event, approximately half of all cells will have an active paternal X and half will have an active maternal X. Thus, all female mammals are a patchwork of paternal and maternal traits coded for by the X chromosome. If the genotype is homozygous dominant of recessive, it is of no consequence. However, if the genotype is heterozygous, … ?