Mendel and the Gene Idea

1. Mendel and the Gene Idea Chapter 14

2. Father of Modern Genetics • After failing to qualify as a biology teacher, the Austrian monk Gregor Johann Mendel (1822-1884) began to research heredity in pea plants (1860’s). • He was the first to take a scientific, experimental approach and to quantify his data. • Particulate theory of heredity -- parents transmit separate inheritable factors (now called genes) to their offspring; replaced the blending hypothesis. • Why peas? : • 1. There were many varieties. • 2. Peas do not cross-fertilize, so he could have strict control over mating. • 3. Easy to grow many generations in a short amount of time.

3. Mendel’s Work • Mendel chose to study seven characters, each of which occurred in two contrasting traits: • 1. Flower color: purple(d) or white(r) • 2. Flower position: axial(d) or terminal(r) • 3. Seed color: yellow(d) or green(r) • 4. Seed shape: round(d) or wrinkled(r) • 5. Pod shape: inflated(d) or constricted(r) • 6. Pod color: green(d) or yellow(r) • 7. Stem length: tall(d) or dwarf(r)

4. Some vocab… • True breeding -- Always producing offspring with the same traitsas the parents (self-pollination). • P (parental) generation -- true-breeding parent plants. • F1 (first filial) generation --hybrid (cross-pollinated) offspring of the P generation. • F2 (second filial) generation – offspring of self-pollinated F1 generation. • Genotype – combination of genes in an organism. • Phenotype – expressed traits in an organism. • Allele – different forms of an gene for the same character.

5. Give peas a chance • When Mendel crossed true-breeding plants with different characters, the traits did not blend. • P generation: purple (PP) x white (pp) • [homozygous dominant] [homozygous recessive] • F1 generation: all purple (Pp) x (Pp) • [heterozygous] • F2 generation: purple(PP) purple(Pp) purple(Pp) white(pp) • Mendel found similar 3:1 ratios with the other 6 characters.

6. Mendel’s Principles • 1. Organisms inherit two alleles for a trait, one from each parent. • 2. (Law of dominance) If the two alleles differ, one is expressed (dominant) and the other is masked (recessive). • 3. (Law of segregation) Two alleles for each character separate during gamete production; homologous chromosomes separate during meiosis. • • If different alleles are present in the parent, there is a 50% chance that a gamete will receive the dominant allele, and a 50% chance that it will receive the recessive allele.

7. Punnett Square • A chart used to predict probabilities of possible genetic outcomes. • Rule of multiplication -- probability that independent events will occur simultaneously is the product of their individual probabilities. • Monohybrid cross: a mating with reference to one character.

8. Mendel’s Principles (cont.) • 4. (Law of independent assortment) -- Each allele pair segregates independently of other gene pairs during gamete formation; one gene does not influence the inheritance of a different gene. • Dihybrid cross: a mating with reference to two characters.

9. Test Cross • Crossing a parent with unknown genotype with a homozygous recessive parent.

10. Other patterns of inheritance (exceptions to Mendel’s rules) • 1. Incomplete Dominance -- one allele is not completely dominant over the other; heterozygote’s phenotype is intermediate. • Snapdragon: red x white = pink • SrSr SwSw SrSw • Incomplete dominance is not support for the blending theory of inheritance, because alleles maintain their traits.

11. Other patterns of inheritance (cont.) • 2. Codominance -- both alleles in the heterozygote are fully expressed. • Human ABO blood groups • Three alleles possible (multiple alleles), but you only inherit two (one from each parent). • IA IB i

12. Type A IA IA or IAi Type B IB IB or IBi Type O ii Type AB IA IB rh + RR or Rr rh - rr A anti- B B anti- A None anti- A & B A & B none rh + none None anti + phenotype/genotype rbc antigen/ serum antibody

13. Other Patterns of Inheritance (cont.) • 3. Pleiotropy -- The ability of a single gene to have multiple effects. • In some cats, a fur pigmentation gene also influences connections between cat's eyes and brain. • 4. Epistasis – one gene alters the expression of a second gene. • The phenotypic ratio resulting from a dihybrid cross will deviate from the 9:3:3:1 Mendelian ratio. • In mice, the pigment production gene(C) is epistatic to the pigment color gene(B). • BB or Bb = black pigment; bb = brown pigment. • CC or Cc = normal pigment; cc = no pigment • Dihybrid cross will result in 9 black mice (CCBB, CCBb, CcBB, CcBb), 3 brown mice (CCbb, Ccbb) and 4 white mice (ccBB, ccBb, ccbb).

15. Other Patterns of Inheritance (cont.) • 5. Polygenic Inheritance – more than one gene determines a single character. • Produces quantitative characters that vary on a continuum. • Skin pigmentation is controlled by at least three separately inherited genes (A, B, and C). • AABBCC = very dark person; aabbcc = very light person. • AaBbCc = intermediate shade. • Environmental factors, such as sun exposure, could also affect the phenotype (nature vs. nuture).

17. Pedigrees • Our understanding of Mendelian inheritance in humans is based on the analysis of family pedigrees (a family tree that shows the inheritance pattern of a particular character among parents and children). • Squares symbolize males and circles represent females. • A horizontal line connecting a male and female indicates a mating; offspring are listed below in birth order, from left to right. • Shaded symbols indicate individuals showing the trait being traced. • Pedigrees can be used to predict probabilities.

18. Dominant Disorder

19. Autosomal Recessive Disorders • Caused by defective recessive alleles (not on sex chromosomes) that code for either a malfunctional protein or no protein at all. • Can be non-lethal (albinism) or lethal (cystic fibrosis). • Disorders occur only in homozygotes (aa) who inherit one recessive allele from each parent. • Heterozygotes (Aa) are normal, but carriers (can transmit the allele to offspring). • Most people with recessive disorders are born to normal parents, both of whom are carriers. • Probability is 1/4 that a mating of two carriers (Aa x Aa) will produce a homozygous recessive zygote. • 2/3 chance that a normal child will be a carrier. • Some of these disorders are more common in certain ethnic groups: cystic fibrosis (caucasians),Tay-Sachs disease (central European Jews), and sickle-cell disease (African descent).

21. Autosomal Dominant Disorders • Caused by defective dominant alleles; only takes one copy to cause disorder. • Lethal dominant alleles are rarer in populations than lethal recessives. • In achondroplasia (dwarfism), AA is lethal in the fetus; Aa affects 1 in 10,000 people. • Huntington's disease (degenerative nerve disease) is caused by a late-acting lethal dominant allele; effects do not appear until 35 to 40 years of age, often after individuals have reproduced. • Children of an afflicted parent have a 50% chance of inheriting the dominant allele; a test can detect the Huntington's allele before disease symptoms appear.

23. Multifactorial Disorders • Diseases that have both genetic and environmental influences (heart disease, diabetes, cancer, alcoholism, and some forms of mental illness). • Traits are often polygenic and poorly understood.

24. Genetic Testing and Counseling • Carrier testing – identify genes in prospective parents and in embryos. • Fetal testing – blood tests, ultrasound, CVS (8-10 weeks), amniocentesis (14-16 weeks) and karyotyping. • Newborn screening – most states do routine blood test for phenylketonuria (PKU).