Ch. 14: Mendel and the Gene Idea

1. Ch. 14: Mendel and the Gene Idea

2. Introduction • A. Heritable traits (brown eyes, green eyes, • blue eyes) are passed down from parents • to offspring. • B. Blending hypothesis: Offspring should • have a blend of parental traits. • (Yellow + Blue = Green)  This blending hypothesis is incorrect. • Particulate inheritance: parental genes • retain their separate identities, and are • sorted and then passed on to future • generations.

3. Gregor Mendel documented particulate • inheritance by studying sweet pea plants. • Gregor Mendel • A. An Augustinian monk who studied • the sciences and math. His studies • influenced his experimentation in genetics. • In 1857, Mendel began breeding garden • peas to study inheritance. • Peas were an ideal subject to study: • a. Many heritable traits (color, height, • pod shape, etc.) • b. Easy to control mating

4. - Self-pollination - Cross-pollination

5. Mendel’s experiment: • 1. Mendel cross-pollinated to hybridize • two contrasting, true-breeding pea • varieties. • - True-breeding: P generation • - Offspring: F1 generation • Mendel then allowed for the F1 • generation to self-pollinate to produce • the F2 generation. • Mendel quantitative analysis of the • F2 generation enabled him to come up • with two important principles of heredity:

6. The law of segregation • The law of independent assortment • Law of segregation: two alleles for a • trait are packaged into separate gametes. • When the P generation cross-pollinated, • the F1 generation were all purple. • When the F1 • generation • self-pollinated, • the F2 • generation had • both purple • and white.

7. The ratio between purple and white flowers in the F2 generation were 3:1. • Mendel reasoned that though the F1 generation had no white flowers, they must have carried the heritable trait for white flower color. • Mendel said that the purple color gene was “dominant” and the white color gene was “recessive.”

8. Mendel found similar 3:1 ratios of traits • in the F2 generation when he conducted • crosses for these traits: Example: Round v. Wrinkled pea seeds F1 = 100% Round F2 = 75% Round, 25% Wrinkled

10. Mendel stated that different genes • (different alleles) account for variations • in inherited characteristics. Ex. Purple and white color genes have different DNA sequences.

11. Mendel also stated that an organism • inherits two alleles for each trait, one • from each parent. • -Diploid • -Homologous chromosomes • If two alleles inherited are different, then • the dominant allele will be expressed. • The recessive allele will have no effect. • The two alleles segregate during gamete • production; homologues will separate • when gametes are made. •  This is the Law of Segregation

12. Mendel’s Law of Segregation accounts • for his observation of 3:1 in the F2 • generation: • a.The F1 generation will create two kinds • of gametes = half will have the white • allele, half will have the purple allele. • During self-pollination, the gametes • will combine randomly to form four • combinations. • A Punnett Square can be made to • predict the results of a genetic cross:

14. An organism with two identical alleles • for a character is homozygous for that • trait. • An organism with two different alleles • for a character is heterozygous for that • trait. • An organisms genetic makeup is called • its genotype. • An organisms physical traits is called • its phenotype. •  Two organisms can have the same • phenotype but different genotypes.

15. The only way to produce a white offspring is to have two recessive traits (homozygous recessive).

16. You cannot predict correctly the • genotype of an organism with a • dominant phenotype. •  A purple flower could be PP, or Pp. • The only way to know the genotype of • an organism with • a dominant traits • is by doing a • “test-cross.”

17. Law of Independent Assortment: Each • pair of alleles segregates into gametes • independently. • Mendel did other experiments that • followed the inheritance of two different • characters in a dihybrid cross • (v. monohybrid). • Mendel studied seed shape and color: • Y = Yellow, y = Green • R = Round, r = wrinkled

18. Mendel crossed true-breeding plants • that had yellow, round seeds (YYRR) • with true-breeding plants that has • green, wrinkled seeds (yyrr). • One possibility is that the two • characteristics could be transmitted as • a package. •  F1 = Yellow, Round •  F2 = 3 Yellow, Round: • 1 Green, Wrinkled ** However, this was not the results of Mendel’s experiment.

19. Instead, the two characteristics • segregate independently of one another. • F1 = Yellow, Round Their gametes can have four combinations: YR, Yr, yR, yr Therefore, the ratios for the F2 generation is: F2 = 9 Yellow, Round (Yy, Rr): 3 Yellow, Wrinkled (Yy, rr): 3 Green, Round (yy, Rr): 1 Green, Wrinkled (yy, rr) ** Whenever Mendel did a dihybrid cross, he always got the 9:3:3:1 ratio. This can be explained as the result of the “Law of Independent Assortment.”

20. Extending Mendelian Genetics • Mendel chose to study pea characteristics • that had simple genetic; each phenotypic • characteristic was determined by one gene. • However, in most cases, the relationship • between phenotype and genotype is • rarely simple. • Incomplete • Dominance: F1 hybrids (heterozygotes) • show a intermediate phenotype:

21. Example: Snapdragons P = Red, White F1 = 100% Pink F2 = 25% Red 50% Pink 25% White • Mendel’s peas showed that heterozygous and homozygous dominant plants had the same phenotype. This is complete dominance.

22. Codominance: two alleles affect the • phenotype in distinguishable ways. • Example: Blood types A, B, AB, and O • Tay-Sach’s Disease • Because an allele is dominant does not • necessarily mean that it is more common • in a population than the recessive allele. • Example: Polydactyly is a dominant gene. • However, the recessive allele is far more • prevalent. • Multiple Alleles: Most genes have more • than one allele in a population. • 1. Example: Blood alleles A, B, and O • with four possible phenotypes for blood • type.

23. A and B alleles are codominant. • Both A and B alleles are dominant to • O. • A type blood = genotype AA or AO. • A type blood has oligosaccharides on • the surface of the RBCs. A type blood • also produces antibodies against B • type blood. • B type blood = genotype BB or BO. • B type blood has oligosaccharides on • the surface of RBCs. B type blood • also produces antibodies against A • type blood.

24. Individuals with O type blood have • neither A or B type oligosaccharides • on their RBCs. They do however • produce antibodies against both A • and B blood types. • Matching compatibility for blood type • is crucial for transfusions. • - O type blood = universal donor • - AB type blood = universal recipient • Pleiotropy: Genes have multiple effects. • Example: Sickle Cell gene has multiple of • effects.

25. Epistasis: a gene on one locus can alter • the effects of another gene on a different • locus. • Example: Coat color in mice is determined • by two genes. • The epistatic gene, determines whether • or not pigment will be deposited into • hair. • Presence of pigment ( C ) is dominant • to no presence ( c ). • Pigment color black (B) is dominant to • brown pigment (b).

26. A mouse with cc will be an albino • regardless of genotypes for hair color • black or brown. • A cross between • two mice that • are heterozygous • for both genes • (Cc, Bb) follows • the law of • independent • assortment: However, the ratio will be 9 black:3 brown: 4 white

27. Polygenic Inheritance: an additive effect • of two or more genes on a single • phenotypic character. • 1. Example: Human skin color is due to • more than 3 separate genes. For • simplicity sake, however, we will • consider just 3 genes for determining • skin color. • A, B, and C are the 3 different genes, • all incompletely dominant to the • alleles (a, b, and c). • An individual with AABBCC genotype • will be very dark. An individual with • AaBbCc genotype will have an • intermediate shade.

28. Because the alleles have a cumulative • effect, an individual with genotype • AaBbCc will have the same skin color • as an individual with genotype AABbcc. • A cross between • two individuals • with AaBbCc • genotypes • would result • in a bell- • shaped • curve, • called a • normal • distribution.

29. The environmental impact on phenotype: • 1. Example: Nutrition can influence height. • Exercise influences build. Sun exposure • influences skin color. Practice improves • intelligence tests. •  Identical twins? • The products of a genotype can be a • wide range of variation. This phenotypic • range due to the environment is called • the norm of reaction. Norm of reaction: colors of hydrangea flowers, range from blue to pink, depending on the acidity of soil.

30. Mendelian Inheritance in Humans • While peas are an easy subject to study • genetics, humans are not. • 1. Human generation span is too long. • 2. Parents produce few offspring. • 3. Breeding experiments is socially • unacceptable. • Pedigree analysis reveals Mendelian • patterns in human inheritance. • 1. Phenotypic information is gathered from • as many members of a family across • generations. • The information can then be mapped • onto a family tree.

31. Example: Widow’s peak (W) is dominant • over a straight hairline (w). We can try • and predict the genotypes of individuals • in a family tree. • If an individual has no widow’s peak • but both his/her parents have a • widow’s peak, we can predict that both • parents are heterozygous. • A pedigree can help us understand the • past and predict the future.

33. Many human disorders follow Mendelian • patterns of inheritance. • Thousands of genetic disorders can be • inherited as recessive traits. • Genetic disorders can range from mild • to life-threatening. • Heterozygous individuals are • phenotypically normal but are “carriers” • of the disorder. • Most individuals that are born with a • disorder are born to carriers with normal • phenotypes. • a. Two carriers have ¼ probability of • having a child with the disorder, ½ • probability of having carriers, and ¼ • free.

34. Some genetic disorders are found more • commonly among people of certain • ethnic backgrounds. • Cystic fibrosis: 1 in 2,500 caucasians • of european descent. 1 in 25 • caucasians are carriers. CF is caused • by a defective Cl- membrane protein • that causes build up of mucus in the • lungs, pancreas, & digestive system.) • Tay-Sachs: 1 in 3,600 births in • Ashkenazic Jews. T-S is caused by • a defective enzyme that cannot break • down certain lipids in the brain. This • causes brain degeneration.

35. Sickle-Cell Anemia: • 1 in 400 African • Americans. It is • caused by a • substitution in one • amino acid of the • hemoglobin protein. • When oxygen levels • are low in blood, red • blood cells deform • into a sickle shape. This sickling can cause a number of results. This sickling has pleiotropic effects.

36. The non-sickling allele is incompletely dominant to the sickle-cell allele. Heterozygous individuals are carriers and can suffer some symptoms of the disease under blood oxygen stress. Both normal and abnormal hemoglobins are synthesized. Individuals that are heterozygous are also resistant to malaria, a parasite that spends part of its life cycle inside RBCs.

37. Dominant Inherited Disorders: • Achondroplasia, a form of dwarfism, • has an incidence of one case in • 10,000 people.

38. Lethal dominant alleles are much less • common than lethal recessives • because an offspring with a lethal • dominant will die before passing the • allele on to future generations. • A lethal dominant allele can be passed • down to generations if the onset of • the disease is later in life. • Example: Huntington’s Disease, a • degenerative brain disorder; onset • bt. 35-45 years of age.

39. -Offspring born to a parent who has the allele for Huntington’s disease has a 50% chance of inheriting the disease and the disorder. -Molecule geneticists have recently discovered that the gene for HD is found on the tip of chromosome #4.

40. Polydactyly: a rare dominant phenotype

41. Multifactorial Diseases: Diseases caused • by genetics and the environment. • Example: Heart disease, diabetes, • cancer, alcoholism, schizophrenia, and • manic-depressive disorder.

42. Albinism is a rare condition that is inherited as a recessive phenotype in many animals, including humans.

43. Technology is providing new tools for • genetic testing and counseling. • Many hospitals have genetic counselors • that can provide information to • prospective parents who are concerned • about a family history of a specific • disease. • Using Mendelian probability (Punnett • Squares), one can determine the risks • of passing on lethal genes. • Tests determine in utero if a child has a • disorder. One technique is called an • amniocentesis.

44. -can be done after 14-16 weeks of pregnancy. -fetal cells are extracted and karyotyped. -other disorders can be detected from chemicals in the amniotic fluid.

45. Chorionic villus sampling (CVS) can • allow faster karyotyping and can be • performed as early as the eighth to • tenth week of pregnancy. -Fetal tissue is extracted from the chrionic villi of the placenta.

46. Ultrasound and fetoscopy, allow fetal • health to be assessed visually in utero. -Both fetoscopy and amniocentesis cause complications in about 1% of cases. They can cause maternal bleeding or fetal death. -These techniques are usually reserved for cases in which the risk of a genetic disorder or other type of birth defect is relatively great. • After the results of a test are revealed, the parents must face the difficult decision of terminating the pregnancy or preparing to care for a child with a genetic disorder.

47. Some genetics tests can be done after • the child is born. • -Example: PKU - phenyketonuria -1 in 10,000-15,000 births -causes a build-up of the amino acid phenylalanine, and its derivative phenypyruvate in the blood to toxic levels. -this build-up can cause mental retardation. -if the genetic test is given at birth, a child can be given a special diet low in phenylalanine, which usually promotes normal growth.