Genetics (chapter 14)

1. Genetics (chapter 14)

2. One of the great ideas of biology begs the question: why do offspring resemble their parents. AGenetics is a field of biology dealing with the study of heritable characteristics. Gregor Mendel conducted studies (breeding garden peas) and greatly increased our knowledge of inheritance • In 1865, Mendel had worked out the rules or laws of inheritance (see below) 2. In 1903, W. S. Sutton and T. Boveri formulated the chromosomal theory of inheritance that contends that the process of meiosis caused the patterns of inheritance Mendel observed and asserts that the hereditary factors called genes are located on chromosomes.

3. Background information I Genes are units of heredity (composed of DNA). The location of each gene on a chromosome is referred to as its locus. Each diploid individual has a pair of genes for each trait. One is inherited from the mother & the other is inherited from the father. Alleles are one or more alternative states of a gene.

4. Background information II If the two alleles for the trait code for the same protein, then we say that they are homozygous. If the two alleles code for different proteins, then we say that they are heterozygous. Dominant genes mask the expression of genes that are recessive. Homozygous dominant (AA), Heterozygous (Aa), or Homozygous recessive (aa) are the possible genotypes of individuals

6. Alleles

7. Background Information III The genotype of an individual is the actual genetic makeup. The phenotype is the observable trait that is controlled by the genotype. if red is dominant to white (red and white are phenotypes), then RR (or homozygous dominant individuals) = red, Rr (heterozygotes) = red, and rr (homozygous recessives) = white. But there are exceptions (see below).

8. Background Information IV One of the tools that we use to determine the possible genotypes and phenotypes of offspring is the Punnett square. In crosses, P = parental generation, F1 = the first generation of offspring, and F2 = the second generation of offspring.

9. Mendel’s Laws are based on his work with garden peas, Pisum sativum (why garden peas? earlier studies had shown that hybrids could be produced, large numbers of true breeding varieties were available, they are small and easy to grow, and they have a short generation time). Mendel set out address the question: what are the basic patterns in the transmission of traits from parents to offspring

10. Law (Principle) of Segregation.Diploid organisms inherit a pair of genes for each trait and these genes segregate (separate) during meiosis and end up in different gametes. Four ideas of Mendel involving the Law of Segregation 1. Alternate versions of genes (different alleles) account for variations in inherited characteristics. 2. For each character, an organism inherits two alleles (one from each parent) 3. If the two alleles differ, then one, the dominant allele is fully expressed in the organism’s appearance, the other, the recessive allele, has no noticeable effect on the organism’s appearance. 4. The two alleles for each character segregate (separate) during gamete production.

11. Law of Segregation

12. Law (Principle) of Independent Assortment. Each gene pair tends to assort into gametes independently of other gene pairs located on other homologous pairs of chromosomes. Mendel determined this law by following two traits, a dihybrid cross.

13. Law of Independent Assortment

14. Sources of genetic variation include: independent assortment, crossing over, and random fertilization

15. Break for monohybrid and dihybrid crosses on board

16. Not all situations are as simple as the examples. There are variations. Part I Pleiotropy is the influence of a single gene on unrelated traits. Epistasis is where two alleles of a gene mask the alleles of another gene and as a consequence, the expected phenotypes associated with the latter are not present. Polygenic inheritance or continuous variationoccurs when multiple genes act jointly to determine a trait such as height or weight and thus it is difficult to determine the contribution of an individual gene.

17. Variations. Part I Incomplete dominance involves the ability of two alleles to produce a heterozygous phenotype that is different from either homozygous phenotype. Codominance is a situation where both alleles are expressed.

18. More board work

19. Human Genetics

20. Autosomal Recessive Inheritance. Conditions associated with this type of inheritance occur when an individual has two copies of a recessive allele (they are homozygous recessives)

21. Cystic fibrosis is a disease where mucus clogs the lungs, liver and pancreas, that results from the failure of a chloride ion transport mechanism. The frequency is 1/2500 Caucasions and it is rare in African Americans. One in 20 people is a carrier

23. Phenylketonuria (when undetected) results in brain damage due to the presence of a defective form of an enzyme associated with the amino acid, phenylalanine. Frequency 1/12,000. All babies born in the US are screened after birth.

25. Sickle-cell anemia is a unique case (Co-dominance). Remember that we talked about the heterozygotes for hgb. S being at an advantage in areas where malaria is endemic? These heterozygotes possess both types of hgb. So actually, hgb. A (normal hgb) is codominant with hgb. S (in heterozygotes) But two copies of hgb. S results in sickle-cell anemia. An interesting phenomenon is that 9% of African Americans are heterozygous and 0.2% are homozygous, whereas 45% of some Africans are heterozygous and 6% are homozygous. why? because of malaria and the increase in survival for heterozygotes over either homozygote

28. Brachydactyly is a condition that occurs in people who are heterozygous for a lethal gene. The normal gene is incompletely dominant over this recessive lethal gene. Thus, individuals who are heterozygous possess a “blended” phenotype. Children who are born with two copies of this recessive lethal gene die early in life due to skeletal deformities

29. Almost absent fingers (brachydactyly) on one hand in a boy of India

30. Tay-Sachs is a disease that results in the death of children due to a nonfunctional form of the enzyme, hexosaminidase A (it normally breaks down lipids in brain cells, but the nonfunctional form does not and the lipids build up). This is rare in U.S. one in 300,000 births but in Central Europe it occurs in 1 in 3,500 births (in Ashkenazi Jews)

32. More board work

33. Sex-linked inheritance. This involves situations where a recessive allele is expressed in males even though the male possesses only one copy of the allele. This occurs because the allele is present on the X chromosome and the Y chromosome does not have a complimentary allele (and thus there is no dominant or possibility of a dominant allele to mask the recessive allele).

34. Classic hemophilia results from a Factor VIII deficiency. The frequency is 1/10,000 Caucasian (generally males). Females can be carriers

36. Muscular dystrophy (Duchenne) is associated with a degeneration of muscle tissue that results from the degradation of myelin coating of nerves that stimulate muscles (1/3,700).

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39. Autosomal Dominant Inheritance Some diseases are caused by dominant alleles that occur on autosomal chromosomes. Thus, it does not matter if the person inherits one or two copies of the allele.

40. Huntington’s disease involves a gradual deterioration of brain tissue that begins after age 30, it results from the production of an inhibitor of brain cell metabolism (1/24,000)

41. Figure 1. Samples of coronal and sagittal magnetic resonance imaging from a patient with Huntington's disease (top row) and a normal control (bottom row) showing the outlines of caudate and putamen (left), cerebral (center) and cerebellar volumes (right).

42. http://video.google.com/videoplay?docid=-7617888490585459224&q=huntington%27s+disease&total=46&start=0&num=10&so=0&type=search&plindex=6http://video.google.com/videoplay?docid=-7617888490585459224&q=huntington%27s+disease&total=46&start=0&num=10&so=0&type=search&plindex=6

43. Hypercholesterolemia is associated with an excessively high level of cholesterol in the blood that leads to heart disease. It is the result of an abnormal form of cholesterol surface receptors. (1/500). Heterozygotes often develop Coronary disease later in life (but not necessarily old age) but homozygous individuals often die extremely early in life (as children).

45. Achondroplasia is a form of dwarfism with a frequency of 1/10,000. Thus 99.99% of the population is homozygous recessive

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47. ABO blood groups Red blood cells have surface markers (often referred to as antigens). Although there are many kinds of these markers, three of them are of major clinical importance resulting in the ABO and Rh blood groups.

48. Concerning the ABO blood groups, A and B are both dominant over O but A and B are codominant with each other. Thus the possible ABO blood types are A, B, O, and AB. Note that a person can have either two A alleles or one A allele and one O allele and still be type A.

49. Table for board

50. The second major blood group is the Rh blood group. Most individuals possess the D surface marker (Rh antigen) and thus would not make anti-D. About 15% of caucasians do not have the D surface marker and upon exposure to Rh+ blood, they will produce anti-D. Thus, if one of the Rh - persons were to get two transfusions with Rh+ blood they could have a transfusion reaction. An additional problem occurs when an Rh- mom is pregnant with an Rh+ fetus