Patterns of Inheritance

1. Patterns of Inheritance Chapter 9 Part II

2. Structure of a Pea Flower • The carpel produces egg cells • The stamens make pollen, which carries sperm Figure 9.3

3. 1. To prevent self-fertilization, Mendel cut off the stamens from an immature flower before they produced pollen - stamenless plant would be the female parent 2. To cross-fertilize this female, he dusted its carpel with pollen from another plant. After pollination, 3. the Carpel developed into a pod, contianing seeds (peas) 4. He planted the seeds, and they grew into offspring plants Fig 9.4

4. Mendel’s Law of Segregation • Mendel performed many experiments in which he tracked the inheritance of characteristics - characteristics, that occur as 2 alternative traits • Mendel created true-breeding varieties of plants - then crossed 2 different true-breeding varieties • His results led him to formulate several hypotheses about inheritance

5. 7 characteristics studied by Mendel • Each comes in 2 alternative forms • One alternative is dominant • One is recessive Figure 9.5

6. The law of segregation • Crossing true-breeding purple-flowered plants with true-breeding white-flowered plants • Produced F1 plants with purple flowers • Self-fertilization of F1 plants • Produced 929 F2: 709 with purple flowers 224 with white flowers Figure 9.6a

7. Explanation of the results in part a (uppercase letter – dominant allele) • Parental plants – true breeding • Gametes (circled) have only one allele • Union of the parental gametes produced F1 hybrids (Pp) • 2 alleles segregated, ½ the gametes received the P allele, other ½ a p allele • Punnet square: F2 plants – 3 : 1 Fig 9.6b

8. Genetic Alleles and Homologous Chromosomes Figure 9.7

9. Mendel’s Law of Independent Assortment • Mendel also studied seed shape and seed color - either round or wrinkled and yellow or green - tracked these characteristics one at a time in monohybrid crosses - R = round was dominant to r = wrinkled - Y = yellow was dominant to y = green • Dihybrid cross – mating of parental varieties differing in 2 characteristics – phenotype? - Mendel crossed homozygous plants having round yellow seeds (genotype RRYY) with plants having wrinkled green seeds (genotype rryy)

10. Union of RY and ry gametes from the P generation yielded heterozygous hybrids - RrYy • Question - were the dihybrids 2 characteristics transmitted from the parent to offspring together - or was each characteristic inherited independently? • 2 hypotheses for gene assortment in a dihybrid cross: Dependent assortment- characteristics inherited as a packet or Independent assortment – characteristics are independent of each other

11. Mendel allowed fertilization to occur among the F1 plants - if the 2 characteristics were inherited together the F1 hybrids would produce the same 2 types of gametes received from their parents - if the 2 characteristics sorted independently, the F1 generation would produce 4 gamete genotypes: RY, rY, Ry, and ry in equal quantities • Punnett square – all possible combinations of alleles in the F2 generation or only 2 - 3:1 phenotypic ratio (3 plants with round yellow seeds for every 1 with wrinkled green seeds) - nine different genotypes - 9:3:3:1

12. a) 1 hypothesis - predict every plant in the F2 generation will have seeds exactly like one of the parents b) Other hypothesis, independent assortment predicts F2 plants with 4 different seed phenotypes Figure 9.8

13. Results from Mendel’s experiments supported the hypothesis that each pair of alleles assorts independently of the other pairs of alleles during gamete formation - inheritance of one characteristic has no effect on the inheritance of another • Mendel’s law of independent assortment states that – each pair of alleles segregates independently of the other pairs during gamete formation

14. Figure 9.9 a) Black vs chocolate; normal vision vs blindness, controlled by separate genes; allele B – densely packed granules of a dark pigment (dominant); allele n recessive to N (nl vision) b) Mate 2 doubly HTZ (BbNn) labs – phenotype ratio 9:3:3:1; results demonstrate genes are inherited independently

15. Using a Testcross to Determine an Unknown Genotype • A testcross is a mating between an individual of dominant phenotype but unknown genotype and a homozygous recessive individual • Mendel used testcrosses to determine whether he had true-breeding varieties of plants - testcross continues to be an important tool

16. Determine genotype of a black Lab (B_): cross with a chocolate Lab (bb); all offspring are black, the black parent must be BB; half are chocolate the black parent must be heterozygous (Bb) Fig 9.10

17. The Rules of Probability • Mendel understood that the segregation of allele pairs during gamete formation and re-forming of pairs at fertilization obey the rules of probability - tossing of coins, probability of heads is 1/2 - 2 coins tossed simutaneously, outcome for each coin is an independent event, unaffected by the other coin: 1/2 X 1/2 • The rule of multiplication states that - the probability of a compound event is the product of the separate probabilities of the independent events

18. Segregation of alleles and fertilization as chance events: - when a heterozygote (Bb) forms gametes, segregation of alleles is like the toss of a coin: a gamete, an oocyte or sperm has a 50% chance of receiving the recessive allele - to determine probability an offspring will inherit the dominant allele from both parents, multiply the probabilities of each required event: ½ X ½ = ¼ Figure 9.11

19. Inherited Traits in Humans • Mendel’s laws apply to the inheritance of many human traits Figure 9.12

20. Dominant allele of a gene A, dominant phenotype results from either the homozygous genotype AA or the heterozygous genotype Aa - recessive phenotypes are always the result of a homozygous genotype (aa) • Dominant does not imply a phenotype is normal or more common than a recessive phenotype - means a heterozygote carrying only a single copy displays the dominant phenotype • Wild-type traits – those seen most often in nature are not necessarily specified by dominant alleles

21. Family Pedigrees • A family pedigree – shows the history of a familial trait - allows geneticist to analyze human traits - to analyze the pedigree, the geneticist uses Mendel’s concept of dominant and recessive alleles and his law of segregation • Individuals who have one copy of the allele for a recessive disorder and do not exhibit symptoms are called carriers of the disorder

22. Pedigree from Martha’s Vineyard A family pedigree showing inheritance of deafness – only those who are HMZ for the recessive allele are deaf Figure 9.13

23. Human Disorders Controlled by a Single Gene • Many human traits show simple inheritance patterns - controlled by genes on autosomes • Most human genetic disorders are recessive - individuals can be carriers of these diseases • Majority of individuals afflicted with recessive disorders are born to phenotypically normal parents - both parents are carriers for the recessive allele

25. Most common lethal genetic disease in the US is cystic fibrous - 1:17,000 African Americans; 1:90,000 Asian Americans; 1:2500 Caucasian births - cystic fibrous allele is carried by 1:25 Eur-Americans • Most genetic disorders are not evenly distributed across all ethnic groups - geographic isolation of a population (inbreeding) - Martha’s Vineyard inhabitants between 1700 and 1900 • Wildlife biologist have observed increased incidence of harmful recessive traits among inbred animals and in some endangered species

26. Recessive Disorders • Using Mendel’s laws, we can predict the fraction of affected offspring likely to result from a marriage between 2 carriers

27. Predicted Offspring When Both Parents are Carriers of a Recessive Disorder • Carriers are HTZ with only one copy of the recessive allele • Carriers have a dominant allele so do not display the disorder • ¼ of their offspring would be affected Figure 9.14

28. Dominant Disorders • A number of human genetic disorders are caused by dominant alleles - some are nonlethal such as polydactyl - one serious disorder is achondroplasia which affects about 1:25,000 people and in the homozygous state is a prenatal lethal - a person with achondroplasia has a 50% chance of passing the condition on to any offspring - 99.99% of the population are homozygous for the recessive allele • Genetic Lethal – affected individual is born but does not survive long enough to reproduce

29. Achondroplasia –A Dominant Trait Dr. Michael Ain, a pediatric orthopedic surgion at the Johns Hopkins Children’s center, specializes in the repair of bone defects caused by achondroplasia and related disorders Figure 9.15

30. Variations on Mendel’s Laws • Mendel’s 2 laws explain inheritance in terms of discrete genes passed from generation to generation – according to the rules of probability • Some patterns of genetic inheritance are not explained by Mendel’s laws - cases of inherited characteristics that exist in more than 2 distinct variants - observed inheritance patterns are usually more complex

31. Incomplete Dominance in Plants and People • The F1 offspring in Mendel’s pea crosses always looked like one of the 2 parent plants - the dominant allele had the same effect on the phenotype whether present in 1 or 2 copies • In incomplete dominance F1 hybrids have an appearance in between the phenotypes of the 2 parents - snapdragon color - hypercholesterolemia characterized by dangerously high levels of blood cholesterol (HH; Hh; hh)

32. Incomplete Dominance in Snapdragons • A heterozygote has a phenotype in between the phenotypes of the 2 kinds of homozygotes • The phenotypic ratios in the F2 generation are the same as the genotypic ratios, 1:2:1 Figure 9.16

33. Dominant allele H - normal individuals carry HH • Specifies for a LDL receptor – binds LDL particles from the blood and promotes their uptake by cells that break down the cholesterol; prevents accumulation of cholesterol in arteries Fig 9.17

34. ABO Blood Type: An Example of Multiple Alleles and Codominance • Most genes occur in more than 2 forms, known as multiple alleles - individuals carry at most only 2 different alleles for a particular gene - more than 2 possible alleles exist in the population • The ABO blood groups in humans are an example of multiple alleles and codominance - 4 phenotypes: O, A, B, or AB - IA and IB both alleles are expressed

35. 3 versions of the gene responsible for blood type may produce carbohydrate A; carbohydrate B, or neither carbohydrate • Each person carries 2 alleles, 6 genotypes are possible that result in 4 different phenotypes Fig 9.18

36. Matching compatible blood groups is critical for safe blood transfusions • Our immune system produces blood proteins called antibodies that bind specifically to the carbohydrate (antigen) we lack - if a donor’s blood cells have an antigen is foreign, the recipient’s antibodies will cause the blood cells to clump (agglutinate) and eventually to lyse

37. Figure 9.19

38. Pleiotropy and Sickle-Cell Disease • Pleiotropy is the impact of a single gene on more than one characteristic • Sickle-cell disease – disorder characterized by a diverse set of symptoms - direct effect of the sickle-cell allele is to make RBCs produce abnormal hemoglobin molecules that tend to link together and crystallize - cascade of symptoms such as weakness, pain, organ damage, and paralysis - there is no cure (kills ~100,000/year worldwide)

39. Figure 9.20

40. Polygenic Inheritance • Mendel studied genetic characteristics that could be classified on an either or basis - purple color or white color • Many characteristics, such as human skin color and height, vary along a continuum in a population and result from the additive effects of 2 or more genes on a single phenotype - skin pigmentation model

41. A Model for Polygenic Inheritance of Skin Color • 3 separately inherited genes – each with a dark-skin and light-skin allele affect the darkness of skin • Each dominant allele brings 1 ‘unit’ of skin pigmentation • Punnett square shows all possible genotypes of F2 offspring – the row of squares shows the 7 skin phenotypes • The graph bars depict the relative numbers of each penotype • Bell-shaped curve indicates the distribution of a greater variety that may result from the combination of heredity and environmental effects Figure 9.21

42. The Role of Environment • Many human characteristics result from a combination of heredity and environment Figure 9.22

43. The Chromosomal Basis of Inheritance • The chromosome theory of inheritance states that: - genes are located at specific positions on chromosomes - the behavior of chromosomes during meiosis and fertilization accounts for inheritance patterns

44. Figure 9.23

45. Linked Genes • Are located close together on a chromosome • Tend to be inherited together as a set • Alleles that start out together on the same chromosome would be expected to travel together during meiosis and fertilization

46. The Process of Science: Are Some Genes Linked? • Using the fruit fly Drosophila melanogaster, Thomas Hunt Morgan determined: • some genes were linked based on the inheritance patterns of their traits • meaning that meiosis in the heterozygous fruit flies would yield gametes with just 2 genotypes

47. Morgan performed a dihybrid testcross on Drosophila fruit flies that were heterozygous for body color (Gg) and wing shape (Ll) • When Morgan tabulated the phenotypes of 2300 offspring, he observed more than the expected number of parental phenotypes (gray/long and black/short) and fewer then expected nonparental phenotypes (gray/short and black/long) Fig 9.24

48. Genetic Recombination: Crossing Over • Two linked genes - can give rise to 4 different gamete genotypes - can sometimes cross over producing recombinant gametes - Morgan hypothesized that crossing over accounted for the observed nonparental offspring (gray/short; black/long) - the percentage of recombinant offspring among the total is called the recombination frequency (sum of the recombinant flies 206 + 185, divided by 2300 = 17%)

49. Crossing over can produce recombinant gametes Fig 9.25

50. Figure 9.26