1 / 98

Human Genetics

Human Genetics. Mendelian Genetics. Inheritance. Parents and offspring often share observable traits. Grandparents and grandchildren may share traits not seen in parents. Why do traits disappear in one generation and reappear in another?

noblem
Download Presentation

Human Genetics

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Human Genetics Mendelian Genetics

  2. Inheritance • Parents and offspring often share observable traits. • Grandparents and grandchildren may share traits not seen in parents. • Why do traits disappear in one generation and reappear in another? • Why do we still keep talking about Mendel and his peas?

  3. Darwin Prior to Mendel Scientists looked for rules to explain continuous variation. The “blending” hypothesis: genetic material from the two parents blends together Head size, height, longevity – all continuous variations - support the blending hypothesis The “particulate” hypothesis: parents pass on discrete heritable units (genes) Mendel’s experiments suggested that inherited traits were discrete and constant

  4. http://www.mendel-museum.org/eng/1online/experiment.htm Why did Mendel succeed in seeing something that nobody else saw? He counted Chose a good system Chose true-breeding characters Gregor Mendel

  5. The field of genetics started with a single paper!

  6. Mendel is as important as Darwin in 19th century science • Mendel did experiments and analyzed the results mathematically. His research required him to identify variables, isolate their effects, measure these variables painstakingly and then subject the data to mathematical analysis. • He was influenced by his study of physics and having an interest in meteorology. His mathematical and statistical approach was also favored by plant breeders at the time.

  7. Mendel used an Experimental, Quantitative Approach • Advantages of pea plants for genetic study: • There are many varieties with distinct heritable features, or characters (such as color); character variations are called traits • Mating of plants can be controlled • Each pea plant has sperm-producing organs (stamens) and egg-producing organs (carpels) • Cross-pollination (fertilization between different plants) can be achieved by dusting one plant with pollen from another

  8. Self-fertilization Cross-pollination

  9. Mendel Planned Experiments Carefully • Mendel chose to track only those characters that varied in an “either-or” manner • He also used varieties that were “true-breeding” (plants that produce offspring of the same variety when they self-pollinate) • He spent 2 years getting “true” breeding plants to study • At least three of his traits were available in seed catalogs of the day

  10. Mendel studied true breeding pea traits with two distinct forms

  11. Terminology of Breeding P1 (parental) - pure breeding strain F1 (filial) – offspring from a parental cross They are also referred to as hybrids – because they are the offspring of two 2 pure-breeding parents F2 - produced by self-fertilizing the F1 plants

  12. Genotype Phenotype PP (homozygous Purple 1 LE 14-6 Pp (heterozygous 3 Purple 2 Pp (heterozygous Purple pp (homozygous White 1 1 Ratio 1:2:1 Ratio 3:1

  13. P Generation Purple flowers PP White flowers pp Appearance: Genetic makeup: p P Gametes F1 Generation Appearance: Genetic makeup: Purple flowers Pp Gametes: 1 1 p P 2 2 F1 sperm P p F2 Generation P PP Pp F1 eggs p Pp pp 3 : 1

  14. The Testcross • How can we tell the genotype of an individual with the dominant phenotype? • This individual must have one dominant allele, but could be either homozygous dominant or heterozygous • The answer is to carry out a testcross: breeding themystery individual with a homozygous recessive individual • If any offspring display the recessive phenotype, the mystery parent must be heterozygous

  15. Dominant phenotype, unknown genotype: PP or Pp? Recessive phenotype, known genotype: pp LE 14-7 If Pp, then 1 2 offspring purple and 1 2 offspring white: If PP, then all offspring purple: p p p p P P Pp Pp Pp Pp P P pp pp Pp Pp

  16. Mendel’s Second Law: The Law of Independent Assortment • Mendel derived the law of segregation by following a single character • The F1 offspring produced in this cross were all heterozygous for that one character • A cross between such heterozygotes is called a monohybrid cross

  17. Mendel identified his second law of inheritance by following two characters at the same time • Crossing two, true-breeding parents differing in two characters produces dihybrids in the F1 generation, heterozygous for both characters • A dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently

  18. P Generation YYRR yyrr Gametes yr YR YyRr F1 Generation Hypothesis of dependent assortment Hypothesis of independent assortment LE 14-8 Sperm YR Yr yR yr 1 1 1 1 4 4 4 4 Sperm Eggs YR yr 1 1 2 2 YR 1 4 Eggs YYRR YYRr YyRR YyRr YR 1 2 F2 Generation (predicted offspring) YYRR YyRr Yr 1 4 YYRr YYrr YyRr Yyrr yr 1 2 YyRr yyrr yR 1 4 YyRR YyRr yyRR yyRr 3 1 4 4 yr 1 4 Phenotypic ratio 3:1 YyRr Yyrr yyRr yyrr 9 3 3 3 16 16 16 16 Phenotypic ratio 9:3:3:1

  19. The law of independent assortment states that each pair of alleles segregates independently of other pairs of alleles during gamete formation • Strictly speaking, this law applies only to genes on different, nonhomologous chromosomes • Genes located near each other on the same chromosome tend to be inherited together

  20. Probability • Ranges from 0 to 1 • Probabilities of all possible events must add up to 1 • Rule of multiplication: The probability that independent events will occur simultaneously is the product of their individual probabilities. • Rule of addition: The probability of an event that can occur in two or more independent ways is the sum of the different ways.

  21. Multiplication and Addition Rules Applied to Monohybrid Crosses • The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities • Probability in an F1 monohybrid cross can be determined using the multiplication rule • Segregation in a heterozygous plant is like flipping a coin: Each gamete has a 1/2 chance of carrying the dominant allele and a 1/2 chance of carrying the recessive allele

  22. ½ chance of P and ½ chance of p allele results in ¼ chance of each homozygous genotype. There are two ways to get the heterozygous genotype so it is ¼ + ¼ = ½ Three genotypes give the same phenotype.

  23. Solving Complex Genetics Problems with the Rules of Probability • We can apply the rules of multiplication and addition to predict the outcome of crosses involving multiple characters • A dihybrid or other multicharacter cross is equivalent to two or more independent monohybrid crosses occurring simultaneously • In calculating the chances for various genotypes, each character is considered separately, and then the individual probabilities are multiplied together

  24. YYRR yyrr Female Gametes YyRr ¼ ¼ ¼ ¼ YyRr YR Yr yR yr ¼ ¼ ¼ ¼ YR Yr yR yr YYRR YYRr YyRR YyRr YyRr Male gametes YYrR YYrr YyRr Yyrr 9/16 YyRR YyRr yyRR yyRr 3/16 3/16 YyRr Yyrr yyRr yyrr 1/16

  25. For a dihybrid cross – the chance that 2 independent events occur together is the product of their chances of occurring separately. • The chance of yellow (YY or Yy) seeds= 3/4 (the dominant trait) • The chance of round (RR or Rr) seeds = 3/4 (the dominant trait) • The chance of green (yy) seeds= 1/4 (the recessive trait) • The chance of wrinkled (rr) seeds= 1/4 (the recessive trait) Therefore: The chance of yellow and round= 3/4 x 3/4 = 9/16 The chance of yellow and wrinkled= 3/4 x 1/4 = 3/16 The chance of green and round= 1/4 x 3/4 = 3/16 The chance of green and wrinkled= 1/4 x 1/4 = 1/16

  26. Inheritance patterns are often more complex than predicted by Mendel • The relationship between genotype and phenotype is rarely as simple as in the pea plant characters Mendel studied • Many heritable characters are not determined by only one gene with two alleles • However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance

  27. Extending Mendelian Genetics for a Single Gene • Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations: • When alleles are not completely dominant or recessive • When a gene produces multiple phenotypes • When a gene has more than two alleles • The forensic characteristics usually have more than two alleles

  28. The Spectrum of Dominance • Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical • In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties • In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways • Forensic Traits are codominant

  29. P Generation Red CRCR White CWCW Gametes CR CW Pink CRCW F1 Generation 1 1 Gametes CR CW 2 2 Sperm 1 1 CR CW 2 2 Eggs F2 Generation 1 CR 2 CRCR CRCW 1 CW 2 CRCW CWCW

  30. The Relation Between Dominance and Phenotype • A dominant allele does not subdue a recessive allele; alleles don’t interact • Alleles are simply variations in a gene’s nucleotide sequence • For any gene, dominance/recessiveness relationships of alleles depend on the level at which we examine the phenotype • If you look directly at DNA, you can always detect codominance.

  31. Frequency of Dominant Alleles • Dominant alleles are not always more common in populations than recessive alleles • For example, one baby out of 400 in the USA is born with extra fingers or toes • The allele for this trait is dominant to the allele for the more common trait of five digits per appendage • In this example, the recessive allele is far more prevalent than the dominant allele in the population

  32. Multiple Alleles • Most genes exist in populations in more than two allelic forms • For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i.

  33. Polygenic Inheritance • Quantitative characters are those that vary in the population along a continuum • Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype • Skin color in humans is an example of polygenic inheritance

  34. AaBbCc AaBbCc aabbcc Aabbcc AaBbcc AaBbCc AABbCc AABBCc AABBCC LE 14-12 20/64 15/64 Fraction of progeny 6/64 1/64

  35. Nature and Nurture: The Environmental Impact on Phenotype

  36. Law of Segregation Pairs of characteristics (alleles) separate during gamete formation Each cell has two sets of chromosomes that are divided to one set per gamete. Law of Independent Assortment The inheritance of an allele of one gene does not influence the allele inherited at a second gene. Genes on different chromosomes segregate their alleles independently. Relating Mendel’s Laws to Cells

  37. Offspring acquire genes from parents by inheriting chromosomes • Children do not inherit particular physical traits from their parents • It is genes that are actually inherited • Genes are carried on chromosomes. • Mendel identified 7 sets of characters- One per each of the 7 chromosomes in peas, so his law worked out perfectly. • Two characters on the same chromosome are linked together and would have messed up his law.

  38. Inheritance of Genes • Genes are the units of heredity • Genes are segments of DNA • Each gene has a specific locus on a certain chromosome • One set of chromosomes is inherited from each parent • Reproductive cells called gametes (sperm and eggs) unite, passing genes to the next generation

  39. Sexual Reproduction • Two parents give rise to offspring that have unique combinations of genes inherited from the two parents. • All humans arise from the joining of 1 egg and 1 sperm cell • 100% of a person’s DNA is the same within and throughout a human being’s body. • Whether you look at the cells of a person’s blood, skin, semen, saliva or hair, the DNA and genes will be the same.

  40. Chromosomes Come in Sets • Each human cell (except gametes) has 46 chromosomes arranged in pairs in its nucleus • The two chromosomes in each pair are called homologous chromosomes • One of each pair came from your mother and the other came from your father. • Both chromosomes in a pair carry genes controlling the same inherited characteristics

  41. The sex chromosomes are called X and Y • Human females have a homologous pair of X chromosomes (XX) • Human males have one X and one Y chromosome • The 22 pairs of chromosomes that do not determine sex are called autosomes

  42. Each pair of homologous chromosomes includes one chromosome from each parent • The 46 chromosomes in a human somatic cell are two sets of 23: one from the mother and one from the father • The number of chromosomes in a single set is represented by n • A cell with two sets is called diploid (2n) • For humans, the diploid number is 46 (2n = 46)

  43. Meiosis reduces of chromosome number from diploid to haploid The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation

  44. Meiosis is preceded by the replication of chromosomes Meiosis takes place in two sets of cell divisions, called meiosis I and meiosis II The two cell divisions result in four daughter cells Each daughter cell has only half as many chromosomes as the parent cell

  45. Key Maternal set of chromosomes Possibility 2 Possibility 1 Paternal set of chromosomes Two equally probable arrangements of chromosomes at metaphase I Metaphase II Daughter cells Combination 3 Combination 2 Combination 4 Combination 1

  46. 8 Gamete Combinations vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv Maternal set of chromosomes (n = 3) 2n = 6 Paternal set of chromosomes (n = 3) Two sister chromatids of one replicated chromosomes Centromere Two nonsister chromatids in a homologous pair Pair of homologous chromosomes (one from each set)

  47. Key Haploid gametes (n = 23) Haploid (n) Ovum (n) Diploid (2n) Sperm cell (n) LE 13-5 MEIOSIS FERTILIZATION Ovary Testis Diploid zygote (2n = 46) Mitosis and development Multicellular diploid adults (2n = 46)

  48. Homologous pairs of chromosomes orient randomly at metaphase I of meiosis • In independent assortment, each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs • The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number • For humans (n = 23), there are more than 8 million (223) possible combinations of chromosomes

  49. Random Fertilization • Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg) • The fusion of gametes produces a zygote with any of about 64 trillion diploid combinations • Crossing over adds even more variation • Each zygote has a unique genetic identity

More Related