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Human Genetics Mendelian Genetics. Inheritance  Parents and offspring often share observable traits.  Grandparents and grandchildren may share traits.

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Presentation on theme: "Human Genetics Mendelian Genetics. Inheritance  Parents and offspring often share observable traits.  Grandparents and grandchildren may share traits."— 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 Gregor Mendel Why did Mendel succeed in seeing something that nobody else saw? 1.He counted 2.Chose a good system 3.Chose true-breeding characters http://www.mendel- ent.htm

5 The field of genetics started with a single paper!

6 Mendel is as important as Darwin in 19 th 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 1.Self-fertilization 2.Cross-pollination

9  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 Mendel Planned Experiments Carefully

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

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

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 the mystery individual with a homozygous recessive individual  If any offspring display the recessive phenotype, the mystery parent must be heterozygous

15 Mendel’s Model  Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F 2 offspring  Four related concepts make up this model  These concepts can be related to what we now know about genes and chromosomes

16  Alternative versions of genes account for variations in inherited characters  For example, the gene for flower color in pea plants exists in two versions, one for purple flowers and the other for white flowers  These alternative versions of a gene are now called alleles  Each gene resides at a specific locus on a specific chromosome The First Concept

17 Allele for purple color Homologous pair of chromosomes Allele for white color Locus for flower color gene

18  For each character, an organism inherits two alleles, one from each parent  Mendel made this deduction without knowing about the role of chromosomes  The two alleles at a locus on a chromosome may be identical, as in the true-breeding plants of Mendel’s P generation  Alternatively, the two alleles at a locus may differ, as in the F 1 hybrids The Second Concept

19 The Third Concept  If the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance  In the flower-color example, the F 1 plants had purple flowers because the allele for that trait is dominant

20 The Fourth Concept  Known as “the law of segregation”  Two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes  Thus, an egg or a sperm gets only one of the two alleles that are present in the somatic cells of an organism  This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis

21 Mendel’s Laws Explain his Data  Mendel’s segregation model accounts for the 3:1 ratio he observed in the F 2 generation of his numerous crosses  The possible combinations of sperm and egg can be shown using a Punnett square, a diagram for predicting the results of a genetic cross between individuals of known genetic makeup  A capital letter represents a dominant allele, and a lowercase letter represents a recessive allele

22 Two types of “states”  Homozygous:  an individual or a locus carries identical alleles of a given gene.  Heterozygous:  an individual or a locus carries different alleles of a given gene

23 Mendel’s Law of Segregation  Members of a gene pairs (alleles) separate from each other during gamete formation.  The underlying mechanism is separation and then segregation of homologous chromosomes during meiosis.  Key terms:  dominant and recessive traits  genotype versus phenotype

24 Inborn errors of metabolism Hint: much more common in first cousin marriages Garrod, 1902 - human traits followed Mendelian rules

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

26 Mendel’s Second Law: The Law of Independent Assortment  Mendel derived the law of segregation by following a single character  The F 1 offspring produced in this cross were all heterozygous for that one character  A cross between such heterozygotes is called a monohybrid cross

27  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 F 1 generation, heterozygous for both characters  A dihybrid cross, a cross between F 1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently

28 LE 14-8 P Generation F 1 Generation YYRR Gametes YR yr yyrr YyRr Hypothesis of dependent assortment Hypothesis of independent assortment Sperm Eggs YR Yr yrYR yr Eggs YYRRYyRr yyrr yR yr Phenotypic ratio 3:1 F 2 Generation (predicted offspring) YYRR YYRrYyRRYyRr YYRrYYrrYyRrYyrr YyRRYyRryyRRyyRr YyRrYyrryyRryyrr Phenotypic ratio 9:3:3:1 YRYryRyr Sperm 1 2 1 4 1 4 1 4 1 4 1 4 3 4 1 2 1 2 1 2 1 4 9 16 3 3 3 1 4 1 4 1 4

29  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

30 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.

31 Probability: The likelihood that an event will occur No chance of event probability = 0 (e.g. chance of rolling 8 on a six-sided die) Event always occurs probability = 1 (chance of rolling 1,2,3,4,5,or 6 on a six-sided die) # on dieprobability 11/6 2 3 4 5 6 The probability of an event = # of chance of event total possible events The probabilities of all the possible events add up to 1.

32 Independent Events The probability of independent events is calculated by multiplying the probability of each event. In two rolls of a die, the chance of rolling the number 3 twice: Probability of rolling 3 with the first die = 1/6 Probability of rolling 3 with the second die = 1/6 Probability of rolling 3 twice = 1/6 x 1/6 or 1/36

33 Independent events  What is the chance of an offspring having the homozygous recessive genotype when both parents are doubly heterozygous?

34 Independent Events

35 Dependent Events The probability of dependent events is calculated by adding the probability of each event. In one roll of a die, what is the probability of rolling either the number 5 or an even number? Probability of rolling 5 or an even number = 1/6 + 3/6 or 4/6 Probability of rolling the number 5 = 1/6 Probability of rolling an even number = 3/6

36  Parents are heterozygous for a trait, R.  What is the chance that their child carries at least one dominant R allele? Probability of child carrying RR = 1/4 Probability of child carrying Rr = 1/2 Probability of child carrying R = 1/4 + 1/2 = 3/4 Dependent Events

37 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 F 1 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

38 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.

39 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

40 YYRR yyrr YyRr Male gametes Female Gametes YyRr ¼¼¼¼¼¼¼¼ ¼ ¼ YR Yr yR yr YR Yr yR yr YYRRYYRr YYrRYYrr YyRRYyRr Yyrr YyRRYyRr YyrryyRr yyrr yyRR 9/16 3/16 1/16

41 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

42 1/2 So, Rr genotype = (1/2 x 1/2) x 2 = 1/2 RR genotype is (1/2 x 1/2) = ¼ Add these to get the combined probability. R r R r RR Rr rr Can use to solve more complicated problems: AaBBccDdEeFFGghhIiJJKk x aaBbCCDdEEffggHhIIjjKk

43 Crossing Double Heterozygotes

44 Fig 3.10b


46 Fig 3.11

47 Dihybrid Cross  From the 16 possible fertilization events  - 9 genotypes  - 4 phenotypes9:3:3:1

48 Dihybrid cross Tetrahybrid cross!

49 Statistical Analysis Simple cross:purple x white F1: all purple F2: 2850 purple, 1150 white Use Χ 2 test to determine likelihood of getting this result by chance Χ 2 = total of (observed-expected) 2 /expected over all classes "Expected" is from null hypothesis - data fit a 3:1 ratio (2850-3000) 2 /3000 + (1150-1000) 2 /1000 = 30! 10x as many, but same ratio P is < { "@context": "", "@type": "ImageObject", "contentUrl": "", "name": "Statistical Analysis Simple cross:purple x white F1: all purple F2: 2850 purple, 1150 white Use Χ 2 test to determine likelihood of getting this result by chance Χ 2 = total of (observed-expected) 2 /expected over all classes Expected is from null hypothesis - data fit a 3:1 ratio (2850-3000) 2 /3000 + (1150-1000) 2 /1000 = 30.", "description": "10x as many, but same ratio P is <

50 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

51 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

52 Mendel’s Third Law: Law of Dominance  Some genes mask the effect of other genes, which then can pass on unchanged to reemerge when they are combined with another recessive gene.

53 The Spectrum of Dominance  Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical  In incomplete dominance, the phenotype of F 1 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

54 Red C R Gametes P Generation CRCR CWCW White C W Pink C R C W CRCR Gametes CWCW F 1 Generation F 2 Generation Eggs CRCR CWCW CRCR CRCRCRCR CRCWCRCW CRCWCRCW CWCWCWCW CWCW Sperm 1 2 1 2 1 2 1 2 1 2 1 2

55 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.

56 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

57 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: I A, I B, and i.


59 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

60 LE 14-12 aabbccAabbccAaBbccAaBbCcAABbCcAABBCcAABBCC AaBbCc 20 / 64 15 / 64 6 / 64 1 / 64 Fraction of progeny

61 Nature and Nurture: The Environmental Impact on Phenotype

62 Relating Mendel’s Laws to Cells 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.

63 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.

64 Terminology Review  Genes are the units of heredity  Genes are segments of DNA  Each gene has a specific locus on a certain chromosome  Genetic variants at the same locus are alleles of one another.  SNPs (Single Nucleotide Polymorphisms) are alleles of the same gene.

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

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

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

68 Recall Meiosis  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 2 n, where n is the haploid number  For humans (n = 23), there are more than 8 million (2 23 ) possible combinations of chromosomes

69 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

70 LE 13-11 Prophase I of meiosis Tetrad Nonsister chromatids Chiasma, site of crossing over Recombinant chromosomes Metaphase I Metaphase II Daughter cells

71 Monohybrid Cross: - cross involving only one character.

72 Results from Crosses  F1 offspring -  the trait expressed was the same as that of one of the parental lines  traits did not blend  F2 offspring  the traits from both parents were expressed in a 3:1 ratio  while the trait had not been expressedin the F1, it remained unchanged as it was passed from the P1 to the F1 and then to the F2 generation.  CONCLUSION  Traits are inherited as discrete, separate units.

73 Mendel’s Conclusions 1.Factors (genes) that determine traits can be hidden or unexpressed.  Dominant traits have a factor (gene) that is expressed in the F1 offspring  Recessive traits have a factor (gene) that is not expressed in the F1 offspring

74 Mendel’s Conclusions 2. Despite P1 and F1 generations appearing identical, they must be genetically different.  Phenotype- observed properties of a trait  Genotype- the genetic makeup of a trait  PP1 and F1 seeds have the same phenotype but different genotypes

75 3. Since the F1 offspring had factors (genes) for both smooth and wrinkled - then there must be at least 2 factors for every trait. Alleles- alternative forms of a gene Genotype- indicates the combination of alleles present Phenotype- indicates the trait observed These terms differentiate the observed form and the underlying alleles present at a particular gene. Mendel’s Conclusions

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