Mendelian Genetics 1

Mendelian Genetics 1

Genetic Terminology Trait - any characteristic that can be passed from parent to offspring Heredity - passing of traits from parent to offspring Genetics - study of heredity 2

Gregor Johann Mendel Between 1856 and 1863, Mendel cultivated and tested some 28,000 pea plants He found that the plants' offspring retained traits of the parents Nicknamed the Father of Genetics" 3

Gregor Johann Mendel Austrian monk Studied the inheritance of traits in pea plants Developed the laws of inheritance Mendel's work was not recognized until the turn of the 20th century 4

Site of Gregor Mendel s experimental garden in the Czech Republic 5

Genes Alleles - two forms of a gene (dominant & recessive) Occur on homologous pairs of chromosomes Dominant - stronger of two genes expressed in the hybrid; represented by a capital letter (R) Recessive - gene that shows up less often in a cross; represented by a lowercase letter (r) 6

More Terminology Genotype - allele combination for a trait (e.g. RR, Rr, rr) Phenotype - the physical feature resulting from a genotype (e.g. red, white) 7

Genotypes Homozygous genotype - gene combination involving 2 dominant or 2 recessive genes (e.g. RR or rr); also called pure Heterozygous genotype - gene combination of one dominant & one recessive allele (e.g. Rr); also called hybrid 8

Genotype & Phenotype in Flowers Genotype of alleles: R = red flower r = yellow flower All genes occur in pairs, so 2 alleles affect a characteristic Possible combinations are: Genotypes RR Rr rr Phenotypes RED RED YELLOW 9

Homozygous Dominant { Homo = same Dominant = Uppercase { Example Homozygous Dominant for Green Color: {? { Homozygous Recessive { Homo = same Recessive = Lowercase { Example Homozygous Recessive for yellow color: {? { Heterozygous { Hetero = different { Example Heterozygous for Green color: 10 {?

Inventory of Autosomal Traits Please open your packet to page 9. 11

Genes and Environment Determine Characteristics 12

Mendel s Pea Plant Experiments 13

Why peas, Pisum sativum? Can be grown in a small area Produce lots of offspring Produce pure plants when allowed to self-pollinate several generations Can be artificially cross-pollinated (by hand) 14

Eight Pea Plant Traits Seed shape --- Round (R) or Wrinkled (r) Seed Color ---- Yellow (Y) or Green (y) Pod Shape --- Smooth (S) or wrinkled (s) Pod Color --- Green (G) or Yellow (g) Seed Coat Color ---Gray (G) or White (g) Flower position---axial (A) or Terminal (a) Plant Height --- Tall (T) or Short (t) Flower color --- Purple (P) or white (p) 15

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Mendel s Experimental Results 18

Did the observed ratio match the theoretical ratio? The theoretical or expected ratio of plants producing round or wrinkled seeds is 3 round :1 wrinkled Mendel s observed ratio was 2.96:1 The discrepancy is due to statistical error The larger the sample the more nearly the results approximate to the 19 theoretical ratio

Generation Gap Parental P 1 Generation = the parental generation in a breeding experiment. F 1 generation = the first-generation offspring in a breeding experiment. (1st filial generation) { From breeding individuals from the P 1 generation F 2 generation = the second-generation offspring in a breeding experiment. (2nd filial generation) { From breeding individuals from the F 1 generation 20

Following the Generations Cross 2 Pure Plants TT x tt Results in all Hybrids Tt 21 Cross 2 Hybrids get 3 Tall & 1 Short TT, Tt, tt

Types of Genetic Crosses Monohybrid cross - cross involving a single trait e.g. flower color Dihybrid cross - cross involving two traits e.g. flower color & plant height 22

Punnett Square Used to help solve genetics problems 23

Breed the P 1 generation tall (TT) x dwarf (tt) pea plants 24

Solution: tall (TT) vs. dwarf (tt) pea plants t t T Tt Tt produces the F 1 generation T Tt Tt All Tt = tall (heterozygous tall) 25

Breed the F 1 generation tall (Tt) vs. tall (Tt) pea plants 26

Solution: T t tall (Tt) x tall (Tt) pea plants T TT Tt t Tt tt produces the F 2 generation 1/4 (25%) = TT 1/2 (50%) = Tt 1/4 (25%) = tt 1:2:1 genotype 3:1 phenotype 27

Monohybrid Crosses 28

P 1 Monohybrid Cross Trait: Seed Shape Alleles: R Round r Wrinkled Cross: Round seeds x Wrinkled seeds RR x rr Genotype: Rr R r Rr r Rr Phenotype: Round Genotypic Probability: 100% R Rr Rr 29 Phenotypic Probability: 100%

P 1 Monohybrid Cross Review Homozygous dominant x Homozygous recessive Offspring all Heterozygous (hybrids) Offspring called F 1 generation Genotypic & Phenotypic ratio is ALL ALIKE 30

F 1 Monohybrid Cross Trait: Seed Shape Alleles: R Round r Wrinkled Cross: Round seeds x Round seeds Rr x Rr Genotype: RR, Rr, rr R R RR r Rr Phenotype: Round & wrinkled G.Ratio: 1:2:1 r Rr rr P.Ratio: 3:1 31

F 1 Monohybrid Cross Review Heterozygous x heterozygous Offspring: 25% Homozygous dominant RR 50% Heterozygous Rr 25% Homozygous Recessive rr Offspring called F 2 generation Genotypic ratio is 1:2:1 Phenotypic Ratio is 3:1 32

Monohybrid Problems A heterozygous man with Widows peak is crossed with a female without widows peak. Cross the individuals Give Genotype, Phenotype and Probability for each Genotype: : Phenotype: widow s 33

1. In pigs, white Practice color is Problems dominant & black color is recessive. Show the results of the following crosses. Provide genotype & phenotype. a. A homozygous male white pig is mated with a homozygous black pig. b. A male heterozygous white pig is crossed with a female heterozygous white pig. c. A male heterozygous white pig is mated with a homozygous black pig. 2. The gene for brown eye color is dominant to the allele for blue eyes. A man with blue eyes marries a brown eyed woman whose mother 34 had blue eyes. What are the genotypes &

Monohybrid Quiz 1. In leghorn chickens, colored feathers are due to a dominant allele, white feathers are recessive. Cross a homozygous white chicken with a heterozygous chicken. Provide the ratio of possible genotypes & phenotypes. 2. The hornless trait in cattle is dominant & the horned trait is recessive. A hornless bull is mated to 3 cows. Cow A is horned & gives birth to a hornless calf. Cow B is horned & gives birth to a horned calf. Cow C is hornless & gives birth to a horned calf. What are the genotypes of the bull, the 3 cows, & the 3 calves? (You do not need to show a punnett square, but you do need to explain. 35

36 Mendel s Laws

Results of Monohybrid Crosses Inheritable factors or genes are responsible for all heritable characteristics Phenotype is based on Genotype Each trait is based on two genes, one from the mother and the other from the father True-breeding (purebred) individuals are homozygous ( both 37 alleles) are the same

Law of Dominance In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation. All the offspring will be heterozygous and express only the dominant trait. RR x rr yields all Rr (round seeds) 38

Law of Dominance 39

Law of Segregation During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other. If mom is Tt, her egg can have a T or a t but not both at once. Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring. 40

Applying the Law of Segregation 41

Law of Independent Assortment Alleles for different traits are distributed to sex cells (& offspring) independently of one another. This law can be illustrated using dihybrid crosses. 42

Dihybrid Cross A breeding experiment that tracks the inheritance of two traits. Mendel s Law of Independent Assortment a. Each pair of alleles segregates independently during gamete formation b. Formula: 2 n (n = # of heterozygotes) 43

Question: How many gametes will be produced for the following allele arrangements? Remember: heterozygotes) 2 n (n = # of 1. RrYy 44 2. AaBbCCDd

Answer: 1. RrYy: 2 n = 2 2 = 4 gametes RY Ry ry ry 2. AaBbCCDd: 2 n = 2 3 = 8 gametes ABCD ABCd AbCD AbCd abcd abcd abcd abcd 3. MmNnOoPPQQRrssTtQq: 2 n = 2 6 = 64 gametes 45

Dihybrid Cross Traits: Seed shape & Seed color Alleles: R round r wrinkled Y yellow y green RrYy x RrYy RY Ry ry ry RY Ry ry ry All possible gamete combinations 46

Dihybrid Cross RY Ry ry ry RY Ry ry ry 47

RY Dihybrid Cross RY Ry ry ry RRYY RRYy RrYY RrYy Genotypes 1 RRYY 2 RRYy 2 RrYY 4 RrYy 1 RRyy 2 Rryy 1 rryy 2 rryy 1 rryy Ry ry RRYy RrYY RRyy RrYy RrYy rryy Rryy rryy Phenotypes 9 round, yellow 3 round, green 3 wrinkled, yellow 1 wrinkled, green 56% 19% 19% 6% ry RrYy Rryy rryy rryy 48

Cross Two heterozygous Pea Plants TtGg x TtGg Will use the FOIL method from algebra to set-up the punnett square Dihybrid Phenotypic Ratio 9:3:3:1 Always this ratio for two 49

Summary of Mendel s laws LAW PARENT CROSS OFFSPRING DOMINANCE TT x tt tall x short 100% Tt tall SEGREGATION Tt x Tt tall x tall 75% tall 25% short INDEPENDENT ASSORTMENT RrGg x RrGg round & green x round & green 50 9/16 round seeds & green pods 3/16 round seeds & yellow pods 3/16 wrinkled seeds & green pods 1/16 wrinkled seeds & yellow pods

Test Cross A mating between an individual of unknown genotype and a homozygous recessive individual. Example: bbc x bbcc BB = brown eyes Bb = brown eyes bc b bb = blue eyes bc CC = curly hair 51

Possible results: Test Cross bc b C bc b c bc bbcc bbcc or bc bbcc bbcc 52

Incomplete Dominance and Codominance 53

Incomplete Dominance F1 hybrids have an appearance somewhat in between the phenotypes of the two parental varieties. Example: snapdragons (flower) red (RR) x white (rr) R r r RR = red flower R 54 rr = white flower

Incomplete Dominance r r R Rr Rr produces the F 1 generation R Rr Rr All Rr = pink (heterozygous pink) 55

Incomplete Dominance 56

Codominance = situation where both alleles of a gene will be expressed in the phenotype Examples: 1. Cattle red hair is codominant to white hair. Produces a roan cow that seems pinkish brown because it is speckled with white & red 57

Chickens black & white feather color is codominant resulting in a speckled chicken 58

Codominance Human Blood Types * there are 2 codominant alleles for blood type plus 1 recessive allele I A, I B, i * these alleles correspond to the blood types found in humans 1. type A = I A I A or I A i 59 2. type B = I B I B or I B i

Codominance Problem Example: homozygous male Type B (I B I B ) I A female Type A (I A i) i x heterozygous I B I B I A I B I A I B I B i I B i 60 1/2 = I A I B 1/2 = I B i

Another Codominance Problem Example: male Type O (ii) x female type AB (I A I B ) I A I B i I A i I B i 1/2 = I A i 1/2 = I B i i I A i I B i 61

Codominance Question: If a boy has a blood type O and his sister has blood type AB, what are the genotypes and phenotypes of their parents? boy - type O (ii) X girl - type AB (I A I B ) 62

Codominance Answer: I A i I B i I A I B ii Parents: genotypes = I A i and I B i phenotypes = A and B 63

Blood Type Monohybrid Crosses 1. A woman who is heterozygous type B marries a man who is type O. What are the possible blood types that their children could be? 2. Suzy has type B blood and her husband has type AB blood. They have 2 children. One child, Bobby, has type A & Robbie, the oldest has type B. What is the genotype of the mother? 64

Question - codominance Henry Anonymous, a film star, was involved in a paternity case. The woman bringing suit had two children, on whose blood type was A and the other whose blood type was B. Her blood type was O, the same as Henry s! The judge in the case awarded damages to the woman, saying that Henry had to be the father of at least one of the children. Obviously, the judge should be sentenced to Biology. For Henry to have been the father of both children, his blood type would have had to be what? 65

Answer Obviously, the judge should be sentenced to Biology. For Henry to have been the father of both children, his blood type would have had to be what? I A I B Answer i I A i I B i 66 i I A i I B i

Sex-linked Traits Traits (genes) located on the sex chromosomes, NOT the autosomes Sex chromosomes are X and Y XX genotype for females XY genotype for males Many sex-linked traits carried on X chromosome 67

Sex-linked Traits Example: Eye color in fruit flies Sex Chromosomes fruit fly eye color XX chromosome - female Xy chromosome - male 68

Sex-linked Trait Problem Example: Eye color in fruit flies (red-eyed male) x (white-eyed female) X R Y x X r X r Remember: the Y chromosome in males does not carry traits. RR = red eyed X R X r X r Rr = red eyed rr = white eyed Y XY = male 69

Sex-linked Trait Solution: X r X r X R Y X R X r X r Y X R X r X r Y 50% red eyed female 50% white eyed male 70

The most common COLOR forms of BLINDNESS congenital defective color vision, the red-green deficiencies, are due to "sexlinked X chromosomes" and "simple recessive hereditary traits". Men are mainly affected because women have two X chromosomes and men have only one X and a Y chromosome. If a man's one X chromosome is color defective he will be color deficient, however, a woman must inherit two color defective X chromosomes to be color deficient. For a woman to be color deficient, her father must be colorblind and her mother must be colorblind or be a carrier. 71

A pedigree for colorblindness 72

73 TEST

Question: Two normal sighted people have a colorblind son. What are the genotypes of the parents? X C X _? x X C Y What are the genotypes 74 and

Phenotypes: 2 normal females, 1 75 normal male, 1 colorblind male Answer: X C Y parents X C X C X C X C Y X c X C X c X c Y Genotypes: 1X C X C, 1X C X c, 1X C Y, 1X c Y

Question: A couple has a colorblind daughter. What are the possible genotypes and phenotypes of the parents and the daughter? 76

X c Y Answer: X C X C X c X C Y X c X c X c X c Y parents: X c Y and X C X c or X c X c father colorblind 77 mother carrier or colorblind

Pedigree Charts The family tree of genetics 78

Pedigrees Suppose you wanted to learn about an inherited trait present in your family. One method could be using a pedigree. A pedigree is a chart of symbols showing the relationships within a family & a specific trait that each member may 79 exhibit.

Pedigree Symbols 80

Connecting Pedigree Symbols Examples of connected symbols: Married Couple Siblings 81

What does a pedigree chart look like? 82

Interpreting a Pedigree Chart Determine if the pedigree chart shows an autosomal or X-linked disease. { If most of the males in the pedigree are affected the disorder is X-linked { If it is a 50/50 ratio between men and women the disorder is autosomal. 83

Example of Pedigree Charts Is it Autosomal or X-linked? 84

Interpreting a Pedigree Chart Determine whether the disorder is dominant or recessive. { If the disorder is dominant, one of the parents must have the disorder. { If the disorder is recessive, neither parent has to have the disorder because they can be heterozygous. 85

Example of Pedigree Charts Dominant or Recessive? 86

Example of Pedigree Charts Dominant or Recessive? 87

Summary Pedigrees are family trees that explain your genetic history. Pedigrees are used to find out the probability of a child having a disorder in a particular family. To begin to interpret a pedigree, determine if the disease or condition is autosomal or X-linked and dominant or recessive. 88

Female Carriers 89

Mr. and Mrs. Smith had 3 children Susie, Sandra, & Steven. When the children were still young, Mr. Smith was killed in an automobile accident. Three years after the accident, Mrs. Smith married Mr. Jones. Mr. Jones had 1 daughter, Joy. Mr. Jones & Joy s mother had been divorced when Joy was 6. Draw a pedigree for each of the following families: 1. The Smith family before the automobile accident. 2. The Smith-Jones family after the new marriage. Some doctors include pedigrees in patients medical charts. List at least 3 types of things that a doctor might include in a pedigree that most people would probably not include in a family tree. Imagine you are a genetic counselor. The pedigree shown illustrates albinism. A couple from the family have come to you for advise about how the trait is inherited. Your task is to determine whether the allele for albinism is dominant or recessive. Choose an inherited trait & create a pedigree word problem that will be drawn by a fellow biology student. Include 4 generations. 90