Inheritance Mendelian Genetics The Modern Genetics View Alleles are versions of a gene Gene for flower color Alleles for purple or white flowers Two alleles per trait 2 chromosomes, each with 1 gene The dominant allele can mask the recessive allele. The dominant allele is designated with a capital letter (e.g. F ) The recessive allele is designated with a lowercase letter (e.g. f ). Mom (egg) Dad (sperm) 1
Genotype Versus Phenotype The genotype of an organism describes the two alleles that are present FF, homozygous dominant for flower color Ff, heterozygous for flower color ff, homozygous recessive for flower color Genotype Versus Phenotype The phenotype of an organism refers to the physical appearance. Purple flowers (genotype FF or Ff) White flowers (genotype ff) 2
Genotype Versus Phenotype Organisms with different genotypes can have the same phenotype Flower color F Purple flowers dominant allele f white flowers recessive allele FF and Ff geneotype result in purple flowers. It is impossible to distinguish the heterozygotes from the homozygous dominants ff - white flowers If you have the recessive phenotype (white) you KNOW you MUST have the recessive genotype (ff)!!! One-Trait Inheritance Naming the generations The original parents P generation. The first generation First filial (F 1 ) generation. The second generation Second filial (F 2 ) generation. F 1 F 2 P 3
One-Trait Inheritance Punnett square Predict the results of a genetic cross All possible gametes of each parent are arranged on the horizontal and vertical axes. The squares represent every possible combination of gametes that could combine to form a zygote. One-Trait Inheritance True breeding means homozygous Either dominant or recessive True-breeding tall plants were crossed with true-breeding short plants (TT x tt) The F 1 generation all tall (100% Tt). gametes T T t Tt Tt t Tt Tt What happened to the short plants in the F 1 generation? The F 1 plants are all heterozygous (Tt) The dominant allele is masking the recessive allele 4
One-Trait Inheritance True breeding means homozygous Either dominant or recessive True-breeding tall plants were crossed with true-breeding short plants (TT x tt) The F 1 generation all tall (100% Tt). gametes T T t Tt Tt t Tt Tt F 1 generation self-pollinated (Tt x Tt) gametes T t T TT Tt t Tt tt ¾ of the F 2 were tall and ¼ were short (3:1) One-Trait Inheritance F 1 generation self-pollinated (Tt x Tt) ¾ of the F 2 were tall and ¼ were short (3:1) How was the short phenotype restored in the F 2 generation? The genes of the heterozygotes (Tt) separated when gametes were formed during meiosis Gametes either the tall (T) or short (t) allele When random fertilization occurred, the combinations were brought together in a 3:1 ratio 5
Mendel s Laws From the results of Mendel s experiments with pea plants, he derived the law of segregation Each gene in an individual is represented by two alleles that segregate during meiosis and recombine during fertilization One-Trait Testcross A testcross is used to determine if an individual carries a recessive factor. Dominant phenotype, what s the genotype? TT or Tt? Cross the unknown dominant individual (T-) with a known recessive individual (tt) to observe the distribution of pheotypes in the progeny. TT or Tt??? Definitely tt!!! 6
If the unknown individual is a heterozygote: Progeny will exhibit a 1:1 ratio of tall:short gametes T t t Tt tt t Tt tt One-Trait Testcross If the unknown individual is a homozygote: One-Trait Testcross 100% of progeny will be tall gametes T T t Tt Tt t Tt Tt 7
Important Ratios To Remember Heterozygote x Heterozygote Ss X Ss 3:1 Dominant phenotype : recessive phenotype 1:2:1 Genotypic ratio SS:Ss:ss Heterozygote x Recessive Homozygote Ss X ss 1:1 Dominant phenotype : recessive phenotype 1:1 Genotypic ratio Ss:ss gametes T t T TT Tt t Tt tt gametes T t t Tt tt t Tt tt You cross a homozygous dominant plant that has red flowers with a homozygous recessive plant that has white flowers. What phenotypes will you see in the F1 generation? A. Ff B. Red C. White D. Pink E. Red and white 8
You cross a homozygous dominant plant that has red flowers with a homozygous recessive plant that has white flowers. What genotypes will you see in the F1 generation? A. Ff B. FF C. ff D. Red E. A, B, and C Red flowers are dominant to white. You cross a red flowered plant of unknown genotype to a white flowered plant. The F1 generation has both red and white flowered plants. What is the genotype of the red plant? A. FF B. Ff C. ff D. Red E. White 9
Two-Trait Inheritance Two traits are examined simultaneously! P generation cross plants and examine two different traits: Smooth, yellow seeds Homozygous dominant for both traits (SSGG). Wrinkled, greed seeds Homozygous recessive for both traits (ssgg). What will be the genotype of the F 1 generation from a cross of: Smooth, yellow seeds (SSGG) Wrinkled, greed seeds (ssgg)? A. SSGG B. ssgg C. SSgg D. ssgg E. SsGg gametes S S s Ss Ss s Ss Ss gametes G G g g 10
What will be the phenotype of the F 1 generation from a cross of: Smooth, yellow seeds (SSGG) Wrinkled, greed seeds (ssgg) A. Smooth, green seeds B. Wrinkled, green seeds C. Smooth, yellow seeds D. Wrinkled, yellow seeds Two-Trait Inheritance Two traits are examined simultaneously! P generation cross plants and examine two different traits: Smooth, yellow seeds Homozygous dominant for both traits (SSGG). Wrinkled, greed seeds Homozygous recessive for both traits (ssgg) F 1 generation 100% smooth, yellow (SsGg) 11
Two-Trait Inheritance The progeny from the F 1 generation (SsGg) were selfpollinated to generate the F 2 generation. The results of the cross of the F 2 generation produced four phenotypes Smooth, yellow seeds Smooth, green seeds Wrinkled, yellow seeds Wrinkled, green seeds Two-Trait Inheritance The two factors segregated independently (huh?) Count all the yellow peas 12 Count all the green peas 4 What does the ratio 12:4 reduce to? 3:1 Count all the smooth peas 12 Count all the wrinkled peas 4 What does the ratio 12:4 reduce to? 3:1 12
Two-Trait Inheritance The two factors segregated independently (oh!) If I just look at one trait, I get the same ratio as in one-trait inheritance! The law of independent assortment. Each pair of alleles segregates (assorts) independently of other genes. All possible combinations of alleles occur in the gametes Unless the genes are found on the same chromosome! Linked genes Two-Trait Inheritance Important Ratio to Remember Two-trait heterozygote cross (SsGg x SsGg) 9:3:3:1 Phenotypic ratio S_G_ : S_gg : ssg_ : ssgg 13
Which of the following is a test cross? A. TTxTT B. tt x tt C. Tt x tt D. Tt x Tt What percent of the offspring have the dominant phenotype: Tt x tt? A. 25% B. 50% C. 75% D. All dominant E. All recessive 14
What is the phenotypic ratio of the following cross: AaTt x AaTt? A. 3:1 B. 1:1 C. 9:3:3:1 D. All dominant E. All recessive Let s Try This Another Way! Punnett squares are swell, but come on Fork line method uses principles of probability If two events occur simultaneously, and they are NOT independent, ADD their probabilities Probability of getting this OR that Probability I will have a boy OR a girl If two events occur simultaneously, and they are independent of each other, MULTIPLY their probabilities Probability of getting this AND that Probability child 1 is and boy and child 2 is a girl 15
A tall plant with purple flowers is heterozygous for both traits. It is crossed with a short plant with purple flowers that is heterozygous for flower color. Tall is dominant to short and purple is dominant to white. What is the probability of each genotype and phenotype in the F1 generation? 1. Define the problem: P purple T tall p white t short PpTt x Pptt A tall plant with purple flowers is heterozygous for both traits. It is crossed with a short plant with purple flowers that is heterozygous for flower color. Tall is dominant to short and purple is dominant to white. What is the probability of each genotype and phenotype in the F1 generation? 2. Possible gamete: PpTt x Pptt Pp X Pp Parent 1: ½ P and ½ p Parent 2: ½ P and ½ p ¼ PP:¼ Pp:¼ pp:¼ pp ¼ PP : ½ Pp : ¼ pp 16
A tall plant with purple flowers is heterozygous for both traits. It is crossed with a short plant with purple flowers that is heterozygous for flower color. Tall is dominant to short and purple is dominant to white. What is the probability of each genotype and phenotype in the F1 generation? 2. Possible gamete: PpTt x Pptt Tt x tt Parent 1: ½ T and ½ t Parent 2: 1 t ½ Tt : ½ tt Probability of 1 st trait: PpTt x Pptt Pp x Pp ¼ Purple (PP) ½ Purple (Pp) ¼ white (pp) Probability of 2nd trait: PpTt x Pptt Tt x tt ½ Tall (Tt) = ¼ x ½ =1/8 PurpleTall (PPTt) ½ Short (tt) = ¼ x ½ =1/8 PurpleShort (PPtt) ½ Tall (Tt) = ½ x ½ = ¼ Purple Tall (PpTt) ½ Short (tt) = ½ x ½ = ¼ Purple Short(Pptt) ½ Tall (Tt) = ¼ x ½ = 1/8 WhiteTall (pptt) ½ Short (tt) = ¼ x ½ = 1/8 WhiteShort (pptt) 17
Phenotypes are additive, because each genotype is mutually exclusive i.e. you can t be PPTt AND PpTt Therefore the are NOT independent events Purple Tall 1/8 PPTt + ¼ PpTt = 3/8 Purple Short 1/8 PPtt + ¼ PpTt = 3/8 ½ Tall (Tt) = ¼ x ½ =1/8 PurpleTall (PPTt) ½ Short (tt) = ¼ x ½ =1/8 PurpleShort (PPtt) ½ Tall (Tt) = ½ x ½ = ¼ Purple Tall (PpTt) ½ Short (tt) = ½ x ½ = ¼ Purple Short(Pptt) ½ Tall (Tt) = ¼ x ½ = 1/8 WhiteTall (pptt) ½ Short (tt) = ¼ x ½ = 1/8 WhiteShort (pptt) A tall plant with purple flowers is heterozygous for both traits. It is crossed with a short plant with purple flowers that is heterozygous for flower color. Tall is dominant to short and purple is dominant to white. What is the probability of breeding a TALL plant with PURPLE flowers? 1. Define the problem: P purple T tall p white t short PpTt x Pptt 18
Probability of 1 st trait: PpTt x Pptt Pp x Pp ¼ Purple (PP) Probability of 2nd trait: PpTt x Pptt Tt x tt ½ Tall (Tt) = ¼ x ½ =1/8 PurpleTall (PPTt) ½ Purple (Pp) ½ Tall (Tt) = ½ x ½ = ¼ Purple Tall (PpTt) 1/8 PPTt + ¼ PpTt = 3/8 Purple Tall Now You Try 1. Define the problem Define alleles Define parental genotypes 2. Possibe gametes 3. Fork it! In Border Collies, black coat color is dominant to red coat color and brown eyes are dominant to blue eyes. You cross a male dog that is heterozygous for both coat and eye color with a female that has a red coat and is homozygous dominant for brown eyes. What is the probability of all genotypes and all phenotypes of the offspring of this mating? 19
Two-Trait Testcross Fruit fly Drosophila melanogaster Two recessive mutations displayed by this fly are wing length and body color. Wild type (normal) flies have long wings (L) and brown bodies (G). Mutant flies can have short wings (l), ebony bodies (g), or both. Two-Trait Testcross Fruit flies that have long wings (L) and brown bodies (B) could be heterozygous or homozygous dominant for each trait. L B : one dominant allele, the other unknown. A two-trait testcross is used to determine genotype Unknown, dominant L B fly is crossed with a recessive fly (known genotype - llbb). 20
Two-Trait Testcross If the unknown fly is heterozygous for both traits (LlBb): Four possible gametes LB Lb lb Lb The homozygous recessive fly can only form gametes lb Offspring should be present in a 1:1:1:1 ratio gametes LB Lb lb lb lb LlBb Llbb llbb llbb Two-Trait Testcross If the unknown fly is homozygous dominant (LLBB), Only on possible gamete: LB. The homozygous recessive fly can only form gametes containing lb All offspring would have dominant phenotypes (genotype LlBb). 21
Pedigree Analysis Pedigree a chart of family history with regard to a particular genetic trait Symbols Males square Females circle Shaded affected Not shaded normal Line between circle and square union (marriage) Vertical line down children Autosomal Recessive Disorder Autosomes are any chromosome that is NOT a sex chromosome (X or Y) Recessive traits are passed on to children from both parents An individual who is affected may have parents who are not affected All the children of two affected individuals are affected In pedigrees involving rare traits, the unaffected parents of an affected individual may be related to each other. Incest 22
What is the genotype of individual #3 in generation II, assuming this is an autosomal recessive trait A. AA B. Aa C. Aa D. It is impossible to determine from the information Autosomal Dominant Disorder A dominant trait is passed on to a son or daughter from only one parent Every affected individual has at least one affected parent Affected individuals who mate with unaffected individuals have a 50% chance of transmitting the trait to each child Two affected individuals may have unaffected children. 23
What is the genotype of individual #2 in generation II, assuming this is an autosomal dominant trait A. AA B. Aa C. Aa D. It is impossible to determine from the information Determining Probability Determine the parents genotypes and follow Mendelian rules to determine the frequency of the mutant phenotype The pedigree traces an autosomal recessive trait. What are the genotypes of each individual? What are the probabilities associated with the possible genotypes of the unaffected son? 24
Determining Probability Determine the parents genotypes and follow Mendelian rules to determine the frequency of the mutant phenotype The pedigree traces an autosomal recessive trait. What are the genotypes of each individual? aa Determining Probability Determine the parents genotypes and follow Mendelian rules to determine the frequency of the mutant phenotype The pedigree traces an autosomal recessive trait. What are the genotypes of each individual? Aa Aa aa 25
Determining Probability Determine the parents genotypes and follow Mendelian rules to determine the frequency of the mutant phenotype The pedigree traces an autosomal recessive trait. What are the probabilities associated with the possible genotypes of the unaffected son? gametes A a A AA Aa Aa Aa a Aa aa 1/3 AA or 2/3 Aa 26