HCS2 Ms. Levensailor

 1857 Mendel began breeding garden peas to study inheritance. Available in many varieties. Used the term character for heritable features that vary among individuals.  Example: flower color Each variant for a character is called a trait.  Example: purple or white

 Mendel had strict control over which plants mated: Sex organs of a pea plant are in the flower. Each flower has both male and female organs.  Stamens and carpels  Pollen from the stamens land on the carpel and fertilizes the ova in the carpel.

 Cross-pollination:

 Mendel started his experiments with only true breeding plants: When a plant self pollinates all their offspring are of the same variety. Example: A plant with purple flowers will only give rise to plants with purple flowers.

 Cross pollinate two contrasting, true- breeding pea varieties. Example: purple flowered X white flowered This is called hybridization.  True breeding parents are referred to as P generation.  Hybrid offspring: F 1 generation  F 1 hybrids self pollinate and produce F 2 generation.

Ratio 3:1

 Eliminates blended model of inheritance: F 1 hybrid does not produce intermediate Purple X White ≠ pale purple flowers  F 1 hybrids produce all purple flowers. What happened to the white flowers genetic contribution? Was it lost? If so F 2 would only produce purple flowers. However, white flowers reappeared in F 2 generation.

 Mendel used large sample size and kept accurate records. Results: F 2 plants 705 purple & 224 white. Ratio of 3 purple to 1 white.  Mendel determined purple was dominant to white (recessive) flower color in these hybrids.  Saw the same pattern in 7 different characters.

 All F 2 generations 3:1 ratio

 Different alleles account for variations in inherited characters. The gene for flower color exists in 2 forms: purple and white. Alternative versions of a gene are called alleles. Each gene resides at a specific locus on a specific chromosome.

 Recall that diploid organisms have homologous pairs of chromosomes. Once chromosome of each pair inherited from each parent. A genetic locus is represented twice in a diploid cell. Homologous loci may have identical or different alleles.

 One is the dominant allele Fully expressed in the organism’s appearance  One is recessive No noticeable effect on the organism’s appearance

 2 alleles for each character segregate during gamete production. An ovum and a sperm each get only 1 of the 2 alleles that are present in the somatic cells of the organism. Law of segregation: separation of alleles into separate gametes.

 Illustrates the predicted results of a genetic cross between individuals of known genotype. Capital letter symbolizes dominant allele. Lowercase letter represents recessive allele.  What percent have the genotype: PP? Pp? pp?

 Homozygous: having identical alleles for a gene. PP or pp  Heterozygous: having 2 different alleles for a gene. Pp  Phenotype: organism’s traits  Genotype: genetic makeup

 If we have a pea plant that has purple flowers we cannot tell the genotype. Could be PP or Pp  To determine genotype we can cross with a white flower pea plant. Can only be pp  Appearance of the offspring will reveal the genotype of the purple parent.

 All purple offspring = homozygous dominant parent (PP)  If both white and purple offspring appear = heterozygous parent (Pp)  Testcross = breeding of a recessive homozygote with an organism of dominant phenotype but unknown gentotype.

 Monohybrids = looks at a single character (i.e. flower color).  What if we follow 2 characters at the same time? (i.e. seed color and shape) Cross Yellow Round with a Green Wrinkled  F 1 will be Dihybrids = heterozygous for both characters.  Are these characters transmitted as a package? Or inherited independently?

 Each character is independently inherited.  Results in a 9:3:3:1 phenotypic ratio in the F 2 generation.  The 2 characters considered separately have a 3:1 phenotypic ratio. Same as a monohybrid cross  Called law of independent assortment.

 Probability scale range is 0 to 1. 0 certain to not occur. 1 certain to occur. Example tossing a coin: chance of tossing heads is ½ and the chance of tossing tails is ½.  The outcome of any particular toss is unaffected by what has happened on previous trials.  Independent events!

 What is the chance that 2 coins tossed simultaneously will land heads up?  1. Compute the probability for each independent event.  2. Multiply these individual probabilities to obtain the overall probability of these events occurring together. ½ X ½ = ¼

 Given F 1 plant genotype is Pp, what is the probability that a particular F 2 plant will have white flowers (pp)?  F 2 is the result of Pp X Pp Both ovum and sperm must carry the p allele What is the probability of a ovum having the p allele? (1/2) What is the probability of a sperm having the p allele? (1/2)  The probability that two p alleles will come together at fertilization is ½ X ½ = ¼

 Applies to dihybrid crosses: Parent YyRr Possible gametes: YR, Yr, yR, yr Probability that gamete will carry the Y and R alleles is ¼. Probability F 2 generation will have genotype YYRR is... YR Ovum (1/4) X YR Sperm (1/4) = 1/16

 What is the probability that an F 2 plant from a monohybrid will be heterozygous? The dominant allele can come from the ovum and the recessive from the sperm, or vice versa.  Probability that an event can occur in 2 or more ways is the sum of the separate probabilities. F 2 heterozygote is ¼ + ¼ = ½

 PpYyRr X Ppyyrr  What fraction of offspring will exhibit the recessive phenotypes for at least 2 of the 3 traits. 1. List all genotypes that fulfill this condition.  ppyyRr, ppYyrr, Ppyyrr, PPyyrr, and ppyyrr 2. Use the rule of multiplication to calculate the probability for each of these genotypes. 3. Use the rule of addition to pool the probabilities for fulfilling the condition.

ppyyRr1/4 (probability of pp) X 1/2 (yy) X 1/2 (Rr) = 1/16 ppYyrr1/4 X 1/2 X 1/2 = 1/16 Ppyyrr1/2 X 1/2 X 1/2 =2/16 PPyyrr1/4 X 1/2 X 1/2 = 1/16 ppyyrr1/4 X 1/2 X 1/2 = 1/16 Chance of at least 2 recessive traits =6/16 or 3/8

 Lucky Mendel used pea plants because they follow simple genetic patterns: One gene per characteristic Only 2 alleles for those characteristics One is dominant to the other  Most hereditary patterns are much more complex.

1. Ranges from complete dominance, incomplete dominance, to codominance. 2. Reflects the mechanisms by which specific alleles are expressed in phenotype. Do not involve the ability of one allele to subdue another. 3. They do not determine or correlate with the relative abundance of alleles in a population.

 F 1 hybrids have an appearance somewhere in between the phenotypes.  Pink results from heterozygotes having less red pigment.

 NOT BLENDED.  Red & White traits can be retrieved.  Seen in F 2 offspring Ration 1:2:1

 Brain cells of a baby are unable to metabolize gangliosides (a type of lipid) because a enzyme does not work properly.  Only children who inherit 2 copies of the Tay-sachs allele have the disease.

 Incomplete dominance: Heterozygotes have intermediate enzyme function. Lack symptoms because amount of enzyme is sufficient to prevent lipid accumulation.

 Complete dominance: phenotypes of heterozygotes and dominant homozygote are indistinguishable.  Codominance: 2 alleles affect the phenotype in separate, distinguishable ways. Example: human blood typing. A, B, AB AB is characterized by both molecules being present.

 Does not mean it is more common in a population.  1 in 400 babies in U.S. are born with extra fingers or toes.  This is the dominant allele.  399/400 people are homozygous recessive.

 Most genes exist in more than 2 allelic forms.  ABO blood groups 4 possible phenotype Note AB is an example of codominance.

 Ability of a gene to affect an organism in many ways. One gene = impact on multiple characteristics.  A gene on one locus alters the phenotypic expression of a gene at a second locus.

 Characters vary along a continuum: Skin color Height  Called quantitative characters.  Result of an additive effect of 2 or more genes on a single phenotypic character.

 Phenotype also depends on environment: Nutrition, exercise... Twins accumulate phenotypic differences as a result of their unique experiences. Norm of reaction = phenotypic range  Called multifactorial: many factors both genetic and environmental collectively influence phenotype.

 Chromosomes theory of inheritance: Menedlian genes have specific loci on chromosomes and it is the chromosomes that undergo segregation and independent assortment.

 Genes located on sex chromosomes are called sex-linked genes.  Important vocabulary: Wild type: phenotype most common in natural populations. Mutant: alleles assumed to have originated as changes or mutations.

 Fruit Fly: Wild type- Red eyes Mutant- White eyes