BIO 100_CH. 10. Mendelian Genetics

BIO 100_CH. 10. Mendelian Genetics Dominance Monohybrid Cross Dihybrid Cross Incomplete Dominance Codominance Quantitative Genetics Additive Inheritance Importance of Environment

Terms: Mendelian Genetics, blending hypothesis vs. particulate hypothesis, pure lines, qualitative traits, law of segregation, law of independent assortment, allele, dominance, dominant, recessive, homozygous, heterozygous, genotype, phenotype, incomplete dominance, codominance, monohybrid cross, Punnet square, dihybrid cross, additive inheritance, polygenic inheritance, quantitative trait, environment, PKU, Huntington’s disease-

Questions: 1. How many kinds of gametes are possible with each of the following genotypes: a. Aa b. AaBB c. AaBb 2. What is the probability of getting the gamete ab from each of the following genotypes: a. aabb b. Aabb c. AaBb 3. If an offspring has the genotype Aa, what possible combinations of parental genotypes can exist? List them all. 4. In certain pea plants, the allele T for tallness is dominant over t for shortness. If a homozygous tall and homozygous short plant are crossed, what will be the phenotypic and genotypic ratios of the offspring? Use a Punnet’s square to aid you. 5. In certain pea plants, the allele T for tallness is dominant over t for shortness. If both individuals are heterozygous, what will be the phenotypic and genotypic ratios? Use a Punnet’s square. 6. Discuss the importance of genotype and environment in the expression of a phenotype. Do 1 - 5 as homework for second class of this lecture. Show your work.

Competing Hypotheses of Heredity (Pre-Mendel): Blending Hypothesis: Theory suggests that genetic material from parents is blended in offspring (Mixing of two paints as an analogy). Particulate Hypothesis: Parents pass on discrete particulate heritable units that retain their separate identities. Gregor Mendel: Provided evidence for the particulate hypothesis. Mendel formulated some of the basic laws of genetics.

Mendelian genetic analysis: (1) traits affected by single gene of major effect (2) present only in one of two alleles: dominant or recessive. Alleles: alternative forms of a gene Dominant: an allele that expresses itself and masks the effects of the allele(s) for the trait. recessive: an allele that does not express itself in the phenotype when it is paired with a dominant allele.

Mendel worked with Garden Pea: 1. Plants can self-pollinate, so can make pure lines (homogeneous) for a given trait. 2. Many traits of peas are qualitative traits—expressed as either one phenotype or a second phenotype. 3. Can make crosses between pure lines.

Genotype refers to an individual’s genes Phenotype refers to the expression of the trait--an individual’s appearance. homozygous dominant: having two dominant alleles:AA homozygous recessive: having two recessive alleles: aa heterozygous: having one dominant and one recessive allele, Aa

Law of Segregation: each organism contains two alleles for each trait (diploid), and the alleles segregate during the formation of gametes. Each gamete contains only one allele. Traits retain their individuality—not blended. Mendel: reproduction between heterozygous individuals (Aa) resulted in dominant and recessive phenotypes, even though both parents’ phenotypes expressed the dominant phenotype.

Monohybrid Cross: genetic cross that tracks the inheritance of a single character P generation: Parental Generation. F1 generation: First filial generation (i.e. first generation after parents). F2 generation: Generation produced from self- pollination of F1 generation.

Punnet square: 1. Assign a symbol for each allele (will depend which allele is dominant. 2. Determine the genotype of each parent. 3. Determine the two possible kinds of gametes each parent can make. 4. Determine the gene combinations. 5. Determine the phenotypes of each potential offspring. 6. Calculate the genotypic and phenotypic probabilities.

Note: We could have either Parent 1 (female - GG) parent be the dominant and recessive parent, but the green pod is dominant to the yellow G G (Law of segregation) pod. G = green (dominant) G = yellow (recessive) g Parent 1 (male - gg) g Imaging crossing two pure lines - one that makes green pea pods and one that makes yellow pea pods. What will the genotypes of the offspring be? What will the phenotypes be?

Parent (female - GG) G G Parent (male - gg) g Gg Gg g Gg Gg Genotypes: 4 Gg Phenotypes: 100% green pea pods.

Punnet square: Calculate the genotypic and phenotypic probabilities.

G g G g Imaging crossing the two heterozygotes - both make green pea pods, but both have alleles for yellow pea pods. What will the genotypes of the offspring be? What will the phenotypes be?

G g G GG Gg g Gg gg Genotypes: 1 GG, 2 Gg, 1 gg - 1:2:1 Phenotypes: 75% green pea pods; 25% yellow pea pods. - 3:1

G g g g Imaging backcrossing the a heterozygote with the recessive pure line. What will the genotypes of the offspring be? What will the phenotypes be?

Parent 1 (female - Gg) G g g Parent 1 (male - gg) Gg gg g Gg gg Genotypes: 2 Gg, 2 gg, 1:1 Phenotypes: 50% green pea pods; 50% yellow pea pods. - 1:1

Note: Purple flowers are Parent (female) dominant to white flowers. Parent (male) Imaging crossing the two heterozygotes - but for flower color. What will the genotypes of the offspring be? What will the phenotypes be?

Parent (female - Pp) P p Parent (male - Pp) P PP Pp p Pp pp Genotypes: 1 PP, 2 Pp, 1 pp - 1:2:1 Phenotypes: 75% purple flowers; 25% white flowers. - 3:1

Dihybrid Cross: genetic cross that tracks the inheritance of two characters simultaneously. Imaging crossing two heterozygotes that produce green pea pods and purple flowers. What is the genotype of the heterozygote? What will the genotypes of the offspring be? What will the phenotypes be? At what ratios? Law of Independent Assortment: members of an allelic pair segregate independently from members of another allelic pair. Heterozygote: GgPp Gametes can be:GP, Gp, gP, gp.

Punnet Square: GP Gp gP gp GPGP or GP GGPP Gp GpgP or gP GgPp gp

GP Gp gP gp GGPP GGPp GgPP GgPp GP GGPp GGpp GgPp Ggpp Gp GgPP GgPp ggPP ggPp gP GgPp Ggpp ggPp ggpp gp Phenotypes: 3:1 (12:4) and 3:1 (12:4) for each trait; Combined: 9 green/purple; 3 green/white; 3 yellow/purple; 1 yellow/white

Alternative forms of inheritance: Incomplete dominance: when heterozygous condition has intermediate phenotype to the homozygous conditions. (Why is this not evidence of blending?)

Snapdragons: Incomplete Dominance FR FW FRFR FRFW FR RFW F WFW FW F Genotypes: 1 FRFR, 2 FRFW, 1 FWFW - 1:2:1 Phenotypes: 25% red flowers; 50% pink flowers; 25% white flowers. - 1:2:1

Alternative forms of inheritance: Codominance: both alleles are expressed by the heterozygote. Blood groups - three different alleles. IA: codes for enzyme that puts A carbohydrate on RBC’s cell membrane. IB: codes for enzyme that puts B carbohydrate on RBC’s cell membrane. i: codes for neither carbohydrate (recessive to IA and IB). also, Multiple alleles - three different alleles.

Alternative forms of inheritance: Polygenic Inheritance: when a number of different pairs of alleles at several loci are important for expression of a trait. Such traits are typically quantitative in nature, not qualitative as seen in dominance/recessive relationships.

At least three different genes at three loci (locations on chromosomes) affect skin color. Each gene may have two alleles that affect melanin expression, with one allele (for melanin) dominant to the other allele.

Two types of Genetics: Mendelian Genetics: qualitative traits; single genes of major effect. Quantitative Genetics: quantitative traits; polygenic inheritance. Student examples of each type of trait.

Environment: can have a dramatic effect phenotype. Pigmentation (freckles or skin color) can increase in the presence of sunlight. (Some medications—for malaria—prevent tanning, so chemical environment can also affect expression.). For quantitative genetic traits: Phenotype = Genotype + Environment

Human diseases and disorders: recessive PKU: Phenylketonuria; Maternal PKU; Treatment of PKU

Human diseases and disorders: dominant Huntington’s disease: Progressive disorder involving degeneration of nerve cells in the brain. It is inherited as a single faulty gene on chromosome #4. Symptoms do not usually appear until adulthood, typically between ages 35 and 50 years old. Loss of mental function and loss of cognitive functions.

THE END.

BIO 100_CH. 10. Mendelian Genetics

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