2Gregor Mendel Generally considered the ‘Father of Modern Genetics’ Worked with pea plants, keeping careful records of his experimentsUsed statistical analysis to establish several important genetic principles
3Mendelian Genetics Things he knew before he started: How to control pea plant reproductionThat he had true-breeding plant strainsThat hybrids between these strains did NOT breed true
4Mendelian Genetics… Things he didn’t know: What DNA or chromosomes wereAnything about mitosis or meiosisThat traits in hybrids did not always ‘blend’ as was the idea of the time
5Mendelian Genetics… What he figured out: Blending of two distinct traits in the parents did not always occur in the hybrid offspringEach kind of inherited feature in an organism is controlled by 2 factors that behave like distinct particlesThat some of these factors can mask others (that is, some are dominant while others are recessive)The Principle of Segregation and The Principle of Independent Assortment
6The Principle of Segregation Alleles – alternate gene forms – are located on corresponding loci on homologous chromosomesDuring gamete formation in meiosis, homologous chromosomes separate (when?)During sexual reproduction, offspring receive one of these homologous chromosomes from each parent
8Monohybrid cross practice 1.) A TT (tall) pea plant is crossed with a tt (short) pea plant
9Monohybrid cross practice… 2.) A Tt pea plant is crossed with a Tt pea plant.
10Monohybrid cross practice… 3.) A heterozygous round seeded pea plant (Rr) is crossed with a homozygous round seeded pea plant (RR).
11Monohybrid cross practice… 4.) A homozygous round seeded pea plant is crossed with a homozygous wrinkled pea seeded plant.
12Monohybrid cross practice… 5.) In pea plants purple flowers are dominant to white flowers. Cross two white flowered plants.
13Monohybrid cross practice… 6.) A white flowered pea plant is crossed with a pea plant that is heterozygous for the trait.
14Monohybrid cross practice… 7.) Two pea plants, both heterozygous for the gene that controls flower color, are crossed.
15Monohybrid cross practice… 8.) In guinea pigs, short hair is dominant over long hair. Show the cross for a pure breeding short haired guinea pig and a long haired guinea pig.
16Monohybrid cross practice… 9.) Show the cross for two heterozygous guinea pigs. What percentage of the offspring will have short hair? ________ What percentage of the offspring will have long hair? _______
17Monohybrid cross practice… 10.) Two short haired guinea pigs are mated several times. Out of 100 offspring, 25 of them have long hair. What are the probable genotypes of the parents? Show the cross to prove it!
18Dihybrid cross practice IN PEAS:R = round T = tall Y = yellow peas P = purple flowersr = wrinkled t = short y = green peas p = white flowers1.) Homozygous tall, round parent X pure short, wrinkled parent
19Dihybrid cross practice… IN PEAS:R = round T = tall Y = yellow peas P = purple flowersr = wrinkled t = short y = green peas p = white flowers2.) Heterozygous for both height and flower color parent X short, white flowers parent
20Dihybrid cross practice… IN PEAS:R = round T = tall Y = yellow peas P = purple flowersr = wrinkled t = short y = green peas p = white flowers3.) Green peas, short plant X Heterozygous for yellow peas, homozygous for tall parent
21Dihybrid cross practice… IN PEAS:R = round T = tall Y = yellow peas P = purple flowersr = wrinkled t = short y = green peas p = white flowers4.) Heterozygous round, green peas X wrinkled peas, Heterozygous yellow peas
22Dihybrid cross practice… IN PEAS:R = round T = tall Y = yellow peas P = purple flowersr = wrinkled t = short y = green peas p = white flowers5.) Both parents heterozygous for height and flower color
23The Principle of Independent Assortment Another of Mendel’s ideasExplains the results of these types of dihybrid crossesEach different trait is inherited independently from the otherNow we know that this is due to meiosis – homologous chromosomes separate independently (again, when?)
24Probability Rules The product rule: Predicts the combined probability of 2 independent eventsIf two or more events are independent of each other, the probability of both occurring is the product of their individual probabilitiesExample: coin toss – heads two times in a row½ X ½ = ¼ or one chance in 4Also: Bb X Bb parents – producing a bb child½ b X ½ b = ¼ or one chance in 4
25Probability Rules… The sum rule Predicts the combined probabilities of mutually exclusive eventsIf there is more than one way to get a result, we combine the probabilities by summingExample: Bb X Bb parents chance of Bb child2 possibilities: B egg + b sperm; b egg + B spermB egg (½) X b sperm (½) = ¼And b egg (½) X B sperm (½) = ¼Then … ¼ + ¼ = ½
26Probability Rules… Most important – Chance has no memory! If events are truly independent, past events have no influence on the probability of future events…Even though we don’t like this idea – ‘my luck is bound to change…’
27Things Mendel didn’t know Linked genes - inherited together because they are located on the same chromosomeLinked genes do not undergo segregation or independent assortmentThe rates of crossing over can be used to determine the relative positions of genes on a chromosomeHigher crossing over rates indicate greater separation of genes on a chromosomeEach %age of crossing over rate = one map unit
28Things Mendel didn’t know… Sex chromosomes:Female = XX - Male = XYMale produces the sex determining gameteIn humans, Y chromosome has the SRY gene (sex reversal gene on the Y) – this acts as a genetic switch to cause testes to developDeveloping testes produce testosterone which determines other sexual characteristicsEveryone has at least one X – female is the ‘default’ sex… need a Y to develop as a male
29Things Mendel didn’t know… Sex-linked genes:Located on the X chromosome onlyInclude genes for color perception and blood clotting – things all humans needFemales get two copies – can be either homozygous or heterozygousMales only get one copy – they are hemizygousDefects in these traits arise more in males than in females
31Things Mendel didn’t know… Dosage compensation:Makes equivalent the female’s 2 ‘doses’ of the genes on the X chromosome to the male’s 1One X chromosome in each female cell is inactivated – called a Barr bodyIndividuals with heterozygous X-linked genes will often have a variegated phenotype as random X chromosomes are inactivated in the body
32Calico cats – an example of X chromosome inactivation in action
33Things Mendel didn’t know… Incomplete dominanceThe phenotype of the heterozygous individual is a blending of the two genes
34Things Mendel didn’t know… Codominance:The phenotype of the heterozygous individual expresses both genes, but without blendingABO Blood Types:A and B alleles are codominant to each otherBoth are dominant to type O allele
35Things Mendel didn’t know… Multiple alleles:Three or more alleles exist in the population, even though each individual only has twoExamples:ABO blood types in humansCoat color in rabbits
36Things Mendel didn’t know… PleiotrophyOne gene with many effectsOften found in genetic diseasesExample:Cystic fibrosis in humansHomozygous individuals produce abnormally thick mucus in many body systems
37Things Mendel didn’t know… EpistasisThe presence of one allele can prevent or mask the expression of a gene at another lociExample:Coat color in Labrador retrieversPigment gene is either B (black) or b (brown)Recessive ee blocks the expression of eitherBlack Brown YellowBBEE bbEE BBeeBbEE bbEe BbeeBBEe bbeeBbEe
38Things Mendel didn’t know… Polygenic inheritanceMulitple independent pairs of genes have similar and additive effects on the phenotypeThe phenotypes in a population will generally show a normal distribution curveExamples:Human skin and eye color
39Things Mendel didn’t know… Environmental interactionGenetically identical individuals show different phenotypes based on environmental factorsExample:Human height and intelligenceNature versus nurture questionsProblems with experimental methods to answer these questions in humans….