Monohybrid Cross for Flower Color Parentals X Purple White 100% Purple F 1 Generation Purple and White Flowered Plants F 2 Generation X 705 purple and 224 white 3:1 Ratio 1.Parents differ for one trait 2.Parents are true breeding 3.Analyze parents for two generations, F1 and F2 4.Examine large number of progeny
Mendel’s Postulates (based on monohybrid crosses) Alternative versions of genes (alleles) account for variation in characteristics Each individual has two alleles for each gene, one inherited from each parent If the individual has two different alleles, one allele is dominant to the other which is recessive The two alleles randomly segregate during gamete formation with 50% of the gametes having one allele and 50% having the other allele.
Important Terminology Genotype – the list of alleles an individual possesses. Phenotype – a measurable characteristic displayed by an individual Homozygote – an individual possessing two identical alleles for a gene Heterozygote – an individual possessing two different alleles for a gene
Fig Allele for purple flowers Homologous pair of chromosomes Locus for flower-color gene Allele for white flowers
Independent Assortment vs Linkage P1 F1F1 Genotype of F 1 Gametes YRYryRyr YRyr Parental Combination of Alleles Parental Only 50% Parental 50% Recombinant
Law of Independent Assortment: Each pair of alleles segregates independently of other pairs of alleles during gamete formation Linked Independent Assort YRYryRyrYRyr
Dihybrid Testcross to Detect Independent Assort YYRR X yyrr YrRr X yyrr YRYryRyr yyRf Yyrr yyrr YyRr Eggs Sperm yr YR yr Yr yR Phenotypic ratio 1:1:1:1 Ratio of parental:Recombinant 1:1 Dihybrid
Independent Assortment and Meiosis
What is Mendel’s Law of independent assortment? A gene is distributed into gametes independently of how other genes are distributed. Take home message:
Variations on Mendel Allelic Interactions Sex Linkage Complete/Partial Linkage Organellular Genes Mendel focused on traits based clearly on dominant and recessive alleles. However, the expression pattern of genes for some traits are not so straight forward.
Complete Dominance – the heterozygote is indistinguishable from one of the two homozygotes. Rh factor – a protein found on the surface of the blood cells in some people. Two Alleles – Rh + - possess the Rh factor – Rh - - lacks the Rh factor Heterozygote Rh +,Rh - - possess the Rh factor and has positive type blood.
Partial Dominance Hypercholesterolemia – defect in LDL receptor mediated endocytosis. Two alleles Hc+ - functional LDL receptor Hc- defective LDL receptor Homozygotes Hc-,Hc- super high serum cholesterol early onset of heart disease. Hc+,Hc+ normal serum cholesterol- typical risk of heart disease. Hc+,Hc- high serum cholesterol – high risks of heart disease.
Multiple Alleles – More than two alleles in the population, although any individual still has only two alleles. ABO Blood type – carbohydrate attached to surface protein of blood cells. Three Alleles – I A – A type blood – I B – B type blood – I O – O type blood Six genotypes HomozygotesHeterozygotes I A,I A I A,I B I B,I B I A,I O I O,I O I B,I O A type B type AB type O type Codominance – Heterozygote expresses both alleles (with no blending as in partial dominance) Two non-identical alleles of a gene are both expressed in heterozygotes, so neither is dominant or recessive. Co-dominance may occur in multiple allele systems.
Pleiotropy – one gene may influence multiple traits. Cystic Fibrosis Two Alleles CF – healthy allele –functional Cl transporter (dominant) cf – cystic fibrosis allele – non functional Cl transporter (recessive) Homozygotes for cf exhibit several symptoms - mucus filled lungs - kidney failure - digestive problems - salty sweat
Genes encoding products used throughout the body are the ones most likely to be pleiotropic. Mutations in the fibrillin gene cause a disorder called Marfan syndrome. Fibrillin (long fibers) impart elasticity to the heart skin, blood vessels, tendons and other parts of the body. In Marfan syndrome the fibrillin are defective or not present. The aorta is particularly affected, can rupture during exercise. Hard to diagnose, affected people are typically, tall, thin, loose jointed. 1 in 5000 Sergei Rachmaninoff
Marfan Syndrome. Become Aware Haris Charalambous, a 6’10’’ Centre, died during a light training session at Toledo University on 9 October He was 21 years of age. He began his basketball career at Manchester Magic at age 13 and left the club at 18 to take up his scholarship at Toledo. He was later diagnosed with Marfan Syndrome, which is a genetic disease associated with abnormally tall people. At the time, neither the club nor Toledo knew of this situation Syndrome are a tall thin physique, disproportionately long limbs, fingers and toes, flat feet, spinal curvature and abnormally shaped narrow chest.
Multigenic Traits – traits influenced by more than one gene. Epistasis – Alleles at one gene mask the expression of alleles at a second gene. Example – black/ brown gene of mice- albino gene B – black is dominantC – allows pigment (dominant) b – brown is recessivec – prevents pigment (recessive)
Fig BbCc Sperm Eggs BCbC Bcbc BC bC Bc bc BBCC 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 BbCC BBCc BbCc BbCC bbCC BbCc bbCc BBCcBbCc bbCc BBccBbcc bbcc 9: 3 : 4
Polygenic Inheritance – many genes influencing a single trait Results in a continuum of variation (height in humans, skin pigmentation in humans, Fat content of milk cows, yield of kernels in corn) # dark alleles /64 6/64 15/64 30/64 15/64 6/64 1/64
Multifactoral traits – traits influenced by both the genetic and environment. Examples Flower color in hydrangea Skin pigmentation in humans Milk yield in cows
Fig P Generation F1F1 F2F2 All offspring had red eyes Sperm Eggs F1F1 F2F2 P Sperm Eggs X X X Y CONCLUSION EXPERIMENT RESULTS w w w w w w w w w w ww w w w w w
Reciprocal Crosses Fruit flies use XY sex chromosomes X-linked white eye gene R – normal red eyes (dominant) r - white eyes (recessive) White eye X Red eyeRed eye X White eye ♂ ♀ ♂ ♀ Assume flies came from true breeding cultures
White eye X Red eyeRed eye X White eye ♂ ♀ ♂ ♀ rY RRRYrr RrRY RrRY Sperm Eggs r Y RRRR 50% Red female 50% Red male RrrY RrrY Sperm Eggs R Y rrrr 50% Red female 50% White male