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Explore the fundamentals of genetics, including recombination, genetic terms, Punnett Squares, alleles, and independent assortment. Learn about genetic variation mechanisms and Mendel's model for inheritance patterns.
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Lecture 18 Genetics
Outline • Recombination – crossing over • Basic Genetic concepts • Genetic terms (Genotype, Phenotype, F1…) • Genetic Tools (Punnett Squares, Probabilities, Pedigrees)
Review • Alleles – different versions of the same gene • Maternal Allele – the version of the gene from your mother • Paternal Allele – the version of the gene from your father
Independent Assortment • Homologous pairs of chromosomes orient randomly at metaphase I of meiosis • Each pair of chromosomes sorts maternal and paternal homologs into daughter cells independently of every other pair
Independent Assortment • The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number • For humans (n = 23), there are more than 8 million (223) possible combinations of chromosomes
Figure 13.10-1 Possibility 2 Possibility 1 Two equally probablearrangements ofchromosomes atmetaphase I
Figure 13.10-2 Possibility 2 Possibility 1 Two equally probablearrangements ofchromosomes atmetaphase I Metaphase II
Figure 13.10-3 Possibility 2 Possibility 1 Two equally probablearrangements ofchromosomes atmetaphase I Metaphase II Daughtercells Combination 1 Combination 2 Combination 3 Combination 4 Followed by Random Fertilization
Crossing Over • Crossing over produces recombinant chromosomes, which combine DNA inherited from each parent • Crossing over begins very early in prophase I, as homologous chromosomes pair up gene by gene
Crossing Over • In crossing over, homologous portions of two nonsisterchromatids trade places • Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome
Figure 13.11-1 Prophase Iof meiosis Nonsister chromatidsheld togetherduring synapsis Pair of homologs
Figure 13.11-2 Prophase Iof meiosis Nonsister chromatidsheld togetherduring synapsis Pair of homologs Chiasma Centromere TEM
Figure 13.11-3 Prophase Iof meiosis Nonsister chromatidsheld togetherduring synapsis Pair of homologs Chiasma Centromere TEM Anaphase I
Figure 13.11-4 Prophase Iof meiosis Nonsister chromatidsheld togetherduring synapsis Pair of homologs Chiasma Centromere TEM Anaphase I Anaphase II
Figure 13.11-5 Prophase Iof meiosis Nonsister chromatidsheld togetherduring synapsis Pair of homologs Chiasma Centromere TEM Anaphase I Anaphase II Daughtercells Recombinant chromosomes
Summary of genetic variation • Three mechanisms contribute to genetic variation • Independent assortment of chromosomes • Crossing over • Random fertilization
Figure 13.7-3 2 1 Interphase Pair of homologouschromosomes indiploid parent cell Chromosomesduplicate Duplicated pairof homologouschromosomes Sisterchromatids Diploid cell withduplicatedchromosomes Meiosis I Homologouschromosomes separate Haploid cells withduplicated chromosomes Meiosis II Sister chromatidsseparate Haploid cells with unduplicated chromosomes
Figure 14.2 3 2 1 4 5 TECHNIQUE Parentalgeneration(P) Stamens Carpel RESULTS First filialgenerationoffspring(F1)
Figure 14.3-1 EXPERIMENT P Generation (true-breedingparents) Purpleflowers Whiteflowers
Figure 14.3-2 EXPERIMENT P Generation (true-breedingparents) Purpleflowers Whiteflowers F1 Generation(hybrids) All plants had purple flowers Self- or cross-pollination
Figure 14.3-3 EXPERIMENT P Generation (true-breedingparents) Purpleflowers Whiteflowers F1 Generation(hybrids) All plants had purple flowers Self- or cross-pollination F2 Generation 705 purple-floweredplants 224 whitefloweredplants
Terms • Trait/Phenotype/Genotype • Generations: Parental, F1, F2 • Self pollination vs Cross pollination • True breeding • Hybrid
Mendel’s Model • Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2 offspring • Four related concepts make up this model • We now know the molecular explanation for this model
1st Concept To Explain 3:1 Pattern in F2 generation • First: alternative versions of genes account for variations in inherited characters • One Gene: Purple flower – White Flower • These alternative versions of a gene are alleles • Each gene resides at a specific locus on a specific chromosome
Figure 14.4 Allele for purple flowers Pair ofhomologouschromosomes Locus for flower-color gene Allele for white flowers
2nd Concept To Explain 3:1 Pattern in F2 generation • Second: for each character (phenotype), an organism inherits two alleles, one from each parent • The two alleles at a particular locus may be identical, as in the true-breeding plants of Mendel’s P generation • Alternatively, the two alleles at a locus may differ, as in the F1 hybrids
3rd Concept To Explain 3:1 Pattern in F2 generation • Third: if the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance • In the flower-color example, the F1 plants had purple flowers because the allele for that trait is dominant
4th Concept To Explain 3:1 Pattern in F2 generation • Fourth: Thelaw of independent segregation: the two alleles for a heritable characteristic (phenotype) separate (segregate) during gamete formation and end up in different gametes • An egg or a sperm get only one of the two alleles • Allele segregation is because homologous chromosomes segregate during meiosis
TECHNIQUE Figure 14.7 Dominant phenotype,unknown genotype:PP or Pp? Recessive phenotype,known genotype:pp Predictions If purple-floweredparent is PP If purple-floweredparent is Pp or Sperm Sperm p p p p P P Pp Pp Pp Pp Eggs Eggs P p pp pp Pp Pp RESULTS or All offspring purple 1/2 offspring purple and1/2 offspring white
Rr Rr Figure 14.9 Segregation ofalleles into sperm Segregation ofalleles into eggs Sperm r 1/2 1/2 R R R r R R 1/2 1/4 1/4 Eggs r r r R r 1/2 1/4 1/4
Figure 14.8 EXPERIMENT YYRR yyrr P Generation Gametes yr YR F1 Generation YyRr Hypothesis ofdependent assortment Predictions Hypothesis ofindependent assortment Sperm or Predictedoffspring ofF2 generation 1/4 1/4 1/4 1/4 yR yr Yr YR Sperm 1/2 YR 1/2 yr 1/4 YR YYRR YYRr YyRR YyRr 1/2 YR YyRr YYRR 1/4 Yr Eggs YYRr YYrr Yyrr YyRr Eggs 1/2 yr YyRr yyrr 1/4 yR YyRr yyRr YyRR yyRR 3/4 1/4 yr 1/4 Phenotypic ratio 3:1 Yyrr yyRr YyRr yyrr 3/16 3/16 1/16 9/16 Phenotypic ratio 9:3:3:1 RESULTS 108 101 315 Phenotypic ratio approximately 9:3:3:1 32
Figure 14.UN02 1/4 (probability of pp) 1/2 (yy) 1/2 (Rr) 1/16 ppyyRr 1/16 ppYyrr 1/41/21/2 2/16 Ppyyrr 1/21/21/2 1/16 1/41/21/2 PPyyrr ppyyrr 1/16 1/41/21/2 6/16 or 3/8 Chance of at least two recessive traits
The ability to curl your tongue up on the sides (T, tongue rolling) is dominant to not being able to roll your tongue. A woman who can roll her tongue marries a man who cannot. Their first child has his father's phenotype. What are the genotypes of the mother, father, and child? • What is the probability that a second child won't be a tongue roller?
Often inheritance patterns are more complicated • Many heritable characters are not determined by only one gene with two alleles • Basic principles of segregation and independent assortment apply even to more complex patterns of inheritance
Examples of single gene not following Mendelian patterns • Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations: • When alleles are not completely dominant or recessive • When a gene has more than two alleles • When a gene produces multiple phenotypes
Degrees of Dominance • Complete dominance: phenotypes of the heterozygote and dominant homozygote are identical • Incomplete dominance, the phenotype of F1 hybrids is in between the phenotypes of the two parental varieties • Codominance, two dominant alleles affect the phenotype in separate, distinguishable ways
Figure 14.10-1 P Generation White Red CWCW CRCR Gametes CW CR
Figure 14.10-2 P Generation White Red CWCW CRCR Gametes CW CR F1 Generation Pink CRCW 1/2 1/2 CR Gametes CW
Figure 14.10-3 P Generation White Red CWCW CRCR Gametes CW CR F1 Generation Pink CRCW 1/2 1/2 CR CW Gametes Sperm F2 Generation 1/2 1/2 CW CR 1/2 CR CRCR CRCW Eggs 1/2 CW CRCW CWCW
Tay-Sachsdisease is fatal; a dysfunctional enzyme causes an accumulation of lipids in the brain • At the organismal level, the allele is recessive • At the biochemical level, the phenotype (i.e., the enzyme activity level) is incompletely dominant • At the molecular level, the alleles are codominant
Multiple Alleles • Most genes exist in populations in more than two allelic forms • The ABO blood group in humans are determined by three alleles • Single Gene codes for an enzyme that attaches a specific carbohydrate to the surface of the RBC • IA allele – The enzyme adds the A carbohydrate • IB allele – The enzyme adds the B carbohydrate • i allele – Adds neither
Figure 14.11 (a) The three alleles for the ABO blood groups and their carbohydrates Allele IA IB i none Carbohydrate B A (b) Blood group genotypes and phenotypes Genotype ii IAIA or IAi IBIB or IBi IAIB Red blood cellappearance Phenotype(blood group) A AB O B
Pleotrophy • Most genes have multiple phenotypic effects, a property called pleiotropy • Pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease • Some traits may be determined by two or more genes
Epistasis • In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus • Labrador retrievers and many other mammals, coat color depends on two genes • One gene determines the pigment color (with alleles B for black and b for brown) • The other gene (with alleles C for color and c for no color) determines whether the pigment will be deposited in the hair
Figure 14.12 BbEe BbEe Sperm 1/4 1/4 1/4 1/4 Be BE be bE Eggs 1/4 BE BbEE BBEe BbEe BBEE 1/4 bE BbEE bbEe bbEE BbEe 1/4 Be BBEe BBee Bbee BbEe 1/4 be BbEe bbEe bbee Bbee : 3 9 : 4
Polygenic Inheritance • Quantitative characters are those that vary in the population along a continuum • Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype • Skin color in humans is an example of polygenic inheritance