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 2 n, where n is the haploid number For humans (n = 23), there are more than 8 million (2 23 ) possible combinations of chromosomes

Figure Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I

Figure Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II

Figure Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Daughter cells Combination 1 Combination 2Combination 3Combination 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 nonsister chromatids trade places Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome

Figure Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs

Figure Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM

Figure Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM Anaphase I

Figure Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM Anaphase I Anaphase II

Figure Prophase I of meiosis Nonsister chromatids held together during synapsis Pair of homologs Chiasma Centromere TEM Anaphase I Anaphase II Daughter cells Recombinant chromosomes

Summary of genetic variation Three mechanisms contribute to genetic variation ◦Independent assortment of chromosomes ◦Crossing over ◦Random fertilization

Figure Pair of homologous chromosomes in diploid parent cell Duplicated pair of homologous chromosomes Chromosomes duplicate Sister chromatids Diploid cell with duplicated chromosomes Homologous chromosomes separate Haploid cells with duplicated chromosomes Sister chromatids separate Haploid cells with unduplicated chromosomes Interphase Meiosis I Meiosis II 21

Figure 14.2 Parental generation (P) Stamens Carpel First filial generation offspring (F 1 ) TECHNIQUE RESULTS

Figure P Generation EXPERIMENT (true-breeding parents) Purple flowers White flowers

Figure P Generation EXPERIMENT (true-breeding parents) F 1 Generation (hybrids) Purple flowers White flowers All plants had purple flowers Self- or cross-pollination

Figure P Generation EXPERIMENT (true-breeding parents) F 1 Generation (hybrids) F 2 Generation Purple flowers White flowers All plants had purple flowers Self- or cross-pollination 705 purple- flowered plants 224 white flowered plants

Table 14.1

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 F 2 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 Locus for flower-color gene Allele for white flowers Pair of homologous chromosomes

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 F 1 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 F 1 plants had purple flowers because the allele for that trait is dominant

4th Concept To Explain 3:1 Pattern in F2 generation Fourth: The law 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

Figure 14.7 Dominant phenotype, unknown genotype: PP or Pp ? Recessive phenotype, known genotype: pp Predictions If purple-flowered parent is PP If purple-flowered parent is Pp or Sperm Eggs or All offspring purple 1 / 2 offspring purple and 1 / 2 offspring white Pp pp p ppp P P P p TECHNIQUE RESULTS

Figure 14.9 Segregation of alleles into eggs Segregation of alleles into sperm Sperm Eggs 1/21/2 1/21/2 1/21/2 1/21/2 1/41/4 1/41/4 1/41/4 1/41/4 Rr R R R R R R r r r r r  r

Figure 14.8 P Generation F 1 Generation Predictions Gametes EXPERIMENT RESULTS YYRR yyrr yr YR YyRr Hypothesis of dependent assortment Hypothesis of independent assortment Predicted offspring of F 2 generation Sperm or Eggs Phenotypic ratio 3:1 Phenotypic ratio 9:3:3:1 Phenotypic ratio approximately 9:3:3: /21/2 1/21/2 1/21/2 1/21/2 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 9 / 16 3 / 16 1 / 16 YR yr 1/41/4 3/43/4 Yr yR YYRR YyRr yyrr YYRRYYRrYyRR YyRr YYRrYYrr YyRr Yyrr YyRR YyRr yyRR yyRr YyRr YyrryyRr yyrr

Figure 14.UN02 Chance of at least two recessive traits ppyyRr ppYyrr Ppyyrr PPyyrr ppyyrr 1 / 4 (probability of pp)  1 / 2 (yy)  1 / 2 (Rr) 1/4  1/2  1/21/4  1/2  1/2 1/2  1/2  1/21/2  1/2  1/2 1/4  1/2  1/21/4  1/2  1/2 1/4  1/2  1/21/4  1/2  1/2  1 / 16  2 / 16  1 / 16  6 / 16 or 3 / 8

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?dominantphenotypegenotypes 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 F 1 hybrids is in between the phenotypes of the two parental varieties Codominance, two dominant alleles affect the phenotype in separate, distinguishable ways

Figure P Generation Red White Gametes CWCWCWCW CRCRCRCR CRCR CWCW

Figure P Generation F 1 Generation 1/21/2 1/21/2 Red White Gametes Pink Gametes CWCWCWCW CRCRCRCR CRCR CWCW CRCWCRCW CRCR CWCW

Figure P Generation F 1 Generation F 2 Generation 1/21/2 1/21/2 1/21/2 1/21/2 1/21/2 1/21/2 Red White Gametes Pink Gametes Sperm Eggs CWCWCWCW CRCRCRCR CRCR CWCW CRCWCRCW CRCR CWCW CWCW CRCR CRCR CWCW CRCRCRCR CRCWCRCW CRCWCRCW CWCWCWCW

Tay-Sachs disease 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 ◦I A allele – The enzyme adds the A carbohydrate ◦I B allele – The enzyme adds the B carbohydrate ◦i allele – Adds neither

Figure Carbohydrate Allele (a) The three alleles for the ABO blood groups and their carbohydrates (b) Blood group genotypes and phenotypes Genotype Red blood cell appearance Phenotype (blood group) A A B B AB none O IAIA IBIB i ii IAIBIAIB I A I A or I A i I B I B or I B i

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 Sperm Eggs 9 : 3 : 4 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 1/41/4 BbEe BE bE Be be BBEE BbEE BBEeBbEe BbEE bbEEBbEe bbEe BBEe BbEe BBeeBbee BbEebbEe Bbee bbee

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

Nature vs. Nurture