Unit 3: Genetics The Cell Cycle + DNA structure/function Mitosis and Meiosis Mendelian Genetics (aka - fun with Punnett squares) DNA replication

MITOSISMEIOSIS preceded by replication of chromosomes? yes # of rounds of cell division12 # of daughter cells24 # of chromosomes in daughter cells compared to parent cell same as parent cellhalf of parent cell daughter cells genetically identical to parent cell? yesno sister cells thus produced identical to one another? yesno happens in diploid cells, haploid cells, both, or neither? both (depending on organism) diploid crossing over (synapsis)?noyes Yesterday’s Exit Ticket

Today’s Agenda Where does variation come from? Mendelian Genetics, Part One

Mutations (changes in an organism’s DNA) are the original source of all genetic variation Mutations create different versions of genes called alleles Sources of genetic variation

Clarity check: homologous chromosomes SAME gene, different ALLELES Gene for hair color; Allele for blonde hair Gene for hair color; Allele for blonde hair Gene for hair color; allele for brown hair Gene for hair color; allele for brown hair

The behavior of chromosomes during meiosis and fertilization reshuffles alleles and chromosomes every generation Three mechanisms contribute to genetic variation: a) Independent assortment of chromosomes b) Crossing over c) Random fertilization Sources of genetic variation

Fig. 13-8b Metaphase I of meiosis I a) Independent assortment Sources of genetic variation Homologous pairs of chromosomes orient randomly during Meiosis I  maternal and paternal homologs assort into daughter cells independently of the other pairs Blue can be on top or bottom

Fig Possibility 1 Possibility 2 with n = 2 there are 4 possibilities for the lineup during Meiosis II  4 possible assortments of chromosomes in the gametes a) Independent assortment Sources of genetic variation

Fig Possibility 1 Possibility 2 Metaphase II Daughter cells Combination 1Combination 2Combination 3Combination 4 a) Independent assortment Sources of genetic variation

“2 n rule”: the number of possible chromosome sorting combinations = 2 n  For humans (n = 23), there are 2 23 = 8,388,608 possible combinations of chromosomes based on independent assortment alone! a) Independent assortment Sources of genetic variation

homologous chromosomes pair up gene by gene and exchange homologous segments This combines alleles that originated from two (grand)parents into a single chromosome b) Crossing over (Prophase of Meiosis I) Sources of genetic variation blond hair from G’pa blue eyes from G’pa Mom’s ovary cell red hair from G’ma brown eyes from G’ma red hair from G’ma blue eyes from G’pa red hair from G’pa brown eyes from G’ma

Pair of homologs Nonsister chromatids held together during synapsis during Meiosis I (at anaphase I) during Meiosis II (at anaphase II) Daughter cells Recombinant chromosomes A single crossing over event leads to 4 genetically unique daughter cells! b) crossing overSources of genetic variation Early in Meiosis I

What is n for the cells shown here? A.1 B.2 C.3 D.4 E.5 Human cells → n = 23

Which cells in this picture are haploid? A.all B.none C.those above line #1 D.those below line #1 E.only those below line #2 1 2

A detailed look at meiosis FIRST CELL DIVISION = “MEIOSIS I” 2 nd CELL DIVISION = “MEIOSIS II”

c) Random fertilization Sources of genetic variation 8.4 million possible gametes > 70 trillion possible offspring!!!

Today’s Agenda Where does variation come from? Mendelian Genetics, Part One

Foundations of Genetics Chapter 14

Outline 1.The work of Gregor Mendel 2.Probability and genetic outcomes 3.Ah, if only it were so simple: complications on genes and traits

Fig. 14-2a Stamens Carpel Parental generation (P) TECHNIQUE: “crossing” or “hybridizing” true-breeding varieties a) The scientific method 1. Mendel

Fig EXPERIMENT P Generation (true-breeding parents) Purple flowers White flowers F 1 Generation (hybrids) All plants had purple flowers F 2 Generation 705 purple-flowered plants 224 white-flowered plants 1. Mendel

Making sense of the data: Why were ALL the F 1 flowers purple? Why were some F 2 flowers white? Why was the ratio in the F 2 generation 3:1?  To explain the data, Mendel developed a model

Mendel’s Model: 4 related hypotheses (remember, DNA had not yet been discovered!) 1.Alternative versions of heritable “particles” (i.e., different alleles of the same gene) Mendel’s explanatory framework 1. Mendel

Mendel’s Model: 4 related hypotheses 1.Alternative versions of heritable “factors” (i.e., alleles) 2. For each character an organism inherits two alleles, one from each parent Mendel’s explanatory framework 1. Mendel

Fig Allele for purple flowers Homologous pair of chromosomes Location of lower color gene Allele for white flowers Diploid organisms Mendel’s explanatory framework 1. Mendel

Mendel’s Model: 4 related hypotheses 1.Alternative versions of heritable “factors” (i.e., alleles) 2. For each character an organism inherits two alleles, one from each parent (i)all F 1 purple (ii)some F 2 white, (iii)F 2 purple:white ratio 3:1 (i)all F 1 purple (ii)some F 2 white, (iii)F 2 purple:white ratio 3:1 Mendel’s explanatory framework 1. Mendel

Mendel’s Model: 4 related hypotheses 1.Alternative versions of heritable “factors” (i.e., alleles) 2. For each character an organism inherits two alleles, one from each parent 3. 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 (i)all F 1 purple (ii)some F 2 white, (iii)F 2 purple:white ratio 3:1 (i)all F 1 purple (ii)some F 2 white, (iii)F 2 purple:white ratio 3:1 Mendel’s explanatory framework 1. Mendel

Mendel’s Model: 4 related hypotheses 1.Alternative versions of heritable “factors” (i.e., alleles) 2. For each character an organism inherits two alleles, one from each parent 3. Some alleles are “dominant”, others “recessive” Mendel’s explanatory framework 1. Mendel 4. “Law of segregation” = the two alleles for a character are separated (segregated) during gamete formation and end up in different gametes

Mendel’s Model: 4 related hypotheses 1.Alternative versions of heritable “factors” (i.e., alleles) account for variations in inherited characters 2. For each character an organism inherits two alleles, one from each parent 3. Some alleles are “dominant”, others “recessive” b) Mendel’s explanatory framework 1. Mendel 4. “Law of segregation” (i)all F 1 purple (ii)some F 2 white, (iii)F 2 purple:white ratio 3:1 (i)all F 1 purple (ii)some F 2 white, (iii)F 2 purple:white ratio 3:1

Outline 1.The work of Gregor Mendel 2.Probability and genetic outcomes 3.Ah, if only it were so simple: complications on genes and traits

F 1 individuals and their gametes 2. Probability and genetic outcomes EXPERIMENT P Generation (true-breeding parents) Purple flowers White flowers  F 1 Generation (hybrids) All plants had purple flowers RRrr homozygous

F 1 individuals and their gametes 2. Probability and genetic outcomes F 1 Generation (hybrids) All plants had purple flowers Possible gamete types (with respect to flower color)?

Fig P Generation Appearance: Genetic makeup: Gametes: Purple flowers White flowers RR R rr r F 1 Generation Gametes: Genetic makeup: Appearance: Purple flowers Rr R r 1/21/2 1/21/2 F 2 Generation Sperm Eggs R R RRRr r r rr 31 R R r r Rr heterozygous

Fig Mendel’s “Law” of segregation is used to construct a “Punnett square”  this simple square tells you the expected frequencies of genotypes and phenotypes from a particular cross

Fig P Generation Appearance: Genetic makeup: Gametes: Purple flowers White flowers RR R rr r F 1 Generation Gametes: Genetic makeup: Appearance: Purple flowers Rr R r 1/21/2 1/21/2 F 2 Generation Sperm Eggs R R RRRr r r rr 31 Reviewing the numbers with respect to this flower color gene:  2 alleles x 2 alleles = 4 outcomes  only 3 distinct genetic types, or genotypes, 1:2:1  only two distinct traits, or phenotypes, 3:1

Testcross: a useful tool How can we figure out the GENOTYPE of a purple flower?  could be PP or Pp

Testcross: a useful tool How can we figure out the GENOTYPE of a purple flower? x PP or Pp? PP Pp pp (A) (B) (C) What do we cross the purple flower with?

Today’s Exit Ticket Create and complete two Punnet squares: 1)A testcross of a heterozygote (rr x Rr) 2)A testcross of a homozygous dominant individual (rr x RR) Explain why using a homozygous recessive individual is useful for distinguishing between Rr and RR.