Unit 3: Genetics

Unit 3: Genetics

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

• DNA replication

MITOSIS MEIOSIS

preceded by replication of chromosomes?

yes yes

# of rounds of cell division 1 2

# of daughter cells 2 4

# of chromosomes in daughter cells compared to parent cell

same as parent cell half of parent cell

daughter cells genetically identical to parent cell?

yes no

sister cells thus produced identical to one another?

yes no

happens in diploid cells, haploid cells, both, or neither?

both(depending on organism)

diploid

crossing over (synapsis)? no yes

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; allelefor brown hair

Gene for hair color; allelefor 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 chromosomesb) Crossing overc) Random fertilization


Fig. 13-8b

Metaphase Iof meiosis I

a) Independent assortmentSources 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. 13-11-2

Possibility 1 Possibility 2

with n = 2there are

4 possibilitiesfor the lineupduring

Meiosis II

4 possible assortments of chromosomes in the gametes


Fig. 13-11-3

Possibility 1 Possibility 2

Metaphase II

Daughtercells

Combination 1 Combination 2 Combination 3 Combination 4


• “2n rule”: the number of possible chromosome sorting combinations = 2n

For humans (n = 23), there are 223 = 8,388,608 possible combinations of chromosomes based on independent assortment alone!


• 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)


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 ofhomologs

Nonsisterchromatidsheld togetherduring synapsis

during Meiosis I(at anaphase I)

during Meiosis II(at anaphase II)

Daughtercells

Recombinant chromosomes

A single crossing over event leads to 4 genetically unique daughter cells!

b) crossing overSources of genetic variation

Early inMeiosis I

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

Human cells → n = 23

Which cells in this picture are haploid?A.allB.noneC.those above line #1D.those below line #1E.only those below line #2

1

2

A detailed look at meiosis

FIRST CELL DIVISION = “MEIOSIS I”

2nd CELL DIVISION = “MEIOSIS II”

c) Random fertilization


8.4 million possible gametes

8.4 million possible gametes

> 70 trillion possible offspring!!!

Today’s Agenda

• Where does variation come from?• Mendelian Genetics, Part One

Foundations of GeneticsChapter 14

Outline

1. The work of Gregor Mendel2. Probability and genetic outcomes3. Ah, if only it were so simple: complications

on genes and traits

Fig. 14-2a

StamensCarpel

Parentalgeneration(P)

TECHNIQUE: “crossing” or “hybridizing” true-breeding varieties

1

2

3

4

a) The scientific method1. Mendel

Fig. 14-3-3

EXPERIMENT

P Generation

(true-breeding parents) Purple

flowers Whiteflowers

F1 Generation

(hybrids) All plants hadpurple flowers

F2 Generation

705 purple-floweredplants

224 white-floweredplants

1. Mendel

1. Mendel

Making sense of the data:

Why were ALL the F1 flowers purple?

Why were some F2 flowers white?

Why was the ratio in the F2 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 frameworkMendel’s explanatory framework1. 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


Fig. 14-4

Allele for purple flowers

Homologouspair ofchromosomes

Location of lower color gene

Allele for white flowers

Diploid organisms





(i) all F1 purple (ii) some F2 white,

(iii) F2 purple:white ratio 3:1







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









3. Some alleles are “dominant”, others “recessive”


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 hypotheses1. Alternative versions of heritable “factors” (i.e., alleles) account for

variations in inherited characters


3. Some alleles are “dominant”, others “recessive”

b) Mendel’s explanatory frameworkb) Mendel’s explanatory framework1. Mendel

4. “Law of segregation”





Outline

1. The work of Gregor Mendel2. Probability and genetic outcomes3. Ah, if only it were so simple: complications

on genes and traits

F1 individuals and their gametes

2. Probability and genetic outcomes2. Probability and genetic outcomes

EXPERIMENT

P Generation

(true-breeding parents) Purple

flowers Whiteflowers

F1 Generation


RR rr

homozygous

F1 individuals and their gametes

2. Probability and genetic outcomes2. Probability and genetic outcomes

F1 Generation


Possible gamete types (with respect to flower color)?

Fig. 14-5-3

P Generation

Appearance:Genetic makeup:

Gametes:

Purple flowers White flowersRR

R

rr

r

F1 Generation

Gametes:

Genetic makeup:Appearance: Purple flowers

Rr

R r1/21/2

F2 Generation

Sperm

Eggs

R

RRR Rr

r

rRr rr

3 1

R R

r

r

Rr Rr

Rr Rr

heterozygous

Fig. 14-5-3

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. 14-5-3

P Generation

Appearance:Genetic makeup:

Gametes:

Purple flowers White flowersRR

R

rr

r

F1 Generation

Gametes:

Genetic makeup:Appearance: Purple flowers

Rr

R r1/21/2

F2 Generation

Sperm

Eggs

R

RRR Rr

r

rRr rr

3 1

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 toolHow can we figure out the GENOTYPE of a purple flower?

could be PP or Pp

Testcross: a useful toolHow 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.

Unit 3: Genetics

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