heredity – passing traits to offspring

Heredity – Heredity – passing traits to offspringpassing traits to offspringChapters 11, 12

Kevin BleierMilton HS, GA

How produce offspring?How produce offspring?Two major modes of reproduction

1)Asexual reproduction

2)Sexual reproduction

section 11.1

Asexual reproductionAsexual reproductionOne parent making exact genetic

copy of itself (offspring are clone of parent)

Advantages: quick, no need for mate, both males and females can produce offspring

Disadvantages: no genetic diversity in offspring

Asexual reproductionAsexual reproductionMany organisms can do this – plants,

fungi, protists, some animals, bacteria

What type of eukaryotic cell division creates exact copies of cells?

What type of prokaryotic cell division?

mitosis

binary fission

Sexual reproductionSexual reproductionTwo parents both contribute half

of their genes to offspring

Which half they contribute can be different each time = genetically DIFFERENT offspring

Sexual reproductionSexual reproductionAdvantages: genetic diversity

Why is this important?

Disadvantages: finding a mate, only females bear children, more energy expense

more to come in evolution unit next

Sexual reproductionSexual reproductionWho reproduces sexually?

Most multicellular eukaryotes can reproduce sexually

(protists, fungi, plants, animals)

Sexual reproductionSexual reproductionWhat type of cell division produces

sperm and egg cells needed for sex?

meiosis (focus of section 11.2)

germ line cells gametes(sperm / egg)

(beginning of meiosis) (end of meiosis)

Sexual vs. asexualSexual vs. asexualBacteria can ONLY reproduce

asexually

Some organisms can ONLY reproduce sexually (like us humans)

MANY organisms can reproduce both ways … so when might they use one method?

Sexual vs. asexualSexual vs. asexualSexual reproduction: generating

genetic diversity needed to overcome a challenge

(threatening, dangerous, unstable environments)

Asexual reproduction: producing many offspring when conditions are stable

(safe, stable environment with lots of resources)

Preparation for meiosis Preparation for meiosis discussiondiscussionOur goal: how do we make cells

in preparation for sexual reproduction?

How does this process generate the genetic diversity important for sexual reproduction?

Some vocabulary reviewSome vocabulary review

Some vocabulary reviewSome vocabulary reviewDNA – chemical code (order of letters)

gene – small segment of DNA letters that codes for a specific protein (that leads to a specific trait)

chromosome – entire strand of DNA that is packed up for cell division (carries 100s to 1000s of genes)

Some vocabulary reviewSome vocabulary review

R

chromosome

gene

T A C G G T

A AT G C C

DNA code

Some new vocabularySome new vocabularyOrganisms’ body

cells (somatic cells) have chromosomes that come in pairs

Pairs called homologous chromosomes (carry same genes, may carry different versions of gene = alleles)

R R

R r r r

gene = seed shape

2 alleles:R = round seedr = wrinkled seed

Some new vocabularySome new vocabularyCells that contain homologous

chromosome pairs = diploid

Somatic cells are diploid, as are germ-line cells that start meiosis

Gamete cells (sperm and egg) are haploid (only one of each chromosome, not pairs)

Why haploid gametes?Why haploid gametes?

R R r r

parent 1’s diploid germ-line cell

parent 2’s diploid germ-line cell

Rparent 1’s haploid gamete (sperm)

meiosis meiosis

rparent 2’s haploid gamete (egg)

sexual intercourse

results in diploid zygote (which grows into new offspring)

so sexual reproduction overall = meiosis + sexual intercourse

Different species = Different species = different chromosome #different chromosome #

Diploid = 2n

Haploid = n

2n for humans is 46

n for dogs is 39

Types of chromosomesTypes of chromosomesHuman karyotype

22 of 23 pairsall sexes have samegenes (autosomes)

23rd pair determinessex (sex chromosomes)

Males = XY Females = XX

Meiosis – creating Meiosis – creating gametesgametesTwo questions to answer while

studying meiosis:

1)How does a diploid germ-line cell eventually become haploid gametes?

2)How does the process generate genetic variety? (making every gamete different)

section 11.2

Meiosis simulationMeiosis simulationWe will work with a diploid germ-

line cell where 2n = 8 (or 4 pairs of homologous chromosomes)

So haploid number (n = ___ )4

Meiosis simulationMeiosis simulation

diploid germ-line cell

(2n = 8)

meiosis

haploid gamete (n = 4)

MeiosisMeiosisCopies all DNA at the beginning

(like all cell divisions)

Two cell divisions and DNA divisions yields haploid cells

2 divisions in meiosis2 divisions in meiosis

beginning of meiosis – diploid germ-line cell(here, 2n = 8)

first step of any cell division – copy the DNA

homologous pairs exist, just not organized or close together yet

exact copies

First meiotic cell divisionFirst meiotic cell division

in meiosis I, put homologous pairs together

exact copies

homologous pairs

First meiotic cell divisionFirst meiotic cell division

in meiosis I, put homologous pairs together

now, cell lines up pairs together (2 lines of Xs … DIFFERENT than mitosis!)

First meiotic cell divisionFirst meiotic cell division

in meiosis I, put homologous pairs together

now, cell lines up pairs together (2 lines of Xs … DIFFERENT than mitosis!)any division of DNA must be symmetrical – here, meiosis I splits the homologous pairs

Finishing meiosis IFinishing meiosis I

this cell has divided its DNA up equally, and now splits into 2 cells

but meiosis is not finished yet, as both of these cells will divide again

Second meiotic cell divisionSecond meiotic cell division

Looks disorganized because chromosomes unpacked at end of meiosis I, then repacked for meiosis II

Line up into 1 line of Xs (just like mitosis)

Second meiotic cell divisionSecond meiotic cell division

Meiosis II must reorganize chromosomes

Line up into 1 line of Xs (just like mitosis)

Meiosis II splits DNA evenly by splitting exact copies

Each cell splits into 2, creating 4 haploid gametes at the end

Summary of first questionSummary of first questionHow does a diploid germ-line cell

eventually become haploid gametes?

Diploid germ-line cell copies all DNA

Then divides DNA in two separate divisions◦Meiosis I separates homologous pairs◦Meiosis II separates exact copies

Meiosis – creating Meiosis – creating gametesgametesTwo questions to answer while

studying meiosis:

1)How does a diploid germ-line cell eventually become haploid gametes?

2)How does the process generate genetic variety? (making every gamete different)

independent assortment and crossover

b

r

T p

Rt

P

Back to early meiosis IBack to early meiosis I

Let’s label one gene on each homologous pair

We also assume all heterozygous here –organisms can be homozygous (RR or rr)

B

PP

tt RR

ppTT

rr

bb

BB

gene = flower color gene = height gene = seed shape gene = seed color P = purple allele T = tall allele R = round allele

B = yellow seedp = white allele t = short allele r = wrinkled allele b = green seed

Remember, there are actually 100s / 1000s of genes on each pair

b

Independent assortmentIndependent assortment

When homologous chromosomes pair up, they do so randomly

B

PP

tt RR

ppTT

rr

b

B

gene = flower color gene = height gene = seed shape gene = seed color P = purple allele T = tall allele R = round allele

B = yellow seedp = white allele t = short allele r = wrinkled allele b = green seed

Ultimate lineup will be different in every instance of meiosis

b

Independent assortmentIndependent assortment

Assuming this particular lineup …

B

PP

tt

RR

pp

TT

rr

bB

gene = flower color gene = height gene = seed shape gene = seed color P = purple allele T = tall allele R = round allele

B = yellow seedp = white allele t = short allele r = wrinkled allele b = green seed

We will get gametes carrying these particular alleles

gamete 1: pTbrgamete 2: PtBR

b

Independent assortmentIndependent assortment

What if another round of meiosis had the “R” chromosomes lineup differently?

B

PP

tt

RR

pp

TT

rr

bB

gene = flower color gene = height gene = seed shape gene = seed color P = purple allele T = tall allele R = round allele

B = yellow seedp = white allele t = short allele r = wrinkled allele b = green seed

We will get gametes carrying these particular allelesgamete 1: pTbr

gamete 2: PtBRgamete 3: PtBrgamete 4: pTbR

b

Independent assortmentIndependent assortment

How many different gametes are possible if every pair can line up two different ways?

B

PP

tt

RR

pp

TT

rr

bB

2

2

2

2

x

x

x

16

Independent assortmentIndependent assortmentHow many homologous

chromosome pairs do human germ line cells have?

So how many different gametes can every human make by lining them up differently every time?

23 pairs

= 223 ~ 8,000,000 different gametes

Independent assortmentIndependent assortmentRecall though that sexual

reproduction requires 2 parents both making gametes

And we cannot choose which gametes fertilize – that’s also random~ 8,000,000 possible sperm x 8,000,000

possible eggs~ 64,000,000,000,000 possible zygotes for 2 parents~ 6,500,000,000 people on Earth

b

Crossing overCrossing overBefore meiosis I lineup of homologous pairs

B

PP

tt

RR

pp

TT

rr

b B

Inner, non-sister chromatids can get so close that tips of chromosomes exchange

Whether or not this occurs in each pair is random every time meiosis occurs

b

Results of crossover eventResults of crossover event

B

PP

tt

RR

pp

TT

rr

bB

All four gametes will be genetically different

PtBR

PTBR

ptbr pTbr

Meiosis – creating Meiosis – creating gametesgametesTwo questions to answer while

studying meiosis:

1)How does a diploid germ-line cell eventually become haploid gametes?

2)How does the process generate genetic variety? (making every gamete different)

independent assortment and crossover

Meiosis summaryMeiosis summaryStarts with 1 diploid germ-line cell,

ends with 4 haploid gametes

Each gamete has one of the chromosome pairs, all genetically different in alleles that they carry

Sperm and egg gametes must combine to complete sexual reproduction

Errors in meiosis = Errors in meiosis = nondisjunctionnondisjunction

Diploid germ line cell 2n = 4

Haploidgametesshouldben = 2

Some gametes have 1 extra (offspring would have too many chromosomes)

Some gametes are missing 1 (offspring would have too few chromosomes)

Nondisjunction effectsNondisjunction effectsExample: Down

syndrome (trisomy 21)

Extra or missing chromosome in all somatic cells has large effect on phenotype

Many trisomies / monosomies result in inviable embryo

Overall human life cycleOverall human life cycle

section 11.3

diploid somatic cells(multicellular human)(2n = 46)

diploid germ-line cells (within sex organs)

haploid gametes(n = 23)

meiosis

sperm

egg(ovum)

diploid zygote(single-cell)(2n = 46)

fertilization

mitosis and development

possible haploid gametes

Where we are headed … Where we are headed … Punnett squares show all the

possibilities of gametes that can be made in meiosis

P p P P

parent 1 parent 2

PP pp PP PP

possible gametes:

P, ppossible gametes: PPp

P p

PP

Pp

Pp

PPPP

P

P

or

PpP p

PP

P

original diploid germ-line cells

possible diploid zygotes formed by fertilization of specific sperm and egg

PpPP

chapter 12

Two – trait heredity Two – trait heredity analysisanalysisWe will assume that genes are on

different chromosome pairs

PP

tt

pp

TT

Parent 1: PpTt Parent 2: PPtt

PP

t t t t

P P

possible gametes: PT, pt

or

PP pp

TTtt

, Pt, pT

possible gamete: Ptrules for making gametes:

1) half of what you started with 2) one of each chromosome pair (one of each letter)

Two – trait Punnett squareTwo – trait Punnett squareParent 1: PpTt Parent 2:

PPttpossible gametes: PT, pt, Pt, pT

possible gamete: Pt

PpTt

PT

Pt pT

pt

PPtt

Pt

Pt

Pt

Pt

PPTt

PPTt

PPTt

PPTt

PPtt

PPtt

PPtt

PPtt

PpTt

PpTt

PpTt

PpTt

Pptt

Pptt

Pptt

Pptt

PT

Pt pT

pt

Pt PPTt PPtt PpTt Pptt

or

Two – trait Punnett Two – trait Punnett squaressquaresPlease don’t do this

PpTtP p T t

PPtt

P

P

t

t

PP

PP

Pt

Pt

Pp

Pp

pt

pt

tt

tt

PT

PT

Tt

Tt

Pt

Pt

Researching in mid 19th century

No idea about chromosomes / DNA / meiosis

Quantitative approach yields basic principles of heredity

Mendel’s worldMendel’s world

section 12.1

Pea plants

1.Quick reproduction produces many offspring

2.Can control parents easily (also self and cross-pollination possibilities)

3.Many simple traits

Mendel’s modelMendel’s model

Mendel could only see phenotypes, inferred genotype from his results

A reminderA reminder

Some plants were true-breedingAll

Some

Some plants were hybridsSome

Mendel’s observationsMendel’s observations

+

Crosses:

Mendel’s observationsMendel’s observations

xpurebred purebred

hybrid

purebred hybrid hybrid purebred

1) All organisms have 2 copies of a gene

Some conclusionsSome conclusions

R R

Modern understanding: homologous pairs both carry copies of gene

section 12.2

2) Each parent passes 1 of their copies to offspring

Some conclusionsSome conclusions

R

Modern understanding: meiosis splits homologous pairs – sends one to each gamete

3) Complete dominance – one trait completely dominates the other when both are together

hybrid organism has same phenotype as purebred dominant organism

Some conclusionsSome conclusions

True of many traitsTrue of many traits

Crossing two traits at Crossing two traits at onceonce

Independent assortment – traits can be considered independently of each other

ConclusionConclusion

R r

Modern understanding: assumes that genes are carried on separate chromosome pairs

independent assortment of homologues

Y y

rR

Y y

Possible gametes:

RY ry

Y Yyy

Ry rY

Assumes:

1)One gene determines trait, two alleles

2)Complete dominance – one dominant allele, one recessive allele

3)Genes always on different chromosomes, always independent of each other (“unlinked”)

Mendelian model of Mendelian model of geneticsgenetics

Testcross differentiates the two

How tell organisms apart?How tell organisms apart?

section 12.3

B = black furb = brown fur

possible genotypes:BB

Bbbb

cannot distinguish“unknowns”

known – brown fur

Determine genotype of unknown organism by crossing with known brown fur organism

TestcrossTestcrossBbB b

BBB B

bb

b

b

b

bbb

Bb

Bb

Bb

Bb

BbBb

bbbb

How does this help you distinguish between homozygous dominant and heterozygous?

Few human traits follow Mendel’s simple model

Rolling tongue, widow’s peak

Certain human genetic disorders (sickle cell anemia, Huntington’s, cystic fibrosis)

Mendelian traits in Mendelian traits in humanshumans

Assumes:

1)One gene determines trait, two alleles

2)Complete dominance – one dominant allele, one recessive allele

3)Genes always on different chromosomes, always independent of each other (“unlinked”)

Mendelian model of Mendelian model of geneticsgenetics

Many organisms have inheritance patterns that do NOT follow Mendel’s assumptions

Updating Mendel’s modelUpdating Mendel’s model

section 12.4

Assumption: complete dominance

Exception: incomplete dominance

Updating Mendel’s modelUpdating Mendel’s model

Assumption: complete dominance

Exception: codominance

Updating Mendel’s modelUpdating Mendel’s model

Assumption: one gene determines one trait

Exception: polygenic inheritance

Updating Mendel’s modelUpdating Mendel’s model

Assumption: there are only 2 alleles for a gene

Exception: multiple alleles

Updating Mendel’s modelUpdating Mendel’s model

Assumption: genes determine a phenotype

Exception: gene / environment interactions

Updating Mendel’s modelUpdating Mendel’s model

acidic soil

neutral or basic soil

Updating Mendel’s modelUpdating Mendel’s model

Assumption: genes are unlinked (always on different chromosome pairs)

Exception: linked genes