11 1 2012-h

dna dna+protein chromatin copied chromatin chromosome

DNA is copied when mitosis or meiosis begins

Chromosomes & Chromatids

Chapter 11Introduction to genetics

Genetics (遗传学 )

• The study of heredity

Some terms in genetics

• Traits （性状）– The features inherited from the

parents– May have different forms– Determined by genes

Red flowers

White flowers


• Genes （基因）– Unit of heredity– Definition: A heritable factor

that controls a specific characteristic gene


• Locus (plural: loci)– The position of the gene on the

chromosome

B b


• Alleles (等位基因 )– Different versions of the same

gene– Eg: B- brown eye, b- blue eye

B b

Some terms in genetics• Homologous chromosomes (同源染色体 )• Two chromosomes, one from each parent, that look the same. • They carry the same genes but not necessarily the same alleles.

Homologous chromosomes& Sister chromatids

A pair of homologous chromosomes

A gene locus

A pair of alleles

Three pairs of genes

Some terms in genetics• Some traits mask others

– purple & white flower colors are separate traits that do not blend • purple x white ≠ light purple• purple masked white

– dominant allele – masks other alleles – recessive allele – shows effect when dominantallele is absent homologous

chromosomes

I’ll speak for both of us!

Dominant allele leads to production ofpurple pigment

mutant allele leads to no pigment

Some terms in genetics • Homozygous = same alleles on the homologous

chromosomes= PP, pp• Heterozygous = different alleles on the homologous

chromosomes = Pp

homozygousdominant

homozygousrecessive

heterozygous

Some terms in genetics • Phenotype = The characteristics of an organisms• Genotype = The alleles of an organism

Discussion: What is the relationship between phenotype and genotype?

Phenotype vs. genotype

• 2 organisms can have the same phenotype but have different genotypes

homozygous dominantPPpurple

Pp heterozygouspurple


• P - Parent generation • ♀- female, ♂- male• F1 - First-generation offspring• F2 - Second-generation offspring

P F1 F2 F3


• Self-pollination• The anther opens and the

pollen lands on the stigma of the same flower.

• Cross pollination• Anthers open on one flower

and a vector (insects, wind, or animals) moves pollen to the stigma of another flower.

Discussion: why a lot of plants develop strategies to prevent self-pollination?


• True breeding plant (纯种 ) – If self-pollinated, the offspring will always be same as

parents

P F1 F2 F3


• Hybrid (杂种 ) – The offspring of a cross between parents with different traits

father

mother

hybrid


• Gamete (配子 ) = sex cells– Sperm and egg

11-1 The Work of Gregor Mendel

2006-2007

Genetics&

The Work of Mendel

Gregor Mendel

• Modern genetics began in the mid-1800s in an abbey garden, where a monk named Gregor Mendel documented inheritance in peas– used experimental method– used quantitative analysis• collected data & counted them

– excellent example of scientific method

Pollen transferred from white flower to stigma of purple flower

anthersremoved

all purple flowers result

Mendel’s work

F1

P

F2

self-pollinate

• Bred pea plants– cross-pollinate

true breeding parents (P)

– raised seed & then observed traits (F1)

– allowed offspring to self-pollinate & observed next generation (F2)

Mendel collected data for 7 pea traits

Looking closer at Mendel’s work

P

100%F1generation(hybrids)

100%purple-flower peas

Xtrue-breedingpurple-flower peas

true-breeding white-flower peas

self-pollinate

F2generation

3:175%purple-flower peas

25%white-flower peas

Traits are inherited as discrete units• For each characteristic, an organism

inherits 2 alleles, 1 from each parent– diploid• inherits 2 sets of chromosomes (2n),

1 from each parent

• haploid 1 set of chromosomes (n)

What are theadvantages ofbeing diploid?

Making crosses• Can represent alleles as letters– flower color alleles P or p– true-breeding purple-flower peas PP– true-breeding white-flower peas pp

___ x ___

___F1

P X

purple white

all purple

Looking closer at Mendel’s work

F2generation

3:1

P

self-pollinate

Xtrue-breedingpurple-flower peas

true-breeding white-flower peas


25%white-flower peas

___ ___



___ ___ ___ ___

????

phenotype

genotype

Punnett squaresPp x Pp

male / sperm

fem

ale

/ eg

gs

PP

___

___

___

___

___

___

____

%genotype

%phenotype

pp

Pp

Pp

F1generation(hybrids)

Aaaaah,phenotype & genotypecan have different ratios

Test cross• Breed the dominant phenotype —

the unknown genotype — with a homozygous recessive (pp) to determine the identity of the unknown allele

ppis itPP or Pp?

x How does that work?

PP pp

How does a Test cross work?

Pp pp

x x

100% purple 50% purple:50% white or 1:1

Mendel’s 1st law of heredity• Law of segregation – during production of gametes(meiosis),

alleles segregate• homologous chromosomes separate

– each allele for a trait is packaged into a separate gamete

PP

P

P

pp

p

p

Pp

P

p

Law of Segregation• Which stage of

meiosis creates the law of segregation?

Whoa!And Mendeldidn’t even knowDNA or genesexisted!

Metaphase 1

Monohybrid cross• Some of Mendel’s experiments followed

the inheritance of single characters – flower color– seed color – monohybrid crosses

Dihybrid cross• Other of Mendel’s

experiments followed the inheritance of 2 different characters – seed color and

seed shape– dihybrid crosses

Mendelwas working outmany of the genetic rules!

Dihybrid crosstrue-breedingyellow, round peas

true-breedinggreen, wrinkled peasx

_____ _____P

_____


yellow, round peas

9:3:3:1

Y = yellowR = round

y = greenr = wrinkled

self-pollinate

F2generation 9/16

yellowround peas

3/16greenround peas

3/16yellowwrinkledpeas

1/16greenwrinkledpeas

What’s going on here?• The genes for different characters are on same or

different chromosomes?– how do they assort in the gametes?– together or independently?

YyRr

YR yr

YyRr

Yr yRYR yr

On same chromosome

On different chromosomes

Which systemexplains the data?

9/16yellowround

3/16greenround

3/16yellowwrinkled

1/16greenwrinkled

Dihybrid crossYyRr YyRr

YR yr

YR

yr

x

YyRr

Yr yRYR yr

YyRr

YR yr

or

Dihybrid crossYyRr YyRr

YR Yr yR yr

YR

Yr

yR

yr

x9/16yellowround

3/16greenround

3/16yellowwrinkled

1/16greenwrinkled

YyRr

Yr yRYR yr

YyRr

YR yr

or

Mendel’s 2nd law of heredity

roundwrinkled

• Law of independent assortment• non-homologous chromosomes align and assort

independently during meiosis• classes of gametes produced in equal amounts– YR = Yr = yR = yr

yellowgreen

: 1 1 : 1:1Yr

Yr yR yR YR YR yr yr

YyRr

Law of Independent Assortment Which stage of meiosis creates the law of

independent assortment?

The chromosomal basis of Mendel’s laws…

Trace the genetic events through meiosis, gamete formation & fertilization to offspring

Review: Mendel’s laws of heredity • Law of segregation– monohybrid cross

• single trait

– each allele segregates into separate gametes• established by Meiosis 1

• Law of independent assortment– dihybrid (or more) cross

• 2 or more traits

– genes on separate chromosomes assort into gametes independently• established by Meiosis 1

metaphase1

Mendel chose peas wisely• Pea plants are good for genetic research– available in many varieties with distinct heritable

features with different variations• flower color, seed color, seed shape, etc.

– Mendel had strict control over which plants mated with which• each pea plant has male & female

structures• pea plants can self-fertilize• Mendel could also cross-pollinate

plants: moving pollen from one plant to another

Mendel chose peas luckily• Pea plants are good for genetic research– relatively simple genetically• most characters are controlled by a single gene with

each gene having only 2 alleles, – one completely dominant over

the other

2006-2007

Any Questions??

11 1 2012-h

Education

Transcript of 11 1 2012-h