chapter 15: the chromosomal basis of inheritance

Chapter 15: The Chromosomal Basis of Inheritance

Let’s review- Ch 13 - Meiosis makes gametes – sperm & egg- Ch 14 – Mendel studied peas

- gametes pass on traits- unknown what was in the gametes

-Ch 15 connects meiosis with Mendel’s observations of genetics!

Yellow-roundseeds (YYRR)

Green-wrinkledseeds (yyrr)

Meiosis

Fertilization

Gametes

All F1 plants produceyellow-round seeds (YyRr)

P Generation

F1 Generation

Meiosis

Two equallyprobable

arrangementsof chromosomesat metaphase I

LAW OF SEGREGATION LAW OF INDEPENDENT ASSORTMENT

Anaphase I

Metaphase II

Fertilization among the F1 plants

9 : 3 : 3 : 1

14

14

14

14

YR yr Yr yR

Gametes

Y

RRY

y

r

r

y

R Y y r

Ry

Y

r

Ry

Y

r

R

Y

r

y

r R

Y y

R

Y

r

y

R

Y

Y

R R

Y

r

y

r

Y

R

y

r

y

r

y

r

Y

r

Y

R

y

R

y

R

y

r

Y

F2 Generation

Starting with two true-breeding pea plants,we follow two genes through the F1 and F2 generations. The two genes specify seed color (allele Y for yellow and allele y forgreen) and seed shape (allele R for round and allele r for wrinkled). These two genes are on different chromosomes. (Peas have seven chromosome pairs, but only two pairs are illustrated here.)

The R and r alleles segregate at anaphase I, yielding two types of daughter cells for this locus.

1

Each gamete gets one long chromosome with either the R or r allele.

2

Fertilizationrecombines the R and r alleles at random.

3

Alleles at both loci segregatein anaphase I, yielding four types of daughter cells depending on the chromosomearrangement at metaphase I. Compare the arrangement of the R and r alleles in the cellson the left and right

1

Each gamete gets a long and a short chromosome in one of four allele combinations.

2

Fertilization results in the 9:3:3:1 phenotypic ratio in the F2 generation.

3

Figure 15.2 The chromosomal basis of Mendel’s laws


1. How was it determined that chromosomes carry genes?- Thomas Hunt Morgan - 1st to trace a specific gene to a specific chromosome- Noticed a fly with white eyes (wild-type is red)- Wild-type – phenotype most common in the natural population (+)- Mutants – alternative trait to the wild-type

PGeneration

F1

Generation

X

Figure 15.4 In a cross between a wild-type female fruit fly and a mutant white-eyed male, what color eyes will the F1 and F2 offspring have?

F2

Generation

Expected 3:1Observed 3:1Problem!!!!!!Only males had white eyes!

PGeneration

F1

Generation

X

Figure 15.4 In a cross between a wild-type female fruit fly and a mutant white-eyed male, what color eyes will the F1 and F2 offspring have?

F2

Generation

PGeneration

F1

Generation

F2

Generation

Ova(eggs)

Ova(eggs)

Sperm

Sperm

XX X

XY

WW+

W+

W

W+W+ W+

W+

W+

W+

W+

W+

W

W+

W W

W


1. How was it determined that chromosomes carry genes?2. Morgan’s next cross showed that linked genes are inherited together.

EXPERIMENT

Wild type(gray body,

normal wings)

P Generation(homozygous)

b+ b+ vg+ vg+

x Double mutant(black body,vestigial wings)

b b vg vg

F1 dihybrid(wild type)(gray body, normal wings)

b+ b vg+ vg

Double mutant(black body,vestigial wings)

b b vg vg

TESTCROSSx

b+vg+ b vg b+ vg b vg+

b vg

b+ b vg+ vg b b vg vg b+ b vg vgb b vg+ vg

965Wild type

(gray-normal)

944Black-

vestigial

206Gray-

vestigial

185Black-normal

Sperm

Parental-typeoffspring

Recombinant (nonparental-type)offspring

RESULTS

Morgan first mated true-breeding wild-type flies with black, vestigial-winged flies to produce heterozygous F1 dihybrids, all of which are wild-type in appearance. He then mated wild-type F1 dihybrid females with black, vestigial-winged males, producing 2,300 F2 offspring, which he “scored” (classified according to phenotype).

- Noticed a disproportionatelylarge number with same phenotype as parents- Deduced 2 genes must beon the same chromosome- Crossing over accounts forthe recombinant phenotypes


1. How was it determined that chromosomes carry genes?2. Morgan’s next cross showed that linked genes are inherited together.3. What if the genes were unlinked…meaning independent assortment?

P generation: YyRr x yyrr

Gametes from green-wrinkled homozygousrecessive parent (yyrr)

Gametes from yellow-roundheterozygous parent (YyRr)

Parental-typeoffspring

Recombinantoffspring

YyRr yyrr Yyrr yyRr

YR yr Yr yR

yr

50% 50%


1. How was it determined that chromosomes carry genes?2. Morgan’s next cross showed that linked genes are inherited together.3. What if the genes were unlinked…meaning independent assortment?4. How often will recombination occur…frequency??

Testcrossparents

Gray body,normal wings(F1 dihybrid)

b+ vg+

b vg

Replication ofchromosomes

b+ vg

b+vg+

b

vg

vgMeiosis I: Crossingover between b and vgloci produces new allelecombinations.

Meiosis II: Segregationof chromatids producesrecombinant gameteswith the new allelecombinations.

Recombinantchromosome

b+vg+ b vg b+ vg b vg+

b vg

Sperm

b vg

b vg

Replication ofchromosomesvg

vg

b

b

bvg

b vg

Meiosis I and II:Even if crossing overoccurs, no new allelecombinations areproduced.

OvaGametes

Testcrossoffspring

Sperm

b+ vg+ b vg b+ vg b vg+

965Wild type

(gray-normal)b+ vg+

b vg b vg b vg b vg

b vg+b+ vgb vg

944Black-

vestigial

206Gray-

vestigial

185Black-normal Recombination

frequency =391 recombinants

2,300 total offspring 100 = 17%

Parental-type offspring Recombinant offspring

Ova

b vg

Black body,vestigial wings(double mutant)

b

Figure 15.6 Chromosomal basis for recombination of linked genes

Sturtevant – developed a genetic linkage map from recombination frequencies


1. How was it determined that chromosomes carry genes?2. Morgan’s next cross showed that linked genes are inherited together.3. What if the genes were unlinked…meaning independent assortment?4. How often will recombination occur…frequency??5. How can a genetic map be created from recombination frequencies?

Recombinationfrequencies

9% 9.5%

17%

b cn vgChromosome

bcn 9%cnvg 9.5%bvg 17%

-1% RF = 1 map unit (m.u.) -Some linked genes are so far apart that crossovers occur very often.-50% RF is MAX-50% is seen with unlinked genes

Mutant phenotypes

Short aristae

Black body

Cinnabareyes

Vestigialwings

Brown eyes

Long aristae(appendageson head)

Gray body

Redeyes

Normalwings

Redeyes

Wild-type phenotypes

IIY

I

X IVIII

0 48.5 57.5 67.0 104.5

Figure 15.8 A partial genetic (linkage) map of a Drosophila chromosome


1. How was it determined that chromosomes carry genes?2. Morgan’s next cross showed that linked genes are inherited together.3. What if the genes were unlinked…meaning independent assortment?4. How often will recombination occur…frequency??5. How can a genetic map be created from recombination frequencies?6. What determines male or female in utero?

- SRY – sex-determining region of Y- w/ SRY – gonads develop into testes- w/o SRY – gonads develop into ovaries- X – has genes not associated w/ sex characteristics- Sex-linked is usually X-linked- Fathers pass X-linked alleles to daughters (XX)- Moms pass X-linked alleles to sons or daughters- If X-linked allele is recessive

- ♀ shows phenotype when homozygous- ♂ shows phenotype when hemizygous – more males affected


1. How was it determined that chromosomes carry genes?2. Morgan’s next cross showed that linked genes are inherited together.3. What if the genes were unlinked…meaning independent assortment?4. How often will recombination occur…frequency??5. How can a genetic map be created from recombination frequencies?6. What determines male or female in utero?7. How are sex-linked alleles transmitted?

XAXA XaY

Xa Y

XAXa XAY

XAYXAxa

XA

XA

Ova

Sperm

XAXa XAY

Ova XA

Xa

XAXA XAY

XaYXAxa

XA YSperm

XAXa XaY

Ova

Xa Y

XAXa XAY

XaYXaxa

XA

Xa

A father with the disorder will transmit the mutant allele to all daughters but to no sons. When the mother is a dominant homozygote, the daughters will have the normal phenotype but will be carriers of the mutation.

If a carrier mates with a male of normal phenotype, there is a 50% chance that each daughter will be a carrier like her mother, and a 50% chance that each son will have the disorder.

If a carrier mates with a male who has the disorder, there is a 50% chance that each child born to them will have the disorder, regardless of sex. Daughters who do not have the disorder will be carriers, where as males without the disorder will be completely free of the recessive allele.

(a)

(b)

(c)

Sperm

Figure 15.10 The transmission of sex-linked recessive traits


1. How was it determined that chromosomes carry genes?2. Morgan’s next cross showed that linked genes are inherited together.3. What if the genes were unlinked…meaning independent assortment?4. How often will recombination occur…frequency??5. How can a genetic map be created from recombination frequencies?6. What determines male or female in utero?7. How are sex-linked alleles transmitted?8. What are some sex-linked alleles in humans?

- Duchenne’s muscular dystrophy- dystrophin – key muscle protein is absent- Progressive weakening of muscles & loss of coordination- 1 in 3500 ♂ - rarely live past early 20s

- Hemophilia- Protein needed for blood clotting

- Color blindness


1. How was it determined that chromosomes carry genes?2. Morgan’s next cross showed that linked genes are inherited together.3. What if the genes were unlinked…meaning independent assortment?4. How often will recombination occur…frequency??5. How can a genetic map be created from recombination frequencies?6. What determines male or female in utero?7. How are sex-linked alleles transmitted?8. What are some sex-linked alleles in humans?9. What are Barr bodies?

- 1 of the 2 Xs becomes almost completely inactive during embryonic development

- Inactive X in each ♀ cell condenses into a Barr body- Most genes on the Barr body are not expressed - Barr body chromosomes are reactivated in ovary cells that

give rise to eggs- Tortoiseshell cats

Two cell populationsin adult cat:

Active X

Orangefur

Inactive X

Early embryo:X chromosomes

Allele forblack fur

Cell divisionand X

chromosomeinactivation

Active X

Blackfur

Inactive X

Allele fororange fur

Figure 15.11 X inactivation and the tortoiseshell (calico) cat


1. How was it determined that chromosomes carry genes?2. Morgan’s next cross showed that linked genes are inherited together.3. What if the genes were unlinked…meaning independent assortment?4. How often will recombination occur…frequency??5. How can a genetic map be created from recombination frequencies?6. What determines male or female in utero?7. How are sex-linked alleles transmitted?8. What are some sex-linked alleles in humans?9. What are Barr bodies?10. What are some chromosomal errors & exceptions?

- Nondisjunction- Homologous chromosomes fail to separate during meiosis

- Chromosomal rearrangements

Meiosis I

Nondisjunction

Meiosis II

Nondisjunction

Gametes

n + 1n + 1 n 1 n – 1 n + 1 n –1 n nNumber of chromosomes

Nondisjunction of homologouschromosomes in meiosis I

Nondisjunction of sisterchromatids in meiosis II

(a) (b)

Figure 15.12 Meiotic nondisjunction

Aneuploidy – an offspring that has an abnormal # of chromosomes (formed from a nondisjunction gamete)

Trisomic – 2n + 1, Monosomic – 2n – 1 Polyploidy – more than 2 complete chromosome SETS: 3n – triploid, 4n - tetraploid

Sometimes, crossing over is NOT exact (Figure 19.18)

This leads to deletions & duplications.

A B C D E F G HDeletion

A B C E G HF

A B C D E F G HDuplication

A B C B D EC F G H

A

A

M N O P Q R

B C D E F G H

B C D E F G HInversion

Reciprocaltranslocation

A B P Q R

M N O C D E F G H

A D C B E F HG

(a) A deletion removes a chromosomal segment.

(b) A duplication repeats a segment.

(c) An inversion reverses a segment within a chromosome.

(d) A translocation moves a segment fromone chromosome to another,nonhomologous one. In a reciprocal

translocation, the most common type,nonhomologous chromosomes exchangefragments. Nonreciprocal translocationsalso occur, in which a chromosome transfers a fragment without receiving afragment in return.

Figure 15.14 Alterations of chromosome structure


1. How was it determined that chromosomes carry genes?2. Morgan’s next cross showed that linked genes are inherited together.3. What if the genes were unlinked…meaning independent assortment?4. How often will recombination occur…frequency??5. How can a genetic map be created from recombination frequencies?6. What determines male or female in utero?7. How are sex-linked alleles transmitted?8. What are some sex-linked alleles in humans?9. What are Barr bodies?10. What are some chromosomal errors & exceptions?11. What are some human disorders due to chromosomal alterations?

- Down syndrome- Trisomy 21 aka nondisjunction of 21st chromosome- Each cell has 47 chromosomes

Figure 15.15 Down syndrome


1. How was it determined that chromosomes carry genes?2. Morgan’s next cross showed that linked genes are inherited together.3. What if the genes were unlinked…meaning independent assortment?4. How often will recombination occur…frequency??5. How can a genetic map be created from recombination frequencies?6. What determines male or female in utero?7. How are sex-linked alleles transmitted?8. What are some sex-linked alleles in humans?9. What are Barr bodies?10. What are some chromosomal errors & exceptions?11. What are some human disorders due to chromosomal alterations?

- Down syndrome- Nondisjunction of sex chromosomes

- Klinefelter syndrome – XXY – 1 in 2000 ♂- XXX - 1 in 1000 ♀- Turner syndrome - XO – monosomy X – 1 in 5000 ♀

Chapter 15: The Chromosomal Basis of Inheritance--EXCEPTIONS

• Only DNA located within the nucleus follows chromosomal inheritance rules!

• Mitochondrial DNA (containing genes coding for production of ETC proteins, ATP synthase, etc.) is primarily inherited only from the maternal parent.

chapter 15: the chromosomal basis of inheritance

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