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THE CHROMOSOMAL BASIS OF INHERITANCE

Copyright © 2009 Pearson Education, Inc.

Review of Mendel’s laws

Mendel’s Laws correlate with chromosome separation in meiosis

• The law of segregation depends on separation of homologous chromosomes in anaphase I

• The law of independent assortment depends on alternative orientations of chromosomes in metaphase I


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F1 generationR

Metaphase Iof meiosis

(alternativearrangements)

r

Y

y

Rr

Y y

R r

Y y

All round yellow seeds(RrYy)


F1 generationR



r

Y

y

Rr

Y y

R r

Y y


Anaphase Iof meiosis

Metaphase IIof meiosis

R

y

r

Y

r

y

R

Y

R r

Y y

Rr

Y y


F1 generationR



r

Y

y

Rr

Y y

R r

Y y


Anaphase Iof meiosis

Metaphase IIof meiosis

R

y

r

Y

r

y

R

Y

R r

Y y

Rr

Y y

1–4

R

y

Ry

R

y

r

Y

1–4 rY

r

Y

1–4 ry

r

y

1–4 RY

R

Y

R

YGametes

Fertilization among the F1 plants

:39 :3 :1F2 generation

r

y


Genes on the same chromosome tend to go together

Linked Genes

• Are located close together on the same chromosome

• Tend to be inherited together

Example studied by Bateson and Punnett

• Parental generation: plants with purple flowers, long pollen crossed to plants with red flowers, round pollen

• The F2 generation did not show a 9:3:3:1 ratio

• Most F2 individuals had purple flowers, long pollen or red flowers, round pollen



Purple long

Purple round

Red long

Red round

Explanation: linked genes

Parentaldiploid cellPpLl

Experiment

Purple flower

PpLl Long pollenPpLl

Prediction(9:3:3:1)

ObservedoffspringPhenotypes

284

21

21

55

215

71

71

24

Mostgametes

Meiosis

PL

pl

PL

PL pl

pl

Fertilization

Sperm

Mostoffspring Eggs

3 purple long : 1 red roundNot accounted for: purple round and red long

PL PL

PL

PL

plPL

pl

pl

pl

pl


Purple longPurple roundRed longRed round

Experiment

Purple flower

PpLl Long pollenPpLl

Prediction(9:3:3:1)

ObservedoffspringPhenotypes

284212155

215717124


Explanation: linked genes

Parentaldiploid cellPpLl

Mostgametes

Meiosis

PL

pl

PL

PL pl

pl

Fertilization

Sperm

Mostoffspring Eggs

3 purple long : 1 red roundNot accounted for: purple round and red long

PL PL

PL

PL

plPL

pl

pl

pl

pl


9.18 Crossing over produces new combinations of alleles

Linked alleles can be separated by crossing over

• Recombinant chromosomes are formed

• Thomas Hunt Morgan demonstrated this in early experiments

• Geneticists measure genetic distance by recombination frequency



Gametes

Tetrad Crossing over

Ba baa b

A BA B A b


Experiment

Parentalphenotypes

Recombination frequency =

Black vestigial

Black body,vestigial wings

GgLl

Offspring

Female Male

Gray long

965 944 206 185

ggll

Gray vestigial Black long

Gray body,long wings(wild type)

Recombinantphenotypes

391 recombinants2,300 total offspring

= 0.17 or 17%


Explanation

G L

g l g l

g lGgLl

(female)ggll

(male)

G L g l g L

g l

g l

g l g l

g l

g l

G L

SpermEggs

Offspring

g L

G l

G l


Chromosome

9.5%

Recombinationfrequencies

9%

17%

g c l

Genetic maps • Show the order of genes on chromosomes

• Arrange genes into linkage groups representing individual chromosomes

Crossover data to map genes


Mutant phenotypes

Shortaristae

Blackbody(g)

Cinnabareyes(c)

Vestigialwings(l)

Browneyes

Long aristae(appendageson head)

Graybody(G)

Redeyes(C)

Normalwings(L)

Redeyes

Wild-type phenotypes


SEX CHROMOSOMES

AND

SEX-LINKED GENES


X X-Y system in

mammals,

fruit flies

XX = female

XY = male

Chromosomes determine sex


(male)

Sperm

(female)

44+

XY

Parents’diploid

cells

44+

XX

22+X

22+Y

22+X

44+

XY

44+

XX

Egg

Offspring(diploid)


22+X

22+

XX


X-O system in grasshoppers and roaches

XX = female; XO = male


76+

ZZ

76+

ZW


Z-W in system in birds, butterflies, and some fishes

ZW = female, ZZ = male


1632


Chromosome number in ants and bees

Diploid = female; haploid = male


Sex-linked genes are located on either of the sex chromosomes

• X-linked genes are passed from mother to son and mother to daughter

• X-linked genes are passed from father to daughter

• Y-linked genes are passed from father to son

9.21 Sex-linked genes exhibit a unique pattern of inheritance



Female Male

XR XR Xr Y

XR YXR Xr

YXr

XR

Sperm

Eggs

R = red-eye alleler = white-eye allele


Female Male

XR Xr XR Y

XR YXR XR

YXR

XR

Sperm

Eggs

Xr XR Xr YXr


Female Male

XR Xr Xr Y

XR YXR Xr

YXr

XR

Sperm

Eggs

Xr Xr Xr YXr


9.22 CONNECTION: Sex-linked disorders affect mostly males

Males express X-linked disorders such as the following when recessive alleles are present in one copy

• Hemophilia

• Colorblindness

• Duchenne muscular dystrophy



QueenVictoria

Albert

Alice Louis

Alexandra CzarNicholas IIof Russia

Alexis


9.23 EVOLUTION CONNECTION: The Y chromosome provides clues about human male evolution

Similarities in Y chromosome sequences

• Show a significant percentage of men related to the same male parent

• Demonstrate a connection between people living in distant locations



1. Explain and apply Mendel’s laws of segregation and independent assortment

2. Distinguish between terms in the following groups: allele—gene; dominant—recessive; genotype—phenotype; F1—F2; heterozygous—homozygous; incomplete dominance—codominance

3. Explain the meaning of the terms locus, multiple alleles, pedigree, pleiotropy, polygenic inheritance

You should now be able to



4. Describe the difference in inheritance patterns for linked genes and explain how recombination can be used to estimate gene distances

5. Describe how sex is inherited in humans and identify the pattern of inheritance observed for sex-linked genes

6. Solve genetics problems involving monohybrid and dihybrid crosses for autosomal and sex-linked traits, with variations on Mendel’s laws

You should now be able to



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