GENETICS:

Observing Patterns

in

Inherited Traits

Genes

• Units of information about specific traits

• Passed from parents to offspring

• Each has a specific location (locus) on a

chromosome

Alleles

• Different molecular forms of a gene

found on homologous chromosomes

• Arise by mutation

• Dominant allele masks a recessive

allele that is paired with it

Allele Combinations

• Homozygous – having two identical alleles

– Homozygous dominant, AA

– Homozygous recessive, aa

• Heterozygous – having two different alleles

– Aa

Genotype & Phenotype

• Genotype refers to particular genes an

individual carries (RR or Rr or rr)

• Phenotype refers to an individual’s

observable traits (flower color, seed

shape, etc)

Other Definitions

• Dominant allele – in a heterozygous individual, a trait that is fully expressed in the phenotype

• Recessive allele – in a heterozygous individual, a trait that is completely masked by the expression of the dominant allele

• Pure (true) breeding – a population with only one type of allele for a given trait

• Self cross – when individuals of a generation fertilize themselves (e.g., self-fertilized flower).

A pair of homologous chromosomes, each in the unduplicated state (most often, one from a male parent and its partner from a female parent)

A gene locus (plural, loci), the location for a specific gene on a specific type of chromosome

A pair of alleles (each being a certain molecular form of a gene) at corresponding loci on a pair of homologous chromosomes

Three pairs of genes (at three loci on this pair of homologous chromosomes); same thing as three pairs of alleles

Fig. 8-1, p.113

Chromosomes

Gregor Mendel

1822-1884

• Father of Genetics

• Austrian Monk

• Strong background in

mathematics

• observed evidence of

how parents transmit

genes to offspring

• Unaware of cells,

chromosomes or

genes

Fig. 10-2, p.152

Mendel studied the Garden Pea

• Experimentally cross-pollinated

• Mendel began by examining varieties of

peas suitable for study

– Character- an observable feature, such

as flower color

– Trait – actual flower color, such as purple

or white

– Heritable trait – is this character passed

on to progeny ?

Mendel’s methods

Mendel’s

Monohybrid

Cross Results

Credit: © Wally Eberhart

Mendel crossed

round x wrinkle seeded plants

• P (parental generation)

round x wrinkled

• F1 (1st filial generation

offspring)

round

• F2 (2nd filial generation

offspring)

round & wrinkled

round x wrinkled

Dominant / Recessive Traits

• Mendel observed

each parent carried

two “units” for a given

trait

• We know these “units”

are genes on

chromosomes

• Dominant traits –

show up each

generation

• Recessive traits –

may be masked by

dominant traits

True-breeding

homozygous recessive

parent plant

True-breeding

homozygous dominant

parent plant

F1

PHENOTYPES

aa

Aa Aa

Aa Aa

Aa Aa

Aa AaA

A

AA

a a

Fig. 10-7b1, p.155

F1 Results of One Monohybrid Cross

Monohybrid Cross

Experimental intercross between

two F1 heterozygotes

AA X aa Aa (F1 monohybrids)

Aa X Aa ?

A

Monohybrid

Cross

True-breeding

homozygous recessive

parent plant

True-breeding

homozygous dominant

parent plant

An F1 plant

self-fertilizes

and produces

gametes:

F1

PHENOTYPES

F2

PHENOTYPES

aa

Aa

AA

aaAa

Aa

Aa Aa

Aa Aa

Aa Aa

Aa Aa

Aa

Aa

AA

aa

A

A

A

A

a a

a

a

AA

An F1 plant self-fertilizes

and produces gametes: F2

PHENOTYPES

Aa

AA Aa

Aa aa

AA Aa

Aa aaa

A

A a

Fig. 10-7b2, p.155

F2 Results of Monohybrid Cross

Average F2 dominant-to-recessive ratio for all of the traits studied:

Dominant Form Recessive Form

FLOWER

COLOR

FLOWER

POSITION

STEM

LENGTH

3.15:1

3:1

2.84:1787 tall

705 purple 224 white

227 dwarf

651 along stem 207 at tip 3.14:1

Fig. 8-5, p.115

female gametes

male

gam

ete

s aA

a aa

A

aA

aaAa

A

a

aA

aaAa

AaA

a

aA

aaAa

AaAAA

a

Fig. 8-6a, p.115

Probability and the Punnett Square

POSSIBLE EVENT PROBABLE OUTCOME

sperm A meets egg A

sperm A meets egg a

sperm a meets egg A

sperm a meets egg a

1/4 AA offspring

1/4 Aa

1/4 Aa

1/4 aa

p.115

Mendel’s Theory

of Segregation

• Individual inherits a unit of information

(allele) for a trait from each parent

• During gamete formation, the alleles

segregate from each other

fertilization

produces

heterozygous

offspring

meiosis

II

meiosis

I

(chromosomes

duplicated

before meiosis)

homozygous

dominant parent

homozygous

recessive parent

(gametes) (gametes)

Fig. 8-4, p.114

Dihybrid Cross

AB X ab

Experimental cross between

individuals that are homozygous for

different versions of two traits

Dihybrid Cross: F1 Results

AABB aabbx

AaBb

AB AB ab ab

TRUE-

BREEDING

PARENTS:

GAMETES:

F1 HYBRID

OFFSPRING:

purple

flowers,

tall

white

flowers,

dwarf

all purple-flowered, tall

AB ab

1

AABB

purple-

flowered,

tall parent

(homozygous

dominant)

2

aabb

white-

flowered,

dwarf parent

(homozygous

recessive)

3 F1 OUTCOME: All of the F1 plants are purple-flowered, tall

(AaBb heterozygotes)

X

Fig. 8-7, p.116

AaBb AaBb

meiosis,

gamete formation

meiosis,

gamete formation

Fig. 8-7, p.116

1/16aaBB

1/16aaBb

1/16aaBb

1/16Aabb

1/16Aabb

1/16AAbb

1/16AABB

1/16AABb

1/16AaBB

1/16AaBb

1/16AABb

1/16

AaBb

1/16

AaBB

1/16AaBb

1/16AaBb

1/4 AB 1/4 Ab 1/4 aB 1/4 ab

1/16aabb

1/4 AB

1/4 Ab

1/4 aB

1/4 ab

AaBb AaBbX

1/16 white-flowered, dwarf

3/16 white-flowered, tall

3/16 purple-flowered, dwarf

9/16 purple-flowered, tall

Dihybrid Cross: F2 Results

Independent Assortment

• “Units” for one trait were assorted into gametes independently of the “units” for the other trait

• Members of each pair of homologous chromosomes are randomly sorted into gametes during meiosis

Independent Assortment

Metaphase I:

Metaphase II:

Gametes:

1/4 AB 1/4 ab 1/4 Ab 1/4 aB

A A A A

A A A A

AAAA

B B

B B

BB

B B

BBBB

a a a a

aa aa

aaaa

bb b b

bb b b

b b b b

OR

Tremendous Variation

Number of genotypes possible in offspring as

a result of independent assortment and

hybrid crossing is

3n

(n is the number of gene loci

at which the parents differ)

Dominance Relations

Complete dominance

Incomplete dominance

Codominance

Codominance: ABO Blood Types

• Gene that controls ABO type

codes for enzyme that

determines structure of a

glycolipid on blood cells

• Two alleles (IA and IB) are

codominant when paired

• Third allele (i) is recessive to

others

ABO Blood Type:

A Multiple Allele System

Range of genotypes:

Blood

types:

IA IA

IA i IA IB IB i

IB IB

ii

A AB B O

or or

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 14.10

ABO and Transfusions

• Type O is universal donor – neither type

A nor type B antigens produced

• Type AB is universal receiver – no

immune response to A or B antigens

Incomplete

Dominance

Xhomozygous

parent

homozygous

parent

All F1 are

heterozygous

X

F2 shows three

phenotypes in

1:2:1 ratio

Incomplete

Dominance

Xhomozygous parent homozygous parent

All F1 offspring

heterozygous for

flower color:

Cross two of the F1

plants and the F2

offspring will show

three phenotypes in

a 1:2:1 ratio: Fig. 8-10, p.118

Dominance Relations

Complete dominance

Incomplete dominance

Codominance

Pleiotropy • Alleles at a single locus may affect two or

more traits

– Marfan syndrome

• Protein: fibrillin-1

– Cystic fibrosis

• Protein: cystic fibrosis transmembrane

conductance regulator (CFTR)

– Color and crossed eyes in Siamese

cats

– Sickle Cell Anemia

Gene interactions and phenotypic

expression

• Genes may interact with each other:

one gene influences phenotypic

expression of others

• Complex variations: phenotype

influenced by gene interactions and/or

environmental conditions

Interactions among Gene Pairs

• Common among genes for hair

color in mammals

BLACK LABRADOR YELLOW LABRADOR CHOCOLATE LABRADOR

Genetics of Coat Color in

Labrador Retrievers

• Two genes involved- One gene influences melanin production

• Two alleles - B (black) is dominant over b

(brown)

- Other gene influences melanin deposition• Two alleles - E promotes pigment deposition

and is dominant over e

Epistasis: Phenotypic expression of one

gene is governed by another

• Black color – dominant B & E must be present

• Yellow color – recessive e & either B or b

• Chocolate color – dominant E & recessive b

Continuous Variation

• A continuous range of small differences in a given trait among individuals

• The greater the number of genes and environmental factors that affect a trait, the more continuous the variation in that trait

Fig. 10-14, p.160

dark brown

medium brown

dark blue, grey or green

light blue

Controlled by

more than one

gene

• Two genes

A or a

and

B or b

4 dominants

3 dominants

2 dominants

0 dominants

1 dominants

light brown or hazel

X AB Ab aB ab

AB AABB AABb AaBB AaBb

dark brown med. brown med. brown hazel or lt br.

Ab AABb AAbb AaBb Aabb

med. brown hazel or lt br. hazel or lt br. gray, grn, dk. bl.

aB AaBB AaBb aaBB aaBb

med. brown hazel or lt br. hazel or lt br. gray, grn, dk. bl.

ab AaBb Aabb aaBb aabb

hazel or lt br. gray, grn, dk. bl. gray, grn, dk. bl. pale blue

The line of a

bell-shaped

curve reveals

continuous

variation in the

population

Range of values for the trait

Nu

mb

er

of

ind

ivid

ua

ls w

ith

so

me

va

lue

of

the

tra

it

Fig. 8-14a, p.120

Plotting Variation

Range of values

for the trait

Nu

mb

er

of

ind

ivid

uals

wit

h

so

me v

alu

e o

f th

e t

rait

Fig. 8-14b, p.120

Fig. 8-15, p.121

• Skin Color in humans:

three genes

with multiple alleles

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 14.12

Continuous Variation

Environmental Effects on

Phenotype

• Genotype and environment can interact

to affect phenotype

– Himalayan rabbit ice pack experiment

– Transplantation of plant cuttings to different

elevations

– Human depression

Environmental Effects

on Phenotype

Soil pH or trace elements?

Hydrangeas and Soil

Phenotypic Plasticity

• Phenotype change

in response to the

environment.

• Examples:– Humans tan in

response to sun

exposure; increased

melanin protects

cells from harmful

solar radiation

Phenotypic Plasticity

• Phenotype change

in response to the

environment.

• Examples:– Mussels exposed to

seastar “scents”

develop stronger

adductor muscles

– Mussels exposed to

dog whelk “scent”

develop thicker

shells

Phenotypic Plasticity

• Phenotype change

in response to the

environment.

• Predator mediated

Human Genetics and Linkages

• Autosome Linkages

• Sex chromosome linkages

• Linkage group; all of the genes along

the length of a chromosome

• Full linkages stay together after cross-

over

• Incomplete linkages separate at

crossover

Sex

Determination

X

X Y

X

XX

XY

XX

XY

X X

Y

X

x

x

eggs sperm

female(XX)

male(XY)

10 weeks 10 weeks

Y chromosome present

Y chromosome absent

birth approaching

penisvaginal opening

At seven weeks, appearance ofstructures that will give rise to

external genitalia

At seven weeks, appearance of “uncommitted”

duct system of embryo

Y chromosome present

Y chromosome absent

penis

testis

ovary

uterusvagina

testes ovaries

Embryonic

Development

The Y Chromosome

• Small, with few genes

• Master gene for male sex determination

– SRY gene (sex-determining region of Y)

• SRY present, testes form

• SRY absent, ovaries form

The X Chromosome

• Carries more than 2,000 genes

• Most genes deal with nonsexual traits

• Genes on X chromosome can be

expressed in both males and females

Crossover Frequency

Proportional to the distance between

genes

A B C D

Crossing over will disrupt linkage between

A and B more often than C and D

AB

50%

AB

Parents:

F1 offspring:

Equal

ratios of

two types

of gametes:

ab

x

50%

ab

meiosis, gamete formation

All AaBb

AB

AB

ab

ab

AB

ab

Fig. 8-20a, p.125

Full Linkage

ACParents:

F1 offspring:

Unequal

ratios of

four types

of gametes:

ac

x

meiosis, gamete formation

All AaCc

Most gametes have

parental genotypes

A smaller number have

recombinant genotypes

A

CA

C

a

ca

c

AC

ac

Ac

aC

Fig. 8-20b, p.125

Incomplete Linkage

Genetic Abnormality

• A rare, uncommon version of a trait

• Polydactyly

– Unusual number of toes or fingers

– Does not cause health problems

– View of trait as disfiguring is subjective

Pedigree for Polydactyly

Genetic Disorder

• Inherited conditions that cause mild to

severe medical problems

• Why don’t they disappear?

– Mutation introduces new rare alleles

– In heterozygotes, harmful allele is masked,

so it can still be passed on to offspring

Autosomal

Dominant Inheritance

Trait typically

appears in

every

generation

Achondroplasia

• Autosomal dominant

inheritance

• Homozygous form

usually leads

to stillbirth

• Heterozygotes display a

type of dwarfism

Autosomal Recessive Inheritance

Patterns

• If parents are

both

heterozygous,

child will have a

25% chance of

being affected

Autosomal Recessive Galactosemia

X-Linked Recessive Inheritance

• Males show

disorder more

than females

• Son cannot inherit

disorder from his

father

Examples of X-Linked Traits

• Color blindness

– Inability to distinguish among some

or all colors

• Hemophilia

– Blood-clotting disorder

– 1/7,000 males has allele for hemophilia A

– Was common in European royal families

Color Blindness

Fig. 8-27, p.128

Hemophilia

Structural Changes in Chromosomes

• Duplication

• Deletion

• Inversion

• Translocation

Duplication

normal chromosome

one segment

repeated

three repeats

Deletion

• Loss of some segment of a chromosome

• Most are lethal or cause serious disorder

Inversion

A linear stretch of DNA is reversed

within the chromosome

segments

G, H, I

become

inverted

In-text figure

Page 206

Translocation

one chromosome

a nonhomologous

chromosome

nonreciprocal translocation

Changes in Chromosome Number

• Aneuploidy

• Polyploidy

• Most changes in chromosome

number are due to nondisjuction

Aneuploidy

• Individuals have one extra or one less

chromosome (2n + 1 or 2n - 1)

• Major cause of human

reproductive failure

• Most human miscarriages are

aneuploids

Polyploidy

• Individuals have three or more of each

type of chromosome (3n, 4n)

• Common in flowering plants

• Lethal for humans

– 99% die before birth

– Newborns die soon after birth

Nondisjunction

n + 1

n + 1

n - 1

n - 1chromosome

alignments at

metaphase Inondisjunction

at anaphase I

alignments at

metaphase II anaphase II

Down Syndrome

• Trisomy of chromosome 21

• Mental impairment and a variety of

additional defects

• Can be detected before birth

• Risk of Down syndrome increases

dramatically when mothers are over age 35

Down Syndrome

• Trisomy of

chromosome 21

Down Syndrome

Turner Syndrome

• Inheritance of only one X (XO)

• 98% spontaneously aborted

• Survivors are short, infertile

females

– No functional ovaries

– Secondary sexual traits reduced

– May be treated with hormones,

surgery

Klinefelter Syndrome

• XXY condition

• Results mainly from nondisjunction in

mother (67%)

• Phenotype is tall males– Sterile or nearly so

– Feminized traits (sparse facial hair,

somewhat enlarged breasts)

– Treated with testosterone injections

XYY Condition

• Taller than average males

• Most otherwise phenotypically normal

• Some mentally impaired

• Once mistakenly associated with

criminal behavior

X Chromosome

Inactivation

• One X inactivated in

each cell of female

• Creates a “mosaic”

for X chromosomes

• Dosage compensation

Mosaic Expression