23.1 Mendel’s Laws Chapter 23 Gregor Mendel Patterns of

1

Chapter 23Patterns of Gene

Inheritance

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

23.1 Mendel’s Laws

• Gregor Mendel

– Investigated

inheritance at the

organism level

(1860’s)

– Concluded that plants

transmit distinct

factors to offspring

• Now called genes found

on chromosomes


• In humans, chromosomes come in pairs called

homologous chromosomes

• One member of the pair is inherited from the

mother, while the other member is inherited from

the father

• Homologous chromosomes

– Both members have same length and centromere

location

– Both carry the same genes for the same traits in the

same order

– Alleles – alternate forms of a gene

Homologous ChromosomesCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

G

R

S

t

g

r

s

T

alleles at a gene locus


• The Law of Segregation

– Each individual has two factors (alleles) for

each trait

– The factors segregate (separate) during the

formation of gametes

– Each gamete contains only one factor from

each pair of factors

– Fertilization gives each new individual two

factors for each trait


• The Inheritance of a Single Trait

– Phenotype: Individual’s actual appearance

– Genotype: Alleles the chromosomes carry

that are responsible for a given trait

• Two alleles for a trait, one on each chromosome

• A capital letter symbolizes a dominant allele (W)

• A lowercase letter symbolizes a recessive allele

(w)

• Dominant refers to the allele that will mask the

expression of the alternate (recessive) allele

23.1 Mendel’s Laws 23.1 Mendel’s Laws

• Gamete Formation

– During meiosis, homologous chromosomes separate

so there is only one member of each pair in a gamete

– There is one allele for each trait, such as hairline, in

each gamete

– If parental genotype is Ww, then gametes from this

individual will contain either a W or a w


• One-Trait Cross

– A homozygous

man with a

widow’s peak

reproduces with a

woman with a

straight hairline


gametes

meiosis

Offspring

!Parents

W w

widow’s peak

WW

Ww

widow’s peak

straight hairline

ww


• One-Trait Cross

– Two individuals who

are both Ww

– A Punnett Square is

useful to solve this

problem


3

Straight hairline

Phenotypic Ratio

Straight hairline

Key

oocytes

sp

erm

Parents

!Ww Ww

WW Ww

Ww ww

wW

W

w

Offspring

Widow’s peak

Widow’s peak

W = Widow’s peak

w = Straight hairline

1

! "

"!


• One-Trait Crosses and Probability– The chance of two or more independent events

occurring together is the product of their chance of

occurring separately

– In the cross Ww X Ww, what is the chance of

obtaining either a W or a w from a parent?

• Chance of W = ! and the chance of w = !

– Probability of having these genotypes is as follows

1.Chance of WW = ! X ! = "

2.Chance of Ww = ! X ! = "

3.Chance of wW = ! X ! = "

4.Chance of ww = ! X ! = "


• The One-Trait Testcross

– Breeders of plants and animals may do a test

cross to determine the likely genotype of an

individual with the dominant phenotype

• Cross with a recessive individual (has a known

genotype)

• If there are any offspring produced with the

recessive phenotype, then the dominant parent

must be heterozygous


a.

All

Phenotypic Ratio

Keyoocytes

sp

erm

Parents

!WW ww

Ww Ww

Ww Ww

ww

W

W

Offspring

W = Widow’s peakw = Straight hairline

Widow’s peak

Straight hairline

Widow’s peak

"!

! "

b.

1

Straight hairline

Phenotypic Ratio

Straight hairline

oocytes

sp

erm

Parents

Ww ww

Ww Ww

wwww

ww

W

w

Offspring

!


Widow’s peak

Widow’s peak

1

Key

"!

! "



• The Inheritance of Two Traits

– The Law of Independent Assortment

• Each pair of factors (alleles) assorts

independently (without regard to how the

others separate)

• All possible combinations of factors can

occur in the gametes


MEIOSIS I

MEIOSIS II

either or

one pair

one pair

S

S

S

W

W

W

s

s

s

w w

w

W Ww w

W W

W W W W

sWSwswSW

w w w w

WW w ww w

s s

s ss

s s s s

s

S S

S

S S S S

SS S

Allele Key


S = Short fingerss = Long fingers

Cell has

two pairs of

homologues.

The Inheritance of Two Traits


• Two-Trait Crosses (Dihybrid Cross)

• WwSs (X) WwSs

–Phenotypic Ratio:

• 9 widow’s peak, short fingers

• 3 widow’s peak, long fingers

• 3 straight hairline, short fingers

• 1 straight hairline, long fingers

• 9:3:3:1 phenotypic ratio is always expectedfor a dihybrid cross when simpledominance is present

Two-Trait Crosses (Dihybrid Cross)

!

wsWS

wwssWWSS

WwSs

P generation

P gametes

F1 generation

F1 gametes

F2 generation

Allele Key Phenotypic Ratio

9

3

Straight hairline, short fingers

Straight hairline, long fingers

Widow’s peak, short fingers

Widow’s peak, long fingers

31

W = Widow’s peak

w = Straight hairline

S = Short fingers

s = Long fingers

"!

WwSs

WWSS

Offspring

oocytes

sp

erm

WS Ws

WS

Ws

ws

wS

wS ws

WWSs WwSS WwSs

WWSs Wwss

WwSS wwSS wwSs

WwSs Wwss wwSs

WWss

wwSs

WwSS

"

!


• Two-Trait Crosses and Probability– Probability Laws

• Probability of widow’s peak = #

• Probability of short fingers= #

• Probability of straight hairline= "

• Probability of long fingers= "

– Using the Product Rule

• Probability of widow’s peak and short fingers = # X # = 9/16

• Probability of widow’s peak and long fingers = # X " = 3/16

• Probability of straight hairline and short fingers = " X # = 3/16

• Probability of straight hairline and long fingers = " X " = 1/16

23.2 Pedigree Analysis

and Genetic Disorders

• Pedigree

– A chart of family’s history with regard to a

particular genetic trait

• Pedigree for an autosomal disorder

– Decide if inherited condition due to autosomal

dominant or an autosomal recessive allele by

studying a pedigree


Pattern I

Key

= affected

= unaffected



• In this pattern, the child is affected, but neither parent is

• This can happen only if the disorder is recessive and the

parents are heterozygotes

• Notice that the parents are carriers because they are

unaffected but are capable of having a child with the

genetic disorder

Autosomal Recessive PedigreeCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Heterozygotes (Aa) have an unaffected phenotype.

Both males and females are affected with equal frequency.

•

•

•

•

•

•

aa aa

Aa Aa

Aa

A? A?

A?

Aa

*

aa A?

A?A?

Autosomal recessive disorders

Key

aa = affected

Aa = carrier (unaffected)

AA = unaffected

A? =

I

II

III

IV

Relatives

unaffected

(one allele unknown)Most affected children have unaffected

parents.

Two affected parents will always have affected children.

Affected individuals with homozygous unaffected mates will have

unaffected children.

Close relatives who reproduce are more likely to have

affected children.

"!


and Genetic DisordersCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Pattern II

Key

= affected

= unaffected

• In this pattern, the child is unaffected, but the parents are

both affected

• This can happen if the condition is autosomal dominant

and the parents are heterozygotes

• This pedigree also illustrates that when both parents are

unaffected, all their children are unaffected

– Neither parent has a dominant gene that passes the condition on

Autosomal Dominant PedigreeCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Aa

aa aa aa

aaaaaaaaAa

aa

Aa

Aa A?

Aa

Autosomal dominant disorders

*

I

II

III

•

• Heterozygotes (Aa) are affected.

•

•

• Both males and females are affected with equal frequency.

Key

= affected

Aa = affected

A?

(one allele unknown)

= affected

AA

aa = unaffectedAffected children will usually have

an affected parent.

Two affected parents can produce an unaffected child.

Two unaffected parents will not have affected children.

" !



• Autosomal Recessive Disorders

– Cystic Fibrosis (CF)

• Most common lethal genetic disorder among Caucasians in

the United States

• Chloride ions fail to pass through a plasma membrane

channel protein in cells

• Causes abnormally thick mucus in bronchial tubes and

pancreatic ducts

– Sickle cell disease

• Red blood cells are sickle shaped due to abnormal

hemoglobin

• Differs from normal hemoglobin by one amino acid

23.3 Beyond Simple Inheritance

Patterns

• Incomplete Dominance

– Occurs when the heterozygote is intermediate

between the two homozygotes

• Codominance

– Occurs when alleles are equally expressed in a

heterozygote

– Blood type AB is an example

• Red blood cells have both Type A and Type B surface

antigens

Offspring

oocytes

sp

erm

!Straight hair

Curly hair

Key

1

2

1

Phenotypic Ratio

A person with naturally curly

hair (H2H2)

A person with straight hair

(H1H1)

Heterozygous

Parents (H1 H2)

H1 H2 H1 H2

H1 H1

H1 H2 H2 H2

H1 H2H1

H2

H1 H2

Wavy hair

"!

"!

Incomplete Dominance 23.3 Beyond Simple Inheritance

Patterns

• Multiple Allele Inheritance

– A trait is controlled by multiple alleles, the

gene exists in several allelic forms

• Each person has only two of the possible alleles


Patterns

• Multiple Allele Inheritance

– ABO Blood Types

• IA = A antigens on red blood cells

• IB = B antigens on red blood cells

• i = has neither A nor B antigens on red blood cells

• Both IA and IB are dominant over i, IA and IB are codominant

Phenotype Genotype

A IAIA or IAi

B IBIB or IBi

AB IAIB

O ii

Inheritance of Blood Types


Key

1

1

1

1

Phenotypic Ratio

Offspring

oocytes

sp

erm

Parents

!

IAi ii

iIA

IB

i

Blood type A

Blood type B

Blood type AB

Blood type O

IBi IAi

IAIB IBi

"!

"!


Patterns

• Polygenic Inheritance

– Occurs when a trait is governed by two or

more genes (sets of alleles)

– Dominant alleles have a quantitative effect on

the phenotype, and these effects are additive

– Skin color

Polygenic Inheritance


Nu

mb

er

of

Peo

ple

Skin Color

23.4 Environmental Influences

• Environmental factors can influence the

expression of genetic traits

– In the case of height, differences in nutrition are one

of the factors that bring about a bell-shaped curve

Coat Color in Himalayan Rabbits


© Jane Burton/Bruce Coleman, Inc.

Temperature can also affect the phenotypes of plants and animals

Documents

23.1 Mendel’s Laws Chapter 23 Gregor Mendel Patterns of