Mendel and the Gene Idea

•  Particulate Hypothesis- parents pass on discrete heritable units, called genes, that retain their separate identities in offspring.

Fig. 14-3

EXPERIMENT

P Generation (true-breeding parents) Purple

flowers White flowers

×

F1 Generation (hybrids) All plants had

purple flowers

F2 Generation

705 purple-flowered plants

224 white-flowered plants

Mendel’s Model

•  Alternative versions of genes account for variations in inherited characters. These alternative versions of a gene are called alleles.

•  Law of Segregation- the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes.

Fig. 14-4

Allele for purple flowers

Homologous pair of chromosomes

Locus for flower-color gene

Allele for white flowers

Mendel’s Terminology

•  Genotype- the genetic makeup •  Phenotype- the appearance of observable traits •  Monohybrid Cross- of a single character •  Dihybrid Cross- with two characters •  Law of Independent Assortment- each pair of

alleles segregates independently of each other pair of alleles during gamete formation. (This applies only to genes located on different, non-homologous chromosomes).

Monohybrid Cross

•  A brown dog is homozygous for the gene that controls coat color. The brown dog is mated with an albino (all white) dog. The dogs have many puppies. All of the puppies have brown coat color. –  Draw a punnett square for this cross and give the

expected genotypic and phenotypic outcomes. –  What are the dominant and recessive alleles?

Provide symbols for both alleles. –  What would be the results if these offspring mated

with an albino dog? A homozygous brown dog? A heterozygote?

Dihybrid Cross •  About 70% of Americans perceive a bitter taste from the

chemical phenylthiocarbamide (PTC). The ability to taste this chemical results from a dominant allele (T) and not being able to taste PTC is the result of having two recessive alleles (t). Albinism is also a single locus trait with normal pigment being dominant (A) and the lack of pigment being recessive (a). A normally pigmented woman who is heterozygous for PTC tasting, has a father who is homozygous for both albinism and PTC tasting. She marries a heterozygous, normally pigmented man who is a taster but who has a mother that does not taste PTC –  Give the phenotypic and genotypic ratios of the offspring.

•  A blue-eyed, left-handed woman marries a brown-eyed, right handed man who is heterozygous for both traits. Blue eyes and left-handedness are recessive. –  Give the phenotypic and genotypic ratios of the offspring.

The Laws of Probability •  The Multiplication Rule- used to determine the

probability that two or more independent events will occur together in some specific combination. –  Multiply the probability of one event by the probability

of the other event. •  The Addition Rule- used to determine the

probability that any one of two or more mutually exclusive events will occur. –  Calculated by adding their individual probabilities.

Degrees of Dominance

•  Complete Dominance- the phenotypes of the heterozygote and the dominant homozygote are indistinguishable.

•  Incomplete Dominance- neither allele is completely dominant and the F1 hybrids have a phenotype somewhere between those of the two parental varieties.

Fig. 14-10-3

Red

P Generation

Gametes

White CRCR CWCW

CR CW

F1 Generation Pink CRCW

CR CW Gametes 1/2 1/2

F2 Generation

Sperm

Eggs

CR

CR

CW

CW

CRCR CRCW

CRCW CWCW

1/2 1/2

1/2

1/2

Degrees of Dominance

•  Codominance- the two alleles both affect the phenotype in separate, distinguishable ways.

Additional Inheritance Patterns

•  Multiple Alleles- most genes exist in populations in more than two allelic forms. –  Ex: blood type

Fig. 14-11

IA

IB

i

A B

none (a) The three alleles for the ABO blood groups and their associated carbohydrates

Allele Carbohydrate

Genotype Red blood cell appearance

Phenotype (blood group)

IAIA or IA i A

B IBIB or IB i

IAIB AB

ii O

(b) Blood group genotypes and phenotypes

Blood Problem

•  Identification bracelets were accidentally removed from three newborn babies. Blood typings were taken to help in the identification procedures. The blood types for the babies and their parents were: Baby 1- type A, Baby 2- type O, Baby 3- type AB –  Mr. Black = type A Mr. Black = type B –  Mr. Green = type AB Mrs. Green = type O –  Mr. White = type O Mrs. White = type O

•  Which baby could belong to Mr. and Mrs. Black? •  Which baby could belong to Mr. and Mrs. Green? •  Which baby could belong to Mr. and Mrs. White?

Additional Inheritance Patterns

•  Pleiotropy- multiple phenotypic effects (exhibited by most genes).

•  Epistasis- a gene at one locus alters the phenotypic expression of a gene at a second locus. Ex: albinism

•  Polygenic Inheritance- an additive effect of two or more genes on a single phenotypic character, which produces quantitative characters that exist on a continuum.

*Environmental Effect

Exit Slip

•  If a man with type AB blood marries a woman with type O blood, what blood types would you expect in their children. (Popcorn)

Homework: •  Read chapter 14, sections 3

JamesPotter

Thomas

Anna LoraineBrackettThomas

Marjorie LoraineThomas Rickard

63

Jean EvonThomasCoffman

80

BillyJohn

Thomas

72

CharlieLouis

Thomas

72

RDRickard

87

WayneLeonAustin

78

FranklinDale

Young

BrandonShaneYoung

TonyaPattyRickard

Jackie KayeKnight YoungUnderw ood

KimberlySutherlinAustin

AndreaLynn Jones

Austin

LauraBeth

Austin

20

KellyJan

Austin

22

StephenMichaelAustin

8

LukeAndrewAustin

6

BenjaminThomasAustin

BryanAdamAustin

25

AshleyMischelle

Austin

23

AlyssaKendallAustin

20

HadleyMischelle

Hardin

4

AustinMichaelMoore

Janet LouiseRickardAustin

57

Susan KayRickardYoung

54

DavidRickard

ElyBrandonYoung

12

GavinLee

Young

8

RogerWayneAustin

54

BryanKeithAustin

50

MichaelShaneAustin

Grace LouiseThomasTotton

76

LynnRickard

SallyRickard

RobertCarn

Woosley

101

Callie BeeMcElw ainWoosley

83

GrundyRickard

JoeRickard

JessicaMarieHale

Martha LeonaWoosleyAustin

76

Guy OrtherWoosley

AubreyWoosley

ClintAustin

Effie ElizabethLindseyAustin

DarrellAustin

HarrellAustin

FriedaAustin

GladysWoosley

MableCrabtree

CharlesAustin

JohnBrackett

Archie CastleHuckleberry

Brackett

RoyBrackett

PearlBrackett

AdamHardin

MichaelMoore

ChristinaMichelleRickard

DavidAndrewRickard

TomGrundy

WaltRickard

CharlesRickard

FrankRickard

LeeRickardCarrico

WilliamAustin

Effie JaneHayesAustin

BerniceAustin

Grimmizon

UraAustin

Brackett

EdSmith

EmuelSmith

PotterDame

DeloresLee Dame

Ferrell

79

AlvinLindsey

88

Ann ArpisineWoosleyLindsey

86

John Jim SeaberryEvaMittie

JohnJack

Woosley

46

RosemanJones

Woosley

60

JamesS.

Jones

Anna ArpaDuvallJones

JosephL.

Woosley

77

Marth E.PressleyWoosley

66

VerdaMorehead

My Family Pedigree

Fig. 14-15b

1st generation (grandparents)

2nd generation (parents, aunts, and uncles)

3rd generation (two sisters)

Widow’s peak No widow’s peak

(a) Is a widow’s peak a dominant or recessive trait?

Ww ww

Ww Ww ww ww

ww

ww Ww

Ww

ww WW

Ww or

You Try It!

•  Jane and Joe Smith have dimples, their daughter, Clarissa, does not. Joe’s dad has dimples, but his mother and his sister do not. Jane’s dad, Mr. Renaldo; her brother, Jorge; and her sister, Emily, do not have dimples, but her mother does.

Recessively Inherited Disorders show up only in individuals homozygous for the allele

Parents

Normal Normal

Normal

Normal (carrier) Albino

Aa Aa

A

AA Aa

a

Aa aa

×

Normal (carrier)

A

a

Dominantly Inherited Disorders Dominant alleles that cause a lethal disease are rare

and arise by mutations.

Parents

Dwarf Normal

Normal

Normal

Dwarf

Dwarf

Dd

×

dd

d D

Dd dd

dd Dd

d

d

Genetic Testing and Counseling

•  Tests for identifying carriers •  Fetal testing

–  Amniocentesis –  Chronic villus sampling (CVS) –  Ultrasound –  Fetoscopy

•  Newborn screening