Mendel’s “Updated” Postulates

• Unit factors in pairs – A pair of genes controls a trait

• Dominance / Recessiveness

For different alleles of a gene, one will be expressed over the other

• Segregation

Alleles are specifically dispersed into gametes

• Independent Assortment

Genes for different traits are assorted independently of each other

during gamete formation

Things don’t always work out…

• Other researchers observed ratios other than the typical “Mendelian” ratios.

• Mendel’s postulate of dominance vs. recessiveness is not always applicable.

– Incomplete Dominance

– Co-Dominance

– Lethal Alleles

Incomplete Dominance

Incomplete Dominance

• Heterozygote phenotype is intermediate between two different homozygous phenotypes

Incomplete Dominance

• Genotypic and

phenotypic ratios

are both 1:2:1

Fig. 4-1

Co-dominance

Codominance

• Joint expression of two alleles which produce two distinct products

– F2 ratios are similar to incomplete dominance

– 1:2:1 for both genotype and phenotype

ABO Blood Typing • A,B: antigens expressed on rbc surface

– O: no antigen is expressed

• Both alleles contribute to phenotype in AB

heterozygotes

Genotype Antigen Phenotype

IAIA A A

IA i A A

IBIB B B

IB i B B

IAIB A, B AB

i i - O

If you cross a yellow frog with a green frog and get

offspring which are green with yellow spots, the

skin color allele is inherited by which pattern?

• A) Incomplete dominance

• B) Co-dominance

Lethal Alleles

Lethal Alleles

• Many gene products essential for survival

– Mutations to these genes may result in nonfunctional gene products

• Typically behave as recessive lethal alleles

– Heterozygotes may survive

– Homozygote recessives will not

• May also result in a distinctive dominant phenotype

Lethal Alleles

• Mutant allele for coat color in mice

– Agouti (brown) = A

– Yellow = AY

Lethal Alleles

Fig. 4-3

Lethal Alleles

• Huntington Disease

– Lethal allele is dominant

• Rare

• Must reproduce before allele expressed otherwise

mutation cannot be maintained in population, thus

age of disease onset ~40yrs.

• Nervous degeneration death

Lethal alleles are usually…

• A) Dominant

• B) Recessive

• C) Codominant

Dihybrid Crosses with Mendelian Deviation

• Ex., inheritance of albinism and blood type

Fig. 4-4

Dihybrid Crosses with Mendelian Deviation

Fig. 4-4

Dihybrid Crosses with Mendelian Deviation

• Multiplication rule only works if genes are

not linked, and if genes do not interact with

each other

Fig. 4-4

Gene Interaction

• Phenotype may be affected by more than

one gene

• Epistasis

– The expression of a gene at one locus masks

or modifies the expression of another gene

Epistasis

• Ex. 1: A dominant allele for one gene

masks the genotype of another gene

• Squash color (white, yellow, green) is due to the

interaction of 2 genes (A, B)

A--- = white

aaB- = yellow

aabb = green

F2 ratio = 12:3:1

Epistasis

• Ex. 2: Homozygous recessive alleles for one gene masks expression of another gene

Bombay Phenotype (ABO blood typing)

Child type O even though one parent was AB

Due to incomplete H substance

Precursor to glycolipids forming A & B antigens

Product of another gene (H)

Result

Can exhibit O phenotype even though genotype is A , B or AB

Bombay Phenotype

Bombay Phenotype

Epistasis

• Ex. 3: Two gene pairs complement each other; one dominant allele is needed at each locus to express a phenotype

Bateson & Punnett

Epistasis

• Bateson & Punnett:

– Two genes for flower color: C, P

C-P- = purple

C-pp = white

ccP- = white

ccpp = white

True or False: A lethal allele cannot participate in

epistasis.

• A) True

• B) False

What is the scientific name for the fruit fly?

• A) drosophila melanogaster

• B) Drosophila melanogaster

• C) Drosophila melanogaster

• D) Drosophila melanogaster

Complementation

Complementation

• For two mutant strains with a similar

phenotype, how can you tell if the

similarity is due to two different mutations

in the same gene, or if two separate genes

are involved in the production of that

phenotype?

31

Genetic Complementation Analysis

• Background:

– A “mutant screen” is an experiment which generates

mutations which affect specific phenotypes

– Mutant screens are used to discover new mutations

that may be due to previously unidentified genes

Complementation Test

Fig. 4-9

33

Genetic Complementation

• Principle of Complementation:

– two recessive allelic mutations yield mutant phenotype

– two non-allelic recessive mutations yield wt phenotype

• The complementation test groups mutants into

allelic classes called complementation groups

If you cross two different mutant strains of

Drosophila which both have white eye color, and

the offspring all have white eyes, this indicates that

the two mutations are in…

• A) the same gene

• B) different genes along the same

biochemical pathway

• C) different genes which are completely

unrelated to each other

X-linked traits

X-linkage

• Occurs in many animal & some plant

species

– Involves “unlike” pair of chromosomes

involved in sex determination

– Ex., XY in humans

• Female = XX

• Male = XY

Fig. 4-10

X-linkage in Drosophila

Fig. 4-10

X-linkage in Drosophila

X-linkage in Humans

• Red-green colorblindness

• Hemophilia types A & B

• Lesch-Nyhan syndrome

– Abnormal nucleic acid metabolism

– Mental retardation, palsy, self mutilation

– Onset: 6-8 months of age

• Duchenne muscular dystrophy

– Progressive muscular wasting

– Onset: 3-5 years of age

If a man with red-green colorblindness and a

woman whose father was also colorblind married

and had children, what would the likelihood be that

their daughter would be colorblind? (Assume the

woman’s mother was homozygous normal.)

• A) 1/1 (i.e., 100%

• B) ½ (50%)

• C) ¼ (25%)

• D) 2/3 (67%)

Sex-limited & Sex-influenced

Traits

• Autosomal

• An allele is dominant in one sex but recessive

in the other

• Hormone production influences gene

expression

• Sex-limited = phenotype limited to one sex

• Sex-influenced = sex influences expression

but not limited to one sex only

Pattern Baldness

Phenotype

Genotype Females Males

BB bald bald

Bb not bald bald

bb not bald not bald

If a trait is sex-influenced, what will the Mendelian

pattern of inheritance be when two heterzygotes

are crossed?

• A) 100% expressed in male, 0% in female

• B) 100% expressed in female, 0% in male

• C) we can’t tell based on this information

Extranuclear Inheritance

• Organelle heredity: mitochondria & chloroplasts

– Contain own DNA

– Self-replicating

– Inherited with maternal cytoplasm

• Maternal effect: Ephestia kuehniella pigment

– Phenotype is determined by mother’s genotype

• Phenotype under control of gene products already

present in the egg prior to fertilization

Environmental Influences on

Phenotypic Expression

• Temperature effects

(conditional mutations)

– Ex., Himalayan rabbit &

Siamese cats

• Enzymes responsible for

pigment production inactive

or less active at higher

temperatures

Environmental Influences on

Phenotypic Expression

Adiponectin mRNA in Microswine Offspring Exposed to

Either Low Protein or Normal Protein Diets in Utero.

Intra-Abdominal Adipose Tissue

Male Female0

1

2

3

*

*AB

D-A

T

Ad

ipo

necti

n /

18s r

RN

A

Subcutaneous Adipose Tissue

Male Female0

1

2

3

****

NPO

LPO

SC

-AT

Ad

ipo

necti

n /

18s r

RN

A

DuPriest, et al. J DOHAD 2011

Environmental Influences on

Phenotypic Expression • “Effects of Pro12Ala mutation in PPAR-

gamma interact with body size at birth”

J. Eriksson, D. Barker, et al.

Clinical Genetics, 2003

True or False: An individual who has an allele for a

particular trait will always express that trait.

• A) True

• B) False

49

Biochemical Genetics

• Variable expressivity refers to genes that are

expressed to different degrees in different

individuals, e.g.: severity of an inherited disease

• Incomplete penetrance means that the

phenotype predicted from a specific genotype is

not always expressed, e.g.: individual inherits

mutant gene but shows no effect

– Penetrance is a percentage of individuals with a

phenotype that express that phenotype

50

Biochemical Genetics

• Many genes code for enzymes which carry out

specific steps in biochemical pathways

• Mutations which alter the structure of genes

block enzyme production if both alleles of the

gene are defective

• Disorders were termed “inborn errors of

metabolism” by Garrod

51

Biochemical Genetics

• Recessive genes often

contain mutations which

block the formation of

gene product (ww)

• Heterozygotes (Ww) produce ½ the amount of

protein as the homozygous dominant (WW)

52

Biochemical Genetics

• Heterozygotes (Ww) may still produce sufficient

gene product to display dominant phenotype

E.g., round seed of Ww genotype = carrier

• For some genes reduction of gene product by ½

in the heterozygote may be physiologically

significant, especially for structural proteins =

dominant disorders

A patient with Marfan Syndrome may express the

disease even though they are heterozygous (a

carrier) because…

• A) the allele is dominant

• B) the allele is recessive

• C) the allele displays incomplete

dominance