Exceptions to Mendelian Genetics

&Pedigrees

Mendel’s work led to the Chromosome Theory

• Genes are located on chromosomes

• Separation of genes during meiosis account for the inheritance patterns discovered by Mendel

However…

Inheritance of traits by a single gene may deviate from simple Mendelian patterns

• The first solid evidence associating a specific gene with a specific chromosome came from Thomas Hunt Morgan

• Morgan’s experiments with fruit flies provided convincing evidence that chromosomes are the location of Mendel’s heritable factors

• Several characteristics make fruit flies a convenient organism for genetic studies: –They breed at a high rate –A generation can be bred every two

weeks–They have only four pairs of

chromosomes–They are easy to keep alive–They don’t need a lot of space

• In one experiment, Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type)

–The F1 generation all had red eyes

–The F2 generation showed the 3:1 red:white eye ratio, but only males had white eyes

• Morgan determined that the white-eyed mutant allele must be located on the X chromosome

Fig. 15-3

Fig. 15-4

PGeneration

Generation

Generation

Generation

Generation

Generation

F1

F2

All offspring had red eyes

Sperm

EggsF1

F2

P

Sperm

Eggs

XX

XY

CONCLUSION

EXPERIMENT

RESULTS

w

w

w

w

ww

w w

+

+

++ +

w

ww w

w

w

w

ww

+

+

+

+ +

+

• Morgan discovered sex-linked genes

• Genes found on the sex chromosomes

• Usually it’s found on the X chromosome

Difference in Human X and Y Chromosomes

• Females are XX, and males are XY

• Each ovum contains an X chromosome, while a sperm may contain either an X or a Y chromosome

• For example, a gene for making a blood clotting protein is found on the X chromosome

• Having a mutation of this gene causes hemophilia

• Males that inherit a mutation of the gene found on their X chromosome, lack another gene on their Y chromosome and have hemophilia

Swelling of the joints in hemophiliac patient

• Duchenne (pseudohypertropic) muscular dystrophy is also sex-linked

Classic signs of Duchenne MD

• 8% of males have some type of color blindness

• 0.4% of females are color blind

• The most common type is distinguishing red from green

Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical

For example: Aa genotype gives the dominant phenotype and the “a” gene is completely hidden

There are exceptions!

In incomplete dominance, the phenotype of heterozygotes is somewhere between the phenotypes of the two parental varieties

NEITHER the dominant or recessive trait shows up – there’s a third phenotype

Such as

inheritance

of petal

color

in

Japanese

4 o’clocks

Fig. 14-10-3

Red

P Generation

Gametes

WhiteCRCR CWCW

CR CW

F1 GenerationPinkCRCW

CR CWGametes 1/21/2

F2 Generation

Sperm

Eggs

CR

CR

CW

CW

CRCR CRCW

CRCW CWCW

1/21/2

1/2

1/2

• The sickle cell gene is also incompletely dominant.

• A person heterozygous has sickle cell trait

In codominance, if two different alleles are present, they will both show their trait

For example, ABO human blood types

• Sometimes there are more than 2 alleles for a trait and are called multiple alleles

• Example: the A, B, and O genes in human blood types

• Polygenic traits are controlled by more than one pair of genes and accounts for a wide variety of a trait

• Skin color in humans is an example of polygenic inheritance

• Alfred Sturtevant, one of Morgan’s students, constructed a genetic map, an ordered list of the genes along a particular chromosome

• He did this through studying how frequently two genes located on the same chromosome separate during crossing over

• Sturtevant discovered that some genes are found on the same chromosome very close together and, if present, are often inherited together

• For example, the genes for hair color and presence of freckles are both found on the same chromosome close to each other and are often inherited together

• This deviates from Mendel’s principle of independent assortment

Even though there are exceptions to Mendel’s principles, basic Mendelian genetics endures as the foundation of genetics

• A pedigree is a family tree that describes the interrelationships of parents and children across generations

• Inheritance patterns of particular traits can be traced and described using pedigrees

Fig. 14-15a

KeyMale

Female

AffectedmaleAffectedfemale

Mating

Offspring, inbirth order(first-born on left)

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 Wwww ww

ww

wwWw

Ww

wwWW

Wwor

Fig. 14-15c

Attached earlobe

1st generation(grandparents)

2nd generation(parents, aunts,and uncles)

3rd generation(two sisters)

Free earlobe

(b) Is an attached earlobe a dominant or recessive trait?

Ff Ff

Ff Ff Ff

ff Ff

ff ff ff

ff

FF or

orFF

Ff