today: some things mendel did not tell us... exam #3 t 12/2 in class, final sat. 12/6

Today: some things Mendel did not tell us...Today: some things Mendel did not tell us...Exam #3 T 12/2 in class, Final Sat. 12/6Exam #3 T 12/2 in class, Final Sat. 12/6

Single genes controlling a single trait are unusual. Inheritance of most genes/traits is much more complex…

Dom. Rec. Rec. Dom.

PhenotypeGenotype

Genes code for proteins (or RNA). These gene products give rise to traits…

It is rarely this simple.

Fig 4.4

Fig4.7

Sickle-cell anemia is caused by a point mutation

Sickle and normal red blood cells Fig4.7

Mom = HS Dad = HS

H or S

H or S

HH

HS SS

HS possible offspring75% Normal25% Sickle-cell

Mom

Dad

S=sickle-cell

H=normal

Sickle-Cell Anemia:A dominant or recessive allele?

Fig4.7

Coincidence of malaria and sickle-cell anemia

Fig 24.14

Mom = HS Dad = HS

H or S

H or S

HH

HS SS

HS

possible offspringOxygen transport:75% Normal25% Sickle-cell

Malaria resistance:75% resistant25% susceptible

Mom

Dad

Sickle-Cell Anemia:A dominant or recessive allele?

S=sickle-cell

H=normal

Fig4.7

The relationship between genes and traits is often complex

Complexities include:

• Complex relationships between alleles

Sex determination is normally inherited by whole chromosomes or by number of chromosomes.

Fig 3.18

X/Y chromosomes in humans

The X chromosome has many genes; the Y chromosome only has genes for maleness.

Human sex chromosomes

(includes Mic2 gene)

Fig 4.14

Sex-linked traits are genes located on the X chromosome

Color Blind Test

Sex-linked traits: Genes on the X chromosome

No one affected, female carriers

A= normal; a= colorblind

colorblindnormal

similar to Fig 4.13

Sex-linked traits: Genes on the X chromosome

50% of males affected, 0 % females affected

A= normal; a= colorblind

normalnormal

similar to Fig 4.13

Sex-linked traits: Genes on the X chromosome

50% males affected, 50% females affected

A= normal; a= colorblind

colorblindnormal

similar to Fig 4.13

Sex-linked traits: Genes on the X chromosome

No one affected, female carriers

50% of males affected, 0 % female affected

50% males affected, 50% females affected

A= normal ; a= colorblind

similar to Fig 4.13

males and females may have different numbers of chromosomes

Fig 3.18

Tbl 7.1

dosage compensation

At an early stage of embryonic development

The epithelial cells derived from this

embryonic cell will produce a patch of

white fur

While those from this will produce a patch of black fur

Fig 7.4

Promotes compaction

Prevents compaction

Mammalian X-inactivation involves the interaction of 2 overlapping genes.

The Barr body is replicated and both

copies remain compacted

Barr body compaction is heritable within an individual

• A few genes on the inactivated X chromosome are expressed in the somatic cells of adult female mammals– Pseudoautosomal genes

(Dosage compensation in this case is unnecessary because these genes are located both on the X and Y)

– Up to a 25% of X genes in humans may escape full inactivation

• The mechanism is not understood

Epigenetics: http://www.pbs.org/wgbh/nova/sciencenow/3411/02.html

Lamarck was right? Sort of…

Image from: http://www.sparknotes.com/biology/evolution/lamarck/section2.rhtml

Genomic Imprinting

• Genomic imprinting is a phenomenon in which expression of a gene depends on whether it is inherited from the male or the female parent

• Imprinted genes follow a non-Mendelian pattern of inheritance

– Depending on how the genes are “marked”, the offspring expresses either the maternally-inherited or the paternally-inherited allele **Not both

Genomic Imprinting:Methylation of genes during gamete production.

A hypothetical example of imprinting

A=curly hair

a=straight hair

B=beady eyes

b=normal

*=methylation

A* in males

B* in females

aB*

aB* A*

bA*b

A hypothetical example of imprinting

A=curly hair

a=straight hair

B=beady eyes

b=normal

*=methylation

A* in males

B* in females

A*abB*

A*abB*

aB*

aB* A*

bA*b

A hypothetical example of imprinting

A=curly hair

a=straight hair

B=beady eyes

b=normal

*=methylation

A* in males

B* in females

A*abB*

A*abB*

A*abB

AabB*

aB*

aB* A*

bA*b

A hypothetical example of imprinting

A=curly hair

a=straight hair

B=beady eyes

b=normal

*=methylation

A* in males

B* in females

A*abB*

A*abB*

A*abB

AabB*

A*b, A*B,ab, aB

Ab, AB*,ab, aB*

aB*

aB* A*

bA*b

similar to Fig 7.10

Thus genomic imprinting is permanent in the somatic cells of an animal

–However, the marking of alleles can be altered from generation to generation

• Genomic imprinting must involve a marking process

• At the molecular level, the imprinting is known to involve differentially methylated regions–They are methylated either in the oocyte or

sperm• Not both

Imprinting and DNA Methylation