Today: some things Mendel did not tell us... Exam #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. Dom.

PhenotypeGenotype Genes code for proteins (or RNA). These gene products give rise to traits… It is rarely this simple.

Fig 4.4

Fig 4.7 Sickle-cell anemia is caused by a point mutation

Sickle and normal red blood cells Fig 4.7

Mom = HSDad = HS H or S HH HSSS HS possible offspring 75% Normal 25% Sickle-cell Mom Dad S=sickle-cell H=normal Sickle-Cell Anemia: A dominant or recessive allele? Fig 4.7

Coincidence of malaria and sickle-cell anemia Fig 24.14

Mom = HSDad = HS H or S HH HSSS HS possible offspring Oxygen transport: 75% Normal 25% Sickle-cell Malaria resistance: 75% resistant 25% susceptible Mom Dad Sickle-Cell Anemia: A dominant or recessive allele? S=sickle-cell H=normal Fig 4.7

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 colorblind normal similar to Fig 4.13

Sex-linked traits: Genes on the X chromosome 50% of males affected, 0 % females affected A= normal; a= colorblind normal similar to Fig 4.13

Sex-linked traits: Genes on the X chromosome 50% males affected, 50% females affected A= normal; a= colorblind colorblind normal 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: Lamarck was right? Sort of… Image from:

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 a B* a B* A* b A* 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*a bB* A*a bB* a B* a B* A* b A* 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*a bB* A*a bB* A*a bB Aa bB* a B* a B* A* b A* 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*a bB* A*a bB* A*a bB Aa bB* A*b, A*B, ab, aB Ab, AB*, ab, aB* a B* a B* A* b A* 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