5. Deviation from mendelian genetics one and two

Non-Mendelian Patterns of Inheritance

PB Red Flower X PB White Flower → 100% Pink flowers → 1 red flower: 2 pink flowers: 1 white flower

What does it remind you of ? the blending hteory but its not

Whats going on?

Incomplete/Partial Dominance- neither allele is truly or fully dominant to the other allele

Result- heterozygous genotype has a distinctive phenotype

Must modify way symbols are assigned to alleles:

Use a letter that is indicative of character

Say c for " colour of flowers"

Use a superscript, a letter or number, to designate different alleles

[c^R red flower allele, c^W white flower allele

Codominance and Multiple Allelism

Codominance: Condition when the phenotypic effects of a gene's alleles are fully and simultaneously expressed in a heterozygote.

Multiple allelism [multiple alleles]: The presence of 3 or more alleles in a population of organisms.

ABO Blood Groups

An example that illustrates the concepts of codominance and multiple allelism.

Most people are able to produce a glycolipid structure called the H substance on the surface of their red blood cells [erythrocytes]

Gene I

The genotype of a Gene I [for isoagglutinogen] determines how the H substance may be modified

The gene encodes for a glycosyltransferase (transfers a sugar onto the H substance)

There are three major alleles for gene I

I^A encoding for a glycosyltransferase that will add a . N-acetyl galactosamine to the H substance (adds N-acetyl galactoseamine to the second galactose???)

I^B encoding for a glycosyltransferase that will add a galactose to the H substance (adds a galatose branch)

i [sometimes IO] that does not encode for a glycosyl transferase

Blood type tables

Blood Type	Genotypes	Antigens produced	Antibodies made	Compatible blood types
O	i/i	none	A and B antibodies	O
A	I^A/I^A or I^A/i^A	A antigen	B antibodies	A,O
B	I^B/I^B or I^B/i^B	B	A antibodies	B,O
AB (codominance)	I^A/I^B	A and B	no antibodies	A,B, O, AB

Blood types diagram

Type O: H antigen only

Type A: H antigen + GalNAc

Type B: H antigen + galactose

Type AB: H antigen + galactose + H antigen +GalNAc

Blood Phenotype

FUT1 gene encodes fucosyl transferase that adds fucose to form a substance.

H- allele encodes for active fucosyl transferase

h- allele encodes for inactive fucosyl transferase

Individual who is h/h will not produce the H substance as fucose not added

Other glycosyl-transferases cannot add to this structure

Example of recessive epistasis

Bombay phenotype info from pedigree slide

if you only have the four sugars you can't add the additional sugar that gives the A or B antigen. h/h is masking I^A/I^B

epistasis- gene expression of one locus masks or modifies the expression of a gene at a second locus

this individual does not have a functional fucosal transferase, the iA alleles are not expressed in this person

Bombay phenotype would die if they got O blood

Diagram slide 22

Figure it out

Lethal Alleles

-allele whose full expression will lead to the death of the individual

Ex: Yellow fur allele in mice -same locus as alleles for Agouti fur colour and black fur colour [multiple allelism]

-unusual:

Cross: Yellow fur mouse with pure-breeding black fur mouse

F1 1Yellow-fur mouse: 1 Black-Fur mouse

Cross Two F1 Yellow mice

-Always get 2 Yellow fur mice: 1 black fur mouse

Never generate pure-breeding line of mice with yellow fur

yellow fur mouse was heterozygous (yellow allele and black fur allele) black was recessive to yellow

2:1 ratio is result of the outcome that Y-yellow y-black

Yellow allele is lethal in homozygous form so the Y/Y mouse is never born. So live births only 2 yellow to one black

Lethal alleles explanation

Why? Allele for yellow fur is dominate for effect on fur colour BUT recessive for lethality.

-allele is pleiotrophic [Pleiotrophy- when a gene has two or more effects on apparently unrelated phenotypic traits.]

-allele for yellow colour also results in mice that are prone to cancer and obesity

A^Y/_ [not A^Y] -yellow fur with tendency to develop cancer and obesity

A^Y/A^Y- dies during embryonic development [no live birth]

Lethal alleles recessive vs dominant

-most [detectable] lethal alleles are recessive

-one wild-type or functional allele

Sufficient gene expression/product to support essential function

Allele can persist in a population in a heterozygous state [carriers]

-dominant lethal alleles rarely persist in a population

if dominant lethal allele results in death during embryological development,

allele is immediately eliminated from population

not passed on to next generation

can only persist if death occurs after reproduction has occurred

Ex. Huntington disease in humans

Huntington Disease

-neurodegenerative disease caused by a dominant lethal autosomal allele for the Huntingtin gene [Chromosome 4, p arm, position 16.3]

-typically, onset in 30-40s when person potentially already has children

-mutant allele has a “trinucleotide expansion” [microsatellite]

-CAg encoding glutamine (Q) is tandemly in specific location in gene (PolyQ tract starts at amino acid position 18)

DNA sequences with an unusual base distrivution and varies in size between individuals

More info on Huntington Disease

-size of trinucleotide expansion critical,

<36 repeats, phenotypically “ normal”, [most people 7-20 repeats]

36 to 39 repeats slow onset, later onset, [age 60+]

40+ repeats typical disease progression,

>60 repeats early onset and progression [before age of 20]

-shows “genetic anticipation”

-whererr genes with 28 or more repeats can undergo additional repeat expansion

-errors in DNA replication caused by DNA slippage,

-more prominent/likely during spermatogenesis

Slips and goes over the same spot multiple times so it creates more repeats

Sex-linked traits

Refers to phenomena associated with the inheritance of genes on the “heteromorphic” sex chromosomes.

X-linkage in Drosophila

Morgan White-eyed male

Cross with wild type [brick red eyed] female

F1- all wild-type colour

F2- 2 wild type female: 1 wild type male: 1 white-eyed male

Back cross F1 female with original white-eyed male

Back cross

white-eyed male X F1 wild-type female

F1: 1 white-eyed male: 1 wild type male: 1 wild-type female : 1 white-eyed female

Concluded: recessive allele for a gene found on the X chromosome and NOT the Y chromsosome

male only has one X chromosome so whatever allele is there is expressed

if you get a chi square test you would reject the null hypothesis. White eye is a partial recessive lethal allele

white eyed allele has a partial lethality

Hemizygous

Hemizygous: having a gene in one dose in an otherwise diploid cell/organism. Usually applies to genes on X chromosome in heterogametic males.

Crisscross pattern of inheritance

Red/green colour blindness in humans

if a human male has an X linked trait he recieved the allele from his mother

If a human female has the condition they have a father that has the condition

NO MALE CARRIERS FOR X LINKED TRAITS! obvious but don't forget it

Sex-limited traits

H- hen feathering H- cock feathering

feathering in chickens

These traits are on somatic cells (not sex cells)

Influence of sex here is by the different ratios of hormones produced in females versus males....high level of estrogen effecting phenotype

Females hen H/H H/h hen hen h/h

Males H/H hen H/h hen h/h cock

Sex-influenced traits

B- bald b- not bald

male pattern baldness

different phenotype due to high levels of estrogen

even if females are bald not as severe and happens later in life

Sex-influenced not limited because it occurs in both sexes

Females: B/B bald B/b not bald b/b not bald

Males: B/B bald B/b bald b/b not bald