GENE INTERACTIONS

GENE INTERACTIONSSlide 2 Meiosis

Slide 3 Crossing Over

Slide 4 Mendel

Slide 5 Punnet squares

Slide 6 Bears

Slide 7 Dihybrid Crosses

Slide 8 Incomplete/Codominant

Slide 9 Multiple Alleles and Lethal Genes

Slide 10 Linked Genes

MEIOSISMeiosis is sex cell division.

It consists of:

1. the DNA replicating normally

2. Homologous chromosomes line up independently (and may cross over).

3. A meiotic cell division.

4. A mitotic cell division.

This has the effect of halving the chromosome number and forming gametes.

Sexual reproduction is important as it greatly increases variation in a species.

CROSSING OVERDuring meiosis, as the homologous chromosomes line up before the first cell division, part of the neighbouring homologues may swap.

The point at which the crossing over occurs is called the chiasma.

Instead of two possible gametes, there are four produced.

Lab manual pages 105/6, (107 opt)

BASIC GENETIC CROSSES

Remember Mendel? And Peas? And Punnet squares?

He found that traits were inherited in chunks, called genes.

Simple monohybrid (one trait) crosses:A purple pea is crossed with a white flowered plant (P generation). All of the offspring (F1 generation) are purple.When the resulting plants were crossed he found that there was always a 3:1 ratio in the offspring (F2 generation).

He correctly deduced that:• the parents are separately donating their information to the offspring• the purple colour is dominant to the white flower (recessive)

The cross of the F1 generation:

Gametes

Offspring

PP is homozygous dominant

pp is homozygous recessive

Pp is heterozygous

Also called “pure breeding”

The genotype is a description of the genes contained in the individual

The phenotype is a description of its physical appearance (e.g. Purple)

In bears, white ears are recessive to black

 Momma bear (white)What is the genotype of Momma Bear?

Momma bear=__________ Poppa bear (black)What genotypes could Poppa Bear have?

Poppa bear=________ or _______

BEARS

This is baby bear (white eared).

What is his genotype?

Baby bear =

Can we say something more about Poppa’s genotype?

Incomplete Dominance and Codominance

Some alleles (gene forms) are not simply dominant or recessive.

In Incomplete Dominance an intermediate phenotype is produced:

In Codominance both alleles are expressed at the same time:

Lab manual pages 114 and 115

Incomplete Dominance

In cases of incomplete dominance, neither allele dominates and the heterozygote is intermediate in phenotype between the two homozygotes.

In crosses involving incomplete dominance, the genotype and phenotype ratios are identical.

Examples of incomplete dominance include flower color in snapdragons (right) and sweet peas, where red and white flowered plants cross to produce pink flowered plants.

CRCRCRCwCwCw

Flower color in snapdragons exhibits incomplete dominance, with red flowered and white flowered plants crossing to produce offspring with pink flowers.

Possiblefertilizations

Flower Color in Snapdragons

GametesCR CR CW CW

Pink

F1 offspring

CRCWCRCWCRCWCRCW

Red flower White flower

Parents

CRCR XCWCW

Pink Pink Pink

CRCw CRCR

Gametes

Possiblefertilizations

Offspring

Snapdragon BackcrossDetermine the phenotype and genotype ratios of the offspring resulting from a backcross of the F1 heterozygote to the red parent.

50% of the offspring are red (RR) and 50% are pink (Rr).

XParents

Pink flowerRed flower

CwCR CR CR

CRCR CRCR CRCw CRCw

Red Red PinkPink

Possiblefertilizations

CodominanceIn cases of codominance, both alleles are independently and equally expressed in the heterozygote. Examples include:

Roan (stippled red and white) coat color in cattle. A cross between a red bull and a white cow produces all roan offspring.

ABO human blood groups.

Roan

Roan

Roan

Roan

CRCW

CRCW

CRCW

CRCWF1 offspring

CR CR CWCWGametes

White cowRed bullCRCR

CWCWParentsX

Possiblefertilizations

WhiteRoanRoanRed

CRCR CRCW CWCWCRCW

Offspring

Codominance in CattleIn a cross between two heterozygous (roan) shorthorn cattle, red, roan, and white offspring are produced in a 1:2:1 ratio.

CWCR CRCWGametes

Roan cowRoan bullCRCWCRC

WParents X

X

Roan cowRed bull

Parents

Possiblefertilizations

Crosses Involving CodominanceIn examples of codominance where a true breeding red or white parent is crossed with a roan parent, the offspring will occur in a 1 : 1 ratio of the parental types (i.e. roan and red, or roan and white)

Offspring

Roan

Roan

RedRed

CRCR CRCW CRCWCRCR

Gametes CWCR CRCR

CRCR CRCW

X

Roan cowWhite bull

Parents

Crosses Involving Codominance

Possiblefertilizations

White

White

RoanRoanOffspring

CRCW CWCW CWCWCRCW

Gametes CWCW

CRCW

CRCW

CWCW

Multiple alleles

It is possible to have more than 2 alleles for a particular trait.

A common example is the ABO blood groups in humans:

O is non-functional

A forms a protein with A antigen

B forms a protein with B antigen

A and B are codominant

Lab manual pages 116/117 and 120

Lethal genes are ones that cause death in the individual. The lethal gene may be dominant or recessive.

In the heterozygous individual there may be some observed difference, e.g. Manx (tailless) cats. Even when dominant the lethal gene may be passed on if it does not have onset until after reproductive age (e.g. Huntington’s).

Lethal Genes

Lethal AllelesLethal alleles are gene mutations that result in a gene product which is not only non-functional, but affects organism survival. Some lethal alleles are fully dominant and are therefore lethal in the heterozygote. Dominant lethal alleles are usually eliminated rapidly, because their expression is fatal.

Exceptions occur when the expression of the allele is delayed until after reproductive maturity, as occurs in Huntington disease.

In other cases (e.g. Manx cats), the allelemutation results in a viable heterozygotewith a recognizable phenotype.

Recessive lethal alleles are fatal only inthe homozygote since their effect ismasked in the heterozygote carrier.

Possiblefertilizations

Offspring

YY Yr yyYr

Not viable

Examples of Lethal AllelesWhen lethal genes prevent full term development of the embryo, offspring are produced in a 2:1 ratio (2 heterozygotes to one normal).

In the inheritance of coat color in yellow mice, offspring phenotype ratios depart from the expected Mendelian 3:1 when yellow mice are mated together.

About two thirds of the offspring are yellow, and one third are non-yellow (right). A testcross reveals the yellow colored mice to be heterozygotes.

Gametes Y yYy

X

Parents

Yy Yy

The average human is heterozygous for 3 to 5 lethal recessive genes.

Example: brachydactyly in humansShortening of the finger bones is caused by a lethal allele; heterozygotes have shorter fingers, but homozygotes for the lethal allele die in infancy from other skeletal defects.

Of the offspring of two brachydactylic people, one in four will die in infancy, one half will show brachydactyly, and one in four will be normal (1:2:1 ratio).

Incidence of Lethal Alleles

X-ray of a normal hand Brachydactyly: note the shortened bones

Cats produce a gene controlling spine length and therefore production of a tail.

The allele for taillessness (ML) is incompletely dominant over the allele for a normal tail (M).

The Manx allele ML interferes with spinal development and heterozygotes (ML M) have no tail (the Manx phenotype).

In ML ML homozygotes, the double dose of the allele produces an abnormal embryo, which does not survive.

Possiblefertilizations

Offspring

MM MML MML MLML

Normal Manx Manx Not viable

The Manx Mutation

Gametes M MLMML

X

Parents

MML MML

Multiple Alleles in BloodThe four common blood groups of the human ABO blood group system are determined by three alleles: A, B, and O (also represented in some texts as IA, IB, IO or just i). This is an example of a multiple allele system for a gene.

ABO antigens consist of sugars attached to the red blood cell surface. These sugars provide the individual antigenic properties. The alleles code for enzymes that join thesesugars together.

Allele O produces a non-functional enzymethat is unable to make changes to the basicantigen (sugar) molecule.

The other two alleles (A, B) each producea different enzyme that adds a different,specific sugar to the basic antigen.

Any one individual possesses only twoalleles and they are expressed equally.

RBC

RBC

Multiple Alleles in BloodPhenotype

(blood group) Genotypes Allele codes for molecule Antigen Antibodies in serum

O OO Precursor

Precursor antigen without extra sugar at end

None(also calleduniversa

ldonor)

A AA, AOacetyl-galactosamine added to precursor

B BB, BOgalactose added to precursor

AB AB Contains both sugars

None(also called

universalrecipient)

Phenotype Genotypes Antibodies in serum

Results from adding RBCs from groupsbelow to serum from groups at left

A AA, AO anti-B

B BB, BO anti-A

AB AB none

O OO anti-A anti-B

Multiple Alleles in BloodBlood donors must be compatible otherwise the red blood cells of the donated blood will clump together (agglutinate) and block capillaries.

A B AB O

X

Blood group: AB

Parent genotypes

Blood group: AB

Gametes B BA A

AB AB

Multiple Alleles in Blood

EXAMPLE 1:For both parents with AB blood type, half of the offspring will be the same as the parents (AB), one quarter will be type A and one quarter will be type B.

BBlood groups

ABA AB

Possible fertilizationsChildren'sgenotypes

AB BBABAA

Blood group: B

Blood group: A

XParent genotypes

Multiple Alleles in Blood

EXAMPLE 2:Two parents with blood groups A and B respectively, may produce offspring with all four possible blood groups: AB, A, B and O.

This may only occur if both parents are carrying the allele for group O.

Possible fertilizationsChildren's' genotypes

AO OOBOAB

Gametes O OB A

BO AO

A OBBlood groups

AB

DIHYBRID CROSSESThis involves two traits that are not linked (not on the same chromosome).

Each of the traits are inherited independently.

Lab manual page 113

In “Quarks” Two eyes (E) is dominant to one eye and Triangular shape (T) is dominant to Pentagonal.

2 Quarks both EeTt are crossed:

LINKED GENESLinked genes are on the same chromosome.

This means that when cell division occurs the 2 genes are very likely to stay together.

So where we might expect a offspring phenotype ratio of 1:1:1:1, we actually get something else.

Two genes B (Bent) and D (Dark) are linked.

For a cross between BbDd and bbdd…

Draw the gametes each could form.

Draw a punnet square for the cross.

Explain these results:

Bent Dark: Bent Light: Straight Dark: Straight Light

Lab manual page 108

bdBD BbDdBd BbddbD bbDdbd bbdd

24 1 3 22B and D (and b and d) are linked. The 1 Bbdd and 3 bbDd individuals are due to crossing over. The different numbers are due to random chance.