Genetics:Genetics:Beyond MendelBeyond Mendel

IB BiologyIB Biology

Mendelian GeneticsMendelian Genetics

This is the term used to describe This is the term used to describe the basic principles of inheritance the basic principles of inheritance for traits that are not inherited in for traits that are not inherited in complicated ways.complicated ways.

There are many exceptions to the There are many exceptions to the principles we have learned with principles we have learned with basic Mendelian genetics.basic Mendelian genetics.

Incomplete DominanceIncomplete Dominance

Sometimes, neither allele for a Sometimes, neither allele for a trait is dominant, so there is a trait is dominant, so there is a blendingblending of phenotypes of phenotypes

Example:Example:– Homozygous red carnations are Homozygous red carnations are

crossed with Homozygous white crossed with Homozygous white carnations and produce 100% pink carnations and produce 100% pink carnation offspringcarnation offspring

Red White

Pink

CodominanceCodominance

Sometimes, both alleles for a trait Sometimes, both alleles for a trait are dominant, resulting in are dominant, resulting in offspring with offspring with bothboth phenotypes. phenotypes.

Example:Example:Crossing a homozygous Crossing a homozygous white horse with a white horse with a homozygous red horse homozygous red horse produces a roan horse with produces a roan horse with a coat of red AND white a coat of red AND white hairs.hairs.

Multiple AllelesMultiple Alleles This term describes traits for which This term describes traits for which

there are more than 2 allelesthere are more than 2 alleles ExampleExample

– There are three alleles for blood type: There are three alleles for blood type: A (IA (IAA), B (I), B (IBB) (codominant), and O (i) ) (codominant), and O (i) (recessive), allowing for 6 possible (recessive), allowing for 6 possible genotypes:genotypes:

– IIA A IIAA, I, IAAi (type A phenotype)i (type A phenotype)– IIBBIIBB, I, IBBi (type B phenotype)i (type B phenotype)– IIA A IIBB (type AB phenotype) (type AB phenotype)– ii (type O phenotype)ii (type O phenotype)

Blood TypesBlood Types

Type A blood produces the A type Type A blood produces the A type glycoprotein on blood cell membranes glycoprotein on blood cell membranes (Type B produces B glycoprotein, type O (Type B produces B glycoprotein, type O produces a carbohydrate with no effect)produces a carbohydrate with no effect)

Giving someone with type A blood a type Giving someone with type A blood a type B transfusion will cause their immune B transfusion will cause their immune system to recognize the type A system to recognize the type A glycoproteins as foreign antigens, attack glycoproteins as foreign antigens, attack them, and cause clotting and usually them, and cause clotting and usually death for the patientdeath for the patient

Blood TypesBlood Types

Those with type AB blood produce both Those with type AB blood produce both glycoproteins (codominant), so they can glycoproteins (codominant), so they can receive A or B transfusions without an receive A or B transfusions without an immune responseimmune response

Those with type O blood are universal Those with type O blood are universal donors because the carbohydrate on donors because the carbohydrate on their cell surfaces do not trigger an their cell surfaces do not trigger an immune response, however, those with immune response, however, those with type O cannot receive any other form of type O cannot receive any other form of bloodblood

EpistasisEpistasis

This occurs when one gene affects This occurs when one gene affects the phenotypic expression of a the phenotypic expression of a second gene.second gene.

Frequently occurs in the Frequently occurs in the expression of pigmentationexpression of pigmentation– One gene turns on (or off) the One gene turns on (or off) the

production of pigment, while a second production of pigment, while a second gene controls either the amount of gene controls either the amount of pigment produced, or the color of the pigment produced, or the color of the pigmentpigment

EpistasisEpistasis

Example: in mice, one gene codes Example: in mice, one gene codes for pigmentation, and another for for pigmentation, and another for the color of the pigment…the color of the pigment…– CC or Cc genotypes produce CC or Cc genotypes produce

pigments, cc produces no pigmentspigments, cc produces no pigments– BB or Bb makes the pigments black, BB or Bb makes the pigments black,

bb makes the pigments brown (if bb makes the pigments brown (if present)present)

– What color would a ccBb mouse be?What color would a ccBb mouse be?

PleiotropyPleiotropy

This occurs when a single gene has This occurs when a single gene has more than one phenotypic expressionmore than one phenotypic expression

Example:Example:– The gene in pea plants that expresses the The gene in pea plants that expresses the

round or wrinkled texture of seeds also round or wrinkled texture of seeds also influences the phenotypic expressions of influences the phenotypic expressions of starch metabolism and water absorptionstarch metabolism and water absorption

– This is like killing 2 (or more) birds with This is like killing 2 (or more) birds with one stoneone stone

Pleiotropy ExamplePleiotropy Example Sickle-cell anemia is an example of a Sickle-cell anemia is an example of a

pleiotropic human blood disease.pleiotropic human blood disease. It is caused by an allele that It is caused by an allele that

incorrectly codes for hemoglobin, incorrectly codes for hemoglobin, causing normally round red blood cells causing normally round red blood cells to become sickle-shaped – leading to a to become sickle-shaped – leading to a painful death when homozygous painful death when homozygous recessive with that allelerecessive with that allele

Heterozygous individuals with that Heterozygous individuals with that allele are more resistant to the allele are more resistant to the mosquito-born pathogen malariamosquito-born pathogen malaria

Polygenic InheritancePolygenic Inheritance Many traits are not expressed in just Many traits are not expressed in just

2 or 3 varieties, such as yellow and 2 or 3 varieties, such as yellow and green pea seeds or A, B, O blood green pea seeds or A, B, O blood typestypes

Your height, for example, is usually Your height, for example, is usually not just short or tall, but can be one not just short or tall, but can be one of a nearly infinite continuum of of a nearly infinite continuum of possibilities within a certain rangepossibilities within a certain range

Many genes are required to shape Many genes are required to shape single complex phenotypes like single complex phenotypes like height.height.

Linked GenesLinked Genes

The law of independent assortment The law of independent assortment only works for genes on different only works for genes on different chromosomeschromosomes

Linked genes are genes that reside on Linked genes are genes that reside on the same chromosome and cannot the same chromosome and cannot therefore segregate independentlytherefore segregate independently

Example:Example:– Body color and wing structure genes in Body color and wing structure genes in

fruit flies are linkedfruit flies are linked

Linked GenesLinked Genes

If the normal fruit fly body color is gray If the normal fruit fly body color is gray (B), while the mutant allele is (B), while the mutant allele is expressed as black (b); and normal expressed as black (b); and normal wings are full (V), while the mutant wings are full (V), while the mutant shriveled wings are vestigial (v)…shriveled wings are vestigial (v)…

A dihybrid cross would typically reveal A dihybrid cross would typically reveal the following cross between this gray, the following cross between this gray, normal-winged male (BbVv) and a normal-winged male (BbVv) and a black, vestigial-winged female (bbvv):black, vestigial-winged female (bbvv):

MaleMale

Female Female BVBV BvBv bVbV bvbv

bvbv BbVvBbVv BbvvBbvv bbVvbbVv bbvvbbvv

Normally: B’s and V’s sort independently

Probabilities: ¼ BbVv, ¼ Bbvv, ¼ bbVv, and ¼ bbvv

Linked: B’s and V’s do not sort so if B is on the same chromosome as V, they do not mix with b or v

MaleMale

Female Female BVBV bvbv

bvbv BbVvBbVv bbvvbbvvProbabilities: ½ BbVv, ½ bbvv

Experimental Probabilities: 41/100 BbVv, 41/100 bbvv, 9/100 Bbvv, 9/100 bbVv

(41:41:9:9 ratio) . . . How?Crossing Over:

Since genes cross over homologous chromosomes in prophase I of meiosis, in this case, about 18% of the time, we do see some of the unexpected combinations above (9% for each = 18% crossover rate). 82% of the time, normal linked combinations are revealed (41% for each expected result = 82%).

Linked GenesLinked Genes The greater the distance between two The greater the distance between two

genes on a chromosome, the more genes on a chromosome, the more places between the genes that the places between the genes that the chromosomes can break and thus the chromosomes can break and thus the more likely the two genes will cross more likely the two genes will cross over during prophase I of meiosis.over during prophase I of meiosis.

So, we can think of every 1% of So, we can think of every 1% of crossover rate as 1 map unit of crossover rate as 1 map unit of distance separating the genes on a distance separating the genes on a chromosomechromosome

This can help us to visualize the This can help us to visualize the arrangement on a chromosome:arrangement on a chromosome:

Linked GenesLinked Genes

Suppose you knew that for a fly Suppose you knew that for a fly with a genotype BBVVAA (where A with a genotype BBVVAA (where A is the apterous, or wingless mutant) is the apterous, or wingless mutant) the crossover frequency between B the crossover frequency between B and V was 18%, between A and V and V was 18%, between A and V was 12%, and between B and A was was 12%, and between B and A was 6%. In what order do the genes lie 6%. In what order do the genes lie on the chromosome, and how far on the chromosome, and how far apart are they?apart are they?

Linked GenesLinked Genes

Hint: think of the 18% crossover Hint: think of the 18% crossover rate between B and V as 18 map rate between B and V as 18 map units apart (making these two the units apart (making these two the farthest apart from each other)farthest apart from each other)

Draw out a possible solution:Draw out a possible solution:

Sex-LinkageSex-Linkage

The one pair of homologous The one pair of homologous chromosomes in animals that chromosomes in animals that does not have exactly the same does not have exactly the same genes are X and Y (sex genes are X and Y (sex chromosomes)chromosomes)

Traits whose genes are located on Traits whose genes are located on X (usually) or Y are determined, in X (usually) or Y are determined, in part, by the sex of the offspringpart, by the sex of the offspring

Sex-LinkageSex-Linkage Example: red-green colorblindness (bb) is Example: red-green colorblindness (bb) is

due to a gene on the X chromosome, normal due to a gene on the X chromosome, normal sight will be represented by (BB or Bb):sight will be represented by (BB or Bb):

XXBB YY

XXBB XXBBXXBB XXBBYY

XXbb XXBBXXbb XXbbYY

Normal Female

Carrier Female

(normal)

Normal Male

Colorblind Male

Because their Y doesn’t have the colorblindness gene at all, males only need one copy to have the recessive phenotype (disorder); so they are much more likely to inherit sex-linked traits.

In the color slideshow, can you see a number hidden in the circle of dots?

X-InactivationX-Inactivation

During fetal development in During fetal development in females, one of the two X females, one of the two X chromosomes will be randomly chromosomes will be randomly inactivatedinactivated– It is formed into a Barr body and its It is formed into a Barr body and its

genes are not expressedgenes are not expressed Daughter cells from a cell which has Daughter cells from a cell which has

inactivated one X will also inactivate inactivated one X will also inactivate the same X.the same X.

X-InactivationX-Inactivation

When X-inactivation begins, some When X-inactivation begins, some cells will inactivate one X, and cells will inactivate one X, and others will inactive the other Xothers will inactive the other X

It is unlikely that the same X will It is unlikely that the same X will be inactivated in all initial embryo be inactivated in all initial embryo cells, but if it happens, then cells, but if it happens, then females can be subject to sex-females can be subject to sex-linked disorders like hemophilia linked disorders like hemophilia with only one copy of the mutated with only one copy of the mutated gene (like men)gene (like men)

NondisjunctionNondisjunction If chromosomes do not separate If chromosomes do not separate

correctly in meiosis, a parent can correctly in meiosis, a parent can donate too many or too few donate too many or too few chromosomes to their offspring:chromosomes to their offspring:

NondisjunctionNondisjunction

Examples:Examples:– Down Syndrome: caused by an extra Down Syndrome: caused by an extra

2121stst chromosome (trisomy 21). chromosome (trisomy 21). Causes mental retardation, heart Causes mental retardation, heart defects, respiratory problems, defects, respiratory problems, deformities, etc…deformities, etc…

– Turner Syndrome: caused by a Turner Syndrome: caused by a missing X chromosome (X0). missing X chromosome (X0). Results in a female with some Results in a female with some physical abnormalities and sterilityphysical abnormalities and sterility