Back to BasicsBack to Basics

                             

A is a dominant characteristic – orange feathers.

                           

a is a recessive characteristic – blue feathers.

This bird has two genes for orange feathers.  Its genotype is AA.

This bird has two genes for blue feathers.  Its genotype is aa.

How would you describe

their genotypes?

What are their

phenotypes?

Punnet SquaresPunnet Squares

How do you know?

Which one is the female?

What if we wanted to look at two What if we wanted to look at two features at once?features at once?

Feather colorFeather color -- ORANGE or blueCrest?Crest? -- CREST or no crest

C – Crest

c – no crest

   

Which alleles are dominant?

What are the possible genotypes of these two

birds?A – Orange

a - blue

Dihybrid CrossDihybrid Cross

Feather colorFeather color -- ORANGE or blueCrest?Crest? -- CREST or no crest

Genotype: Aacc

Possible gametes: Ac; Ac; ac; ac

Genotype: aaCc

Possible gametes:

aC; ac; aC; ac

Dihybrid CrossDihybrid Cross

Feather colorFeather color -- ORANGE or blueCrest?Crest? -- CREST or no crest

Genotype: Aacc

Possible gametes: Ac; Ac; ac; ac

Genotype: aaCc

Possible gametes:

aC; ac; aC; ac

Dihybrid CrossDihybrid CrossWhat are the

possible genotypes of the

offspring?

What are the possible

phenotypes of the offspring?

What are the phenotypic

ratios?

Another exampleAnother exampleMA = lots of

melanin

mA = small amount

of melanin

MB = lots of

melanin

mB = small amount

of melanin

mAmB MAmB MAMB

mAMB

Linked GenesLinked Genes

These are:These are:

• Genes that have Genes that have lociloci that are that are physically closephysically close to each other on to each other on the same chromosomethe same chromosome

• less likely to be less likely to be independently independently assortedassorted (separated from each (separated from each other) during crossing over in other) during crossing over in meiosis meiosis

Linked GenesLinked Genes

These are:These are:

• Genes that have Genes that have lociloci that are that are physically closephysically close to each other on to each other on the same chromosomethe same chromosome

• less likely to be less likely to be independently independently assortedassorted (separated from each (separated from each other) during crossing over in other) during crossing over in meiosis meiosis

Because…Because…

A crossing over A crossing over point is more point is more likely to occur on likely to occur on a chromosome a chromosome between two between two genes that are genes that are widely separated widely separated compared to compared to genes that are genes that are closer togethercloser together

For example:For example:

So what??So what??

This means that:This means that:

• Linked genesLinked genes are almost always are almost always inherited together as a single unitinherited together as a single unit from one of the two parentsfrom one of the two parents

• A dihybrid cross A dihybrid cross cannotcannot be used to be used to estimate the probability of estimate the probability of inheriting different combinations of inheriting different combinations of traitstraits

So how can we tell if genes are So how can we tell if genes are linked?linked?

By looking at the results of a By looking at the results of a test test cross:cross: • ABabABab x x abab abab == ABABabab; ; AbAbabab; ; aBaBabab; ; abababab

• If all offspring occur in If all offspring occur in equalequal amounts, amounts, genes are genes are not linkednot linked

• If linked, the vast majority of the If linked, the vast majority of the offspring will be:offspring will be:

ABABabab or or abababab Because Because ABAB or or abab will be inherited as single will be inherited as single

units from one parent whilst only an units from one parent whilst only an ab ab combination an be inherited from the other.combination an be inherited from the other.

Test crosses always include

one homo-zygous

recessive parent and

one hetero-zygous parent

So how can we predict outcomes So how can we predict outcomes of linked gene crosses?of linked gene crosses?

This depends on the distance This depends on the distance between the linked genes on between the linked genes on the chromosomethe chromosome

This is measured by This is measured by map map unitsunits

The number of map units The number of map units correlates to the percentage correlates to the percentage of offspring that have of offspring that have recombined traitsrecombined traits

For exampleFor example In the cross In the cross

ABabABab x x abababab If A and B are If A and B are 6 6

map unitsmap units apart apart the recombined the recombined alleles alleles AbAb and and aBaB will only will only have a have a probability of probability of making up making up 6%6% of the total of the total offspring (3% offspring (3% each)each)

Parent Gametes

AB ab Ab aB

ab AaBb aabb Aabb aaBb

ab AaBb aabb Aabb aaBb

ab AaBb aabb Aabb aabb

ab AaBb aabb Aabb aabb

Offspring AaBb47%

aabb47%

Aabb3%

aaBb3%

Additional examplesAdditional examples In the cross In the cross CDcdCDcd x x cdcdcdcd 8 map units8 map units apart apart recombined alleles recombined alleles CdCd

and and cDcD will only have a will only have a probability of making up probability of making up 8%8% of the total offspring of the total offspring (4% each)(4% each)

Parent Gametes

CD cd Cd cD

cd CdDd ccdd Ccdd ccDd

Offspring 46% 46% 4% 4%

In the cross In the cross EFefEFef x x efefefef 12 map units12 map units apart apart recombined alleles recombined alleles EfEf

and and eFeF will only have a will only have a probability of making up probability of making up 12%12% of the total of the total offspring (6% each)offspring (6% each)

Parent Gametes

EF ef Ef eF

ef EeFf eeff Eeff eeFf

Offspring 44% 44% 6% 6%

Pedigree ChartsPedigree Charts

Pedigrees are diagrams which Pedigrees are diagrams which demonstrate the inheritance of a demonstrate the inheritance of a particular trait within a familyparticular trait within a family

Pedigrees can indicate the Pedigrees can indicate the inheritance modes of particular inheritance modes of particular traitstraits

Pedigree SymbolsPedigree Symbols

I, II, III, IV – 1st, 2nd, 3rd, 4th generations respectively

Are males and females equally

affected?

Pedigrees of Pedigrees of x-linked x-linked recessive recessive traits:traits: • Males affected Males affected

more more commonlycommonly

• All sons of All sons of affected affected female will female will also be also be affectedaffected

• All children of All children of 2 affected 2 affected individuals individuals will also have will also have the traitthe trait

Are males and females equally

affected?

Do all affected individuals have

at least one affected parent?

Autosomal DominantAutosomal Dominant Pedigrees of autosomal Pedigrees of autosomal

dominant traits:dominant traits:

• Males and females affected equally Males and females affected equally (autosomal)(autosomal)

• Affected individuals must have at Affected individuals must have at least one affected parentleast one affected parent

Traits cannot ‘skip’ generationsTraits cannot ‘skip’ generations

• No carriersNo carriers

Are males and females equally

affected?

Do all affected individuals have

at least one affected parent?

Autosomal RecessiveAutosomal Recessive Pedigrees of autosomal Pedigrees of autosomal

recessive traits:recessive traits:

• All children of 2 affected parents All children of 2 affected parents will also have the traitwill also have the trait

• Some affected individuals might Some affected individuals might have 2 unaffected parents if both have 2 unaffected parents if both are carriers:are carriers:

Can ‘skip’ generationsCan ‘skip’ generations

X-linked DominantX-linked Dominant Pedigrees of x-linked dominant Pedigrees of x-linked dominant

traits:traits:

• All daughters of affected males will All daughters of affected males will also have the traitalso have the trait

• Female with the trait can pass it on Female with the trait can pass it on to both daughters and sonsto both daughters and sons

• No carriersNo carriers