Back 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 Squares How do you know? Which one is the female?

What if we wanted to look at two features at once? Feather color Feather color -- ORANGE or blue Crest? 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 Cross Feather color Feather color -- ORANGE or blue Crest? Crest? -- CREST or no crest Genotype: Aacc Possible gametes: Ac; Ac; ac; ac Genotype: aaCc Possible gametes: aC; ac; aC; ac

Dihybrid Cross Feather color Feather color -- ORANGE or blue Crest? Crest? -- CREST or no crest Genotype: Aacc Possible gametes: Ac; Ac; ac; ac Genotype: aaCc Possible gametes: aC; ac; aC; ac

Dihybrid Cross What are the possible genotypes of the offspring? What are the possible phenotypes of the offspring? What are the phenotypic ratios?

Another example M A = lots of melanin m A = small amount of melanin M B = lots of melanin m B = small amount of melanin m A m B M A m B M A M B m A M B

Linked Genes These are: These are: Genes that have loci that are physically close to each other on the same chromosomeGenes that have loci that are physically close to each other on the same chromosome less likely to be independently assorted (separated from each other) during crossing over in meiosisless likely to be independently assorted (separated from each other) during crossing over in meiosis

Linked Genes These are: These are: Genes that have loci that are physically close to each other on the same chromosomeGenes that have loci that are physically close to each other on the same chromosome less likely to be independently assorted (separated from each other) during crossing over in meiosisless likely to be independently assorted (separated from each other) during crossing over in meiosis

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

For example:

So what?? This means that: This means that: Linked genes are almost always inherited together as a single unit from one of the two parentsLinked genes are almost always inherited together as a single unit from one of the two parents A dihybrid cross cannot be used to estimate the probability of inheriting different combinations of traitsA dihybrid cross cannot be used to estimate the probability of inheriting different combinations of traits

So how can we tell if genes are linked? By looking at the results of a test cross: By looking at the results of a test cross: ABab x abab = ABab; Abab; aBab; ababABab x abab = ABab; Abab; aBab; abab If all offspring occur in equal amounts, genes are not linkedIf all offspring occur in equal amounts, genes are not linked If linked, the vast majority of the offspring will be:If linked, the vast majority of the offspring will be: ABab or abab ABab or abab Because AB or ab will be inherited as single units from one parent whilst only an ab combination an be inherited from the other. Because AB or ab will be inherited as single units from one parent whilst only an ab 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 of linked gene crosses? This depends on the distance between the linked genes on the chromosome This depends on the distance between the linked genes on the chromosome This is measured by map units This is measured by map units The number of map units correlates to the percentage of offspring that have recombined traits The number of map units correlates to the percentage of offspring that have recombined traits

For example In the cross ABab x abab In the cross ABab x abab If A and B are 6 map units apart the recombined alleles Ab and aB will only have a probability of making up 6% of the total offspring (3% each) If A and B are 6 map units apart the recombined alleles Ab and aB will only have a probability of making up 6% of the total offspring (3% each) Parent Gametes ABabAbaB abAaBbaabbAabbaaBb abAaBbaabbAabbaaBb abAaBbaabbAabbaabb abAaBbaabbAabbaabb Offspring AaBb 47% aabb 47% Aabb 3% aaBb 3%

Additional examples In the cross CDcd x cdcd In the cross CDcd x cdcd 8 map units apart 8 map units apart recombined alleles Cd and cD will only have a probability of making up 8% of the total offspring (4% each) recombined alleles Cd and cD will only have a probability of making up 8% of the total offspring (4% each) Parent Gametes CDcdCdcD cdCdDdccddCcddccDd Offspring 46% 4% In the cross EFef x efef In the cross EFef x efef 12 map units apart 12 map units apart recombined alleles Ef and eF will only have a probability of making up 12% of the total offspring (6% each) recombined alleles Ef and eF will only have a probability of making up 12% of the total offspring (6% each) Parent Gametes EFefEfeF efEeFfeeffEeffeeFf Offspring 44% 6%

Pedigree Charts Pedigrees are diagrams which demonstrate the inheritance of a particular trait within a family Pedigrees are diagrams which demonstrate the inheritance of a particular trait within a family Pedigrees can indicate the inheritance modes of particular traits Pedigrees can indicate the inheritance modes of particular traits

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

Are males and females equally affected?

Pedigrees of x-linked recessive traits: Pedigrees of x-linked recessive traits: Males affected more commonlyMales affected more commonly All sons of affected female will also be affectedAll sons of affected female will also be affected All children of 2 affected individuals will also have the traitAll children of 2 affected individuals will also have the trait

Are males and females equally affected? Do all affected individuals have at least one affected parent?

Autosomal Dominant Pedigrees of autosomal dominant traits: Pedigrees of autosomal dominant traits: Males and females affected equally (autosomal)Males and females affected equally (autosomal) Affected individuals must have at least one affected parentAffected individuals must have at least one affected parent Traits cannot ‘skip’ generations Traits cannot ‘skip’ generations No carriersNo carriers

Are males and females equally affected? Do all affected individuals have at least one affected parent?

Autosomal Recessive Pedigrees of autosomal recessive traits: Pedigrees of autosomal recessive traits: All children of 2 affected parents will also have the traitAll children of 2 affected parents will also have the trait Some affected individuals might have 2 unaffected parents if both are carriers:Some affected individuals might have 2 unaffected parents if both are carriers: Can ‘skip’ generations Can ‘skip’ generations

X-linked Dominant Pedigrees of x-linked dominant traits: Pedigrees of x-linked dominant traits: All daughters of affected males will also have the traitAll daughters of affected males will also have the trait Female with the trait can pass it on to both daughters and sonsFemale with the trait can pass it on to both daughters and sons No carriersNo carriers