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TYPES OF GENE ACTION AND MODIFICATION OF MENDELIAN

RATIOS

• Complete dominance – a type of gene action in which one allele at locus completely masks the effect of another allele at the same locus.

• All the allelic pairs that Mendel studied showed complete dominance/complete recessiveness relationships i.e. one allele at a locus is dominant to the other allele, so that the phenotype of the homozygous dominant and heterozygous genotypes are the same.

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• The recessive genotype is phenotypically expressed only when the organism is homozygous.

• However, the inheritance of some traits show exceptions, the allelic pairs do not show complete dominance relationship.

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Complete dominance: Inheritance of black and red colour in cattle

P Generation Female parent x Male parent Phenotype Black Red Genotype BB bb Gametes All B All b F1 Genotype All Bb F1 Phenotype All black

Interbreeding of F1 F1 Generation Female x Male F1 Phenotype Black Black F1 Genotype Bb Bb F1 Gametes ½ B, ½ b ½ B, ½ b

F1 Male gametes F1 Female gametes ½ B ½ b

½ B ¼ BB ¼ Bb ½ b ¼ Bb ¼ bb

F2 Genotypes: ¼ BB, ½ Bb, ¼ bb F2 Phenotypes: ¾ black, ¼ red F2 Phenotypic ratio: 3: 1 for black: red

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Incomplete dominance

• Incomplete (partial) dominance is shown when the dominant allele is not completely dominant to the recessive allele and the heterozygote’s phenotype is closer to the dominant phenotype than to the recessive phenotype, but is not identical to the dominant phenotype.

• Plumage colour in chickens is a good example of incomplete dominance. Crosses between a true-breeding black strain (CBCB) and a true-breeding white strain (CWCW) give F1 progeny with bluish-grey plumage (CBCW).

• Interbreeding the F1 birds produces black, bluish-grey and white fowl in a ratio of 1:2:1.

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Inheritance of plumage colour in chickens

P generation: Female x Male Phenotype:Black plumage White plumage Genotypes: CB/CB CW/CW Gametes: All CB all CW F1 genotypes: All CB/CW F1 Phenotypes: All bluish-grey (due to incomplete dominance) Interbreeding of F1 F1 generation: Female x Male F1 phenotype: Bluish-grey Bluish-grey F1 genotypes: CB/CW CB/CW F1 gametes: ½ CB, ½ CW ½ CB, ½ CW

F1 male gametes F1 female gametes ½ CB ½ CW

½ CB ¼ CB/CB ¼ CB/CW

½ CW ¼ CB/CW ¼ CW/CW

F2 Genotypes ¼ CB/CB, ½ CB/CW, ¼ CW/CW F2 Phenotypes ¼ black, ½ bluish-grey ¼ white F2 Phenotypic ratio is 1:2:1 for black:bluish-grey:white

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• In plants, incomplete dominance is shown by flower colour in the snapdragon. A cross of a red-flowered variety (CRCR) with a white-flowered variety (CWCW) produces heterozygote (CR CW) F1 plants with pink flowers.

• Interbreeding the F1 plants produces an F2 with a 1:2:1 ratio of red:pink:white flowered plants.

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Inheritance of flower colour in snapdragon plants

P generation: Female x Male Phenotype: Red White Genotypes: CR/CR CW/CW Gametes: All CR all CW F1 genotypes: All CR/CW F1 Phenotypes: All pink (due to incomplete dominance)

Interbreeding of F1 F1 generation: Female x Male F1 phenotype: Pink Pink F1 genotypes: CR/CW CR/CW F1 gametes: ½ CR, ½ CW ½ CR, ½ CW

F1 male gametes F1 female gametes ½ CR ½ CW

½ CR ¼ CR/CR ¼ CR/CW

½ CW ¼ CR/CW ¼ CW/CW

F2 Genotypes ¼ CR/CR, ½ CR/CW, ¼ CW/CW F2 Phenotypes ¼ red, ½ pink ¼ white F2 Phenotypic ratio is 1:2:1 for red:pink:white

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Codominance

• In codominance both alleles of a pair are fully expressed in a heterozygote individual, thus the F1 heterozygote exhibits the phenotypes of both homozygote parents.

• There is no dominant or recessive allele. Both alleles contribute equally to the production of the phenotypes, so the heterozygous genotype produces a phenotype that is intermediate between those produced by the two homozygous genotypes.

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• The ABO blood system provides a good example of codominance. Heterozygote IA/IB individuals are blood group AB because both the A antigen (product of the IA allele) and the B antigen (product of the IB allele) are produced. Thus, the IA and IB alleles are codominant.

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Inheritance of ABO blood group in human

P generation Female x Male Phenotype blood group A blood group BGenotype IA/IA IB/IB

Gametes All IA All IB

F1 Genotypes All IA/IB

F1 Phenotypes All blood group AB

Interbreeding F1F1 generation Female x Male F1 Phenotype blood group AB blood group ABF1 Genotype IA/IB IA/IB

F1 Gametes ½ IA, ½ IB ½ IA, ½ IB

F2 genotypes ¼ IA/IA, ½ IA/IB ¼ IB/IB

F2 Phenotypes ¼ blood group A ½ blood group AB ¼ blood group B

F2 Phenotypic ratio 1:2:1 for blood group A: blood group AB : blood group B

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• The inheritance of red, white and roan coat colour in cattle is another example of codominance. The R allele leads to the production of red pigment in the coat hairs while the W allele leads to no pigment production.

• In animals with RR genotype all the hairs are red. Animals with WW genotype appear white. Heterozygote animals (RW) have some pigmented hairs and some unpigmented hairs, and they appear roan.

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Inheritance of red, white and roan colour in cattle

P generation: Female x Male Phenotype: White Red Genotypes: WW RR Gametes: All W all R F1 genotypes: All RW F1 Phenotypes: All roan

Interbreeding of F1 F1 generation: Female x Male F1 phenotype: Roan roan F1 genotypes: RW RW F1 gametes: ½ R, ½ W ½ R, ½ W

F1 male gametes F1 female gametes ½ R ½ W

½ R ¼ RR ¼ RW

½ W ¼ RW ¼ WW

F2 Genotypes ¼ RR, ½ RW, ¼ WW F2 Phenotypes ¼ red, ½ roan ¼ white F2 Phenotypic ratio is 1:2:1 for red:roan:white

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Overdominance

• Overdominance –type of gene action in which the performance of heterozygous animals is more extreme that that of both homozygous.

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Gene interactions that produce new phenotypes• In Mendel’s dihybrid crosses, the genes for different

traits acted independently, e.g. the allelic pair for smooth/wrinkled seeds had no effect on the allelic pair for long/short stem.

• However, there are cases whereby two allelic pairs affect the same phenotypic characteristic and nonallelic genes interact to give new phenotypes and result in a modified phenotypic ratio.

• A classic example of such gene interactions is comb shape in chickens. There are four comb shape phenotypes which result from combinations of alleles at two different loci. Rose comb results from R/- p/p, pea comb results from r/r P/-, walnut comb results from R/- P/- and single comb is a result of r/r p/p genotypes.

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The interaction of genes for comb shape in fowl P generation Female x Male Parental phenotype Rose comb Pea comb Parental genotype R/R p/p r/r P/P Gametes All Rp all rP F1 genotypes R/r P/p F1 phenotypes All walnut Interbreeding of F1 F1 generation Female F1 x Male F1 F1 phenotypes: walnut walnut F1 genotype: R/r P/p R/r P/p F2 phenotypes F2 phenotypes F2 phenotypic For R/r x R/r for P/p x P/p proportions ¾ P/- 9/16 R/- P/- walnult comb ¾ R/- ¼ p/p 3/16 R/- p/p rose comb ¾ P/- 3/16 r/r P/- Pea comb ¼ r/r ¼ p/p 1/16 r/r p/p single comb A crosses of rose-combed bird with a pea combed bird gives all wanult-combed offspring in the F1. When the F1 are interbred, a 9:3:3:1 ratio of walnut:rose:pea:single combed birds occurs in F2.

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Epistasis

• Epistasis is a form of gene interaction in which an allele at one locus interferes or masks the effect of another allele at a different locus.

• A gene that masks another gene’s expression is said to be epistatic and the gene whose expression is masked by a nonallelic gene is said to be hypostatic.

• An example of epistasis is coat colour in rodents. When true-breeding agouti mice are crossed with albino (white) mice, the F1 are all agouti. When the F1 are interbred, the phenotypic ratio in the F2 progeny is 9:3:4 for agouti:black: albino.

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Agouti (banded) hair

Agouti

Non-agouti

Eumelanin

Phaeomelanin

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The interaction of genes for coat colour in rodents P generation Female x Male Parental phenotype Agouti Albino Parental genotype A/A C/C a/a c/c Gametes All AC all ac F1 genotypes A/a C/c F1 phenotypes All agouti Interbreeding of F1 F1 generation Female F1 x Male F1 F1 phenotypes: agouti agouti F1 genotype: A/a C/c A/a C/c F2 ratio F2 ratio F2 phenotypic For A/a x A/a for C/c x C/c proportions ¾ C/- 9/16 A/- C/- agouti ¾ A/- ¼ c/c 3/16 A/- c/c albino ¾ C/- 3/16 a/a C/- black ¼ a/a ¼ c/c 1/16 a/a c/c albino Phenotypically A/- c/c and a/a c/c are albino, giving a 9:3:4 ratio of agouti:black:albino. There is epistasis of c/c over A/-, that is, white hairs are produced in c/c mice regardless of the genotype at the other locus.

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Pleiotropy

• Pleiotropy is shown when an allele at a single locus has an influence on more than one characteristic. A gene which affects several characteristics is said to have pleiotropic effect.

• In several breeds of cattle, a single gene responsible for double muscling is also associated with reduced fertility, lower calf survival and sometimes increased stress susceptibility.

• In goats, the polled condition leads to the development of intersex animals, rather than females.

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Sex-limited genes

• Sex-limited genes are autosomal genes (genes located on autosome chromosomes, i.e. not located on the sex chromosomes) that affect traits which appear only in one sex, but not in the other sex.

• Traits of this kind are called sex-limited traits e.g. milk production in dairy cattle, the formation of breast in human and the ability to produce eggs in chicken. The genes involved in these traits operate in females but not in males.

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Inheritance of sex-limited trait e.g. the feathering pattern in chickens P generation: Female x Male Phenotype: hen-feathered cock-feathered Genotypes: h+/h+ h/h Gametes: All h+ all h F1 genotypes: All h+/h F1 Phenotypes: All hen-feathered

Interbreeding of F1 F1 generation: Female x Male F1 phenotype: hen-feathered hen-feathered F1 genotypes: h+/h h+/h F1 gametes: ½ h+, ½ h ½ h+, ½ h

F1 male gametes F1 female gametes ½ h+ ½ h

½ h+ ¼ h+/h+ ¼ h+/h

½ h ¼ h+/h ¼ h/h

F2 Genotypes ¼ h+/h+, ½ h+/h, ¼ h/h F2 Phenotypes in females h+/h+, h+/h and h/h are hen-feathered in males h+/h+, h+/h are hen-feathered while h/h are cock-feathered.

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Sex-influenced traits

• The expression of some genes may be sex influenced. The traits controlled by these autosomal genes appear in both sexes, but either the frequency of occurrence in the two sexes is different or the relationship between genotype and phenotype is different.

• An example is pattern baldness in humans. The b/b genotype specifies pattern baldness in both males and females and the b+/b+ genotype gives a nonbald phenotype in both sexes. The difference is in the heterozygotes, in males b+/b leads to the bald phenotype but in females it leads to the nonbald phenotype. In other words, the b allele acts as a dominant in males but a recessive in females.

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Sex-linked genes

• Genes are either located on the autosomes (on any chromosome except the sex chromosomes) or sex chromosomes.

• Gene located on the sex chromosomes are said to be sex-linked and have different patterns of inheritance.

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The inheritance of X-linked genes: red eyes and white eyes in Drosophila melanogaster

P generation parent 1 x parent 2 Parental phenotype Female, red -eyes Male, white eyes Parental genotype Xw+ Xw+ Xw Y Gametes all Xw+ ½ Xw , ½ Y F1 generation F1 genotype ½ Xw+

Xw , ½ Xw+ Y

F1 phenotype ½ female, red eyes ½ male, red eyes

Interbreeding F1 generation parent 1 x parent 2 F1 phenotype Female, red eyes Male, red eyes F1 genotype Xw+

Xw Xw+ Y F1 Gametes ½ Xw+

, ½ Xw ½ Xw+ , ½ Y

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F2 generation Male gametes Female gametes ½ Xw+ ½ Y

½ Xw+ ¼ Xw+Xw+ ¼ Xw+

Y

½ Xw ¼ Xw+ Xw ¼ XwY

F2 genotype ¼ Xw+Xw+, ¼ Xw+

Xw, ¼ Xw+ Y, ¼ XwY

F2 phynotype: all females are red eyes,

½ males are red eyes and ½ males are white eyes

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Age of onset

• All genes do not continually function, instead over time, there is programmed activation and deactivation of genes as the organism develops and functions.

• The age of the organism reflects internal environmental changes that can affect gene function.

• Numerous age-dependent genetic traits occur in humans e.g. pattern baldness appears in males between 20 and 30 years of age, duchenne severe muscular dystrophy appears in children between 2 and 5 years of age.

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Genotype x environment interaction

• Genotype x environment interaction occurs when genotypes do not rank the same in different environment or when the advantage of a particular genotype in one environment is smaller or greater than in another environment.

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(i) Genotypes A and B rank differently in the two environments

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Environment1

Environment2

Animal AAnimal B

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(ii) Genotypes A and B rank the same in the two environments i.e B is the best in both

environment, but the advantage of B is much less in environment 2

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101520253035404550

Environment1

Environment2

Animal AAnimal B

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Penetrance and Expressivity

• The frequency with which a dominant or homozygous recessive gene manifests itself in individuals in a population is called the penetrance of the gene.

• Penetrance depends on both the genotype (e.g. the presence of epistatic or other genes) and the environment.

• In some cases not all individuals who are known to have a particular genotype show the phenotype specified by the gene.

• Penetrance is complete (100%) when all the homozygous recessives show one phenotype, when all the homozygous dominants show another phenotype and when all the heterozygotes are alike.

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• If less than than 100% of the individuals with a particular genotype exhibit the phenotype expected, penetrance is incomplete. If , say, 80% of the individuals carrying a particular gene show the corresponding phenotype, we say that there is 80% penetrance.

• Expressivity refers to the degree to which a genotype is phenotypically expressed.

• Expressivity may be described either qualitatively or quantitatively e.g. it may be referred to as severe, intermediate or slight.

• Like penetrance, expressivity depends on the genotype and the internal and external environments, and it may be constant or variable.

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Essential genes and lethal alleles• An allele that results in the death of an organism is

called a lethal allele, and the gene involved is called an essential gene.

• Essential genes are genes which, when they are mutated can result in a lethal phenotype.

• If the mutation is due to a dominant lethal allele, both homozygotes and heterozygotes for that allele will show the lethal phenotype. If the mutation is due to a recessive lethal allele, only homozygotes for that allele will be lethal.

• An example is yellow body colour in mice. The yellow allele has a dominant effect with regard to coat colour, but acts as a recessive allele with respect to the lethality phenotype.

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Inheritance of lethal allele in mice

P generation Female parent x Male parent Parental phenotype Yellow mouse Yellow mouse Parental genotype AY/A AY/A Gametes ½ AY, ½ A ½ AY, ½ A F1 generation

Male gametes Female gametes ½ AY ½ A

½ AY ¼ AY/ AY ¼ AY/A

½ A ¼ AY/A ¼ A/A

F1 genotypes ¼ AY/AY, ½ AY/A , ¼ A/A F1 phenotypes ¼ die, ½ yellow, ¼ nonyellow Birth ratio 2/3 AY/A yellow, 1/3 A/A nonyellow AY allele is a recessive lethal allele because it causes death in the homozygous state.