Genetics Genetics Study of genesStudy of genes

GQ- How does the understanding GQ- How does the understanding of genetics help you interpret the of genetics help you interpret the

variations you come across variations you come across everyday?everyday?

GenesGenes

Units that carry Units that carry hereditary hereditary information for information for specific traitsspecific traits

Genes are a Genes are a sequence of sequence of nucleotides on nucleotides on the DNAthe DNA

Alleles Alleles

An allele is an alternative form of a gene An allele is an alternative form of a gene

Alleles are located at a specific position on Alleles are located at a specific position on a specific chromosome. a specific chromosome.

Homologous chromosomes carry alleles Homologous chromosomes carry alleles on the same location.on the same location.

ExamplesExamples

Gene – Trait – Height of Pea plantsGene – Trait – Height of Pea plants Alleles – Tall and ShortAlleles – Tall and Short

Genes – Colour of flowerGenes – Colour of flower Alleles – Red, White , Purple, ……Alleles – Red, White , Purple, …… more examples….more examples….

Homologous ChromosomeHomologous Chromosome

Homologous ChromosomesHomologous Chromosomes

Allele ManifestationsAllele Manifestations

Complete DominanceComplete Dominance

Incomplete DominanceIncomplete Dominance

Co dominanceCo dominance

Homozygous condition: Both the chromosomes Homozygous condition: Both the chromosomes of the homologous pair has the same allele. TT, of the homologous pair has the same allele. TT, tttt

Heterozygous condition: Each of the Heterozygous condition: Each of the chromosome in the homologous pair has chromosome in the homologous pair has different alleles. Tt different alleles. Tt

Dominant allele : The allele that totally masks Dominant allele : The allele that totally masks the traits of the recessive allele.the traits of the recessive allele.

Recessive allele : Manifests only when present Recessive allele : Manifests only when present in homozygous condition.in homozygous condition.

Phenotype: Physical appearancePhenotype: Physical appearance Genotype: Genetic configurationGenotype: Genetic configuration

I - Complete dominance I - Complete dominance

A kind of A kind of dominancedominance wherein the dominant wherein the dominant allele completely masks the effect of the allele completely masks the effect of the recessive allele in heterozygous recessive allele in heterozygous condition.condition.

TT – Tall plantTT – Tall plant tt – Short plant tt – Short plant Tt- Tall plantTt- Tall plant

Complete dominanceComplete dominance

Mono hybrid crossMono hybrid cross

Cross of one gene Cross of one gene (alleles tall, short)(alleles tall, short)

Parent generationParent generation GametesGametes Punnete SquarePunnete Square Off springsOff springs Genotype : 4 Genotype : 4 Phenotype: 4Phenotype: 4

Gametes

T T

t Tt Tt

t Tt Tt

Test CrossTest Cross

Tt x ttTt x tt [Work it out][Work it out] GametesGametes Punnete SquarePunnete Square Off springsOff springs GenotypeGenotype PhenotypePhenotype

Principles of geneticsPrinciples of genetics Law of segregationLaw of segregation: Alleles separate during : Alleles separate during

the formation of gametes. the formation of gametes. One allele of a One allele of a gene in one gamete.gene in one gamete.

Eg: gene- height of plant,Eg: gene- height of plant, Alleles: Tall (T)Short (t)Alleles: Tall (T)Short (t) Law of Independent assortment: Law of Independent assortment: Alleles of Alleles of

one gene does not influence another allele of one gene does not influence another allele of another gene.another gene.

Eg: gene Eg: gene 1)1) Colour (Yellow –Y, green y) Colour (Yellow –Y, green y) 2)2) Shape (Round R, wrinkled r) Shape (Round R, wrinkled r) Gametes YR, Yr, yR, yr Gametes YR, Yr, yR, yr

II - Incomplete dominanceII - Incomplete dominance

Incomplete dominance is a Incomplete dominance is a type of inheritance in which type of inheritance in which both alleles for a specific trait both alleles for a specific trait are expressed. This results in are expressed. This results in a combined phenotype a combined phenotype (expressed physical trait). (expressed physical trait).

Gametes are written as Gametes are written as Specific trait over trait.Specific trait over trait.

III - Co dominanceIII - Co dominance

A condition in which both alleles of a gene A condition in which both alleles of a gene pair in a heterozygous condition are fully pair in a heterozygous condition are fully expressed, with neither one being expressed, with neither one being dominant or recessive to the other.dominant or recessive to the other.

Gametes are both mentioned in capital Gametes are both mentioned in capital lettersletters

Example …..Example …..

Co dominanceCo dominance

Multiple allelesMultiple alleles

A gene with more than two alleles (caused by A gene with more than two alleles (caused by mutations of the same gene)mutations of the same gene)

Possible blood groupsPossible blood groups

Polygenic inheritance Polygenic inheritance

Trait determined by more than one gene Trait determined by more than one gene Eg, Human eye color

Sex Limited GenesSex Limited Genes

Sex-limited genesSex-limited genes are ones that are are ones that are inherited by both men and women but are inherited by both men and women but are normally only expressed in the phenotype normally only expressed in the phenotype of one of them.  of one of them. 

Genes for beard are present both in male Genes for beard are present both in male and female, but shows in the phenotypes and female, but shows in the phenotypes of males only.of males only.

Genotype Male FemaleGenotype Male Female

BB Beard No beard

Bb Beard No beard

bb No beard No beard

Sex influenced traitSex influenced trait

Sex-influenced traits are that are Sex-influenced traits are that are influenced by the sex.influenced by the sex.

Baldness allele is dominant in male and Baldness allele is dominant in male and recessive in female.recessive in female.

One allele for baldness is enough to show One allele for baldness is enough to show in phenotype in men.in phenotype in men.

ExampleExample

Genotype Male FemaleGenotype Male Female BB Bald Thin hair BB Bald Thin hair Bb Bald NormalBb Bald Normal bb Normal hair Normal hairbb Normal hair Normal hair

One allele for baldness is enough to One allele for baldness is enough to show in phenotype in men.show in phenotype in men.

SummarySummary Traits or characteristics are passed on Traits or characteristics are passed on

from one generation to the next generationfrom one generation to the next generation The traits are controlled by genesThe traits are controlled by genes Organisms inherit genes in pairs, one from Organisms inherit genes in pairs, one from

each parenteach parent Some genes are dominant and other Some genes are dominant and other

genes are recessivegenes are recessive Some genes are neither dominant nor Some genes are neither dominant nor

recessiverecessive

Virtual labVirtual lab

http://www2.edc.org/weblabs/Mendel/http://www2.edc.org/weblabs/Mendel/MendelMenu.htmlMendelMenu.html

Di-hybrid crossDi-hybrid cross

Round Yellow x wrinkled green Round Yellow x wrinkled green

[Work out] RrYy x RrYy[Work out] RrYy x RrYy GametesGametes Punnete SquarePunnete Square Off springsOff springs GenotypeGenotype PhenotypePhenotype