Genetics Study of genes GQ- How does the understanding of genetics help you interpret the variations you come across everyday?

Genes  Units that carry hereditary information for specific traits  Genes are a sequence of nucleotides on the DNA

Alleles  An allele is an alternative form of a gene  Alleles are located at a specific position on a specific chromosome.  Homologous chromosomes carry alleles on the same location.

Examples  Gene – Trait – Height of Pea plants  Alleles – Tall and Short  Genes – Colour of flower  Alleles – Red, White, Purple, ……   more examples…. 

Homologous Chromosome

Homologous Chromosomes

Allele Manifestations  Complete Dominance  Incomplete Dominance  Co dominance

 Homozygous condition: Both the chromosomes of the homologous pair has the same allele. TT, tt  Heterozygous condition: Each of the chromosome in the homologous pair has different alleles. Tt  Dominant allele : The allele that totally masks the traits of the recessive allele.  Recessive allele : Manifests only when present in homozygous condition.  Phenotype: Physical appearance  Genotype: Genetic configuration

I - Complete dominance  A kind of dominance wherein the dominant allele completely masks the effect of the recessive allele in heterozygous condition. dominancedominant a recessive aheterozygousdominancedominant a recessive aheterozygous  TT – Tall plant  tt – Short plant  Tt- Tall plant

Complete dominance

Mono hybrid cross  Cross of one gene (alleles tall, short)  Parent generation  Gametes  Punnete Square  Off springs  Genotype : 4  Phenotype: 4 Gamete s TT tTt t

Test Cross  Tt x tt  [Work it out]  Gametes  Punnete Square  Off springs  Genotype  Phenotype

Principles of genetics  Law of segregation: Alleles separate during the formation of gametes. One allele of a gene in one gamete.  Eg: gene- height of plant,  Alleles: Tall (T)Short (t)  Law of Independent assortment: Alleles of one gene does not influence another allele of another gene.  Eg: gene 1) Colour (Yellow –Y, green y) 2) Shape (Round R, wrinkled r)  Gametes YR, Yr, yR, yr

II - Incomplete dominance

 Incomplete dominance is a type of inheritance in which both alleles for a specific trait are expressed. This results in a combined phenotype (expressed physical trait). allelesphenotypeallelesphenotype  Gametes are written as Specific trait over trait.

III - Co dominance  A condition in which both alleles of a gene pair in a heterozygous condition are fully expressed, with neither one being dominant or recessive to the other.  Gametes are both mentioned in capital letters  Example …..

Co dominance

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

Possible blood groups

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

Sex Limited Genes  Sex-limited genes are ones that are inherited by both men and women but are normally only expressed in the phenotype of one of them.  Sex-limited genes are ones that are inherited by both men and women but are normally only expressed in the phenotype of one of them.  Genes for beard are present both in male and female, but shows in the phenotypes of males only.

 Genotype Male Female BBBeardNo beard BbBeardNo beard bbNo beard

Sex influenced trait  Sex-influenced traits are that are influenced by the sex.  Baldness allele is dominant in male and recessive in female.  One allele for baldness is enough to show in phenotype in men.

Example  Genotype Male Female  BB Bald Thin hair  Bb Bald Normal  bb Normal hair Normal hair  One allele for baldness is enough to show in phenotype in men.

Summary  Traits or characteristics are passed on from one generation to the next generation  The traits are controlled by genes  Organisms inherit genes in pairs, one from each parent  Some genes are dominant and other genes are recessive  Some genes are neither dominant nor recessive

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Di-hybrid cross

Round Yellow x wrinkled green  [Work out] RrYy x RrYy  Gametes  Punnete Square  Off springs  Genotype  Phenotype