LU 4 Genetic Diversity II

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MENDEL & THE GENE IDEA

Discoveries :-

Mathematical approach to study heredity

Peas. Why ?

- Char that is easy to be studied which are heritable features

- He can control his experiments because he can remove the stamen to control mating. There would be absolutely no self-fertilisation. He could mate male & female games as he liked.

- Sperm from 1 plant is taken & is added to the carpel of another plant(stamen removed).

- Trait = purple/white colour. = Flower colour, seed colour, seed shape & stem length.

Character = flower

Trait = purple or white

-Only use true breeding plant. After self fertilisation, trait remain constant. IE - pea plants with genes for white flowers, self fertilise, all offspring have genes for only white flowers. Trait will be constant in each generation.

Definition: A true-breeding plant is one that, when self-fertilized, only produces offspring with the same traits. The alleles for these type of plants are homozygous.

Breeding experiments =

Hybridization - mating of 2 true-breeding individuals

P generation - true breeding parents.

Hybrid offspring are produced and called F1 Generation.

Offspring to F1 generation is called F2 Generation. Produced thru self-pollination.

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2 IMPORTANT PRINCIPLES

Law of Segregation

Law of Independent Assortment

1. Law of Segregation

- Each has multiple versions. Versions are alleles.

- There are 2 alleles each organism obtain for each character. 1 allele from mom, 1 from dad.

- If those 2 alleles are diff, then the dominant allele will be expressed while the recessive allele will not be expressed and will stay unnoticeable on the organism's appearance.

These 2 alleles for each char will be separated during meiosis (gamete production) - segregation.

Punnet Square

Used to predict results of genetic crossing between 2 indiv. of unknown genotype.

Homozygous = identical allele for a char (PP, pp, HH,hh)

Heterozygous = 2 diff allele for a char (Pp, Hh,Bb,)

Phenotype = Organism's traits (purple/white)

Genotype = Organism's genetic make up

Unknown Genotype =

- We can use Punnett Square to find a way to find the genotype of unknown organism by using testcross.

- We'll breed this unknown organism with homozygous recessive indiv. to determine it's genotype. We'll know when we use the ratio of phenotypes in the offspring.

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2. Law of Independent Assortment

- Usually mendel would use 1 character at a time. But we can follow 2 char at a time, to demonstrate the Law of Independent Assortment.

- Segregation of allele pairs are independent from one another. The experiments used are called dihybrid cross.

Factors that make genetics not as straight forward as what Mendel Saw.

Incomplete Dominance.

- If two alleles have ID, the phenotype of the heterozygote will be intermediate between the phenotypes of the 2 homozygotes.

- The individual has intermediate phenotype between 2 parents, no dominant allele.

- Example = 4 o'clock plants.

- A condition where neither member of a contrasting allele is completely expressed.

- A cross between heterozygote will show 3 phenotypes, the parents(mum&dad) and the heterozygote(the blend of both)

F1 hybrids have phenotype in between of the parents such as incomplete dominance in snapdragon colour.

In human = sickle cell anemia.

2. Complete Dominance

- Phenotype of the heterozygote will not be able to be differentiated from the phenotype of the homozygous dominant. Example, Bb won't be any different to BB because both traits are dominant.

3. Co- Dominance

- Similar to incomplete dominance where no allele would be dominant. Incomplete dominance, the phenotype would be a blend of the parents' phenotype. But for co-dominance, both phenotype will be expressed.

Tay-Sachs disease

- A disease where the brain is not able to metabolise certain lipids because of a defective enzyme. So the lipid accumulate in the brain.

RR - Normal

Rr - heterozygotes, enzyme is produced but only in half amount compared to the normal homozygotes dominant. But sufficient for normal development.

rr - homozygous recessive, no functional enzyme is produced to metabolise lipids that accumulate in brain cells, so the cells die. Tay Sachs disease is expressed.

Incomplete Dominance VS Co-dominance

- Both alleles are dominant.

- ID = a heterozygous individual blends the two traits

- CD = a heterozygous indiv. expresses both trait simultaneously without blending.

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3. Multiple Alleles.

Multiple alleles occurs when more than two different alleles exists for the same trait. Usually there would be two.

Humans, 30% di-allelic, 70% mono-allelic, and very few are poly-allelic.

3 alleles = 6 genotypes

4 alleles = 10 genotypes

8 alleles = 36 genotypes

Can be derived. 3 alleles, A B C = AA, AB, AC, BB, BC, CC = 6 genotypes.

Poly-allelic = ABO blood system.

- usually associated with tissue types.

- Important to our immune system, it helps recognise between self & non self. Self - own cells. Non self = microbes

ABO blood system

Tri-allelic gene = 6 genotypes (3 alleles)

Allele is the one that controls the production of antigen at the surface of the red blood cells.

IA,IB = codominant to each other & dominant to i.

IA = Antigen A, IB = Antigen B, i= no antigen

Blood A & B has 2 possible genotypes - homozygous & heterozygous = IA IA, IA i. or IB IB, IB i.

AB & O = 1 genotypes

Drosophila

White gene, the gene determining the white colour of Drosophila. At least 150 allele.

While genotype is caused by null allele, where it causes the gene to produce no functional product. --> No pigment produced.

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Pleitropy

Occurs when a single gene influences multiple phenotypic traits

As a result, a mutation in the gene may have an effect on some/all of the traits simultaneously.

Can also effect unrelated char

Examples

PKU (phenylketonuria) = Causes mental retardation, reduced hair & skin pigmentation. - Caused by any large amount of mutations in a single gene that codes for the enzyme phenylalanine hydroxylase. (converts phenylalanine to tyrosine, another to amino acid)

Untreated PKU =

Eczema

Seizures

Mousy odour

Severe mental retardation

2. Sickle Cell Anemia

Occurs when a person carry 2 defective alleles for blood protein, beta-homoglobin. ---> mutation causes sickle cell anemia

Blood cell curl up into weird shaped and therefore cannot carry any oxygen & also block up blood vessels.

Not enough blood, and blocked vessels can cause heart diseases and damaging of other parts of the body like bones and kidneys.

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Epistasis

Occurs when one gene affects the phenotypic characteristics of another gene. / Allele at 1 genetic locus can alter phenotypic expression of alleles at another locus.

Interaction between 2/more genes to control a single phenotype

Char of epistasis

Not producing new phenotypes

One changes/modify/masks the phenotypic expression of the other

Masks = epistatic

Being masked = hypostatic

6 types of ratios

Dominant epistasis (12:3:1)

Recessive epistasis (9:3:4)

Duplicate Recessive genes (9:7)

Dominant-and-recessive interaction (13:3)

Duplicate genes with cumulative effect (9:6:1)

Duplicate dominant genes (15:1)

Picture below.

Dominant Epistatic

Gene for white colour is B

B is dominant to yellow and yellow is dominant to green

Yellow appear when dominant epistatic B is represented by it's recessive allele (bb)

When hypostatic gene A is also recessive, the colour is green.

Recessive epistatic

Recessive masks dominant when in homozygous

Recessive allele aa masks Bb BB bb

Allele B only expresses themselves when epistatic locus A is dominant allele like AA or Aa

Examples

Coat color of Labrador retriever

ABO Blood groups : Bombay phenotype

Phenotypic ratio

F1 = 9:3:3:1

F2= 9:3:4

Bombay Phenotype = found in Bombay india. Results from the inheritance of 2 rare recessive h genes. Often results from consanguineous matings = parents are blood relatives like first cousins.

- Lack normal expression of A, B or O genes they inherit. They lack of the antigens so can't react with anti-A, anti-b, Anti -A,B & anti H

- Transfusing O would cause immediate cell lysis by the powerful anti-H of bombay indiv. So they can only get blood from another bombay indiv.