Mendelian &Transmission Genetics

Father of Genetics: Gregor Mendel1. Law of Dominance: Dominance is a attribute of a pair

of alleles in relation to a particular phenotype. For eg. all of the F1 progeny exhibit only one of the parent’s traits and the other is not expressed (all yellow seeds).

2. Law of Segregation: The pair of alleles of genes segregate independently (green independent of yellow)eg. F2 generation in dihybrid cross have phenotypic ratio

of 3 yellow : 1 green while genotype ratio is 1:2:1.

3. Law of Independent Assortment: Genes separate in gametes and join randomly in the progeny i.e. presence of one does not effect the assortment of other.

Mendel's Laws 2 & 3

Fig. 2.10

WW

ww

GG gg

WWgg

wwggwwGG

WWGG

Wrinkled phenotype is due to an inborn error in Starch Branching Enzyme I synthesis

Di Hybrid Cross: Looking at two inherited traits at the same time and their inheritance / relationship from the parents to the offspring.

Also called Test Cross or Backcross

Trihybrid Genetic Cross

Trihybrid cross = 3 pairs of traits that assort independently, such as WwGgPp are studied in this kind of cross.

For any pair, phenotypic ratio = 3:1 & For 2 pairs, ratio is = 9:3:3:1

Therefore, theTrihybrid cross pattern of segregation and independent assortment is identical to dihybrid cross

Addition Rule: The probability of the realization of one or the other of two mutually exclusive possibilities, A or B, is the sum of their separate probabilities.

Multiplication Rule: The probability of two independent possibilities, A and B, being realized simultaneously is given by the product of their separate probabilities.

Incomplete Dominance

Incomplete dominance = the phenotype of the heterozygote is intermediate between the phenotypes of the homozygote

Incomplete dominance is often observed when the phenotype is quantitative rather than discrete

ie. The amount of gene product matters to the phenotype rather than just presence or absence of the gene.

Note: the phenotype and gentotype ratios in ID are the same as one can distinguish the heterozygote from the homozygote.

Multiple Alleles/Codominance

People of

blood type O make both anti-A and anti-B antibodies but do not contain either A or B antigens. “Universal donor”

blood type A contains A antigen & makes anti-B antibodies

blood type B contains B antigen make anti-A antibodies

blood type AB contains both A and B antigens and make neither type of antibody. Therefore, they can receive blood from O, A or B blood types but cannot donate to any of them “Universal acceptor”.

The blood types are therefore an example of codominance.

Expressivity and Penetrance

Variation in the phenotypic expression of a particular genotype may happen because other genes modify the phenotype or because the biological processes that produce the phenotype are sensitive to environment (environmental effect).

This is why identical twins even though they have the same genes may have some differences due to the effect of the environment on the expression of the trait.

Variable expressivity refers to genes that are expressed to different degrees in different organisms

Penetrance refers to the proportion of organisms whose phenotype matches their genotype for a given trait. A genotype that is always expressed has a penetrance of 100 percent

Pedigree Analysis

In humans, pedigree analysis is used to determine individual genotypes and to predict the mode of transmission of single gene traits

Epistasis & Complementation

Epistasis refers to any type of gene interaction that results in the F2 dihybrid ratio of 9 : 3 : 3 : 1 being modified into some other ratio due to one gene’s expression masking the expression of the other gene.

Flower color in peas: formation of the purple pigment requires the dominant allele of both the C and P genes: the F2 ratio is modified to 9 purple: 7 white.

Complementation means that the mutations are in different genes

Mutations in the same gene do not complement each other

Complementation group - a group of mutations that fail to complement each other

DNA, Chromosomes and Genes The germ cells, or gametes, are haploid and contain

only one set of chromosomes, consisting of one member of each of the pairs

The haploid gametes unite in fertilization to produce the diploid state of somatic cell

The chromosomes are present in pairs just like Mendel’s factors or genes.

The genes are the protein coding parts of the chromosome and are arranged in linear order on the chromosomes (small % of the total genome actually codes for genes).

Rest of the stuff nobody really knows what it does!

Cell Cycle and Mitosis:

Fig. 3.2

Final Stages of MitosisIn Anaphase, the centromeres divide longitudinally, the

two sister chromatids of each chromosome separate, and move toward opposite poles of the spindle (basis for independent assortment).

In Telophase, a nuclear envelope forms around each compact group of chromosomes, nucleoli are formed, and the spindle disappears

The chromosomes undergo decondensation until they are no longer visible as discrete entities

The two daughter nuclei assume a typical interphase appearance and the cytoplasm of the cell divides (cytokinesis) in two.

Mitosis vs. Meiosis Meiosis produces 4 cells: each contains 1 copy of each

pair of homologous chromosomes = genetically haploid.

The chromatids of a chromosome are not genetically identical to their parents because of crossing-over associated with the formation of chiasmata during prophase of the first division.

Crossing over or recombination occurs due to the exchange of genetic material between the 2 sister chromatids.

Mitosis produces 2 cells containing both members of each pair of homologous chromosomes = genetically diploid

Chromosome Structure

Histones are proteins that are basic in nature and are highly conserved among different organisms (very few mutations are found in these proteins are found as they are not tolerated).

Eukaryotic chromosomes are highly coiled stable complexes of DNA and histone called chromatin. It is believed that the histones regulate/ determine the structure and function of DNA.

The nucleosome is the basic structural unit of chromatin.

Fig. 3.15a

Fig. 3.21 (Micrograph courtesy of T.C. Hsu and Sen Pathak)

Nucleosomes coil to form higher order DNA structure called the 30-nm chromatin fiber

In the nucleus of a nondividing cell, chromatin fibers form discrete chromosome territories

Chromatin Structure

Compact and heavily stained regions of chromatin the heterochromatin mainly consists of highly repeated non-coding DNA sequences—satellite DNA

The rest of the chromatin, which becomes visible only after chromosome condensation in mitosis or meiosis, is called euchromatin.

Non Disjunction in Drosophila Female Flies: Proof of Chromosomal Theory of Heredity

Nondisjunction = chromosomes fail to separate (disjoin) and move to opposite poles of the division spindle, results in loss or gain of a chromosome.

Data analysis

Genetic data analysis makes use of probability and statistics

Progeny of crosses are predicted by the binomial probability

If the probability of possibility A is p and the probability of the alternative possibility B is q, then the probability that, in n trials, A is realized s times and B is realized t times is given by the eqn.

n!psqt

s!t!

Fig. 3.34

Last Slide: Thanks for coming

I am ready to address any other questions that you have.

Thanks, Everyone! It's been fun chatting with you this evening.

I'll see you in discussion!

Good night everyone!