Chapter 10

Mendel’s Pea Plant Experiments

Gregor Johann Mendel Austrian monk Studied the inheritance of traits in pea plants Developed the laws of inheritance He found that the plants' offspring retained traits of the parents He did not know at the time he was observing allele/gene inheritance

Site of Gregor Mendel’s experimental garden in the Czech Republic Mendelian Genetics 5/19/2019 Site of Gregor Mendel’s experimental garden in the Czech Republic Fig. 5.co

Particulate Inheritance Mendel stated that physical traits are inherited as “particles” Mendel did not know that the “particles” were actually chromosomes & DNA

How Mendel Began Mendel produced pure strains by allowing the plants to self-pollinate for several generations

Mendel’s Experimental Results

Did the observed ratio match the theoretical ratio? The theoretical or expected ratio of plants producing round or wrinkled seeds is 3 round :1 wrinkled Yes, as Mendel’s observed ratio was 2.96:1 The discrepancy is due to statistical error The larger the sample, the more the results approximate to the theoretical ratio

Summary of Mendel’s laws PARENT CROSS OFFSPRING DOMINANCE TT x tt tall x short 100% Tt tall SEGREGATION Tt x Tt tall x tall 75% tall 25% short INDEPENDENT ASSORTMENT RrGg x RrGg round & green x round & green 9/16 round seeds & green pods 3/16 round seeds & yellow pods 3/16 wrinkled seeds & green pods 1/16 wrinkled seeds & yellow pods

Law of Dominance This law of Mendel’s states that traits are either dominant or recessive The law does NOT apply to codominant inheritance!!

Law of Segregation During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other and only 1 allele is inherited per gamete. Alleles can be equally passed onto male or female offspring

Applying the Law of Segregation

This law does not consider nondisjunction or sex-linked genes

Law of Independent Assortment Alleles for different traits are distributed to sex cells (& offspring) independently of one another. This law does NOT apply to linked genes and does not consider random reassortment in meiosis.

Traits are controlled by 2 “factors” Since he only examined dominant and recessive traits, he concluded only 2 factors (alleles) control each trait This law does not apply to multiple alleles or polygenic traits (traits coded for by more than one gene…e.g. skin colour)

Other exceptions from Mendels Laws Mutations (the traits he examined remained the same and stable throughout generations)

DNA – Discovered after Mendel Structure Double helix Made of nucleotides Nucleotides joined via bonds between the phosphates and sugars (phosphodiester bond)

DNA has direction DNA has a 5’ and a 3’ end The strands run ANTIPARALLEL (they run in opposite directions to eachother)

Weak hydrogen bonds are between complementary base pairs

Chargaff’s Rule The proportions of the bases A and T are equal The proportions of the bases G and C are equal Indicated that A binds to T, and G binds with C (or vice versa)

Some terms….. Dissociation – the separation of both DNA strands, using heat, to break hydrogen bonds between complementary base pairs Re-association – the binding of both DNA strands to each other again when DNA is cooled Hybridisation – pairing of complementary base pairs from different sources

Mitochondrial DNA Chloroplasts and mitochondria have DNA too. Their DNA is circular and codes for proteins and RNA required for photosynthesis or cellular respiration

Mitochondrial DNA (mtDNA) D – loop: non-coding region of mtDNA. The part where mtDNA replication begins from The part where most mutations occur

mtDNA is passed from the mother to sons and daughters (no paternal mt DNA is in any children)

Gene Sequencing Identifying the order (sequence) of nucleotides in a gene Done using computers Can identify genetic disorders, identify products of genes, compare sequences of different species to determine how closely related they are or which species proteins might be used in humans (e.g. insulin) Human Genome Project – aims to sequence all the human genome

Single Nucleotide Polymorphisms (SNP’s) Not everyone’s genome is identical Many of the differences are due to single base changes (called SNP’s) in non-coding regions of DNA (regions between genes or in non-coding regions within a gene – called introns) Base changes in coding regions could alter protein function

SNP’s

Introns and Exons Introns are NON-CODING regions in a gene. They do not contribute to the code for the protein that the gene codes for.

There are also non-coding regions between genes Regulatory sequence – controls switching genes on or off, and how fast the gene is decoded Coding region – codes for protein

Gene structure Flanking region – regions either side of the coding region. Includes: Upstream region – the region before the coding region - contains the START codon (3 nucleotide sequence signalling the start of the coding region) contains regulatory sequences (control the gene being switched on or off, and how fast it is transcribed) Downstream region – the region after the coding region - contains the STOP codon (3 nucleotide sequence to signal the end of the coding region)

Gene structure Flanking regions (non-coding) Upstream region Downstream region

Comparative Genomics Comparing gene sequences, and chromosome number and structure (karyotypes) between species Can show how closely related, according to evolution, species are to each other Can also show conserved genes – genes present in multiple species and therefore carry out important functions for survival.

More terms…. Also discovered in comparative genomics: Gene duplication – when multiple copies of a gene are present in a species Horizontal gene transfer – a gene from one species transfers to another species

Horizontal gene transfer example