Inheritance cont.. Dihybrid crosses – two genes The crosses discussed so far relate to one trait...
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Transcript of Inheritance cont.. Dihybrid crosses – two genes The crosses discussed so far relate to one trait...
Inheritance cont.
Dihybrid crosses – two genesThe crosses discussed so far relate to one trait
(e.g., flower colour).
Mendel’s second Principle of Inheritance can be illustrated by considering crosses involving two genes that affect two traits.
We can cross pure-breeding strains of flies that differ for two traits: eye colour and body colour and then conduct a dihybrid cross (‘di’ meaning two).
Brown body, yellow eyes—independent assortment The two traits in this example are eye colour
and body colour in Drosophila.
The eye-colour gene in this example is the ‘yellow eye’ gene. It is a different gene from the white eye-colour gene previously discussed and is located on a different chromosome.
The yellow-eye gene is autosomal; the alleles are Y and y.
The wild-type red-eye phenotype (genotypes YY and Yy) is dominant and yellow (genotype yy) is recessive.
Brown body, yellow eyes—independent assortment The second trait is body colour and the gene in
this example is called ‘brown body’.
It is an autosomal gene with two alleles B and b.
The wild-type green body colour phenotype (genotypes BB and Bb) is dominant and brown body (genotype bb) is recessive.
The two genes yellow eye and brown body are on different chromosomes.
F generation
First we set up a cross between a pure-breeding red eye, green body (YY;BB) strain and a yellow eye, brown body (yy;bb) strain.
The homozygous YY;BB strain will produce only Y;B gametes and the homozygous yy;bb strain will produce only y;b gametes.
These gametes fuse to produce F 1 progeny, which are heterozygous for both genes (Yy;Bb).
Following meiosis, a gamete will end up with any of four possible combinations of alleles: Y;B, y;b, y;B or Y;b.
During gamete formation by the F1 fl y, the chance of a sperm or egg cell containing a Y allele is 1/2 (as ½ of the gametes contain a y allele).
The chance that the same gamete will contain a B allele is also ½ .
Therefore the chance of the gamete being Y;B is 1/ 2 × 1/2 = 1/4.
The probability of each of the other three possible gametic combinations is also 1/ 4.
This is because the segregation of alleles of a gene on one chromosome in meiosis is independent of the segregation of alleles of a gene on another chromosome.
F2 generation The heterozygotes generated in the F 1 can be
crossed together (a dihybrid cross) to produce an F 2 generation.
The genotypes and phenotypes of the F progeny are shown in the Punnett square.
The expected ratio of phenotypes in the F 2 generation is –
9 red eye, green body : 3 red eye, brown body : 3 yellow eye, green body : 1 yellow eye, brown body.
If these crosses were actually performed, the phenotypic ratio in the F 2 generation would be close to the 9 : 3 : 3 : 1 ratio.
Dihybrid cross In summary, a phenotypic ratio approximating 9 : 3
: 3 : 1 will be observed in the F 2 generation of a dihybrid cross if the following five conditions apply:
the two genes control two distinct traits
there are two alleles for each of the genes
for each trait one phenotype is dominant
both genes are on autosomes
the two genes assort independently.
In this example, independent assortment occurs because the two genes are on different chromosomes. However, we will see that independent assortment can occur via another mechanism.
For the cross AaBb x AaBbGametes
¼ AB ¼ Ab ¼ Ab ¼ ab
¼ AB AABB AABb AaBB AaBb
¼ Ab AABb AAbb AaBb Aabb
¼ Ab AaBB AaBb aaBB aaBb
¼ ab AaBb Aabb aaBb aabb
The resulting phenotypes show a 9:3:3:1 ratio
Testcrosses and phenotypic ratios If an individual has a dominant phenotype, we
can find out how many genes control the phenotype by carrying out a testcross.
A testcross is a particular type of backcross.
A backcross is a cross between the F 1 heterozygote and either one of the pure- breeding (homozygous) parental strains.
A testcross involves crossing the F 1 heterozygote with the parental strain showing the recessive phenotype(s).
Testcrosses are also used to determine whether an individual of dominant phenotype is homozygous or heterozygous.
An example:Another example can be seen
in your textbook on pg 225.
Also see page 227 for ratios.
So far…. Transmission of heritable characteristics:
Genes as units of inheritance Eukaryote chromosomes, alleles, prokaryote
chromosomes, plasmids
Cell reproduction: Cell cycle, DNA replication, apoptosis, binary fission,
gamete production, inputs and outputs of meiosis. Variation: genotype, phenotype, continuous &
discontinue Patterns of inheritance:
Monohybrid cross – dominance, recessive, codominance
Dihybrid cross Pedigree charts
DNA – the universal molecule of life Genome – total genetic material in a cell
Only 25% of DNA in our cells is involved in coding for biological molecules.
Most of these molecules are proteins but DNA also codes for the production of some RNA molecules.
Only about 1.5% of our DNA actually codes for the production of functioning protein molecules.
The rest is either never transcribed or never translated
Page 243
Human genome
Mitochondrial Genome
Nuclear Genome
RNA genes
Poly peptide genes
Genes 25%
Non coding DNA
Coding DNA 1.5%
Regulatory sequence
introns
Non coding DNAA few hypotheses for their existence:
Just junk Remnant from our past From past infections May protect DNA from mutations
Genes revisited….Basic unit of heredity
More than 30, 000 genes in the human genome
Each chromosome carries many hundred genes
The position of a gene on a chromosome is known as the gene locus (plural – loci)
Gene structure
Upstream region
Exon Exon ExonIntron IntronDown
stream region
Promotes & regulates
the activity of the gene
Coding regionContains the DNA which is transcribed to
pre-mRNA
Contains DNA
sequence which stops transcriptio
n
Types of Genes1. Structural Genes
These genes express structure and/or functional proteins
2. Regulatory Genes
These genes are short nucleotide sequences that express proteins that control the activity of structural genes by feedback mechanisms.
DNA StructureDNA is a complex molecule composed of basic
structural units called nucleotides.
Each nucleotide consists of three components: Deoxyribose Phosphate Nitrogenous base
Deoxyribose5-carbon sugar molecule
Forms part of the backbone of the DNA molecule
PhosphateAn inorganic molecule: PO4 -3
Provide the bond or link between neighboring nucleotides
Nitrogenous BaseTwo main types:
Purines Pyrimidines
Bonds between bases form the links between the two strands of the double helix
Purines always bind to pyrimidines according to specific rules: A – T G - C
PurinesDouble ringed structures
Two of them: Adenine Guanine
A G
PyrimidinesSingle ringed structures
Cytosine Thymine C
T
NucleotideA single nucleotide is composed of a
phosphate, a deoxyribose sugar and a base
A chain of nucleotides form a single DNA strandNulceotides are held by covalent bonds
between the phosphate and sugar molecules
Double strand DNA Is formed from two strands held together by
weak hydrogen bonds between the bases.
Form a double helix structure
DNA ReplicationDNA replication is semi-conservative: each new
molecule consists of one original strand and one new strand
DNA replication is carried out by enzymes.
It is important to know which ones do what job!
The enzymes, helicase and gyrase catalyses the unwinding and unzipping of the parent molecule leaving the bases exposed to act as templates
DNA ReplicationThe enzyme RNA polymerase builds a short
RNA primer sequence using the initial section of DNA as a template
Then the enzymes DNA polymerase extends this primer by adding nucleotides to the growing chain, in a 5’ to 3’ direction
One strand is built as a continuous length (leading strand) while the other strand is built up backwards in short fragments (Okazaki fragments) that are joined together by DNA ligase.
The two strands in DNA are antiparallel
Gene Expression The expression of genetic information is one of
the fundamental activities of all cells. The central role of DNA is to determine what proteins the cell makes.
Instructions stored in DNA are transcribed (copied) and processed into various RNA molecules.
These RNA molecules have specific roles in how the information is translated and expressed as a polypeptide/protein product
Gene ActionInvolves two processes
TranscriptionWhen a gene becomes active it first makes a mobile copy of the coded instruction that it contains.
TranslationIs the process whereby the instructions are decoded
The mobile gene code has to leave the nucleus and move to the cytoplasm.
In the cytoplasm the instruction is decoded. Ribosomes in the cytoplasm make the proteins.
All cellular life forms have about 60 proteins in common!Most of these proteins are involved in
translation as ribosomal proteins and tRNA enzymes.
A couple are involved in transcription.
A few are involved in DNA replication and repair.
This makes sense: all life forms share the same molecular processes.
The DNA codeGenes determine the production of protein
molecules.
The sequence of bases in a gene determines the amino acids which are used in building the protein molecule.
The genetic code is “written” in sequences of 3 bases – called a DNA triplet. Eg. A DNA triplet containing the bases TTA will
code for one amino acid, a DNA triplet containing AGC codes for a different amino acid.
A gene which
codes for a protein
mRNA ribosomes
protein
tRNA & amino acids
transc
ripti
on
Travels to transl
ati
on
TranscriptionOccurs in the nucleus
When the time comes for a particular gene to be expressed, the relevant segment of the appropriate chromosomes unwinds.
The hydrogen bonds between the two nucleotide chains break and the bases become exposed.
Messenger RNA (mRNA) then forms the length of one of the nucleotide chains (the template strand).
Transcription mRNA nucleotide will pair with the bases on the DNA
template according to the base pair rule: A with U T with A C with G G with C
A promoter region of DNA at the start of the gene starts the process.
The enzyme RNA polymerase is needed.
Sequences of unnecessary bases (introns) are removed from the mRNA strands so that only coding bases (exons) remain.
Within genes, bases are found in sets of three called triplets. When mRNA forms, triplets are transcribed to codons.
mRNA leaves the nucleus through pores in the nuclear membrane.
Transcription
A T C G T G
T A G C A C
DNA coding strand
DNA template strand
A U C G U GmRNA RNA polymerase uses the
DNA template strand to produce mRNA that is a transcript of the DNA coding strand.
Remember in RNA:
U substitutes for T
A triplet base code
DNA Replication
Transcription
mRNA codons
TranslationtRNA - anticodons
Polypeptides assembled on ribosomes - rRNA
A minimum combination of 3 bases, a triplet base pair or codon, is needed to code for one amino acid. Codons or triplet codes form the basis of the genetic code.DNA stays in the nucleus, and another molecule, acting as a messenger carries instructions from the nucleus to the cytoplasm.
Players & Places in Translation
Agents Analogy
DNA in the nucleus Master plan with complete set of instructions
mRNA(messenger RNA)
Working copy of one instruction
ribosomes Construction site
tRNA (Transfer RNA)
Carriers of raw material
Amino acids Raw material
Protein chain(Polypeptide)
End product
Translation Occurs in the cytoplasm on the surface ribosomes.
Ribosomes attach to mRNA that has left the nucleus.
Transfer RNA (tRNA) molecules in the cytoplasm bring their specific amino acids to the mRNA for polypeptide synthesis.
Each tRNA molecule has an amino acid binding site and an anticodon.
Each successive codon on the mRNA is paired with an anticodon of a tRNA
The amino acid attached to the tRNA is then released.
Peptide bonds form between the amino acids.
A polypeptide is the end result.
An overview of Gene Structure
Regulatory region
START
STOP
Promoter
region
Terminator region
Coding region
DNA sequence that will be transcribed from the template strand
5
3
3
5
DNA is a double stranded moleculeSections of the DNA act like traffic lights for enzymes.Promoter region of nitrogen bases says start/on.Terminator region of nitrogen bases that say stop/off.It is like walking into a room and switching lights on and off.
One gene can code for more than one protein!The human genome has about 30 000 genes but our
proteome (the total number of different proteins) is much larger approximately 60 000 proteins.
How can this occur?
Many genes can produce more than one protein because the mRNA transcript contains different combinations of exons. This process is called alternative splicing.
Alternative Splicing
Exon 1 Exon 2 Exon 3 Exon 4
Exon 1 Intron Exon 2 Intron Exon 3 Intron Exon 4
Exon 1 Exon 2 Exon 4
Possible mRNA’s using different combinations of exons
Exon 2 Exon 3 Exon 4
Result
When each mRNA is translated, a different protein is produced
Alternative splicing means that the number of outputs from the genetic instructions (genes) in a genome is far greater than the number of genes. A gene can regulate in different ways to produce more than one protein.
At different stages of development it can produce different proteins.
• Intron retention
• Exon juggling
In different types of tissue a gene can produce different types of protein.
The complexity of mammals is not in the number of genes they have but in the processes by which genes can be regulated.
Gene RegulationEach cell contains an entire organism’s
genome.
All cells of an organism have the same genome but can have different phenotypes. For example cells in the eye have the gene for producing fingernail protein (keratin) but this gene is not expressed.
How do genes get switched “on” or “off”?
Why have cells evolved complex mechanisms to regulate their genes?Cells conserve energy and materials by
blocking unneeded gene expression.If a substrate is absent in the
environment why produce the enzyme for that substrate!
Repressor molecules keep the cell from wasting energy by not making mRNA and enzyme molecules that have no use.
The cell can control its metabolism – resources are used only when there is a metabolic need and can be redirected to other metabolic pathways.
Gene Regulation in Prokaryotes Bacteria have groups of genes that are controlled together and are
turned on/off as required
One example, the Lac operon is a set of genes
in bacteria used for lactose metabolism
Some bacteria use the lactose as an energy
source
Bacteria produce the enzymes to break
down lactose to glucose and galactose only
when lactose is present.
Bacteruim
Lactose
The Lac Operon3 catalytic proteins with specific functional roles in the metabolism of lactose
Repressor protein expressed by Lac regulatory gene
ß galactosidasepermease Trans
acetylase
When Lactose is absent
Repressor protein sits on the start/promoter region.
RNA polymerase not happy
that its place is occupied Genes are switched off
Repressor protein
When Lactose is present
Lactose binds to repressor and can’t block promoter site because of change in shape.
Genes are switched onRNA
polymerase
ß galactosidase
permease
Trans acetylase