03 - Chick Check: The Genetics Behind Blue Chicken Eggs
Food for thought. Every cell in our body contains the same genetic material and thus genes. However different cells express different genes to make them unique. So many genes are present but in an “off” state in different tissues, and when aberrantly turned “on” can change the function of those cells. Inheritance Pattern
Figure 4: Using a Punnett Square.
Inheritance is the process through which traits are passed from parents to offspring through genes. Each gene has two versions, called alleles , one inherited from each parent (Figure 4). • A dominant allele will show its trait even if only one copy is present. • A recessive allele is hidden when paired with a dominant allele and will only show its trait when an individual has two recessive copies. Chickens have 39 pairs of chromosomes, 38 auto- somes and two sex chromosomes, Z and W. The blue eggshell trait follows autosomal dominant inheritance :
Genotype: 1/4 PP 1/2 Pp 1/4 pp Phenotype: 3/4 dominant 1/4 recessive
• Homozygous birds (two copies of blue allele): Lay dark blue eggs. • Heterozygous birds (one copy of blue allele): Lay lighter blue eggs. • Homozygous non-blue (no blue alleles): Lay white/brown eggs with no blue biliverdin in the shell. Endogenous Retroviruses (ERVs) ERVs are ancient viral sequences that got “stuck” in animal genomes long ago. While most have lost the ability to make new viruses, their regulatory sequences can still affect nearby genes, often turning them on when they would otherwise be “off” in the affected tissue. In this case, the EAV-HP retrovirus insertion provides new regulatory signals that change where and when SLCO1B3 is expressed. This is an example of how viruses drive pheno- typic changes in animals and fuel diversity and the development of new genetic traits. The 3-Primer PCR Strategy In the lab, you extract chicken DNA to use as a What do I need to perform PCR?
template for the polymerase chain reaction, or PCR. This powerful biotechnology technique quickly and accurately creates large amounts of a specific DNA sequence in vitro . Using a PCR-based genotyping as- say, we can target and amplify specific DNA sequenc - es to determine whether chickens will lay blue eggs. Because PCR is extremely sensitive, it can detect even the smallest amounts of the target DNA. The chicken DNA is our starting template, which is then mixed with primers, Taq polymerase and nucleotides. Once the sample is in the thermal cycler, we heat up the DNA to separate its two strands, like unzipping a zipper. Then, we cool it down so the primers can stick to the DNA, marking the sequence
• Template – the purified, double- stranded piece of DNA we want to copy • Primers – short synthetic DNA molecules that target a specific DNA sequence for amplification • Taq DNA Polymerase – ther- mostable enzyme used to copy DNA • Free nucleotides – the building blocks of DNA
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