These photos are in part collected here for my own records, but I hope they can also be educational when others are pondering the changes they may see between chick fluff and adult feathers. They grow up so quickly...
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Born on St. Paddy's Day, I was very tempted to keep this little one, my first successful 2020 hatchling.
CHICKENS & EGGS
A Mendelian Inheritance Primer
(or Refresher)
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Inheritance refers to how traits are passed from parents to their offspring through genes. Each organism inherits two copies of every gene—one from each parent. These genes are carried on chromosomes, and they determine the organism's traits, like eye color, hair type, and even certain behaviors or diseases.
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Selective breeding is the process by which humans intentionally breed plants or animals to develop specific desirable traits in their offspring. This is achieved by choosing parents that exhibit particular characteristics and mating them in hopes that these traits will be passed on to the next generation.
Having an understanding of how traits are inherited is vital when it comes to raising livestock (or growing many crops), as it helps us decide which animals to breed to produce the next generation.
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Table of Contents
Genes & Chromosomes
Genes & Chromosomes
Genes and chromosomes are fundamental concepts in genetics, playing crucial roles in the inheritance of traits from one generation to the next.​
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Genes
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Genes are segments of DNA that contain the instructions for making proteins, which are the molecules that perform most of the functions in cells and determine the traits of an organism. Each gene carries the code for a specific protein or set of proteins, which influence traits such as eye color, blood type, and even susceptibility to certain diseases. Genes are located on chromosomes within the nucleus of a cell. The specific location of a gene on a chromosome is called a locus.​
When a gene is "expressed," it means the information it contains is being used to produce a protein. The expression of genes is what leads to the development of physical characteristics (phenotypes).
Chromosomes​
Chromosomes are long, thread-like structures made of DNA and proteins. They carry genetic information in the form of genes. A chromosome consists of a single, continuous piece of DNA coiled around proteins called histones, which help keep the DNA organized.
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Chromosomes ensure that DNA is accurately copied and distributed during cell division. During reproduction, chromosomes are passed from parents to offspring, carrying the genes that determine inherited traits
Humans have 46 chromosomes, arranged in 23 pairs. One set of 23 chromosomes is inherited from the mother, and the other set is inherited from the father. Of these, 22 pairs are called autosomes, and the 23rd pair are the sex chromosomes (XX for females, XY for males). The pictures below reveal the number of chromosomes in humans, sheep, chickens, and dogs.​​​​
46
chromosomes
homo sapien
54
chromosomes
ovis aries
78
chromosomes
gallus gallus domesticus
78
chromosomes
canis lupus familiaris
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Genes & Chromosomes Working Together​
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Genes are the individual units of heredity found on chromosomes, and chromosomes are the structures that organize and carry genes within cells. Together, they play a central role in the transmission of genetic information from one generation to the next.
The Role of Alleles in Passing Down Specific Traits
Alleles are different versions of the same gene that determine specific traits in an organism. They are the variations that exist within a gene and can lead to different physical characteristics or phenotypes.
The Relationship between Genes and Alleles:
While a gene is a segment of DNA that codes for a particular trait, such as eye color or blood type, an allele is one of the different forms of a gene that can exist at a specific location (locus) on a chromosome. For example, a gene for eye color might have an allele for brown eyes and another for blue eyes.
Homozygous Traits and Heterozygous Traits
An organism is homozygous for a gene if it has two identical alleles (e.g., AA or aa) for that gene. This means both alleles code for the same version of the trait. An organism is heterozygous for a gene if it has two different alleles (e.g., Aa). In this case, the two alleles may code for different versions of the trait.
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Multiple Alleles
Some genes have more than two possible alleles, but any individual organism can only have two of these alleles (one from each parent). A common example is the ABO blood type system in humans, where three alleles (A, B, and O) determine blood type.
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What are Dominant & Recessive Alleles?
Dominant and Recessive Alleles
A dominant allele is one that expresses its trait even if only one copy is present. For example, if "A" is a dominant allele for a trait, then both "AA" and "Aa" will display the dominant trait. A recessive allele only expresses its trait if two copies are present. If "a" is a recessive allele, the trait will only appear if the genotype is "aa."​​
Codominance and Incomplete Dominance
In codominance, both alleles are fully expressed in the phenotype when present in a heterozygous individual. Example: In certain types of flowers, a cross between a red flower and a white flower might result in offspring with both red and white patches.
In incomplete dominance, the phenotype of a heterozygous individual is a blend of the two alleles. Example: Crossing a red flower with a white flower might result in pink offspring.​
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What Are Punnet Squares?
A Punnett square is a tool used in genetics to predict the possible combinations of alleles that offspring might inherit from their parents. It visually represents the ways in which alleles from the mother and father can combine and shows the probability of the offspring having certain genotypes and phenotypes.
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Punnett Squares in Action
​Let's use a Punnett square to explain how eye color, specifically blue and brown eyes, can be inherited. In this example:
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B represents the dominant allele for brown eyes.
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b represents the recessive allele for blue eyes.
Parent Genotypes:
If a person has brown eyes, their genotype could be either BB (homozygous dominant) or Bb (heterozygous).
If a person has blue eyes, their genotype must be bb (homozygous recessive) since blue is recessive.
B
Parent #1 Genes
b
Example 1: Both Parents Heterozygous (Bb x Bb)
Let’s consider a scenario where both parents are heterozygous (Bb), meaning they both have brown eyes but carry the allele for blue eyes.
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Set up the Punnett square:
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The top left square combines "B" from both parents: BB.
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The top right and bottom left squares combine "B" and "b": Bb.
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The bottom right square combines "b" from both parents: bb.
Results (Genotypes):
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1 BB (homozygous dominant)
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2 Bb (heterozygous)
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1 bb (homozygous recessive)
Results (Phenotypes):
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Brown eyes: 3 out of 4 offspring (BB or Bb).
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Blue eyes: 1 out of 4 offspring (bb).
There is a 75% chance the child will have brown eyes (BB or Bb) and a 25% chance the child will have blue eyes (bb).
Parent #2 Genes
b
B
BB
Bb
Bb
bb
Blue Pigment
Eggshells form around the egg in the egg's last days inside a hen. If the hen carries any blue shell genetics, a blue pigment will be incorporated into the shell as it forms. This is why blue and green eggs are blue on the inside of their shell.
