Genetic Overview By: Elizabeth Izzo & Mia Scott

Gregor Mendel is considered to be the "Father of Genetics". He lived in a monastery where continuously tended to the garden there.

Over time Mendel noticed different colored flowers blooming. He then bred a variety of colored flowers to find the patterns occurring in the offspring. Soon after his experiments he found a pattern, this is what led to the study of genetics and the understanding of heredity traits. Genetics is the studying of heredity traits. The genetic traits that are shown in the DNA is called a genotype, but how a person looks is called their phenotype. Heredity is when parent generations transfer their traits to offspring. Each trait that is passed on is transferred in a unit called a gene, which is composed of two alleles, with one allele coming from the mother and the other from the father. An allele can either be a dominant or recessive. The dominant gene will be shown in the offspring and the recessive gene will be shown only if there is no dominant form present.

Mendel noticed during his experiments that a color of flower would appear, disappear, and eventually reappear again. This led to Mendel creating Mendel’s Principle of Dominance which states that even though a dominant gene may suppress a recessive gene, it could eventually show up in an offspring if paired with another recessive. If a gene is made up of the same allele form, dominant or recessive, it is considered homozygous, but if there is a dominant and recessive allele in the gene, then it is considered a heterozygous. When desiring one certain trait, breeders will use a monohybrid cross to continue that one trait. If there is two desired traits that want to be bred, a dihybrid cross will occur.

All 23 chromosomes are shown in the picture above.

Autosomes are the 22 chromosomes in a human body. The 23 chromosome is called a sex chromosome. XY are used to represent a male, and XX is used to represent a female. A person’s chromosomal profile is their karyotype. Chromosomes are organized going from large to small, with the sex chromosome going behind 1-22 autosomes. Having the chromosomes go from big to small makes it easier for scientists to see mutations or missing chromosomes.

A punnett square is a table that helps to identify possible traits offspring could receive. This is an example of what a punnett square would look like.

In the example above, the punnett square is determining the eye color of the offspring with brown eyed parents, who both carry the recessive blue eye gene. First, label your punnett square, put one of the parents across the top and the other down the side. Next, fill in the squares, the top allele is come down and the side alleles will come across. Finally, determine the genotype and phenotype percentages of each possible offspring. For the genotype, since there is only one square containing two capital B’s, there is a 25% chance the offspring will be a homozygous dominant, there are 2 squares containing one capital B and one lowercase b, there is a 50% chance it will be heterozygous, and finally, there is one square with two lowercase b’s meaning there is a 25% chance that the offspring will be a homozygous recessive. For the phenotype, there is a 75% chance the offspring will have brown eyes and a 25% chance that it will have blue eyes.

By looking at offspring’s phenotype, it is possible to figure out the genotypes of the parents and previous generations.

An example of pedigree chart showing both male and females in the family. The males are represented with a square and the females are represented with a circle. But if the shape is shaded then that signify's that they carry a specific trait.

A pedigree is a chart that keeps record of certain traits throughout generations. By looking at a pedigree chart you can find the male and females throughout the past generations, you can also identify which ones have thee trait. Not every gene is the same, there are many versions of the same gene, which is why every person looks differently. A gene can changed into a different version due to epigenome, which is the top layer of a person’s DNA that carries genetic information. The epigenome also turns on and silences specific genes in the DNA.

When a human has two dominant alleles, there are two possible outcomes, incomplete dominance and codominance. Incomplete dominance is when the two dominants mix together and codominance is when the two dominants continue to compete to be shown, resulting in them both being expressed. You can show codominance and incomplete dominance in punnet squares by putting an apostrophe beside the letter.

This picture shows codominance. The red and blue are both competing to be shown, so they both are expressed.
This picture shows incomplete dominance. The chestnut and white are both dominant and are mixed together to make the Roan Horse.

A carrier is a human that doesn’t show a genetic disorder or disease but carries the allele that could eventually be shown in their offspring. The 23 chromosome is the the sex chromosome and there are specific traits that can be passed through that chromosome called sex- linked traits. Sex- linked traits are more common in men than women because women only give an X chromosome to the offspring while the man can give either an X or Y, since the male determines the sex of the baby.

There are 4 possible blood types that a human can have, A, B, AB, and O. With each blood type, being positive or negative, called the Rhesus Factor or Rh. In a punnett square, blood types are represented in a different way. If someone is a homozygous A, then, on a punnett square, it is shown as I^AI^A, homozygous B is shown I^B I^B. If someone is a heterozygous A, they are represented as I^A i, the same being for heterozygous B, just with a B instead of an A. If a person has AB blood type, they are shown in a punnett square with I^A I^B, and O is represented by ii.

Credits:

Created with images by Libertas Academica - "Figure 1"

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