Classical Genetics
Gregor Mendel is the founder of genetics as we know it. In the 1850s, he bred garden peas in order to study patterns of heredity. He collected data from hundreds of plants across generations and used statistical analysis to predict these patterns.
In his experiments, he studied the law of dominance, the law of segregation, and the law of independent assortment.
Law of Dominance
When two homozygous (purebred) organisms for opposing traits are crossed, the offspring will be hybrid (they will carry two different alleles) but their phenotype will be that of the dominant trait.
Law of Segregation
The law of segregation says that during the formation of alleles, the two traits carried by each parent separate. Thus two hybrid parents, although they may have the phenotype for the dominant allele, can have a baby with the recessive phenotype (phenotype is what the organism looks like, while genotype are the actual alleles of the organism)
Monohybrid Cross
A monohybrid cross is an act of crossing two organisms that are hybrid for a single trait, such as Tt x Tt (T= tall t=short). The result of a monohybrid cross is 1:2:1 where there is a 25% chance the organism will be homozygous dominant, 50% chance the organism will be heterozygous dominant and a 25% chance the organism will be homozygous recessive.
Backcross or Testcross
If you donāt know whether an individual plant or animal is homozygous or heterozygous, it is effective to cross them with a homozygous recessive individual. If all the offspring display the dominant trait, the mystery parent is most likely homozygous dominant. If the offspring have a mix of dominant and recessive traits, the mystery parent is heterozygous dominant.
Law of Independent Assortment
When a cross is carried out between two individuals who are hybrid for two traits on separate chromosomes, during gamete formation, the genes for one trait (for example, height) are not inherited along with the genes for another trait (such as seed colour)
T= tall t=short Y=yellow y=green
The diagram above is of a dihybrid individual. T will be inherited along with Y, and t will be inherited along with y. The only factor that determines how the alleles are inherited is how the homologous pairs line up during metaphase I.
Here is a Punnett square for this dihybrid cross.
The phenotype ratio ends up being 9:3:3:1, as shown above.Ā
Incomplete Dominance
Incomplete dominance leads to the blending of treats. For example, if a long watermelon (LL) crosses with a round watermelon (RR) to produce an oval watermelon (RL), incomplete dominance is at play.
Another example is the Japanese four oāclock flower. Below, 1 red flower and 1 white flower combine to make 4 pink flowers.
Codominance
In codominance, both traits show. For example, the MN blood group in humans is a codominant trait (donāt confuse these with blood types). These are based on distinct molecules on the surface of the cell. A person can be homozygous for M, homozygous for N, or heterozygous for MN, where both molecules appear on the blood cell.
Multiple Alleles
Most genes exist in two allelic forms, etc tall or short. When there are more than two alleles, the gene has multiple alleles. Human blood type is a good example, as there are 4 groups, determined by 3 alleles. Blood type can either be written as its letters A, B, O, and AB, or I^a, I^b, I^a I^b, and ii. āIā stands for immunoglobin.
Polygenic Inheritance
Some traits, like skin and hair colour result from the blending of several separate genes that vary along a continuum. They are controlled by multiple genes, and so are polygenic. This leads to a wide variation in genotype.
(y-axis is labelled: the proportion of the population)
Sex-Linked Genes
When a trait is carried on the X chromosome, it is sex-linked. Since women have 2 X chromosomes, if the trait is a recessive mutation, she will need to carry two mutated genes for it to express itself. If she only has one, she will be a carrier.Ā Meanwhile, since men only have 1 chromosome (XY) they only need 1 gene for it to express itself. Recessive sex-linked traits are much more common that dominant sex-linked traits. This explains why men are much more likely to have sex-linked traits like colourblindness and haemophilia.
Epigenetics
In some scenarios, the environment can alter gene expression. For example, in fruit flies, the expression of the vestigial wings (short, and shrivelled) can repaired in higher temperatures.
Sex-Influenced Inheritance
Sex-influenced traits are not the same as linked traits. For example, baldness is a trait expressed both in men and women, however,Ā it expresses itself very differently in the two sexes.
Karyotypes
Karyotypes are lab procedures, where the size, shape, and number of chromosomes are analyzed. This takes place during metaphase, as this is when the chromosomes are fully condensed. In our 46 chromosomes, we have 44 autosomes (22 pairs) and 2 sex chromosomes.
This karyotype is of an average biological male.
The Pedigree
A pedigree is a family tree that studies the inheritance of a specific trait. Normally, in these graphs, a woman is represented by a circle, and a man by a square. In the pedigree below, black shapes represent deaf people.
On this pedigree, deafness is shown to be autosomally recessive. We know it is not dominant, as when a deaf parent has children as shown in this pedigree, none of their children has the phenotype, and all the affected children have unaffected parents. We also know it isnāt sex-linked, as looking at the two daughters in the F3 generation, neither of their fathers is affected.
Mutations
Mutations are abnormalities within the genome. They can occur in the somatic (body) cells and can cause cancer, or they can occur during gametogenesis, and affect future offspring. (When the somatic cells are impacted, future generations are not.)
Gene mutations- Gene mutations are changes in a DNA sequence.
Chromosome mutations- Chromosome mutations are able to be observed under a light microscope. An example of a chromosome mutation is nondisjunction. Nondisjunction may add an entirely new chromosome. Some other types of chromosomal aberrations include:
Deletion- When a fragment lacking a centromere is lost during cell division
Inversion- When a chromosomal fragment reattaches to its original chromosome, but in the reverse orientation
Translocation- When a fragment of a chromosome becomes attached to a non-homologous chromosome
Nondisjunction
Nondisjunction is an error during meiosis, where homologous chromosomes do not separate properly. When this happens, one gamete has two homologues, while the other doesnāt have any.
An abnormal chromosomal condition is known as aneuploidy. If a chromosome is present in triplicate, the condition is called trisomy. For example, people with down syndrome have 3 #21 chromosomes, and so have trisomy-21. Any organism with an extra set of chromosomes is called a triploid. The cells of the endosperm or cotyledon of seed are triploid. When an organism has more than 3 sets of chromosomes, they are a polyploid. Polyploid plants have abnormally large flowers and fruits.
