Brooklyn College BIOL 1010 : Lab Notes
Patterns of Inheritance
Lets start with some genetic terminology ...
Phenotype
- Phenotype refers to an organism's expressed or physical traits
If we use the flower to the left as our example:
purple would be the phenotype
Genotype
- Genotype is genetic make-up of an organsism
The genotype for our purple flower would be written PP
Alleles
- Alleles are alternative forms of genes (units that determine heritable traits)
- Dominant allele is the allele that is fully expressed and will determine the phenotype of the organism.
Purple-flower color is dominant in our example and it is written with a capital letter to symbolize that it is dominant (P).
- Recessive alleles have no noticeable effect on phenotype and is written in lowercase (p).
In our example, p represents the white-flower (or recessive) allele.
Homozygous
- If an organism has 2 identical alleles for a given gene, it is homozygous (or 'true-breeding')and is written: PP or pp.
Heterozygous
- When the 2 alleles are different, the organism is heterozygous (Pp)
Mendel's cross-fertilizing technique
Mendel chose to work with garden peas because they were easy to grow, were available in many readily distinguishable varieties, and he could control plant matings (i.e. self-fertilization or cross fertilization).
Self-fertilization
- Self-fertilization occurs when pollen is used to fertilize egg in same flower (this occurs naturally in peas).
Cross-fertilization
- Fertilization of one plant with pollen from another plant which allows mating (crossing) between different varieties
Mendels success was also due to his selection of the characteristic to study.
By chosing characteristics that occur in only two distinct forms, he made it easier to categorize and count accurately.
We need a little more terminology now ...
- True-breeding
- Varieties for which self-fertilization produced offspring all identical to the parent (same as pure-breeding).
- Hybrids
- Offspring of two different true-breeding varieties.
- P generation
- Parents in a cross between two true-breeding varieties.
- F1 generation
- Hybrid offspring resulting from mating two true-breeding varieties.
- F2 generation
- Offspring resulting from f1 that self-fertilize.
- Trait
- Each variant for a particular character.
- (ex. purple or white flower color)
- Monohybrid cross
- A genetic cross that tracks the inheritance of a single character.
The Test ...
Mendel performed many experiments in which he tracked the in inheritance of a single characteristic (i.e. monohybrid cross).
He crossed two different true-breeding varieties:
P generation : purple flowers X white flowers
And this is what he found:
In the F1 generation, all plants had purple flowers.
In the F2 generation, 3/4 had purple flowers and 1/4 had white flowers (i.e. 3:1 ratio)
The left column shows the phenotype (visual appearance). When we look at the genotype (right column), we can see why Mendel got the results he did.
Mendel repeated the same experiment with other traits.
All crosses gave similar results: all F1 were identical, and the F2 showed a 3:1 ratio.
Based on these results Mendel developed 4 hypotheses:
- There are alternative forms of genes, the units that determine heritable traits. These alternative forms of genes are called alleles.
- For each inherited characteristic, an organism has two genes, one from each parent. These genes may both be the same allele (homozygous), or they may be different alleles (heterozygous).
- A sperm or egg carries only one allele for each inherited trait, because allele pairs separate (segregate) from each other during the production of gametes.
- When the two genes of a pair are different alleles and one is fully expressed while the other has no noticeable effect on the organism's appearance, the alleles are called dominant allele and recessive allele, respectively.
Mendel's principle of segregation:
The underlying mechanism accounting for this inheritance pattern is stated by Mendel's principle of segregation.
- 1. Alleles segregate during gamete formation
2. The random fusion of gametes at fertilization creates allele pairs again.
Remember this while doing your investigation
An organism's appearance does not always reveal its genetic composition.
Organisms with dominant phenotype can be either homozygous or heterozygous for a trait.