Patterns of Inheritance

Two explanations: Blending Theory of Inheritance vs Particulate Theory of Inheritance

Gregor Mendel - The Father of Genetics

1. Monk who used science and math to establish patterns in how traits were inherited
2. Year 1857
3. He used the garden pea as his test subjects

Some Vocabulary

Character - a heritable feature (ex. flower color)
Trait - a variant of each character (ex. purple or white)
Cross Pollination - one plant fertilizes a different plant
Self Pollination - a plant fertilizes itself
True-Breeding - plants that over several generations only produce plants like themselves

Mendels Experiments - a Monohybrid Cross

*cross involved plants that differed for a single character: tall x short, purple x white, round x wrinkled

P (Parental Generation) True breeding plants
F1 (First Filial) The offspring of the P generation --> they always displayed a single trait, the dominant one
F2 (Second Filial) The offspring of the F1 generation, self fertilized --> always had a 3:1 ratio

Purple x White
Round Seeds vs Wrinkled Seeds
Task: Show the P, F1 and F2 generations
(Click to see Answer)

More Vocabulary

Gene - unit of inheritance
Allele - Alternate form of a gene, symbolized by letters

Homozygous - having two of the same allele (AA or aa); true-breeding
Heterozygous - having two different alleles (Aa)

Monohybrid cross - involves 1 trait (Aa x Aa)
Dihybrid cross - involves 2 traits (AaBb x AaBb)

Dominant - Allele that masks the other allele
Recessive - Allele that is covered up

By convention - Dominant alleles are represented by capital letters, recessive by lowercase - always using the same letter

Genotype - the organisms alleles, represented by letters
Phenotype - the physical appearance of the organism

Mendel's Laws

The Law of Segregation: two alleles separate during gamete formation (a Tt parent can produce both T sperm, and t sperm)

A punnet square is a representation of this law, showing how gametes separate and then come together during fertilization

Locus - spot on the chromosome where an allele (gene) is located.


The Law of Independent Assortment: each pair of alleles segregates independently of other pairs of alleles (this law evidence by dihybrid crosses)

Test Cross: When the genotype of a parent is unknown, parent is crossed with a recessive indiviual. The offspring's phenotypes show the unknown parent:

Ex: In guinnea pigs, short hair is dominant to long. A short haired guinnea pig of unknown genotype is test crossed. Two of the five offspring have short hair and three have long. What was the genotype of the unknown parent?

Dihybrid Crosses

 

TIP: In any cross that is dihybrid (AaBb x AaBb) you will always get a 9:3:3:1 ratio, if you memorize this, you can save the trouble of doing a giant square!

 

A Mathematical Alternative

A punnet square is not needed to determine the ratios of genotypes and phenotypes. Simple statistics and math can save you the trouble of filling out a square.

In a monohybrid cross Pp x Pp, each parent produced P gametes and p gametes

If you wanted to determine how many of the offspring are pp: x =

 

Example 2: H is dominate for long hair (h = short) and B is dominate for black eyes (b = red eyes)

If the parents are: HhBb x hhBb

How many off the offspring will be short haired and red eyed?

 

Task: Use mathematical analysis to determine the number of short haired, black eyed offspring from the cross above.