Gregor Mendel was an Austrian monk who is known as the father of genetics due to his work with garden peas. Since he was into math, he wanted to know if there was a way to calculate the offspring’s traits.
Mendel proved that if you cross a tall plant with a short plant, all of the offspring would be tall. Then if you crossed those tall plants, 75% of them would be tall and 25% would be short.
A gene carries encoded information about specific traits. Each gene has a location on a chromosome called a “locus.”
Alleles are various molecular forms of a gene for the same trait. An allele usually refers to a single letter, “T” for example.
Homozygous = Same letters; AA or aa for example.
Heterozygous – Different capitalization; Aa for example
Homozygous dominant = AA
Homozygous recessive = aa
Heterozygous = Aa
Genotype is the sum of the genes (letters)
Phenotype is how the genes are expressed (words/descriptions)
Mendel’s Law of Dominance
Some traits are dominant and others are recessive.
A Punnett Square is a diagram used to predict the outcome of a cross between individuals.
Mendel’s Law of Segregation
Half of your traits come from one parent and the other half come from the other parent.
In rabbits brown fur is dominant to white fur. Predict the outcome between a homozygous dominant brown rabbit and a white one.
In flowers, red is dominant to white. Predict the outcome between a heterozygous flower and a homozygous dominant flower.
In bears, long fur is dominant to short fur. Predict the outcome between a short furred bear and one that is heterozygous.
In some plants, tall is dominant to short. Predict the outcome between two heterozygous tall plants.
An organism is heterozygous or homozygous dominant they look identical. To determine which is which, you perform a test cross.
If you cross your unknown (either heterozygous or homozygous dominant) with homozygous recessive and ANY ONE of the offspring is recessive, then your unknown organism is heterozygous.
A monohybrid cross involved only 1 trait at a time. A dihybrid cross involves 2 traits.
Mendel’s Law of Independent Assortment
Traits are not linked to one another and you have and equal chance of getting each gene independently of the others.
A dihybrid cross between two heterozygous organisms for two contrasting traits results in a 9:3:3:1 ratio.
In some species of plants, red flowers are dominant to white flowers and tall plants are dominant to short plants. Predict the outcome between 2 heterozygous organisms.
In incomplete dominance, the heterozygous results in a blending of the traits.
RR = Red
WW = White
RW = Pink
In codominance the traits don’t blend but both show up. The heterozygous results in spotted or speckled off spring.
F B F B = Black feathers
F W F W = White feathers
F B F W = Speckled feathers
Where there are more than 2 alternative forms of a trait, it is said to have multiple alleles.
Blood type is a common example of multiple alleles.
Type A = I A I A or I A i
Type B = I B I B or I B i
Type AB = I A I B
Type O = ii
Some traits actually ARE linked by sex. These include baldness, color blindness, and hemophilia.
Males are represented by XY. The X from the mother and the Y from the father. Sperm can carry either an X OR a Y chromosome.
Females are represented by XX. They inherit and X from both parents. All eggs carry an X chromosome.
X A X A – Normal female
X A X a – Normal female (but a carrier)
X a X a – Affected female
X A Y – Normal male
X a Y = Affected male
A pedigree shows the history of a trait in a family.
Circles represent females.
Squares represent males
Horizontal lines represents a mating
Vertical lines represent an offspring
A triangle represents an abortion of miscarriage.
A diamond indicates unknown sex.
Shaded in means affected
A diagonal lines means dead.
All pedigrees must have a title, each generation must be numbered with a roman numeral, and each member must be numbered.