Biology
Chapter 6
Fundamentals of Genetics
I. Patterns of Inheritance
A. History of Genetics
1. Genetics – study of heredity
2. Heredity – passing on of characteristics from parents to offspring
3. Trait – characteristic that can be passed from parent to offspring
4. Gregor Mendel – Austrian monk considered the founder of genetics
B. Mendel’s Experiments
1. Selection of Pea Plant
a. Easy to grow
b. Several contrasting traits
c. Produce numerous seeds
- purebred – an organism that receives the same genetic information
from both parents
- hybrid – an organism that receives different forms of a genetic trait
from it’s parents.
C. Mendel’s Observations
1. Crossed yellow “pea” plant with green “pea” plant
a. P1 – parental generation – original plants
2. Offspring all had yellow peas
a. F1 generation – 1st batch of offspring
3. Crossed the F1 generation with each other and got ¾ yellow and
¼ green
a. F2 generation – second batch of offspring
D. Mendel’s Conclusions
1. Rule of Unit Factors – each organism has two factors that control each
trait(genes)
a. Alleles – genes that have contrasting effects
ex – yellow peas/green peas
tall/short
1) Located on different copies of a chromosome
- one from mother, one from father
2. Principle of Dominance and Recessiveness – one factor in a pair may mask
the other, preventing it from having an effect.
a. Dominant – stronger or more forceful trait
b. Recessive – weaker trait
3. Law of Segregation – two alleles for each trait separate during meiosis.
(during anaphase I)
4. Law of Independent Assortment – genes for different traits are inherited
independently of each other.
ex – hair color and height are unrelated
E. Terms
1. Phenotype – the way an organism or characteristic looks
ex – blonde hair, brown eyes, yellow peas, tall plants
2. Genotype – the actual genes in a cell
ex – bb, BB, Tt
3. Homozygous – when paired genes are the same (purebred)
ex – bb, TT, XX
4. Heterozygous – when paired genes(alleles) are different (hybrid)
II. Genetics and Predictions
A. Probability(P) – the likelihood of an event or the frequency of an occurrence.
1. Can be represented as a fraction (½), percent(50%), or a ratio (1:1)
B. Punnett Square – grid for organizing genetic information
1. Shows probabilities, not actual results
2. Monohybrid cross – study the inheritance of only one trait.
C. Test Cross – the cross of a homozygous recessive with an unknown
genotype
1. Used to check and see if an organism is homozygous (purebred)
or heterozygous (hybrid).
D. Pedigree – chart that shows how a trait is inherited in a family.
1. Carrier – individual who possesses a recessive trait but does not
express it.
III. Difficult Predictions
A. Incomplete Dominance – neither allele of the pair is completely dominant.
1. When both alleles are present a different phenotype occurs
(intermediate)
a. Red + White --- Pink
B. Codominance – when both alleles are present, both are expressed equally.
1. Neither allele is dominant over the other
a. Chickens
Black + White -- Black and White
B + W
BW
C. Multiple Alleles – more than two alleles for a given genetic trait.
1. Blood Type – three human blood type alleles
a. A, B, O
b. A and B are dominant over O, but not each other.(codominance)
D. Polygenic Traits – controlled by two or more pairs of genes.
1. Phenotype is a combination of the genes present
a. Eye color, skin color, hair color
IV. Environmental Effects – the environment can influence how certain genes are
expressed
A. External Environment
1. Light, Temperature, chemicals, nutrition, and infectious agents
B. Internal Environment
1. Internal environments change with age.
2. Males and females have different internal environments.
a. Hormones and structural differences