Download Basic Concepts of Heredity

Basic Concepts of Heredity
Heredity - the transmission of traits from one generation to
another
Gregor Mendel in the 1860's published experiments that laid down the
foundations for genetics. He is known for his garden pea experiments in which
he was able to predict the outcome of the next generation. At this time no one
had knowledge of chromosomes or the role of meiosis.
Monohybrid Cross - simplest cross of two contrasting traits.
Ex.
Tall stems
vs.
Dwarf stems
1st generation (F1)= All tall stemmed plants
When the F1 generation was crossed = 787 tall, 277 dwarf
a ratio of 2.84 : 1
Mendel experimented with other traits and came up with similar findings.
Ex. Seeds - Round vs. Wrinkled
F1 = all round
F2 = 5474 round, 1850 wrinkled
Ratio of 2.96: 1
Flower color - Violet vs. Blue
F1 generation = violet
F2 generation = 705 violet, 224 white
Ratio of 3.15:1
From this data, Mendel came up with 3 principles of Inheritance.
1. Unit Factors in Pairs - genetic characters are controlled by unit factors that
exist in pairs in individual organisms
2. Dominance/Recessive - When 2 unlike unit factors are responsible for a
single characteristic are present in a single individual, one is said to be
dominant and one recessive.
Dominant - the expression of a trait in a heterozygous condition. Ex. Tall
stems, round seeds, violet colored flowers
Recessive - term describes an allele that is not expressed
Ex. dwarf stems, wrinkled seeds, white flowers
3. During the formation of gametes, the paired unit factors separate or
segregate paired unit factors separate or segregate randomly so the gamete
receives one or the other.
- If an individual contains a pair of like unit factors - (ex. both specify for
tall) then all gametes will receive one tall unit factor)
- If individual contains unlike unit factors ( one dwarf and one tall) then
each gamete has a 50% probability of receiving tall or dwarf unit factors.
Example:
Parental Cross
Tall
DD
Fertilize
D
X
X
Dwarf
dd
d
First Generation (F1) Dd
Cross
Dd
Dd
X
D
Second Generation (F2)
DD
F2 Genotype
Tall
Homozygous
d
= all Tall
Dd
D
d
Dd
Dd
Tall
Tall
Heterozygous
dd
Dwarf
Homozygous
Gene- a distinct hereditary unit found on a chromosome.
Phenotype - the physical appearance
- Ex. Tall verses Dwarf, Wrinkled verses Smooth
Allele - single gene that determines the phenotype
Ex. The allele for the phenotype tall vs dwarf is the height of
the plant.
A lowercase letter represents the allele for the recessive trait.
An uppercase letter represents the dominant trait.
Ex.
d = Dwarf
D = Tall
Genotype - when 2 alleles are written in pairs to represent the 2
unit factor and is considered to be the genetic makeup
of an individual (ex. DD, Dd, dd)
Punnett Square - allows you to visualize genotypes and phenotypes
resulting from the recombination of gametes
Example:
Dd
D
D
D
x
Dd
d