Download Intro to Genetics

Intro to Genetics
November 2014
Heredity
• Study of the transmission of
characteristics from parent to
offspring.
• The field of genetics began with
the work of Gregor Mendel
The Work of Gregor Mendel
• About Mendel
– Began his work in 1860s
– Breeded & studied garden pea
plants
– Published paper in 1866 stating
that parents pass on discrete
“heritable factors” (genes) on to
their offspring
• Remember that genes are segments
of DNA that code for traits
– Considered “Father of Genetics”
Plants used in Experiments
• Mendel probably studied pea plants
because of their short generation time,
plentiful offspring & visible varieties
– Traits: Variations of genetic
characteristics.
• These pea plants are usually selfpollinating (sperm cells in pollen
fertilize egg cells located in ovary on
the carpel in the same flower)
– This is an example of Sexual Reproduction
(Male & female gametes fuse to form a
zygote.)
What Mendel Did
• He prevented selfpollination by cutting off
male stamens (w/ pollen)
• Transferred pollen from
other plants onto stigma
located on carpel.
• This is called cross
breeding- crossing
plants w/ different
characteristics
Traits Mendel Studied
Mendel’s
work
• Mendel crossed “true
breeding” (plants that
produced offspring identical
to parents after selfpollinating) tall pea plants
with true-breeding short pea
plants (P generation)
• All of the offspring (F1
generation) were tall
• When he allowed the firstgeneration tall plants to selfpollinate, three-fourths of
the offspring (F2 generation)
were tall and one-fourth
were short.
Pea Plant Generations
• Parental generation (P) - original
crossed plants
• First Filial (F1) generation- offspring of P
generation
• F2 generation- second generation
made when two F1 generation plants
breed
Results of Mendel’s
Experiments
1. Alternate forms of genes (alleles) produce
variations in inherited characters
2. For each character, an organism inherits 2
alleles (one from each parent)
3. Principle of dominance
4. Law of Segregation
5. Law of Independent Assortment
Principle of Dominance
• Principle of Dominance- some alleles
are dominant while others are recessive
– Dominant- when that allele is present that
trait will be exhibited
– Recessive- masked trait when dominant
allele present
Important Vocabulary
• Homozygous- organisms w/ two identical
alleles for a trait (TT or tt) This is also called
purebred.
• Heterozygous- have different alleles for a
trait (Tt). This is called hybrid.
• Genotype- genetic makeup described with
letters (TT, Tt or tt)
• Phenotype- physical characteristics
described with words like tall or short
Law of Segregation
• Applies to sexual
reproduction
• Individuals have two alleles
of each gene. When
gametes are produced,
each gamete receives one
of these alleles.
• During fertilization, these
gametes randomly pair to
produce four combinations
of alleles.
Law of Independent Assortment
• Each pair of alleles assorts
independently of other pairs of
alleles during gamete formation
• The inheritance of one
character has no effect on the
inheritance of another
• For example– Pea plant with genotype AaBb
– A and a will separate from one
another and B and b will separate
from each other
– These alleles can then recombine
in four different ways.
Probability and Punnett
Squares
• Probability- likelihood that a particular
event or trait will occur.
• Probabilities are more accurate with
bigger samples
• Punnett squares- diagram used to
predict gene combinations
Crosses
• Hybrid: Offspring of parents that have different
forms of a trait (Ex. tall & short)
• Punnett squares can represent different types of
crosses
– Monohybrid Cross: Two parents that differ
from each other by a single trait.
– Dihybrid Cross: Cross between two parents
involving two different traits (Ex. Round/Yellow
& Wrinkled/Green)
Punnett Squares
• Dominant allele is represented with a
capital letter. When capital letter is present,
that phenotype is usually exhibited. Always
write dominant letter first.
• Recessive allele is represented with a lower
case letter. Phenotype exhibited when
homozygous recessive (tt)
Setting up a Punnett Square
•
•
•
•
•
Monohybrid Cross (One trait)
Female genotype across top
Male genotype across side
Ex:
Mother Tt, Father Tt
T
T
t
t
Problem: P Generation
Mother is homozygous tall TT
Father is heterozygous tall Tt
T
Genotype: F1
TT
TT
2TT:2Tt or 1/2TT:1/2Tt or
50%TT:50%Tt
Tt
Tt
T
T
t
Phenotype: F1
4 tall or 100% Tall
F1 Cross
Father Tt
Mother Tt
T
T
t
t
TT
Tt
Tt
tt
Genotype: 1TT: 2Tt: 1tt
Phenotype: 3Tall:1short
1/4 TT: 1/2 Tt: 1/4 tt
3/4 Tall: 1/4 short
25%TT: 50%Tt: 25%tt
75%Tall: 25%short