Download Genetic Notes

GENETICS
Genetics
• The study of heredity-passing on
characteristics from parents to
offspring.
• Gregor Mendel (1860’s) discovered the
fundamental principles of genetics by breeding
garden peas. “Father of Genetics”
• Gene – segment of chromosome (DNA) that
codes for a trait
• Trait -specific characteristics
– Green pea color
Generation lingo
•P1 generation - parents, first initial
cross
•F1 generation - first offspring resulting
from crossing the P1 generation
•F2 generation - second set of offspring
that resulted from crossing the F1
generation
Mendel’s Factors
1. Units that determine heritable traits.
2. Versions of gene = Alleles
- Two alleles for every trait
- One allele from each parent
- Dominant and Recessive
Letters used to represent alleles.
Capital letter = Dominant
ex: B
Lowercase letter = recessive
ex: b
Complete (simple) Dominance
• Dominant = masks/covers up the other allele
for a specific characteristic.
• Recessive = can be “masked” by dominant.
Will only see recessive trait if dominant allele
is absent.
Genetic Terms
Homozygous – both alleles are the SAME type
of allele.
Homozygous Dominant – both alleles are
dominant. Will show the dominant trait.
-TT
Homozygous Recessive – both alleles are
recessive. Will show the recessive trait.
-tt
Heterozygous – two DIFFERENT alleles. One
dominant and one recessive. Will show the
dominant trait.
-Tt
Genotype
• Version of gene (alleles) inherited from
parents (one allele from each) .
• Example:
TT
tt
Tt
Phenotype
• Outward appearance
• Physical characteristics
• Examples:
tall pea plant
Freckles
Brown fur
Long tail
Baby Bunny
Activity
Mendel’s findings
• Law of Segregation =alleles are separated during
the formation of gametes.
– ONE allele from EACH PARENT
• Law of Independent Assortment = alleles for
different traits are distributed randomly to gametes
independently.
• These only occur in sexual reproduction
– Produce genetic variety
– Genetic recombination!
Law of Segregation
Meiosis - eye color
B
sperm
B
B
Bb
haploid (n)
b
diploid (2n)
b
b
meiosis I
meiosis II
Homologous Chromosomes
eye color locus
B = brown eyes
eye color locus
b = blue eyes
Paternal Maternal
Law of
independent
assortment – the
eye color location
(locus) of
chromosome is
not the same as
another trait, say
hair color.
Punnett square
• A Punnett square is used to cross two
parents and show the possible offspring
combinations of gametes.
Monohybrid Cross
• A breeding experiment that tracks the inheritance
of a single trait.
• Types of monohybrid cross
– Homozygous x homozygous
– Homozygous x heterozygous
– Heterozygous x heterozygous
Breed the P generation
• tall (TT) vs. dwarf (tt) pea plants
T
t
t
T
tall (TT) vs. dwarf (tt) pea plants
T
T
t
Tt
Tt
produces the
F1 generation
t
Tt
Tt
All Tt = tall
(heterozygous tall)
100% tall – phenotype %
100% Tt – genotype %
Breed the F1 generation
• tall (Tt) vs. tall (Tt) pea plants
T
T
t
t
tall (Tt) vs. tall (Tt) pea plants
T
T
t
TT
Tt
t
Tt
tt
produces the
F2 generation
1/4 (25%) = TT (Tall)
1/2 (50%) = Tt (Tall)
1/4 (25%) = tt (short)
1:2:1 genotype ratio
3:1 phenotype ratio
More terms to know…
• Purebred – PURE = SAME =
Homozygous
– HH
– hh
• Hybrid – Different = Heterozygous
– Hh
Review Monohybrid Cross
• Example:
Cross between two heterozygotes
for brown eyes (Bb)
BB = brown eyes
Bb = brown eyes
bb = blue eyes
B
b
B
Bb x Bb
b
female gametes
male
gametes
Monohybrid Cross
B
b
B
BB
Bb
b
Bb
bb
Bb x Bb
1/4 or 25%
= BB - brown eyed
1/2 or 50%
= Bb - brown
eyed
1/4 or 25%
= bb1:2:1
- blue
eyed ratio
genotype
3:1 phenotype ratio
Sex-linked Traits
• Traits (genes)
located on the sex
chromosomes
• XX = girl
• Xy = boy
• The y chromosome
is smaller than the
X chromosome and
is lacking
corresponding
alleles.
Karyotype
The 23rd pair is the sex chromosomes. Should
be able to determine gender. Notice the Y
chromosome size in comparison to the X.
Sex-linked traits
• Will find predominantly (higher %
occurrence) in MALES.
• Males can not be heterozygous
(carriers) for a sex-linked trait.
• They either have it or not…only get one
allele on the X chromosome.
Queen Victoria of England was a carrier of the gene for hemophilia. She
passed the harmful allele for this X-linked trait on to one of her four sons
and at least two of her five daughters. Her son Leopold had the disease
and died at age 30, while her daughters were only carriers. As a result of
marrying into other European royal families, the princesses Alice and
Beatrice spread hemophilia to Russia, Germany, and Spain. By the early
20th century, ten of Victoria's descendants had hemophilia. All of them
were men, as expected.
Queen Victoria (1819-1901) with her
husband and nine children in 1857
Sex-linked Traits
• Example:
fruit flies
(red-eyed male) X (white-eyed female)
• Remember: the Y chromosome in males
does not carry traits.
RR = red eyed
Rr = red eyed
rr = white eyed
Xy = male
XX = female
XR
Xr
Xr
y
Sex-linked Traits
XR
Xr
XR Xr
y
Xr y
1/2 red eyed and female
1/2 white eyed and male
Xr
XR Xr
Xr y
Incomplete Dominance
• F1 hybrids have an appearance somewhat in
between the phenotypes of the two parental
varieties.
• With incomplete dominance, a cross between organisms
with two different phenotypes produces offspring with a
third phenotype that is a blending of the parental traits.
R
R
Example: snapdragons
(flower)
r
red (RR) x white (rr)
RR = red flower
r
rr = white flower
Incomplete Dominance
R
R
r
Rr
Rr
produces the
F1 generation
r
Rr
Rr
All Rr = pink
(heterozygous pink)
Both alleles expressed
as “mix”
Codominance
• the prefix "co-" is "together"
• Two alleles are expressed (multiple alleles)
in heterozygous individuals.
• Neither of two alleles masks the other
• In COdominance, both traits appear
together equally (at the same time) in
the phenotype of hybrid organisms.
Codominance
• Example: blood
1.
2.
3.
4.
type A
type B
type AB
type O
=
=
=
=
IAIA or IAi
IBIB or IBi
IAIB
ii
Examples
• A very common phenotype used in
questions about codominance is roan
fur in cattle. Cattle can be red (RR = all
red hairs), white (WW = all white hairs),
or roan (RW = red & white hairs
together).
A good example of
codominance.
Mutations
• Mistakes – errors – changes in the DNA
• Can happen during DNA replication and
Protein Synthesis (transcription)
• How?
– DNA breaks apart (bonds broken)
– Nitrogen bases are paired incorrectly
– Nitrogen bases switch spots with
neighboring bases = paired incorrectly
– Nitrogen bases deleted
Why do mutations occur?
• Random
• Genetically inherited from parental
gametes
– in alleles
• Caused by mutagens. What are
mutagens?....(wait for it…)
Mutagens
• Environmental factors that
cause mutations
– DNA molecule to fall apart – bonds are
broken
• High heat & temperature
– Nitrogen base changes
•
•
•
•
UV radiation (sun)
Nuclear radiation
Chemicals – such as formaldehyde
Some viruses
Are all mutations bad?
• Some mutations can result in providing
an organism with an advantage for
survival.
– New variations in species
– Dumbo…bigger than normal ears.
– What if big ears give an advantage?
Applying what you know…
• DNA  RNA  (amino acid) Protein 
Trait
• If you wanted to find out if two
organisms were more closely related,
how would you do that?
– What could you analyze?
DNA Sequence
(order of nitrogen bases)
Which of the two organisms
are most closely related?
•
•
•
•
ATTCCGGCAATG
ATTCCCCCAATG
ATTTAACGTAAC
ATAGGAATACGG
Amino Acid Sequence
Traits
• Remember when you sorted the
organism pictures based on what you
could see (their traits).
• Outward traits (phenotypes) are
determined by genotype.
• More similar traits then more likely to
have more similar DNA