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Genes
Introduction to Genetics
What is DNA?
 DNA
stands for
DeoxyriboNucleic Acid
 It is the hereditary material
in humans and almost all
other organisms.
 Most DNA is located in the
cell nucleus.
 “Blueprint of life”
 The main role of DNA
molecules is the long-term
storage of information.
DNA Structure
 DNA
is a double-stranded molecule .
 The shape of the DNA molecule is a
double-helix (think of a spiral staircase or
like a twisted ladder).
 The sides of the ladder are composed of
alternating sugars (deoxyribose) and
phosphates.
 The rungs of the ladder are composed of
nucleotides.
DNA Structure
 The
structuof DNA was established
by James Watson and Francis Crick.
Base Pairing Rule
Nucleotides (also called Bases)
Adenine, Thymine, , Guanine,
Cytosine or A, T, G, C
Nucleotides pair in a specific
way - called the Base-Pair Rule
Adenine pairs to Thymine AT
Guanine pairs to Cytosine GC
Memory helper - Think "A T
Granite City" - which is where
you live.
What is a gene?
 Gene
- a segment of DNA that codes
for a protein, which in turn codes for
a trait (skin tone, eye color.etc),
 a gene is a stretch of DNA.
 Every person has two copies of each
gene, one inherited from each
parent.
What is a chromosome?
Chromosomes are DNA molecule
packaged into thread-like structures
in the nucleus
How many chromosomes do people have?
 In
humans, each cell normally
contains 23 pairs of chromosomes,
for a total of 46. Twenty-two of these
pairs, called autosomes, look the
same in both males and females.
 Diploid a cell that contains “two sets”
of chromosome 2N
 Haploid a cell that contains “one set”
of chromosome
Sex Determination
 Sex
cell chromosomes determine the
sex of an organism
 Sex cells are the sperm and egg–
gametes
 Females have two X chromosomes,
(XX)
 Males have one X and one Y
chromosome (XY)
 Zygote is the union of the sperm and
egg (46 chromosomes).
 Heredity
is the transmission of
characteristics from parents to
offspring.
 The genes for many traits are passed
down in families from parents to
children.
 Because you come from two
parents, each parent will provide one
half of your genes for any trait. That
is called an "allele." So you have two
alleles for each gene, one from each
parent.
Gregor Mendel
Gregor Mendel
 “Father”
of genetics
 Austrian monk, mid-1800s
 Researched pea plant inheritance
– Easy to grow, fast reproduction
– Studied plant height, pea shape/color,
pod color, etc.
Pea Plant Reproduction
 Self-pollination
– Male gametes (pollen) fertilize egg of
same flower
– Produces pure-bred offspring
– True-breeding: produce offspring
identical to parent when self-pollinated
Pea Plant Reproduction
 Cross-pollination
– Pollen from one plant fertilizes egg of
another plant
– Offspring have two parents
Mendel’s Experiments
P
= Parent generation
 F1 = First filial generation
 F2 = Second filial generation (F1 X
F1)
–Outcome
P
Pure Green X Pure Yellow
F1
All Green
F2
3 Green:1 Yellow
Mendel’s Conclusions
 Law
of Dominance – one allele (form
of a gene) is dominant, one is
recessive
– Recessive trait was hidden in F1
generation
– Green = dominant
– Yellow = recessive
Mendel’s Conclusions
 Law
of Segregation:
alleles for a gene
separate when
gametes form
(meiosis I)
– Each gamete gets
one copy of each
gene
Some Vocab.
 Genotype
– allele combination
 Genotype is the genetic make up
– Capital letter = dominant allele
– Lowercase letter = recessive allele
– Ex – AA, Aa, aa
 Phenotype
– physical appearance
– Ex – green, yellow
Some Vocab.
 Homozygous
– two alleles Same
 Homozygous organisms that have two
identical alleles.
– Homozygous dominant: AA
– Homozygous recessive: aa
 Heterozygous
– two alleles different
 Heterozygous organism with two different
alleles.
– Aa
Dominant and recessive
 Dominant
traits are the expressed
characteristic. It is always
represented with a capital letter
 Recessive traits are the repressed
alleles; the ones that are not
expressed. It is always represented
with a lower case
Probability
 Punnett
squares are used to predict
the probability of certain traits in
offspring of genetic crosses
 Tt X Tt
– ½ chance of getting ‘t’ from mom, ½
chance of getting ‘t’ from dad
– ½ X ½ = ¼ tt in offspring
Punnett Squares
 First
must determine possible
gametes
 Heterozygous tall plant = Tt
– Half of gametes will get ‘T’, other half
will get ‘t’
 Homozygous
tall plant = TT
– All gametes will get ‘T’
Punnett Squares
Tt X Tt

Monohybrid cross
– Cross involving one
trait
Gametes go on the
top and side
 Combine gametes
to find possible
offspring

Punnett Squares
Tt X Tt

Genotype ratio
1TT: 2Tt: 1tt

Phenotype ratio
3 tall: 1 short
Dihybrid Cross

Mendel looked at the inheritance patterns
of two traits
– Seed shape and seed color
Found that the traits were inherited
independently of each other
 Law of Independent Assortment

– Genes on separate chromosomes are inherited
at random
– Due to random chromosome shuffling in
Metaphase I
Independent Assortment
Metaphase I
Non-Mendelian Genetics
 Not
all traits follow Mendel’s Law of
Dominance
 Four Variations
– Incomplete Dominance (blending)
– Codominance (two phenotypes)
– Multiple Alleles
– Polygenic Traits
Incomplete Dominance
 Neither
allele is dominant, both
produce a protein
 Heterozygous phenotype is a blend
of both homozygous phenotypes
 Ex – wavy hair, pink flowers
Incomplete Dominance
Codominance
 Neither
allele dominates the other,
both produce a protein
 Heterozygous phenotype is a
combination of both homozygous
phenotypes
 Ex – checkered chicken, human
blood types,
Multiple Alleles
 Some
genes have more than two
alleles
 Each individual only gets two, but
there are more than two in the
population
 Ex – Rabbit fur color
Multiple Alleles
 Rabbit
fur alleles (in order of
dominance)
– C: dark gray
– cch: chinchilla
– ch: himalayan
– c: albino
Multiple Alleles
Polygenic Traits
 Many
traits are
controlled by more
than one gene
 Traits show wide
variation
 Ex – human height,
IQ, bell pepper
colors
Sources
 http://learn.genetics.utah.edu/conte
nt/begin/traits/activities/index.html