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Study of heredity
• Heredity: passing of traits from parent to
child
• Patterns of relatedness can help predict
offspring characteristics
• Genes are located on chromosomes
• Genes inherited from mom and dad
• Genes come in several forms called alleles
– Allele: Alternative forms
of a gene
– Ex: Flower color
• White allele or Purple
allele
• An allele is any alternative form of a gene
occurring at a specific locus on a chromosome.
– Each parent donates
one allele for every
gene.
– Homozygous
describes two alleles
that are the same at
a specific locus.
– Heterozygous
describes two alleles
that are different at a
specific locus.
Alleles can be represented using letters. Ex: Aa or AA or aa
– A dominant allele is
expressed as a phenotype
when at least one allele is
dominant.
– A recessive allele is
expressed as a phenotype
only when two copies are
present.
– Dominant alleles are
represented by UPPERCASE
letters
– Recessive alleles by
lowercase letters.
Dominant and Recessive Genes
• Gene that prevents the other gene from “showing” –
dominant
• Gene that does NOT “show” even though it is present –
recessive
• Symbol – Dominant gene – upper case letter – T
Recessive gene – lower case letter – t
Dominant
color
Recessive
color
Genotype and Phenotype
• Combination of genes
an organism has (actual
gene makeup) –
GENOTYPE
Ex: TT, Tt, tt
• Physical appearance
resulting from gene
make-up – PHENOTYPE
Ex: hitchhiker’s thumb
or straight thumb
Genetics Vocabulary Review
Choices:
1) YY
2) Tall
3) Yy
4) SS
5) yY
6) gg
7) Smooth
8) Green
9) TT
10) Dwarf
Which choice(s) are examples of:
Genotypes?
1, 3, 4, 5, 6, 9
Phenotypes?
2, 7, 8, 10
Homozygous genotypes?
1, 4, 6, 9
Heterozygous genotypes?
3, 5
Homozygous recessive genotypes?
6
Homozygous dominant genotypes?
1, 4, 9
Dominant vs. Recessive
#1
• If Brown eye color is dominant over blue
eye color, how would you indicate…
– The brown allele?
– The blue allele?
Dominant vs. Recessive
#2
• If dimples in the cheeks are dominant, how would you
indicate…
– The allele for dimples?
– The allele for no dimples?
Dominant vs. Recessive
#3
• If left thumb over right thumb is dominant,
how would you indicate…
– Left over right?
– Right over left?
Mendel laid the groundwork for genetics.
• Traits are distinguishing
characteristics that are
inherited.
• Genetics is the study of
biological inheritance
patterns and variation.
• Gregor Mendel showed that
traits are inherited as
discrete units.
• Many in Mendel’s day
thought traits were blended.
Mendel’s data revealed patterns of
inheritance.
• Mendel made three key decisions in his
experiments.
– use of purebred plants
– control over breeding
– observation of seven
“either-or” traits
• Mendel used pollen to fertilize selected pea plants.
– P generation crossed to produce F1 generation
– interrupted the self-pollination process by removing male
flower parts
Mendel controlled the
fertilization of his pea plants
by removing the male parts,
or stamens.
He then fertilized the female
part, or pistil, with pollen from
a different pea plant.
• Mendel allowed the resulting plants to self-pollinate.
– Among the F1 generation, all plants had purple flowers
– F1 plants are all heterozygous
– Among the F2 generation, some plants had purple
flowers and some had white
Each trait had a
3:1 ratio.
No way that can be
a coincidence!
Mendel’s 3 Conclusions
1. Law of Dominance
• If the two alleles of a particular gene present in an
individual are the same, the individual is said to be
homozygous. (dominant or recessive)
• If the alleles of a particular gene present in an
individual are different, the individual is
heterozygous.
• In heterozygous individuals, only the dominant allele
is expressed; the recessive allele is present but
unexpressed.
Mendel’s 3 Conclusions
2. Traits are inherited as
discrete units.
• Organisms inherit two
copies of each gene
(allele), one from
each parent.
• Genes are inherited
independently of each
other.
purple
white
Mendel’s 3 Conclusions
3. Law of
Segregation
The two copies of
alleles segregate
during gamete
formation.
Punnett
Squares
• Defined: Tools used to
determine genetic
probability
• Crosses:
– Monohybrid cross = cross of
two different true-breeding
strains (homozygotes) that
differ in a single trait.
– Dihybrid cross = cross of two
different true-breeding strains
(homozygotes) that differ in
two traits.
• Probability = likelihood
that a certain event will
happen
Cross a homozygous dominant yellow (YY) plant with
a homozygous recessive green (yy) plant
Key
Y = yellow
y = green
100%
Probability of growing a yellow plant? _____________
0%
Probability of growing a homozygous recessive plant? ____________
0%
Probability of growing a homozygous dominant plant? ____________
Cross a heterozygous dominant yellow (Yy) plant with a
heterozygous dominant yellow (Yy) plant
Key
Y = Yellow
y = green
50%
Probability of growing a heterozygous plant? ________
25%
Probability of growing a green plant? ________
25%
Probability of growing pure yellow? _________
Maria is a heterozygous healthy (Hh) female and Jeff
suffers from recessive cystic fibrosis (hh).
Key
H = healthy
h = cystic
fibrosis
Probability of having a healthy
50%
child? _________
Probability of having a homozygous
0%
dominant child? ________
PEDIGREE
• A pedigree chart is a diagram that shows
the occurrence particular gene/trait from
one generation to the next.