Introduction
to Genetics
Heredity
• The passing on of
characteristics
from parents to
offspring
Traits
• Characteristics
that are
inherited
Genetics
• The study of heredity
–Genes: chemical
factors that determine
traits
• Can occur in 2 different
forms called alleles
Gregor Mendel
• Father of genetics
• 1st to succeed in predicting
how traits would be transferred
from one generation to the next
• Used pea plants
• Pea plants reproduce sexually
– Produce male and female sex
cells (gametes)
– Fertilization: male and female
gametes unite, resulting in a
zygote (fertilized egg)
• Controlled plant crosses by removing male
parts
• Mendel studied only 1 trait at a time (7 total
traits)
– True-breeding pea plants:
• Produced identical offspring
• Ex: tall or short, green or yellow
• Crossed a tall plant with a
short plant
– Parent generation (P1)
– Offspring called hybrids
(F1)
• Have different forms of
a trait
• Showed characteristics
of only 1 parent
– Observed traits are
dominant
– Traits that seem to
disappear are recessive
– Allele for tall plants is
dominant to the allele for
short plants
– Inherit one gene from
mom and one from dad
• Organisms can look alike
but have different
combinations of alleles
• Different forms of a gene
for each variation of an
inherited trait
• Phenotype: way an organism looks and
behaves
– Physical characteristics
– Ex: height (tall, short), skin color (red,
white, blue..etc), head size, eye color…etc
• Genotype: Allele combination an organism
contains
– Genetic make-up
– Assigned a letter to represent the trait
– Ex: TT, Tt, tt
• Homozygous: 2 alleles for
the trait are the same
– Called true-breeding
– Ex: TT, CC, HH, tt, cc,
hh
• Heterozygous: 2 alleles
for the trait differ from
each other
– Called hybrids
– Ex: Tt, Cc, Hh, Ss
Punnett Squares
• Way to find expected proportions or probability of
possible genotypes in the offspring of a cross
• Probability
– Likelihood an event will occur
– Used to predict outcomes of a genetic cross
– Way alleles separate is random
– Cannot give actual outcome
• Monohybrid cross (mono-: means one)
– Focus on 1 trait
– Ex: height T= tall, t= short
texture S= smooth vs. s= rough,
– Crossed a tall plant with a short plant
Mendel’s Crosses
• 1st generation plants (F1) were all
tall
–Self-pollinated to make 2nd
generation plants (F2)
• 2nd generation plants:
–¾ were all tall plants
–¼ were short plants
• ex: First generation plants (F1): all tall

ex: Second generation plants (F2)

Punnett squares are good for showing all
the possible combinations of alleles and
the probability of inheriting a trait
Bell Ringer:
1. Which 2 combinations of alleles could produce a
trait controlled by a dominant allele?
2. What combination of alleles could produce a trait
controlled by a recessive allele?
3. If a heterozygous plant for seed color (Rr) is
crossed with a homozygous recessive plant (rr),
what is the probability of each seed color being
produced? Draw a punnett square to show work.
4. Organisms that have identical alleles for a
particular trait are:
a. heterozygous
c. diploid
b. polygenic
d. homozygous
Dihybrid Crosses (di-: means 2)
• Involve two traits
• Mendel: used seed
color and seed
texture
– Crossed round
yellow seeds (RR
and YY) with
wrinkled green
seeds (rr and yy)
– Round is dominant
to wrinkled
– Yellow is dominant
to green
• 1st generation (F1) plants:
round and yellow (RrYy)
• 2nd generation (F2) plants:
–
–
–
–
round yellow (9)
round green (3)
wrinkled yellow (3)
wrinkled green (1)
Rule of Dominance
•
•
•
•
Some alleles are dominant and others are recessive
Pea plants with 2 alleles for tallness were tall
Pea plants with 2 alleles for shortness were short
Dominant allele: observed trait
– Upper-case letters = Dominant allele (always put capital
letter 1st for the genotype)
– Ex: allele for tallness (T)
• Tall Plants: TT or Tt
• Recessive allele
– Lower-case letters = Recessive allele
– Always listed after the dominant allele
– Will only appear if no dominant allele is present
– Ex: allele for shortness (t)
• tt  short plant
– Disappears
Law of Segregation
• Every individual has 2
alleles of each gene
• When gametes are
produced, each gamete
receives 1 of these alleles
– Randomly pair to produce 4
combinations of alleles
during fertilization
Tall plant:
Short plant:
TT
x
Yellow plant x
YY
x
tt =
Tt
Green plant:
yy = Yy
F1 generation
Tt Tt
T
TT
Tt
X
t
Tt
gametes
T
Tt
F2 generation
t
tt
Law of Independent Assortment
• Ex: Does a gene that
determines whether a seed is
round or wrinkled have
anything to do with seed
color?
• Genes do not have to be
carried together
– Travel independently
• Genes for different traits can
segregate independently
during formation of gametes
– Accounts for the wide variety of
genetic combinations in
offspring
• Genes for different traits are
inherited independently of
each other
Incomplete Dominance
• “blend” or mixing of
traits
• neither allele is
completely dominant
• phenotype of
heterozygous individuals
is intermediate (in the
middle)
• mix of purebred
phenotypes
• ex: Red flowers (RR) x
White flowers (R´R´) =
Pink flowers (RR´)
Codominance
• phenotypes of both homozygotes
are produced
– both are dominant
• both alleles contribute to the
phenotype but do not blend
together
– both alleles are expressed
equally
• ex: chickens
– Black chickens (BB) x White
chickens (WW) = checkered or
speckled chickens (BW)
• ex: red cow (RR) x white cow (WW)
= roan cow (pinkish brown  RW)
• ex: Sickle-Cell anemia
Multiple Alleles
• Genes with more than 2 alleles
• Ex: rabbit coat color, blood type,
eye color
Polygenic Inheritance
• inheritance pattern of a trait that is
controlled by 2 or more genes
• may be on the same or different
chromosome
• each gene can have 2 or more alleles
• ex: skin color, fruit fly eye color
Bell Ringer:
1. According to the principle known as _______, genes that
segregate independently do not influence each other’s
inheritance.
2. The separation of alleles during gamete formation is called
____.
a. segregation
b. true-breeding
c. meiosis
d. crossing-over
3. Which of the following structures assort independently?
a. genes
b. crossovers
c. chromosomes
4. Genes on the same chromosome
a. never separate
c. always separate
b. sometimes separate
d. don’t show linkage
d. genotypes
Applying Mendel’s Principles
• Thomas Hunt Morgan (1900’s)
– Studied fruit flies
– Observed fruit flies produce a lot of
offspring quickly
• Mendel’s principles apply to all living
organisms
Linkage and Gene Maps
• Each chromosome is a group of linked genes
• Chromosomes assort independently (not
individual genes)
• Genes can separate by crossing-over
– Gives genetic diversity
– More likely to be separated the further apart they are on
the chromosome
• Gene map
– Shows
locations of
known genes
on a
chromosome
Simple Dominant Heredity
• Only need to inherit 1 dominant
allele to display the trait
• Tongue rolling
• Free earlobes
• Hitchhiker’s thumb
• Dimples
• Cleft chin
• Freckles
Bell Ringer:
1. Which of the following shows the relative
locations of each known gene in an organism?
a. polygenic trait
b. punnett square
c. gamete
d. gene map
2. Match the type of inheritance with its correct
description:
incomplete dominance
multiple alleles
codominance
polygenic traits
_______ both alleles contribute to the phenotype of the organism
_______ more than two possible alleles for a trait exist in a population
_______ traits controlled by two or more genes
_______ one allele is not completely dominant over the other allele