Heredity
Heredity is the passing of
traits from one generation to
another, or inheritance.
Inherited Traits vs.
Environmental Factors
• Inherited Traits are those that are
passed from parent to offspring
• Environmental Factors can also have an
effect on how you look
– Diet
– Exercise
– Smoking/Drinking
– Sun exposure
Why do I look like my parents?!
You look like your parents because
genetic information (DNA) is passed from
parent to offspring during sexual
reproduction.
Each sex cell (egg or sperm) of the parent
organism (plant or animal) contains onehalf of the genetic material needed to
create a new organism.
What are Chromosomes?
A structure found in the nucleus of a
cell that contains the genetic
information (DNA).
Remember, these are those things you were
drawing that doubled and split in Mitosis and
Meiosis. Humans have 46 in every cell except
sex cells, which have 23.
What Is a Gene?
• A gene is a segment of
DNA found on a
chromosome that
determines the
inheritance of a
particular trait.
• Genes are what make
one individual look
different from another
Principles of Heredity
1. Each trait is governed by two
factors – now called genes.
2. Genes are found in alternative
forms called alleles.
3. Some alleles are dominant and
mask alleles that are recessive.
Dominant Traits
Dominant Trait- a trait that will always be
expressed in the phenotype.
These alleles are represented by a capital
letter.
Ex. Having a widow’s peak is dominant to
not having a widow’s peak.
Recessive Trait
Recessive Trait- a trait that will only be
expressed in the phenotype if two
recessive alleles are present. In the
presence of a dominate trait, the
recessive trait will not be expressed.
These alleles are represented by
lowercase letters.
Ex. No widow’s peak.
Seven Traits used by Mendel in Genetic Studies
Genotype and Phenotype
Genotype: alleles carried by an
individual ex. RR, Rr, rr
Phenotype: physical characteristic
or appearance of an individual
ex. Round, wrinkled
Who do you look like?
Draw a picture of yourself and then label
all of the traits that you inherited from
someone in your family (write the family
member’s name beside that trait)
For example, if your hair is red and your
mother’s hair is red, write her name
beside your hair.
Principles of Heredity
Mendel’s Experiment with Peas
Round seed
RR
Homozygous
Dominant
x Wrinkled seed
rr
Homozygous
Recessive
F1: All round seed coats
Rr
Heterozygous
Homozygous parents can only pass one
form of an allele to their offspring.
R
R
R
R
Heterozygous parents can pass either
of two forms of an allele to their offspring.
R
r
R
r
Principles of Heredity
Mendel was a scientist studying peas as a
way to explore genetics.
He needed to explain:
1. Why one trait seemed to disappear in
the first generation.
2. Why the same trait reappeared in the
second generation in one-fourth of the
offspring.
Genetic Segregation
Parentals:
RR x rr
R
R
R
R
r
r
r
r
Rr
Rr
Rr
F1 x F1:
Rr x Rr
Rr
R
½R
½r
r
R
½R
½ r
¼ RR ¼ Rr
¼ Rr
¼ rr
r
Genetic Segregation
Genotypic Ratio: ¼ RR + ½ Rr + ¼ rr
Phenotypic Ratio: ¾ Round + ¼ Wrinkled
Using Probability in Genetic
Analysis
1. Probability (P) of an event (E) occurring:
P(E) = Number of ways that event E can occur
Total number of possible outcomes
Eg. P(Rr) from cross Rr x Rr
2 ways to get Rr genotype
4 possible outcomes
P(Rr) = 2/4 = 1/2
Sex Determination
Female
XX x
Male
XY
½X
½X
½X
¼ XX
¼ XX
½Y
¼ XY
¼ XY
Phenotypic Ratio of Offspring
½ Female + ½ Male
Colorblindness
Pedigree for Colorblindness,
an X-linked Recessive Trait