Genetics
Chapter 11
Heredity
• Passing of traits
from parent to
offspring
Genetics
• The study of
heredity
• Gregor MendelAustrian monk
known as the
“father of genetics”
Mendel’s Four Hypotheses
1. For each inherited trait, an individual has
a copy of that gene from each parent.
2. There are alternate versions of genes,
called alleles. (ex: freckles or no freckles)
Hypothesis continued
3. When 2 different alleles occur together,
one of them may be completely
expressed (dominant), while the other
may not be observed (recessive).
4. Alleles separate independently, so that
each gamete only carries 1 allele for that
trait.
Monohybrid cross
• Mendel began with 1 trait (monohybrid)
crosses
• He bred pea plants to produce several
generations
P- the parent generation
F1 - the first filial generation
F2 - second filial generation
• Mendel concluded
that each parent
has two separate
“factors” for a
particular trait
• “Factors” are now
called genes.
Mendel’s Laws of Heredity
1. Law of Segregation- two alleles separate
when gametes are formed
2. Law of Independent Assortment- alleles
of different genes separate
independently of one another during
gamete formation
Homozygous
dominant parent
Homozygous
recessive parent
(chromosomes
duplicated
before meiosis)
Mendel’s
Law of
Segregation
meiosis
I
meiosis
II
(gametes)
(gametes)
fertilization
produces
heterozygous
offspring
Fig. 11-5, p.172
Nucleus of a
diploid (2n)
reproductive cell
with two pairs of
homologous
chromosomes
Independent Assortment
Possible alignments
of the two homologous
chromosomes during
metaphase I of meiosis
The resulting
alignments
at metaphase II
Allelic
combinations
possible in
gametes
1/4 AB
1/4 ab
1/4 Ab
1/4 aB
Fig. 11-8, p.174
Some modern genetic terms
• Alleles are
represented by
letters
– Dominant allele
is a capital letter
– Recessive allele
is a lower case
letter
• Homozygous- identical alleles for a
specific trait (BB, FF, rr, tt)
• Heterozygous- alleles are different for a
specific trait (Bb, Ff, Rr, Tt) (aka “hybrid”)
• Genotype- the set of alleles an individual
inherits for a trait (i.e.- Rr, Ww, ff)
• Phenotype- the physical expression of a
trait (i.e. blue eyes, freckles, dimples)
Punnett Square
• Predicts possible
outcomes of traits
• Shows all possible
outcomes of a
genetic cross
• Probability (likelihood
that an event will
occur) of a genetic
outcome can be
predicted
• Ex: cross two
heterozygous
individuals Aa x Aa
predict ratios of
3:1 for phenotype and
1:2:1 for genotype
Practice
• Curly hair is dominant over straight hair. A
man with straight hair and a woman who is
heterozygous for curly hair have a child.
What is the probability that this child will
have straight hair?
• Incomplete Dominance- when an
individual shows a combination of the
inherited alleles.
Ex: red snapdragon x white snapdragon
will produce a pink snapdragon
straight hair x curly hair = wavy hair
• Codominance- 2
dominant alleles
are expressed at
the same time
Ex: Roan horses
show both red and
white hairs in equal
numbers
Patterns of heredity can be very
complex
• Sex-linked trait- a trait whose alleles is
located on the X chromosome (most are
recessive)
• Polygenic trait- trait where several genes
influence the outcome (eye color, hair
color, skin color, height, weight)
• Multiple Alleles- genes with 3 or more
alleles
- Blood type is an example of multiple
alleles because we have A, B and O
alleles and 4 possible blood type
outcomes
(A, B, AB and O)
ABO Blood Type
Range of genotypes:
IAIA
IBIB
or
or
IAi
Blood
Types:
A
IAIB
AB
IBi
ii
B
O
Fig. 11-10a, p.176
Diploid
• Number of chromosomes found in the
body or somatic cells of an organism
• “2n”
Haploid
• Number of chromosomes found in the
gametes of an organism
• “n”
Meiosis
• The type of cell division that produces
gametes
• Gametes = sex cells
– female gamete = egg or ovum (plural = ova)
– male gamete = sperm
Meiosis
Formation of sperm and egg