Genetics
The Study of Heredity
Gregor Mendel
• Discovered many of
•
•
the principles of
modern genetics
Mendel studied the
inheritance of traits
using pea plants
Principles of basic
inheritance are called
Mendelian genetics
Genes
• Genes are segments of
•
•
chromosomes that
determine the traits of an
organism (ex. Eye color)
There may be several
hundred to several
thousand genes on a
chromosome
We inherit 50% of our
genes from our mothers
and 50% from our fathers
50% of
your genes
from dad
50% of
your genes
from mom
Alleles
• Every gene comes in different
variations called alleles
• Most genes have two different
alleles while some have more
than two
• One person can have no more
than two different alleles for the
same gene (we get one from
each parent)
Three people with different
alleles for a gene involved in
eye color
Dominant vs. Recessive Alleles
• Alleles interact to
•
produce traits (eye
color, shape of
skeleton, blood type,
etc.)
Dominant alleles
“overpower” recessive
alleles (see diagram)
Brown
eyes
Brown
eyes
Blue
eyes
Human Karyotype
(picture of all 46 chromosomes)
Each pair is called a homologous pair
Homologous Chromosomes
• Chromosomes that
Mother
Father
are very similar (size,
shape, same type of
genes) except that
they come from two
different individuals
(see diagram)
A pair of homologous chromosomes
from one individual
Monohybrid Crosses
Using probability to determine the
likely inheritance of a trait
ALBINISM
Lack of pigment in skin, hair, and
eyes
Example problem
The allele for normal skin pigment (A)
is dominant over the allele for albinism
(a). If a heterozygous normal
pigmented man (Aa) marries a
homozygous albino woman (aa), will
they have any albino children?
A= normal pigmentation
a= albino
Punnett
Square
DAD’s possible
MOM’s possible
gametes
gametes
A
a
a
Aa
aa
a
Aa
aa
Possible gene combinations (genotypes) of children
What percentage of their
children have normal skin
pigment?
WHY?
The allele for normal pigment
(A) is dominant over the allele
for albinism (a).
Genotype vs. Phenotype
• Genotype is the genetic
•
•
make-up of an organism
(its genes)
Phenotype refers to the
actual physical traits an
organism has as a result
of its genes
The genotype
The genes of the fly give it its
determines the
unique characteristics
phenotype (see picture)
Heterozygous & Homozygous
• Homozygous -
•
having the same
alleles for a given trait
Heterozygous having different alleles
for a trait
Meiosis
• Formation of gametes (sperm
and egg)
• Happens in testes (males) and
ovaries (females)
• Reduces the chromosome
number by half
• Meiosis produces 4 genetically
different cells from the original
cell
Why are the cells produced in
meiosis different?
• When gametes are made, they
receive only one chromosome
from each homologous pair,
making haploid cells (have half
the number of chromosomes as
the original cell)
• Chromosomes are distributed to
gametes randomly resulting in
different combinations of
chromosomes (called
independent assortment)
Independent assortment - random
distribution of chromosomes into the gametes
during meiosis
Why are the cells produced in
meiosis different? (continued)
• Chromosome shuffling
•
•
(independent assortment)
Crossing over homologous
chromosomes exchange
genetic information
forming new combinations
of genes (see diagram)
Mutations - a change in
the DNA (genes) of an
organism
Crossing over during prophase I
of meiosis
Mitosis
 One cell division
 Results in 2 diploid



cells (full set of
chromosomes
Daughter cells are
identical
Happens in body
cells (somatic cells)
For growth and
repair
vs.
Meiosis
• Two cell divisions
• Results in 4 haploid cells
•
•
•
(half # of chromosomes)
Daughter cells are
different
Happens in sex cells
(gametes)
For maintaining the
chromosome # of the
species
Mendel’s Law of Segregation
• Alleles for the same
trait separate from
each other during the
formation of gametes
(see Punnett square
diagram)
Mendel’s Law of Independent
Assortment
• Alleles for different
traits are randomly
distributed to
gametes (unless
they are very closely
linked)
Predicting Genetic Crosses
with Punnett Squares
• Monohybrid crosses
(shows segregation)
• Dihybrid crosses
(shows segregation
and independent
assortment)
Example of a dihybrid cross
Dihybrid cross (example)
A heterozygous guinea pig for black, rough fur
mates with another heterozygous guinea pig
for black, rough fur. What will the genotypes
and phenotypes of their offspring be given the
information below:
B = black fur
b = white fur
R = rough fur
r = smooth fur
Answer to Problem
•
•
•
•
Black, rough fur (B__R__)
Black, smooth fur (B__rr)
White, rough fur (bbR__)
White, smooth fur (bbrr)
9
3
3
1
A 9:3:3:1 ratio occurs when two heterozygous parents
have offspring in a dihybrid cross
Human Inheritance
Chapter 12
Patterns of Inheritance
• Complete dominance
• Incomplete dominance
• Codominance
• Polygenic Inheritance
• Sex-linked inheritance
Incomplete Dominance


Neither allele of a
pair is dominant over
the other
A blend of both traits
results
Genotypes and phenotypes of flowers
showing incomplete dominance for
petal color (R= red and W=white)
Codominance

Both alleles show
dominance causing
both traits to be
expressed in the
phenotype of the
organism

Examples:
1. Coat color in animals
2. Blood type in humans Example of black roan coat
showing codominance
Blood type
3 different alleles for
blood type exist:
 IA and IB are
codominant to each
other
 IA and IB are both
dominant to i
Genotypes of the 4
blood types:
 IAIA or IAi = type A
 IBIB or IBi = type B
 IAIB = type AB
 ii = type O
Possible outcomes of children of
dad with type AB blood and mom with
type O blood
Polygenic Traits
• Traits that are controlled by the combined
action of many genes
• Show a diverse range of phenotypes
• Most traits are polygenic
• Ex. Skin color, hair color, eye color, height,
risk for heart disease, etc.
Sex-linked traits
 Any trait that is
carried on the X or Y
chromosome
 If allele is on the X
chromosome, females
will have two copies
(XX) but males only
one (XY)
 Example: Hemophilia
(blood does not clot
properly)
Independent assortment of
Symbolizing Sex-Linked Traits
Hemophilia is a sex-linked
recessive trait that is carried
on the X chromosome.
H
h
X = normal
X = hemophilia
* Y chromosome has no gene for
hemophilia
Possible genotypes and phenotypes
XH XH = normal woman
H
h
X X = carrier (does not have
disorder but carries the allele)
h
h
X X
H
X Y
h
X Y
= hemophiliac female
= normal man
= hemophiliac male