Transcript Chapter 11

Introduction to
Genetics
Chapter 11 (M)
Genetics and Inheritance
 Genetics
 the study of
heredity that deals with the
transmission of traits or
characteristics from one
generation to another
 Inheritance The reception of
traits by transmission from
parent to offspring
Prehistoric Times
 Little
is known when humans first
recognized the importance of
genetics
 The breeding of horses, cattle, and
various breeds of dogs began around
8000 B.C. and 1000 B.C.
 Plants such as corn, wheat, and rice
was cultivated in Mexico around 5000
B.C.
The History Of Genetics
Pythagoras 500
B.C., a Greek
philosopher, stated
that human life
began with male
and female fluids
The History Of Genetics
Aristotle furthered
this idea and
suggested that
these fluids, or
“semens,” were
actually purified
blood—therefore,
blood must be part
of heredity.
One Bizarre Theory
•The theory of Homunculus
-17th century
•sex cells contained a
complete miniature adult,
perfect in form
•This statement was
popular way into the 18th
century
Small
individual
Blending Hypothesis 1800’s

This stated that both the genetic material from
the mother and father were blended in order to
produce an offspring
Parents Red Flower X Yellow Flower Offspring
Orange (offspring all orange)
 Exceptions Red yellow ,etc
 Theory discarded, If blending occurred  all
extreme characteristics would disappear from
the population
Gregor Mendel-1800’s
•Considered to be the
“father of genetics
•Austrian monk, teacher
and mathematician
•Expt. approach to
genetics
•Particulate Hypothesis
Particulate Hypothesis
 Parents
pass on to their
offspring separate and
distinct factors (genes) that
are responsible for inherited
traits
 Used pea plants to study this
Why peas?
Rapid reproduction
rate
 Presence of
distinctive traits
 Closed structure of
flowers (each pea plant

has male (stamens) and
female (carpal) sexual
allows selffertilization
organs)
Cross-fertilization
 One
plant is
fertilized by
another
Terminology



P Generation 
parents
F1 Generation 
first filial
F2 Generation
second filial
PXP
F1 F1 F1
F2
Allele
Alternative forms of a gene
which determines a trait.
Alleles cont.
 Uppercase
(Capital) letters for
dominant traits
 Lowercase letters for recessive
traits
 Ex: Tall = T short = t,
expressed in pairs TT, Tt, tt
 Phenotype
Physical appearance
 Genotype Genetic makeup
 Dominant trait that is easily
observed
 Recessive  trait that is often
masked
 Homozygous2 alleles for a trait
are identical  TT or tt
 Heterozygous – 2 alleles for a
trait are not identical  Tt
Inheritance Follows Rules of
Chance
 Mendel
began experiments to
track the inheritance of
characters in pea plants
 Results led him to formulate
several hypotheses
Seven Traits Studied by Mendel
Terminology



P Generation 
parents
F1 Generation 
first filial
F2 Generation
second filial
PXP
F1 F1 F1
F2
Mendel’s Experiment


Crossed pure purple
and a pure white
flower (P
generation) =F1
generation
All F1 plants
(purple) are
crossed by self
pollination = F2
generation yields
¾ purple and ¼
yellow
Mendel’s Hypotheses
1.
2.
3.
4.
There are alternate forms of genes
For each character an organism has
two alleles
Alleles are either dominant or
recessive
Alleles segregate during formation
of gametes
Law of Dominance
When organisms pure for
contrasting traits are crossed,
all their offspring will show the
dominant trait
Probability

Fractions or ratios that will predict that an
event will occur
Punnett Square
 Diagram
which
shows the
possible
outcome of a
cross
Monohybrid Cross
Using a single trait –
crossing a pure bred
Tall (TT) with a pure
bred short (tt) plant
Mating 2 heterozygous black (Bb)
rabbits
Test Cross
• Individual with dominant
phenotype  not possible to
predict the genotype run a test
cross with individual with
recessive phenotype to determine
the allele
Dominant Phenotype purple
flower (genotype PP or Pp)
Recessive Phenotype  white
flower ( genotype pp)
Test Cross
Law of Independent
Assortment
Each pair of alleles segregates
into gametes independently
Independent Assortment

Mendel followed the inheritance of two
different characters ( dihybrid cross)
 The
allele for yellow seeds (Y) is dominant to
the allele for green seeds (y).
 The allele for round seeds (R) is dominant to
the allele for wrinkled seeds (r).

He crossed true-breeding plants that had
yellow, round seeds (YYRR) with truebreeding plants that had green, wrinkled
seeds (yyrr).
Dihybrid Cross Crosses Involving
Two Traits:
Color: Yellow, Green
Shape: Round, Wrinkled
Yellow-Round
Yellow-Wrinkled
Green-Round
Green-Wrinkled



The Y and R alleles and
y and r alleles stay
together
F1 offspring would
produce yellow, round
seeds.
The F2 offspring would
produce two
phenotypes
in a 3:1 ratio, just like a
monohybrid cross.



If the two pairs of
alleles segregate
independently of each
other
Four classes of
gametes (YR, Yr, yR,
and yr) would be
produced in equal
amounts.
These combinations
produce four distinct
phenotypes in a
9:3:3:1 ratio
Practice problems
Cross a homozygous yellow, homozygous
round plant with a green, wrinkled plant
 Cross a homozygous yellow, homozygous
round plant with a heterozygous yellow,
wrinkled plant

Other Patterns of
Inheritance
11.3
Variation in Inheritance
 Incomplete
Dominance
 Codominance (multiple
alleles)
 Polygenic inheritance
Incomplete Dominance
Both alleles contribute to a phenotype
of a heterozygous individuals to produce
a trait not like either parent.
 Phenotype intermediate between two
pure traits
Ex: Snapdragons, Andalusian chick.

Snapdragon Flower Color




Alleles often written as capital letters with
superscripts
ex. CRCR (red) x CWCW (white)
Incomplete Dominance revealed in Heterozygous
Individual
A cross between a white-flowered plant
and a red-flowered plant will produce all
pink F1 offspring
Self-pollination of the F1 offspring produces
25% white, 25% red, and 50% pink
offspring.
Codominance (multiple
alleles)
 Two
dominant alleles are
expressed at the same time
 Ex: Roan coat color in horses
C R C R , C WC W, C R C W
R
R
C C
W
W
C C
R
W
C C
(Red)
(White)
(Roan)
If a roan cow (RW) is mated
to a roan bull (RW), what are
the phenotypes of the
offspring?
Multiple Alleles
Most genes have more than two alleles in a
population
 The ABO blood groups in humans are
determined by three alleles, IA, IB, and i.

 Both
the IA and IB alleles are dominant to the
i allele
 The IA and IB alleles are codominant to each
other

Because each individual carries two alleles,
there are six possible genotypes and four
possible blood types.
ABO Blood Group System
 Type
A
 Type B
 Type AB
 Type O
IA IA or
IB IB or
IA IB
ii
IA i
IB i
Problems
A man homozygous for type A blood
marries a woman who is heterozygous
for type B blood. What are the possible
genotypes and phenotypes of their
children?
 A couple has a child with type O blood.
If one parent is type O, what are the
possible genotypes of the other parent?

Polygenic inheritance

When two or more genes effect a single
characteristic
 For
example, skin color in humans is
controlled by at least three different
genes.
 Imagine that each gene has two alleles, one
light and one dark, that demonstrate
incomplete dominance.
 An AABBCC individual is dark and aabbcc is
light


A cross between
two AaBbCc
individuals
(intermediate skin
shade) would
produce offspring
covering a wide
range of shades
The range of
phenotypes
forms a normal
distribution.
Importance of Environment
In some cases environment plays an
important part in the expression of
genes
Ex: Temperature (Siameses cat fur),
Nutrition (height & growth of individual)
 However human blood type is not
influenced by environment.

Influence of Environment
Meiosis
11.4
Meiosis
A
type of cell division that
produces four cells
 Each cell hash half the number
of chromosomes as the parent
cell.
 In animals, meiosis occurs in the
sex organs—the testes in males
and the ovaries in females
Meiosis
 Each
species has its own
number of chromosomes
 Humans  46 or 23 pairs
 Karyotype display of
these 46 chromosomes
 Homologus Chromosomes
chromosomes making up the
pair that carry the genes
for the same trait
Homologus Chromosomes
 Two
general types
Autosomes
22 pairs found both in
male & female
Sex Chromosomes  carry the gene
that determines sex indicated by
“X” or “Y”
Sex Chromosomes

Female 
22 Autosomes + 1 Sex chromosome,
 XX  1 from father , 1 from mother
 Eggs 1X + 22 Autosomes


Male
 XY
X from mother, Y from Father
 Sperm half have 1X+ 22 autosomes, other
half have 1Y+22 autosomes
Diploid and Haploid Cells
 Diploid
Cells Cells with 2 sets of
chromosomes total number of
chromosomes diploid # or 2n
Humans 2n=46 in somatic cells
 Haploid Cells  1n =23
chromosomes  in sex cells, sperm
& egg (Gametes)
Human Life cycle
Meiosis
 Process
by which haploid gametes
are formed
 Alternation of meiosis and
fertilization keeps the # of
chromosomes same from generation
to generation.
 Mitosis 2 offspring w/same # of
chromosomes
Meiosis
Gametes are formed egg & sperm
 4 offspring with ½ the # of
chromosomes
 Exchange of genetic material crossing
over
 Oogenesis production of egg 1 egg +
3 smaller polar bodies
 Spermatogenesis production of
sperm 4 sperm cells

Two Distinct parts of Meiosis
Meiosis I
Meiosis II
Assortment of Chromosomes
Crossing Over/ Genetic
Recombination
Comparison of Mitosis & Meiosis

1.
By three events in Meiosis l
Synapsis and crossing over

2.
Separation of homologues


3.
Homologous chromosomes physically connect and
exchange genetic information
At anaphase I of meiosis, homologous pairs move
toward opposite poles of the cell
In anaphase II of meiosis, the sister chromatids
separate
Tetrads on the metaphase plate

At metaphase I of meiosis, paired homologous
chromosomes (tetrads) are positioned on the
metaphase plates
Comparison of Mitosis & Meiosis