Transcript pea plants

CH 10 Mendel and
Meiosis
What is Heredity?
Heredity

The passing on of characteristics from parents to
offspring through genes
Genes

A section on DNA that carries the information on
what type of protein to make; they influence the
types of traits an organism inherits
What is Heredity?
Traits
 Characteristics that are inherited
Genetics
 The branch of biology that studies
heredity
History of Genetics

Gregor Mendel: an
Austrian monk that
first developed the
study of genetics

Experimented with
pea plants to discover
the general laws of
genetics
History of Genetics

A pea plant has gametes and both male and
female reproductive organs
• Gametes: male and female sex cells
History of Genetics

Pollen
grains
Transfer pollen
Female
part
Male
parts
Cross-pollination
Pollination: transfer of
pollen (male gametes) from
a male reproductive organ to
a female reproductive organ
in a plant
• Mendel allowed pollination to
occur within the same flower
or between different flowers
for his experiments
History of Genetics

Fertilization: the male gamete unites
with the female gamete to form a zygote
(a fertilized cell)
• In the case of pea plants, the zygote becomes
a seed
Pea Plant Traits Tested By
Mendel
Flower
color
Flower
position
Pod
color
Pod
shape
yellow
purple
axial
(side)
green
inflated
tall
green
white
terminal
(tips)
yellow
constricted
short
Seed Seed
shape color
Plant
height
Dominant
trait
round
Recessive
trait
wrinkled
History of Genetics

Conducted crosses between pea plants
to compare traits
• Monohybrid cross:
•
when only one trait is
compared at a time (like eye color)
Dihybrid cross: when two traits are
compared at a time (like eye & hair color)
•
•
•
•
P1 = Parental generation (parents)
F1 = First filial generation (children)
F2 = Second filial generation (grandchildren)
Filial means “family”
Punnett Squares

Used to compare traits

Shorthand way of determining the
probability of having a certain type of
offspring if you know the parents’
genotypes
Punnett Square-Monohybrid cross
Dad

T
t
T
TT
Tt
t
Tt
tt
Mom
Possible
offspring
A Punnett square for this
cross is two boxes tall
and two boxes wide
because each parent
has two kinds of
gametes for this trait, but
will only pass one along
to each offspring
Mendel’s Conclusion
1. The rule of unit factors

Each organism has two factors, or genes,
that control each trait
•

One gene comes from mom, one gene comes from
dad
A single gene can exist in different forms called
alleles
•
Example: Blue, brown, and green are alleles for the
eye color gene
FYI: Alleles
Chromosomes = location of
genes
Gene = section of DNA on
the chromosome that
determines the type of
protein (Ex. for color)
produced
Allele = different forms of a
single gene (protein for a
purple flower or a protein for
a white flower)
Mendel’s Conclusion
2. The rule of dominance
 Offspring inherit an allele from each parent,
but only one trait is observed
 Dominant trait: “stronger” trait that shows up
when the dominant allele is present
•

Represented by a capital letter; T is for tallness
Recessive trait: “weaker” trait that shows up
only when the dominant allele is not present
•
Represented by a lowercase letter; t is for shortness
The Rule of Dominance
Tall
plant
T T
Short
plant
t
T
All tall
plants
T
t
Tt
Tt
t
Tt
Tt
t
t
F1
T
Results: All tall plants
T
t

5 Punnett Squares
• 1 Homozygous Dominant Dad and
•
•
•
Heterozygous Mom
1 Homozygous Dominant Dad and 1
Homozygous Recessive Mom
1 Homozygous Dominant Dad and
Homozygous Dominant Mom
2 doing any matching you want
Mendel’s Law of Segregation



What is means: Every organism has 2
alleles of each gene
When gametes are made, each gamete
receives only one of these alleles
During fertilization, the offspring will
receive one allele for each gene from
each parent
Mendel’s Law of Segregation


Parents: Tt x Tt
Results: 3 tall plants,
1 short plant
T
t
T
TT
Tt
t
Tt
tt
Phenotypes & Genotypes

Phenotype: the physical
appearance of an
organism
• Ex.

Blue skin
Genotype: genetic
makeup of an organism
• Ex.
Alleles for skin color are
B (=blue) or b (= black)
Homozygous vs. Heterozygous

Homozygous: 2 alleles for
a trait are the same
• BB is homozygous
dominant for blue skin
• bb is homozygous
recessive for black skin
• Homozygous offspring are
called purebred because
their alleles are the same

Homozygous vs. Heterozygous
Heterozygous: 2
alleles for a trait are
not the same
• Bb is heterozygous
for blue skin
• Heterozygous
offspring are called
hybrids because
their alleles are
different
A “zeedonk” is a
zebra/donkey hybrid
Mendel’s Law of Independent
Assortment


Genes for different traits are inherited
independently of each other
Example: You have brown hair (Bb) and
freckles (Ff)
• You can pass on to your children one of the
following combinations: B + F,
F,
b+f
B + f,
b+
Genes, Chromosomes,
and Numbers

Diploid cell: has 2 of each kind of
chromosome
• One came from mom, one came from dad
• All body cells, except gametes, are diploid
•
cells
When they divide, this is called mitosis
Genes, Chromosomes, and
Numbers

Haploid cell: has 1 of each kind of
chromosome
• Gametes are haploid cells
• When they divide, this
is called meiosis
Genes, Chromosomes, and
Numbers

Each type of organism has a specific
number of chromosomes (humans
have 46 chromosomes)
Genes, Chromosomes, and
Numbers

Homologous chromosomes: 2 of the
same type of chromosome in a diploid
cell
• Have genes for the same traits, but might
have different alleles for the same gene
• Ex:
Genes are for eye color, but blue eye color
allele is on 1 chromosome and brown eye color
allele is on the other chromosome