Chapter 3 Heredity

1
Chapter 3Heredity
2
Heredity Passing of traits from parent to
offspring.
3
Gregor Mendel Father of Genetics
4
Gregor Mendel

• Lived from July 20, 1822 January 6, 1884)
• Austrian monk
• Worked in monastery garden
• Used pea plants to show that the inheritance of
  traits follows particular laws, which were later
  named after him

5

• Trait a characteristics of an organism
• Heredity the study of how traits are passed
  from parents to offspring

6
Why Mendel used pea plants

• Quick reproduction and growth
• Easily observed traits
• Many observable traits
• Easy cross pollination

7
Self pollinating
Plants usually contain both male and female
reproductive structures
8
During self pollination

• Pollen from anthers (male) is transferred to the
  stigma (female)
• Fertilization occurs when a sperm from the pollen
  travels through the stigma and enters the egg in
  the ovule.

9

• True breeding plants have offspring that always
  show the same form of the trait
• Cross fertilization a process in which one
  plant fertilizes the egg in a flower of a
  different plant
• Pollen tiny grains containing plant sperm cells

10
P generation parental generation F1
generation offspring in first cross hybrids

11

• Offspring from first cross are known as first
  generation.
• Dominant The trait observed when at least one
  dominant allele for a characteristic is
  inherited.
• Recessive A trait that is apparent only when two
  recessive alleles for the same characteristic are
  inherited,

12
Each parent donates one set of instruction to an
offspring known as genes.
13
ALLELES Two forms of the same gene for every
characteristic
14

• Hybrids the offspring of two different true
  breeding plants
• Monohybrid cross a cross between two plants
  that differ in only one trait
• F2 generation the offspring that result when 2
  hybrid plants are crossed

15
Mendels P (Parental) Cross

• true breeder X true breeder
• short long
• Resulted in all long offspring (F1)

16
Mendels F1 (first filial) cross

• Cross pollinated 2 of the long offspring produced
  in the P generation
• F1 long X F1 long
• Results
• 75 long and 25 short

17
Mendels conclusions

• Gene the factor that controls traits
• Allele the possibilities of a gene (e.g. A or
  a)
• Simple dominance one allele is dominant to a
  recessive allele

18
Mendel Vocabulary

• Dominant the allele that masks any other allele
  when there are 2 alleles present (A in Aa)
  (symbolized by the first letter in the dominant
  traits name, always capital)
• Recessive the allele that is masked by another
  allele (a in Aa) (symbolized by the first letter
  in the dominant traits name, always lower case)

19
More Mendel Vocabulary

• Homozygous having two identical alleles for a
  trait (AA or aa) (Mendel called this
  true-breeding)
• Homozygous dominant having two dominant alleles
  for a trait (AA) (Mendel called this true
  breeding dominant)
• Homozygous recessive having two recessive
  alleles for a trait (aa) (Mendel called this
  true breeding recessive)
• Heterozygous having two different alleles for a
  trait (Aa)

20

•  
• Phenotype the visible traits of an organism
  (e.g. long or short)
• Genotype the alleles that an organism carries
  (e.g. Aa or AA or aa)

21
Punnett square a model used to represent
crosses between organisms
Example What are the possible offspring of a
cross between a homozygous dominant green plant
(GG) and a homozygous recessive green plant (gg)?
22
Steps to doing a Punnett Square

• 1. identify the gametes of the parents

Example What are the possible offspring of a
cross between a homozygous dominant green plant
(GG) and a homozygous recessive green plant (gg)?
GG parent will produce all G gametes gg parent
will produce all g gametes
23
Steps to doing a Punnett Square
2. draw a square with 4 boxes
Example What are the possible offspring of a
cross between a homozygous dominant green plant
(GG) and a homozygous recessive green plant (gg)?
GG parent will produce all G gametes gg parent
will produce all g gametes


24
Steps to doing a Punnett Square
3. put the gametes from one parent on the top of
the box and the gametes from the other parent on
the side of the box
GG parent ?all G gametes gg parent ?all g gametes
G
G


g
g
25
Steps to doing a Punnett Square
4. cross multiply to find the genotypes of the
children
G
G


g
g
26
Steps to doing a Punnett Square
4. cross multiply to find the genotypes of the
children
G
G
Gg Gg
Gg Gg
g
g
27
Steps to doing a Punnett Square
5. write the phenotypes of each child in the boxes
G
G
Gg Gg
Gg Gg
g
Green
Green
g
Green
Green
28
Steps to doing a Punnett Square
6. calculate genotypic and phenotypic ratios of
the offspring
List all possible Genotypes GG Gg gg List all
possible Phenotypes green yellow
G
G
Gg Green Gg Green
Gg Green Gg Green
g
g
29
Steps to doing a Punnett Square
6. calculate genotypic and phenotypic ratios of
the offspring
Count how many of each Genotypes GG
Gg gg Phenotypes green yellow
G
G
Gg Green Gg Green
Gg Green Gg Green
0/4 0 4/4 100 0/4 0
g
4/4 100 0/4 0
g
30
Now try this one

• What are the possible offspring of a cross
  between a pea plant which is heterozygous for
  green peas and a pea plant which is homozygous
  recessive for green peas?

31
Another example

• What are the possible offspring of a cross
  between a mother and father who are both
  heterozygous for the ability to roll their
  tongues?

32
Incomplete Dominance

• Sometimes, there are two dominant alleles and no
  recessive alleles.
• Ex flower color

33
In some flowers, red and white are both
dominant. A red flower has the phenotype ______
and the genotype RR. A white flower has the
phenotype ______ and the genotype WW.
RED
WHITE
34
What if you cross a RED (RR) flower with a WHITE
(WW) flower?
R


RW
RW
R
W
RW
RW
W
35
What color are the offspring?
R
R


WHITE
RW
RW
W

RED
RW
RW
W
PINK
36
What is the phenotype of this flower?What is
the genotype of this flower?
PINK
RW
37
So, when a trait is inherited by incomplete
dominance, there are ____ possible phenotypes
and ____ possible genotypes.
3
Red White Pink
3
RR WW RW
38
Multiple Alleles

• In some cases, there are more than 2
  possibilities.
• Ex hair color, eye color, skin color

39
Blood Type

• 2 Dominant alleles A and B
• 1 recessive allele O

Genotype Phenotype
AA A
AO A
AB AB
BB B
BO B
OO O
40
What if you cross a AO parent with a BO parent?
A
O


AB
BO
B
AO
OO
O