chapter 3 heredity heredity: passing of traits from parent to offspring
TRANSCRIPT
Chapter 3Heredity
Heredity: Passing of traits from parent to offspring.
Gregor Mendel “Father of Genetics”
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
Trait – a characteristics of an organism
Heredity – the study of how traits are passed
from parents to offspring
Why Mendel used pea plants:
1. Quick reproduction and growth
2. Easily observed traits3. Many observable traits4. Easy cross pollination
Self pollinating
Plants usually contain both male and female reproductive structures
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.
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 plantPollen – tiny grains containing plant sperm cells
P generation – parental
generation
F1 generation – offspring in first cross =hybrids
• 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,
Each parent donates one set of
instruction to an offspring known as
genes.
ALLELES Two forms of
the same gene for
every characteristic
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
Mendel’s P (Parental) Cross
true breeder X true breedershort long
Resulted in all long offspring (F1)
Mendel’s F1 (first filial) crossCross pollinated 2 of the long offspring
produced in the P generation
F1 long X F1 longResults:
75% long and 25% short
Mendel’s 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
Mendel VocabularyDominant – the allele that masks any other allele when there are 2 alleles present (A in
Aa) (symbolized by the first letter in the dominant trait’s name, always capital)
Recessive – the allele that is masked by another allele (a in Aa) (symbolized by the
first letter in the dominant trait’s name, always lower case)
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)
More Mendel Vocabulary
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)
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)?
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 gametesgg parent will produce all g gametes
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 gametesgg parent will produce all g gametes
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 gametesgg parent all g gametes
G G
g
g
Steps to doing a Punnett Square:4. cross multiply to find the genotypes of the children
G G
g
g
G g
Steps to doing a Punnett Square:4. cross multiply to find the genotypes of the children
Gg Gg
Gg Gg
G G
g
g
Steps to doing a Punnett Square:5. write the phenotypes of each child in the boxes
Gg Gg
Gg Gg
G G
g
g
Green Green
Green Green
Steps to doing a Punnett Square:6. calculate genotypic and phenotypic ratios of the offspring
GgGreen
GgGreen
GgGreen
GgGreen
G G
g
g
List all possible Genotypes:GGGggg
List all possible Phenotypes:greenyellow
Steps to doing a Punnett Square:6. calculate genotypic and phenotypic ratios of the offspring
GgGreen
GgGreen
GgGreen
GgGreen
G G
g
g
Count how many of each: Genotypes:
GG Gggg
Phenotypes:greenyellow
= 0/4 = 0%= 4/4 = 100%= 0/4 = 0%
= 4/4 = 100%= 0/4 = 0%
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?
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?
Incomplete Dominance
• Sometimes, there are two dominant alleles and no recessive alleles.
• Ex: flower color
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
What if you cross a RED (RR) flower with a WHITE (WW)
flower?
R
R
W
W
RW RW
RW RW
What color are the offspring?
R R
W
W
RW RW
RW RW RED
WHITE+
PINK
What is the phenotype of this flower?
What is the genotype of this flower?
PINK
RW
So, when a trait is inherited by incomplete dominance, there are ____ possible phenotypes
and ____ possible genotypes.
3
3
Red WhitePink
RRWWRW
Multiple Alleles
In some cases, there are more than 2 possibilities.
Ex: hair color, eye color, skin color
Blood Type
• 2 Dominant alleles – A and B• 1 recessive allele – O
Genotype Phenotype
AA AAO AAB ABBB BBO BOO O
What if you cross a AO parent with a BO parent?
A O
B
O
AB BO
AO OO