chapter 3 heredity heredity: passing of traits from parent to offspring

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