Monohybrid Crosses

Monohybrid cross used to study only two variations of a single trait

Mendel produced true-breeding pea strains for seven different traits

Flower color, seed color, seed texture, pod color, pod shape, flower position, plant height

Each trait had two variants

THE F, GENERATION

Fi = first filial generation

Offspring produced by crossing two true-breeding Strains

For every trait Mendel studied, all F, plants resembled same parent (for example purple flower

Referred to visible trait as dominant

Alternative trait (for example White flower)was recessive

No plants with intermediate characteristics of parents were produced, meaning no blending inheritance

Ez GENERATION

Fz = second filial generation

Produced from the self-fertilization of F, plants

Although masked in the F, generation, the recessive trait reappeared among some F2 individuals

Counted proportions of traits

Always found about 3: 1 ratio

3: 1 IS ACTUALLY 1: 2: 1

Ez plants

3/4 plants with the dominant form

14 plants with the recessive form

The dominant to recessive ratio was 3: 1

Mendel discovered the ratio is actually:

1 true-breeding dominant plant

2 not-true-breeding dominant plants

I true-breeding recessive plant

THE Ez GENERATION IS A DISGUISED 1: 2: 1 RATIO

In other words, the PHENOTYPIC ratio is 3: 1

The GENOTYPE ratio is 1: 2:1

Also notice: His plants did not show intermediate traits

Each trait was intact? discrete

For each pair, one trait was dominant, the other recessive

Alternative traits were expressed in the E2 generation in the ratio of 314 dominant to "recessive

MENDELS FIVE-ELEMENT MODEL

1. Parents transmit discrete factors (genes

2. Each individual receives one copy of a gene from each parent

3. Not all copies of a gene are identical

· Allele -alternative form of a gene

Homozygous -two of the same allele

· Heterozygous -different alleles

· WILDTYPE -the allele most prevalent in the population

MENDEL'S FIVE-ELEMENT MODEL, CONTINUED

4. Alleles remain discrete -no blending

5. Presence of allele does not guarantee expression

· Dominant allele (B)-expressed

· Recessive allele (b)-hidden by dominant allele

· Genotype (Bb)-an individual's complete set of alleles (it will always be 2 for each gene

Phenotype (Brown Hair)-an individual's physical appearance

Note: many genetics term were adopted after Mendel

QUESTION 1

The observable expression of the genes present in an organism is called its

-a phenotype

QUESTION 2

Mendel demonstrated which of the following about inherited traits? Check all that apply.

A. Traits can be hidden in some generations, but subsequently reappear unchanged in future generations

B. Traits segregate among the offspring of a cross

Certain traits are more likely to appear than their alternatives

& Parents that both have the same trait can have offspring with an alternative trait

QUESTION 3

A diploid organism that has two identical alleles for a given trait is called -For that trait.

-homozygous

Principle of Segregation

Two alleles for a gene segregate during gamete formation (one from each parent)and are rejoined at

random during fertilization

Physical basis for allele segregation is the movement of chromosomes during meiosis

Mendel had no knowledge of chromosomes or meiosis -had not yet been described

Deduced the principle based on trait ratios

PUNNETS SQUARES

dad's side

p + p

PPP Pp babies

moms side X

p. Pp pp

· Note: each parent can only pass on ONE copy of each gene

each baby has 2 copies of every gene -one from each parent

YOU CAN DO THIS WITH NON-PLANTS, TOO!

Many human diseases are caused by recessive mutations in important genes

E. Sickle cell anemia

People who are homozygotes for the dominant allele (SS)do not have sickle cell

People who are heterozygotes (Ss)may have some sickling, but generally not life-threatening

People who are homozygotes for the recessive allele (ss)have sickle cell anemia.

NOW FOR DIHYBRID CROSSES

Dihybrid crosses used to study two variations of two traits

Mendel produced true-breeding lines, each with two traits

RRYY x rryy (Round Yellow -wrinkled green

The Fy generation of a dihybrid cross (Rr Yy)shows only the dominant phenotypes for each

trait

Allow F to self-fertilize to produce F2

DIHYBRID CROSSES PRODUCE A 9: 3: 3: 1 ratio

↑ self fertilizes to produce F2

· Br Yy x Rr Yy

The F2 generation shows all four possible phenotypes in a set ratio

· 9: 3: 3: 1

IR -Y_ ): (R_yy): (rrY_ ): (vryy)-underscore represents dominant or recessive

allele

(Round yellow): (round green): (wrinkled yellow): (wrinkled green)

PRINCIPLE OF INDEPENDENT ASSORTMENT

In a dihybrid cross, the alleles of each gene assort independently

The segregation of different allele pairs is independent -for example seed

shape is independent of seed color