Mendelian Genetics: Principles of Trait Transmission, Study notes of Genetics

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Mendelian Genetics

Mendelian Genetics

™

Also referred to as “transmission genetics”

™

Principles that describe how traits are passed from parentsto offspring.

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Definitions

Monohybrid cross Monohybrid cross

: A cross involving mating two

individuals, each of which expresses only one of a pair ofcontrasting traits (e.g. round/wrinkled peas, yellow/greenpeas, purple/white flowers, tall/dwarf stem)

Parental generation (P Parental generation (P

1 1

: The original parents

First filial generation (F First filial generation (F

11

: Offspring resulting from

parental mating.

Second filial generation (F Second filial generation (F

22

: Offspring resulting from

the self-crossing of individuals from the F1 generation.

Seeds: round/wrinkled

yellow/green

Pods: full/constricted

green/yellow

Flowers: violet/white Stem: axial/terminal

tall/dwarf

Definitions

Reciprocal Crosses Reciprocal Crosses

: Crosses were created such that the

parental source of each trait was monitored and both typeswere represented in crosses of the parental generation.

For example,

tall x

dwarf and

dwarf x

tall.

Note: Mendel observed that it did not matter which P

1

plant

served as the source of pollen and which served as thesource for the ovum. (The traits were not sex-linked)

Outcomes

Transmission genetics studies examine the outcomes

of many matings.

Data are expressed as

ratios

ratios

. For example, in one

study, Mendel examined 1064 F

2

offspring and

observed 787 tall plants and 277 dwarf plants,

giving rise to a ratio of 2.8 tall : 1 dwarf, or about

Mendel’s First Three Postulates

1. Unit factors (genes) in pairs

1. Unit factors (genes) in pairs A specific unit factor exists for each trait (tall gene

and dwarf gene). Each diploid individual has twounit factors, one of which was inherited fromeach parent.

Factors occur in pairs; therefore, three combinations

are possible: two tall, two dwarf or one of each.

The combination inherited determines stem height.

Mendel’s First Three Postulates

2. Dominance/

2. Dominance/

Recessiveness

Recessiveness

When two unlike unit factors responsible for a single

characteristic are present in a single individual, onefactor is dominant to the other, which is recessive.

Expression of the trait that disappeared in the F

1

but

reappeared in the F

2

generation (dwarf) is under

the influence of the trait that appeared regardlessof generation (tall).

The trait that is always

apparent (tall) is dominant to the trait that is notalways apparent (dwarf, recessive).

Monohybrid Cross

P

1

tall plants with identical tall unit factors x P

1

dwarf plants with identical dwarf unit factors.

Tall is dominant to dwarf, so all F

1

plants were tall.

When the gametes form in the F

1

plants, each gamete

will receive

either

the tall gene

or

the dwarf gene.

Monohybrid Cross

When the F

1

are allowed to randomly self-fertilize,

there are four gamete pairings possible in the F

2

generation, each occurring at the same frequency:

tall/tall

tall/dwarf

dwarf/tall

dwarf/dwarf

Monohybrid Cross—Gametes

Contributed

The dominant unit factor is usually assigned a capital

letter (D is tall)

The recessive unit factor is usually assigned a small

letter (d is dwarf)

Terminology

Genotype

Phenotype

™

tall/tall (DD)

(tall)

™

tall/dwarf (Dd)

(tall)

™

dwarf/tall (dD)

(tall)

™

dwarf/dwarf (dd) (dwarf)

Terminology

Homozygous

Homozygous

: An individual possesses two of the

same

allele for a given trait (DD or dd).

Heterozygous

Heterozygous

: An individual possesses two

different

alleles for a given trait (Dd)

(The nouns are

homozygote

and

heterozygote

Punnett Squares

Graphics that help determine the phenotype and

genotype outcomes of gamete fertilization.

Devised by Reginald C. Punnett