Heredity

The passing of traits from parent to offspring

Genetics

The study of heredity

Gregor Mendel is known as the “Father of Genetics”

In 1866 – Gregor Mendel, an Austrian monk,published his findings about the inheritance ingarden pea plants.

Mendel noticed that certain types of gardenpea plants produced specific forms of a trait,generation after generation.

He noticed some plants always produced greenseeds and others always produced yellow seeds.

He called these “true-breeding”.

Mendel performed cross pollination bytransferring the male gametes from the flowerof a yellow-seed plant to the female organ of agreen-seed plant.

Mendel called the green-seed plant and yellowseed plant the Parent Generation (P generation).

XParent Generation:

Yellow Peas (male)

Green Peas (female)

When Mendel grew the seeds from the crossbetween the green-seed and yellow-seed plantsall of the offspring had yellow seeds.

The offspring of the P generation were calledthe first filial generation (F1 generation)

XParent Generation:

Yellow Peas (male)

Green Peas (female)

F1 Generation:

All Yellow Peas

Mendel wanted to know whether the trait wasno longer present, or whether it was hidden ormasked.

He planted the F1 generation of yellow-seedsand allowed them to grow and self-fertilize, thenexamined the seeds from that cross.

The offspring from the F1 generation werecalled the second filial generation (F2generation).

The F2 generation had 6022 yellow seeds and2001 green seeds, which is almost a perfect 3:1ratio.

P Generation:

F1 Generation:

F2 Generation:

X

Yellow Peas (male)

Green Peas (female)

All Yellow Peas

SELF FERTILIZATION

6022 Yellow Peas2001 Green Peas

3:1 Ratio

Mendel concluded that there must be 2 forms ofthe seed trait in the pea plants – yellow seedand green seed – and that each was controlledby a factor, which we now call an allele.

Allele

An alternative form of a single gene passed from generation to generation

ex. The gene for yellow seeds and the gene for green seeds are just different

forms of a single gene.

Mendel said the 3:1 ratio he saw in his pea plantexperiments could be explained if the alleleswhere paired in each of the plants.

Mendel called the form of the trait he saw in theF1 generation dominant (yellow) and the formof the trait that was masked in the F1 generationrecessive (green).

Dominant

Trait that appears in the F1 generation, and masks or hides the expression of the recessive

gene

Indicated by a capital letter.

Recessive

Trait that is masked in the F1 generation by the dominant trait.

Indicated by a lowercase letter.

In Mendel’s experiment the yellow-seed form ofthe trait is dominant, so it is represented by acapital Y.

The green-seed form was recessive, so it isrepresented by a lowercase y.

If an organism has two of the same alleles for atrait it is homozygous.

Ex. Homozygous yellow seeds are YY. Homozygous green seeds are yy.

Homozygous

An organism with two of the SAME alleles for a specific trait.

YY or yy

An organism can have two different alleles for atrait, and when this happens they areheterozygous.

Ex: Yy The dominant trait observed in heterozygous

organisms, which means it will be yellow in this case.

Heterozygous

An organism with two DIFFERENT alleles for a specific trait.

Yy

Were the yellow plants in the F1 generation of Mendel’s experiments homozygous or

heterozygous?

The yellow plants could have been homozygous OR heterozygous.

The outward appearance of an organism does not always indicate which pair of alleles is

present.

Genotype

The genetic make-up of the alleles.

Ex. YY or yy or Yy in Mendel’s plants

Phenotype

The physical appearance or expression of an allele.

Ex. The phenotype of plants with the genotype yy will be green

plants.

Mendel used homozygous yellow seed and homozygous green seed

plants in his P generation.

Each gamete from the yellow plant contains one Y.

Each gamete from the green plant contains one y.

Mendel’s Law of Segregation says that two alleles for each trait separate during Meiosis, and

during fertilization two alleles for that trait unite.

Hybrid

An organism that is heterozygous for a specific trait.

Ex. The F1 generation in Mendel’s experiments were hybrids. (Yy)

When Mendel allowed the Yy plants (F1 generation) to self-

fertilize he performed a monohybrid cross.

Monohybrid Cross

A cross that involves hybrids for a single trait.

Results: Genotypic Ratio – 1:2:1Phenotypic Ratio – 3:1

Punnett Square

A visual summary of the possible alleles for any given trait.

Shows all the possible combinations of genetic traits that results from the crossing of the parent organims.

Once Mendel knew how a single trait was inherited using a monohybrid cross he could look at two or more traits in the same plant.

In Garden Pea Plants round seeds (R) are dominant to wrinkled seeds (r), and yellow seeds (Y) are dominant to green seeds (y).

If Mendel crossed homozygous yellow, round seed pea plants with homozygous green, wrinkled pea plants the cross would result in all YyRr, yellow, round pea plants (F1 generation).

The F1 generation are called DIHYBRIDS because they are heterozygous for both traits.

You can cross dihybrids using a dihybrid cross.

Dihybrid Cross

A cross that involves two dihybrids.

Results in a phenotypic ratio of 9:3:3:1.

From Mendel’s dihybrid cross he came up with the

Law of Independent Assortment.

Law of Independent Assortment

Alleles randomly distribute during gamete formation. Genes on separate chromosomes sort

independently during meiosis.

The law of independent assortment says that any four of these combinations are equally

likely.

Genetic Recombination

The new combination of genes produced by crossing over and

independent assortment.

To calculate the possible combinations of genes due to independent assortment you use the formula 2n.

n = the number of chromosomes

The possible # of combinations after fertilization for humans would be:

223 X 223 = over 70 trillion

Genes that are close to each other on the same chromosome are said

to be “linked”.

They usually travel together during gamete formation.

Gene Linkage

Genes located close to one another on the same chromosome that usually travel

together and do not segregate independently.

Linked genes are an exception to Mendel’s Law of Independent

Assortment.

Most species have diploid cells (2n), but some have polyploidy cells.

Polyploidy

The occurrence of 1 or more extra sets of all chromosomes in an

organism

Ex. Sugar Cane (8n)Oats (6n)

Strawberries (8n)