• Gregor Mendel (1822-1884) experimented with pea plants in his monastery in Austria.

• In a garden pea, the petals enclose the reproductive tissues.

Anthers produce pollen, which will generate sperm.

Carpels can receive the pollen, and deliver the sperm to eggs to form seeds.

These enclosed tissues allows self-fertilization or artificial cross-fertilization ("crossing").

Peas exhibit several characters, or traits, that appear to be inherited.

• Mendel studied seven characters (traits) that can be observed in peas.

He made large numbers of crosses and recorded the traits exhibited by each generation.

• In a monohybrid cross, Mendel examined a single trait, such as flower color.

1. Plants were allowed to self-fertilize until true-breeding lines were established.

2. Plants with different traits were cross-fertilized to yield an offspring generation (F₁).

3. The 2nd generation plants were self-fertilized to yield a 2nd offspring generation (F₂).

• Mendel let each variety self-fertilize for several generations.

Eventually true-breeding lines were obtained that showed the same trait, such as flower color (purple or white).

These true-breeding plants were used as parental (P) plants for further crosses.

• Mendel placed pollen from the anthers of a true-breeding white flower onto the carpel of a true-breeding purple flower.

This allows cross-fertilization to take place, yielding an F₁ (first filial) generation, all of which exhibited purple flowers.

He called traits such as purple flower color dominant; the white flower trait was called recessive: it disappeared in the F₁ generation.

• Mendel let the plants in the F₁ generation self-fertilize.

In the F₂ (second filial) generation, the recessive white flower trait re-appeared.

The ratio was about 3 dominant (purple) to 1 recessive (white) for this monohybrid cross

• Mendel let each variety self-fertilize for several generations.

Eventually true-breeding lines were obtained that showed the same trait, such as flower color (purple or white).

These true-breeding plants were used as parental (P) plants for further crosses.

• Mendel placed pollen from the anthers of a true-breeding white flower onto the carpel of a true-breeding purple flower.

This allows cross-fertilization to take place, yielding an F₁ (first filial) generation, all of which exhibited purple flowers.

He called traits such as purple flower color dominant; the white flower trait was called recessive: it disappeared in the F₁ generation.

• Mendel let the plants in the F₁ generation self-fertilize.

In the F₂ (second filial) generation, the recessive white flower trait re-appeared.

The ratio was about 3 dominant (purple) to 1 recessive (white) for this monohybrid cross

• All of the seven traits Mendel studied exhibited ratios close to 3:1 dominant:recessive.

• The "factors" Mendel studied are genes carried on chromosomes.

Homologous chromosomes carry the same genes, one chromosome inherited from each parent.

Alternative forms of a gene are alleles.

An individual's alleles make up its genotype; which may be observed as a visible trait (phenotype).

If both of the alleles are the same, the individual is homozygous, otherwise the individual is heterozygous, and the dominant allele determines its phenotype.

• Human genes are located on 22 pairs of homologous autosomes and the sex chromosomes.

The full complement of genetic information of an organism is called its genome.

• Alleles.

In the P generation, the purple plants are homozygous for the dominant allele (P), and their genotype is PP.

The white plants are homozygous for the recessive p allele, and their genotype is pp.

The F₁ generation plants are all heterozygous (Pp) and express the dominant trait.

• To analyze a monohybrid cross using a Punnett square, first the possible gametes of the two parents are aligned along two sides of a square.

Then the gametes are combined to show genotypes of potential zygotes in the cells of the square.

In this cross between heterozygous parents, the genotype ratio of the offspring is

• 25% homozygous dominant
• 50% heterozygous
• 25% homozygous recessive

Exercise:

• The frequency of genotypes and phenotypes in the offspring is based on probability.
All the F₁ offspring of a monohybrid cross between true-breeding white (homozygous recessive, pp) and purple (homozygous dominant, PP) parents are purple heterozygotes (Pp).
The F₂ individuals exhibit a dominant : recessive phenotype ratio of 3 : 1.
The genotype ratio is 1 PP : 2 Pp : 1 pp.   Exercise:

• To determine whether an individual exhibiting a dominant phenotype (purple flowers) was homozygous (PP) or heterozygous (Pp), a testcross can be made with an individual exhibiting the recessive phenotype (white flowers, or pp genotype).

• Sex-linked traits.

A white-eyed mutant male fly was crossed with a normal red-eyed female.

The F₁ generation flies all exhibited red eyes, as expected if the white-eyed allele is recessive.

In the F₂ generation, all of the white-eyed flies were male, indicating that the eye-color gene is sex-linked: located on the X chromosome.

Note: "gene" should read "allele" in the diagram.

• Wild-type fruit fly (Drosophila) has red eyes.

A mutation in the eye-color gene results in an allele that causes white eyes.

• Nondisjunction is the failure of chromosomes or chromatids to separate in meiosis.

This can occur in meiosis I when pairs of homologous chromosomes fail to separate during anaphase I.


Nondisjunction can also occur in meiosis II when sister chromatids fail to separate during anaphase II.


Both result in gametes with incorrect number of chromosomes, or aneuploidy.

Meiosis II nondisjunction animation:

• Down syndrome is caused by trisomy 21, the existence of three copies of chromosome 21.
Review:

• Nondisjunction of the X chromosome can produce sex chromosome aneuploidies.

In humans, the presence of a Y chromosome determines gender and makes the individual male.

• XXX individuals are females: the extra X is usually inactivated.

• Turner syndrome (XO) individuals are females: missing the small Y is survivable.

• Klinefelter syndrome (XXY) individuals are males: the extra X is usually inactivated.

• OY individuals are nonviable: missing the large X is fatal.

• Most genetic disorders are caused by recessive alleles.
Homozygous recessive individuals exhibit the disorder; heterozygous individuals are called carriers.
This is a pedigree for the inheritance of a recessive trait.
Males are depicted with squares, females with circles, unaffected persons with empty symbols, carriers (heterozygotes) with half-filled symbols, and affected persons (homozygotes) with solid symbols.

• Sickle-cell is a recessive autosomal disorder. Heterozygotes are carriers who do not suffer from the disease. Note it is possible for a recessive trait to "skip" a generation.

• Huntington's disease is unusual in being a dominant, fatal trait.

It persists in the population because of the late age of onset.









Dominant traits should not "skip" a generation.

In this pedigree, the mother must be heterozygous, since if she were homozygous, all of her children would be affected.