Gregor Mendel: The father of genetics

Mendel’s Inferences

The basic units of genetics are material elements (lengths of DNA)

These elements come in pairs (homologous chromosomes)

These elements maintain their character through many generations (chromosomes copy themselves)

Gene pairs separate during the formation of gametes (meiosis, homologous chromosomes line up and separate into gametes)

Peas are self-pollinating plants.

Humans can cross pollinate pea plants.

Terminology

Recessive – A trait that remains hidden in a certain generation

Dominant – A trait that appears in the same generation

Phenotype – A physical feature, bodily characteristic or behavior

Genotype - The phenotype is determined by the underlying genetic makeup

Allele – the alternative form of a gene

Parental generation

Parental generation or P F1 – the first filial generation, the first generation Fx – the succeeding generation

Results of several of Mendel’s pea crosses

Mendel’s results show a 3:1 phenotypic ratio

Homozygous – When an organism has two of the same alleles for a characteristic.

YY or yy Heterozygous – When the organism has two

different alleles for a characteristic. Yy The characteristic in the heterozygous condition that

is expressed is the dominant one The recessive trait in the heterozygous condition is

not expressed

Monohybrid crosses involve just one character. Dihybrid crosses involve two characters.

The question asked by Mendel was: Are these two characters associated with each other? Do they show up as pairs or are they independent of each other?

Monohybrid cross between homozygous trait pea plants, results in F1

Monohybrid cross between heterozygous trait pea plants, results in F2 and F3

Dihybrid Cross

P = smooth, yellow and wrinkled, green F1 = 100% smooth yellow F2 = 315 smooth yellow

101 wrinkled yellow 108 smooth green

32 wrinkled green

9:3:3:1 ratio

WHY?

Seed shape and seed color are transmitted independently of one another. The transmission of one character does not affect that of another.

Phenotypic ratio 9:3:3:1315 Smooth yellow

101 Wrinkled yellow

108 Smooth green

32 Wrinkled green

3 : 1 yellow/green

3 : 1 wrinkled/smooth

True breeding purple peas are bred to true-breedingwhite peas. All F1 peas have purple flowers.

F1 flower is self-pollinated. The F2 generation has3 purple flowered plants for every 1 white flowered plant

Dihybrid cross

Dihybrid cross: F1

Results of dihybrid cross showing F2 generation(produced by self-crossing heterozygous F1 individual)

Mendel’s Laws

Law of Segregation – Differing characters in organisms result from two genetic elements (alleles) that separate in gamete formation, such that each gamete gets only one of the two alleles.

Law of Independent Assortment - because they are on separate chromosomes, they assort independently at metaphase. This means they are passed independently to future generations.

Incomplete Dominance

A genetic condition in which the heterozygous phenotype is intermediate between either of the two homozygous phenotypes

White + Red = Pink White + Red = Pink SnapdragonsSnapdragons

Incomplete dominance in snapdragons

Codominance

A condition in which two alleles of a given gene have different phenotypic effects, with both effects manifesting in organisms that are heterozygous for the gene.

Human blood types, ABO

This is a blood typing system that refers to the kind of proteins found on the surface of red blood cells. These proteins come in the form of A or B. A person could have two A alleles, two B alleles, or one of each, AB. Two alleles that are inactive are type O.

Both alleles produce proteins.

Polygenic Inheritance

When a genetic character is determined by the interaction of multiple genes, with each gene having an additive effect. This produces a continuous variation of the character.

Multiple alleles – when three or more alleles of the same gene exist.

Human height, weight, eye and skin color

Polygenic inheritance : Eye color

Continuous variation : Infant birth weight

The Bell Curve

A distribution of values that is symmetrical, and largest around the average. Most biological traits manifest this way.

Genes and the Environment

Genetic susceptibility – the genetic makeup of some individuals makes them more susceptible to an environmental influence.

Genes and the environment work together to create an outcome.

Smoking is an environmental factor that influences the development of lung cancer.

Some people are more susceptible to that risk because of their genetic makeup.

Himalayan rabbits have an allele that makes production of melanin (a dark pigment) heat-sensitive. Less melanin

is deposited in warmer areas of the body (such as the trunk) and more in cooler areas (such as ears and limbs).

Effect of Environment on Phenotype

It is possible to demonstrate the phenotypic effect of temperature experimentally by shaving a patch of fur from a rabbit's back and applying an ice pack to the

shaved area.

The fur that grows back in the artificially cooled area is dark. The lowered temperature caused increased melanin

production. A change in an environmental condition(temperature) has altered the phenotype (fur color).

Effect of Environment on Phenotype

Arrowhead plants (Sagittaria sagittifolia) assume very different-looking forms depending on whether they grow

on land or in water.

Genes and environment work together to create the phenotypes we see in living things.

Pleiotropy

One gene has many effects.

Fragile-X Syndrome

An inherited cause of mental retardation. Called this because of the break in the X chromosome, a defect in one single gene.

Symptoms include mental retardation, abnormally long face, protruding ears, large testicles (male only).

Fragile-X occurs in 1 out of 2000 male birthsand 1 in 4000 female births.

Pleiotropic effects of sickle cell anemia

Sickle-cell anemia can cause deformation of the bones of the skull, interfere with normal circulation, dilation of the heart and heart failure, enlargement of the spleen, local failures in blood supply, kidney damage and kidney failure, paralysis, damage

the lungs, damage to the gastrointestinal system and abdominal pain.

Cancerous growth

Characteristics of cancerous cells