HEREDITY

HEREDITY

Heredity is the passing on of traits from parents to offspring. In this family tree, traits such as hair color and poor eyesight are passed from generation to generation.

GENETICS

Genetics is the study of how traits are passed from parent to offspring. Individual animals and plants differ from other individuals of the same species in all sorts of ways, even when they are offspring of the same parents. These differences between individuals of the same species are called variations.

Height is a variation

THE FATHER OF GENETICS

The first recorded scientific study of how traits pass from one generation to the next was done by Gregor Mendel, a Monk. In 1856, he began experimenting with garden peas. He found that it was possible to predict the kinds of flowers and fruit a plant would produce. He realized that information about the parents of the plants was needed before such a prediction could be made.

SCIENTIFIC METHOD

Mendel made extremely careful use of scientific methods in his research. He studied pea plants for eight years! He eventually became known as the Father of Genetics.

MENDEL’S WORK

In nature, bees and other insects pollinate plants as they go from flower to flower. Mendel cross-pollinated the pea plants he studied (he took pollen from the male reproductive structure of flowers of purebred tall plants and placed it on the female reproductive structure of flowers of pure short plants).

GENES

Chromosomes contain genes and are made up of DNA. Genes are small sections of chromosomes that determine traits (physical characteristics of an organism). When pairs of chromosomes separate into sex cells during meiosis, pairs of genes also separate from one another. Each sex cell ends up with one form of a gene for each The study of how traits are inherited through the actions of alleles is the science of genetics.

ALLELESThe different forms that a gene may have for a trait are called alleles. For instance, in pea plants flowers may be purple or white. Purple is dominant. Dominance is when one trait covers over or masks another form of the trait. White is recessive. A trait that is hidden or masked if the dominant form of the trait is present is called a recessive trait.

DOMINANT VS. RECESSIVE

In humans, freckles is the dominant trait, whereas no freckles is the recessive trait.

DOMINANT OR RECESSIVE?

Mendel discovered that tall plants crossed with short plants produced all tall plants. Mendel called the tall height form that appeared the dominant factor, because it dominated or covered up the short height form. He called the form that seemed to disappear the recessive factor.

RECESSIVE REAPPEARS!

Mendel allowed the new tall plants to self-pollinate. When he planted the new seeds he found that the recessive form had reappeared… both tall and short plants grew. He found that for every three tall plants there was one short plant, a 3:1 ratio.

PUREBRED

When an organism has two identical alleles for a trait, it is said to be a pure bred (ex- 2 purple flower alleles). When an organism contains different alleles for a trait it is said to be a hybrid (ex- 1 purple and 1 white flower allele).

USING A PUNNETT SQUARE

A handy tool used to predict results like Mendel did for genetics is called the Punnett square. In a Punnett square dominant and recessive alleles are represented by letters. A capital letter represents the dominant allele and a small letter represents the recessive allele for a trait. The letters show the genetic makeup, or genotype, of an organism.

HOMOZYGOUS OR HETEROZYGOUS?

Most cells in your body have two alleles for a trait.

An organism with two alleles for a trait that are exactly the same is called homozygous. (example: TT or tt)

An organism with two different alleles for a trait is called heterozygous. (example: Tt)

The physical trait that shows the result of a particular genotype is its phenotype (example: tall or short)

GENETICS SINCE MENDEL

While Mendel’s results were true for pea plants, different plants sometimes varied from Mendel’s predictions. When scientists crossed pure red four o’clock plants with pure white four o’clock they expected to get all red flowers… but all the flowers were pink! Neither allele for the flower color seemed dominant. Incomplete dominance is the production of a phenotype that is intermediate to those of the two homozygous parents.

MULTIPLE ALLELES & GENES

Mendel studied traits in peas that were controlled by just two alleles. However, many traits are controlled by more than two alleles- they are controlled by multiple alleles.

Human blood typing is controlled by multiple alleles.

Phenotype Genotype

A AA or AO

B BB or BO

AB AB

O OO

MULTIPLE GENES

Some traits are produced by a combination of many genes. Polygenic inheritance occurs when a group of gene pairs act together to produce a single trait. Eye color and fingerprints are examples. Height, weight, body build, hair, and skin color, are as well.

SEX DETERMINATION

Females produce eggs that have only an X chromosome. Males produce sperm that contain either an X or a Y chromosome. If an egg is fertilized by an X sperm the offspring is XX- a female. If the egg is fertilized by a Y sperm the offspring is XY- a male.

SEX-LINKED DISORDERS

Some inherited conditions are closely linked with the X and Y chromosomes that determine the sex of an individual. Individuals who are red-green color blind have inherited an allele on the X chromosome that prevents them from seeing these colors.

What number do you see?

SEX-LINKED GENE

An allele inherited on a sex chromosome is a sex-linked gene.