Mendel and Heredity Chapter Eight

The passing of characters (traits) from parents to offspring is called heredity.

Genetics is the study of heredity or how genes are passed from parents to their offspring. Much of our understanding of this field is owed to the studies of Gregor Johann Mendel, an Austrian monk who studied garden peas.

In 1842 Gregor Mendel entered a monastery in Brunn, Austria. One of his tasks was to care for the garden. His inquiring mind acquired and stored data about garden peas that he would use later in his studies. In 1851 he entered the University of Vienna where he learned statistics (study of probabilities) in his math courses. These math courses would prove valuable to him when he began to organize his data in his study of statistics.

Mendel repeated the experiments of a British farmer who had crossed a variety of garden peas that had purple blooms and white blooms. All of the offspring of the cross had purple flowers, however, in the next generation, two white blooms resurfaced. Mendel would use his math to study the traits.

The garden pea was an excellent selection for Mendel’s studies: Several characters of garden peas exist in two clear forms with no intermediates such as purple or white flowers. The male and female parts of the flower are found in the same bloom and can easily be cross-pollenated. The garden pea grows on a small plant, matures quickly, and produces many offspring.

Mendel’s Garden Pea Characters

Mendel intended to cross various parent varieties and study the occurrence of one trait. This is a monohybrid cross.

Mendel’s experimental plan was to: All parent plants to develop true breeding. That is, these parents’ offspring always would show the same traits. The groups are sometimes defined as pure. These were called the P generation. Mendel would then cross two P generations to develop the F 1 generation or first filial. He would then cross two F 1 generations to develop an F 2 generation.

Mendel expressed the occurrence of each trait as a ratio. He found that traits always occurred in the F 2 generation in a ratio of 3:1.

Mendel’s Theory Section Two

Before Mendel, it was thought that offspring inherited a blend of their parent’s traits. Mendel did not support this hypothesis. He believed that offspring inherited two factors, one from each parent, that would control their traits.

Mendel developed four hypothesis from his experiments. For each inherited character, an individual has two copies of a gene– one from each parent.(Pair Principle) There are alternative forms of a gene (alleles) that control different traits. When the two alleles occur together, one may be completely expressed (dominant) and one may have no observable effect (recessive).(Law of Dominance) Gametes carry only one allele for each inherited character and, during fertilization, each gamete contributes one allele.

Mendel’s hypothesis have developed into two laws of heredity: Law of Segregation– The two alleles for a trait separate when gametes are formed. Law of Independent Assortment– Alleles of different genes separate independently when gametes are formed.

The two alleles for a gene that an individual carries can be classified in two ways:  Homozygous– The two genes that determine a character are the same.  Heterozygous– The two genes that determine a character are different.

Two terms are used to describe a character:  Genotype is the exact genes that determine a trait  Phenotype describes the appearance of a trait.

Studying Heredity Section Three

A Punnet Square is a diagram that predicts all the outcomes of a cross by considering all the possible combinations of genes. The probability or liklihood that a character will be expressed is stated as a fraction of the whole.

To study a trait in a particular family, geneticists prepare a pedigree which is a history of how the trait has been expressed over several generations.

To study a trait, a geneticist must determine if it is: Autosomal—That is, the trait is carried on autosomes and appears in both sexes equally. Autosomal—That is, the trait is carried on autosomes and appears in both sexes equally. Sex-Linked—Most sex-linked traits are recessive and are carried on the X chromosome. The only way a female would express the trait would be if she inherited it on both of her chromosomes. A male would always express the trait if it was present on his X chromosome. Sex-Linked—Most sex-linked traits are recessive and are carried on the X chromosome. The only way a female would express the trait would be if she inherited it on both of her chromosomes. A male would always express the trait if it was present on his X chromosome.

Complex Patterns of Heredity Section Four

When several genes influence a trait, it is considered polygenic inheritance. Examples include height, weight, eye color, hair color and skin color.

Other polygenic inheritance patterns include: Incomplete dominance as in Japanese Four O’Clocks. Incomplete dominance as in Japanese Four O’Clocks. Multiple alleles as in blood type. Multiple alleles as in blood type. Codominance as in a roan. Codominance as in a roan. Environmentally influenced traits as in the coat color of arctic foxes. Environmentally influenced traits as in the coat color of arctic foxes.

Sometimes mutated genes cause a disease or condition. As these genes are passed to new generations, they are considered genetic disorders. Sickle Cell Anemia — Recessive gene that causes a defect in the red blood cells. Cystic Fibrosis —Recessive gene that keeps certain enzymes from being produced that digest certain foods and mucus. Hemophilia – Sex-linked mutated gene that affects blood clotting. Huntington’s Disease – Dominant gene that caused slow failure of the person’s nervous system.

Genetic disorders cannot be cured. Treatment is possible as with a baby with PKU. A person with a genetic disorder may want to go to genetic counseling with a doctor before becoming a parent.

Gene therapy is a pioneer field of replacing defective genes with healthy ones. For instance, a form of cold virus has been used to transfer healthy genes into lung cells.