Mendel and the Gene Idea

A scientist and monk by the name of Gregor Mendel (1822 or 23 - 1884) contributed significantly to the understanding of genetics in the 1800s by being the first to actually count numbers of offspring in crosses involving pea plants. He is often called the father of genetics.

• Son of a farm family - not wealthy
• Did very well in school but to get more education he became a monk and attended the University of Vienna.
• Over a period of 7 years he bred and counted about 28,000 pea plants.
• He chose the pea plant because others had bred many varieties of peas and there were known characters. Also, because the pea flower has both female and male parts in the same flower and usually self fertilizes. By removing the stamens. he could cross-fertilize them by hand. Fig 14.1
• Mendel chose 7 characters that showed 2 traits for that character, like purple versus white flowers, rather than a blending of the colors. Table 14.1
• Character - a heritable feature.
• Trait - a variant for a character.

Mendel's Experimental Design - Mendel began his crosses with true-breeding varieties (homozygous) that contained only one type of gene for each character. At first, he only looked at one character at a time - a monohybrid cross.The first generation in a succession of crosses is the P or parental generation; their offspring are the F1 generation or first filial generation. Fig 14.2

Offspring of two members of the F1 generation comprise the F2 generation.

Mendel repeatedly came up with the same results when examining seven pairs of contrasting traits.

Mendel called the trait expressed in the F1 plants the dominant trait and the trait not expressed was recessive.

When the F1 plants were allowed to self-fertilize, Mendel found 3:1 dominant to recessive phenotype in the F2 generation. Fig 14.4.

When F2 plants were allowed to self-fertilize, Mendel found a 1:2:1 ratio of true-breeding dominant to not true-breeding dominant to true-breeding recessive.

Today we use these terms:

Each individual has two genes (Mendel's "factors") for each trait.

When both genes are the same, the individual is said to be homozygous for that trait. If the two genes are different, the individual is heterozygous for that trait.

Alternate types of genes for each trait are alleles. Fig. 14.3

Phenotype refers to the outward expression of the genes.

The actual genetic makeup of an individual is the genotype. Fig 14.5

Analyzing Mendel's Results

Punnett Squares - an easy way to express the probabilities of genotypes. image and image 2.

The Testcross - When Mendel did not know the genotype of an individual expressing a dominant trait, he did a test cross by crossing the individual with a homozygous recessive for the trait. Fig. 14.6.

Mendel's Laws (using today's terminology)

Mendel's First Law: Law of Segregation, says that only one of a pair of alleles is passed to a gamete. Fig 14.4

Mendel's Second Law: Law of Independent Assortment was determined when he worked with two traits at a time in dihybrid crosses. This law states that genes located on different chromosomes are inherited independently. Fig 14.7b. Crossing two individuals that are heterozygous for both characters yields a phenotypic ratio of 9:3:3:1. If the genes are located on the same chromosome, they would be linked. Fig 14.7a

Rules of probability

Rules of multiplication

 

Rules of addition

 

Probability and genetics

Exceptions to Mendel's Laws - Remember Mendel did not know about genes or chromosomes. The following are other ways that genes may express themselves.

Incomplete Dominance - when offspring exhibit a phenotype intermediate to that of both parents. Fig 14.9.

Codominance - 2 alleles affect the phenotype in separate distinguishable ways. Ex: MN blood factor.

Multiple Alleles

More than two alleles exist for a given trait in a population of individuals.

In the human ABO blood group, A and B are equally dominant and can occur together as codominants. The O blood group consists of two recessive genes that code for neither A nor B. image and Fig. 14.10

Pleiotropic Effects - when an allele affects more than one trait. Ex: cystic fibrosis

Epistasis is an interaction between the products of two genes in which one of the genes modified the phenotypic expression produced by the other. Fig 14.11

Polygenic (multiple genes) inheritance or continuous variation - when one trait, such as human height, is determined by the action of several genes, it results in a continuous variation for the trait within a population. image of human height variation. Human skin color Fig 14.12 and eye color are also determined by multiple genes.

Environmental Effects - The degree to which an allele is expressed can sometimes depend on the environment
EX: Some alleles are heat-sensitive, resulting in different pigmentation during seasonal weather changes or sensitive to acidity levels. Fig 14.13

Mendelian Inheritance in humans

Some either/or characters in humans include presence or absence of a widow's peak (dominant), attached (recessive) or free earlobes. We can chart a family tree or pedigree based on these either/or traits. This is more commonly done when there is a history of a family disorder, as below.

Autosomal Genetic Disorders

Autosomal Recessive Disorders

Pattern of Inheritance
Both parents must have the allele to have a child born with the condition.
Even if both parents are carriers, they only have a 1-in-4 chance of having an offspring with the condition.

Cystic fibrosis - most common genetic disorder in Caucasian Americans; 1 in 25 is a carrier, 1 in 2500 is affected. Chloride channels that enable the passage of chloride ions are absent or defective so that the chloride ions build up and cause a thickening and accumulation of mucus in the lungs, pancreas and other organs. This hampers breathing and other functions and also allows for a greater chance of infection.

Tay Sachs - most common in Jews from Eastern and Central Europe, 1 in 30 are carriers, 1 in 3,600 are affected. Tay-Sachs disease is an incurable hereditary disorder that progressively destroys the brain of those affected. graph

Sickle-cell anemia Fig 14.15

• Prevalent in populations in or from Africa
• Red blood cells become distorted into sickle shape in low oxygen.
• Mutation is in the gene for the B chain protein of hemoglobin.
• Hemoglobin S has a substitution of one amino acid, causing the chain to coalesce into crystals that distort the red blood cells.
• Persons with one S allele and one normal A allele do not have the condition, but are called carriers because they can pass the gene on to their offspring.
• In the case of sickle-cell anemia, which occurs more frequently in populations from malaria-ravaged sections of Africa, heterozygotes are more resistant to malarial infection. Map
  Malaria is caused by a protozoan parasite transmitted by the Anopheles mosquito. Some of the life cycle of the parasite must be spent in human red blood cells.

Autosomal Dominant Disorders Pattern of Inheritance - single "faulty" allele of a gene causes damage, even with a "good" allele present, because the "faulty" allele is dominant.

Huntington's disease is a dominant lethal condition that does not express itself until later in life, after the trait has been passed on to the next generation. It is characterized by degenerative neural disease. image

ALS, Lou Gehrig's disease, is also a degenerative neural disorder.

Multifactorial disorders

Genetic Counseling and Therapy

Genetic counseling can help couples predict the risk of bearing children with genetic defects. Carrier recognition.

Screening techniques Fig. 14.17

Amniocentesis - a minute amount of amniotic fluid surrounding the fetus is removed and checked for genetic defects. Performed at 4th month of pregnancy. image.

Chorionic villus sampling - removal of a small portion of the chorionic villi of the placenta for genetic testing. Can be done earlier in the pregnancy, is less invasive, and yields results more quickly.

Ultrasound allows for viewing of the fetus without harming it.

Newborn screening - simple test available like the test for PKU