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.
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
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.
Today we use these terms:
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)
Rules of probability
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.
2 alleles affect the phenotype in separate distinguishable ways.
Ex: MN blood factor.
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
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
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.
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