GENETICS
Experimental genetics began in an abbey garden
The modern science of genetics began in the 1860s when a monk named Gregor Mandel deduced the fundamental principles of genetics by breeding garden peas. Mendel lived and worked in an abbey in Austria. Strongly influenced by his study of physics, mathematics, and chemistry at the University of Vienna, his research was both experimentally and mathematically rigorous, and these qualities were largely responsible for his success.
In a paper published in 1866, Mendel correctly argued that parents pass on to their offspring discrete hereditary factors. He stressed that these hereditary factors (today called genes) retained their individuality generation after generation. In other words genes are like marbles of different colors: just as marbles retain their colors permanently and do not blend, no matter how they are mixed, genes permanently retain their identities.
Mendel probably chose to study garden peas because he was familiar with them from his rural upbringing, they were easy to grow, and they came in many readily distinguishable varieties. Perhaps most importantly, Mendel was able to exercise strict control over pea plant matings. The petals of the pea flower almost completely enclose the reproductive organs. Consequently, pea plants usually self-fertilize in nature. That is, pollen grains land on the egg of the same flower. Mendel could ensure self-fertilization by covering a flower with a small bag so that no pollen from another plant could reach the egg. When he wanted cross-fertilization (fertilization of one plant by pollen from a different plant), he used a particular method so that he could be sure of the heritage of the new plants.
Mendel worked with his plants until he was sure he had true breeding varieties-- that is, varieties for which self fertilization produced offspring all identical to the parent In other words, a “pure-bred” plant). For instance, he identified a purple flowered variety that produced offspring plants that all had purple flowers.
Now Mendel was ready to ask what would happen when he crossed his different true breeding varieties with each other. For example, what offspring would result if plants with purple flowers and plants with white flowers were cross fertilized? In the language of the plant and animal breeders and geneticists, the offspring of two different varieties are called hybrids, and the cross-fertilization itself is referred to as hybridization, or simply a cross. The true breeding parental plants are called the P generation and their hybrid offspring are the F1 generation. The offspring of F1 plants are known as the F2 generation.
HEREDITARY PHYSICAL CHARACTERISTICS
Genotype and Phenotype
Genotype means the type of genes a person has, or their genetic make-up.
Those genes that affect the same trait are called alleles.
A dominant allele is given a capital letter, and a recessive allele is given the same letter in lower case. For instance, having an earlobe that is unattached to the face is a dominant trait, so we can call it E. An attached earlobe would then be called e.
Alleles occur in pairs; just as one pair of each type of chromosome is inherited from each parent, so too each pair of alleles inherited from each parent.
The allele which is traditionally indicated by an uppercase (capital) letter is the dominant trait. The allele which is traditionally indicated by a lowercase (small) letter is the recessive trait.
If a sperm cell has e and the egg cell has e, the offspring must have ee. That is called homozygous (pure) recessive. That means the person would have an attached earlobe.
If a sperm cell has E and the egg cell has E, the offspring must have EE. This is called homozygous (pure) dominant. That means the person would have an unattached earlobe.
The term for “pure” is homo. It refers to something being the same. In the old days, you had to shake up milk because the cream would rise to the top. Nowadays, people want less fat, so the cream is removed before you get it; this is called homogenized milk. A homogenized mixture is one that is the same throughout, and requires no periodic mixing. Therefore, when the allele pairs are either EE or ee, they are homozygous.
The opposite of homo is “hetero”, so an allele pair that is “Ee” is heterozygous.
If one of the sex cells has E and the other sex cell has e, what will the offspring have? Ee.
What type of earlobe will they have? Attached. Why? Because the dominant trait is stronger, so if it is present at all, it will manifest.
The physical appearance of a person is called the phenotype.
A person with Ee will therefore be called a heterozygous genotype, with an unattached earlobe phenotype.
What earlobe alleles will a person have who is homozygous recessive? ee
What earlobe alleles will a person have who is homozygous dominant? EE
What earlobe alleles will a person have who is heterozygous? Ee
Figuring the Odds
If one of the parents is homozygous dominant (EE), the chances of their having a child with unattached earlobes is 100 %, because this parent has only a dominant allele (E) to pass on to the offspring. On the other hand, if both parents are homozygous recessive (cc), there is a 100% chance that each of their children will have attached earlobes.
However, if both parents are heterozygous, then what are the chances that their child will have unattached or attached earlobes? To solve a problem of this type, it is customary first make a table of the genotype of the parents and their possible gametes.
E / eE / EE / Ee
e / Ee / ee
That means that when Harry meets Sally, their child has a 25% chance (1:3) of being ee, and 25% chance of being EE, and 50% chance (1:1) of being Ee.
But that’s just the genotype. What about the phenotype (what will the child look like)?
There is a 75% chance (3:1) of having an attached earlobe (ee).
Example test questions:
In crossing a heterozygous parent and a homozygous recessive parent, what are the chances that an offspring will receive a dominant allele? Answer = 50%
What is the ratio for crossing two heterozygous parents for ear lobe attachment
(Ee x Ee): 3:1
Free earlobes (E) are dominant over attached earlobes (e). If two people with homozygous attached earlobes mate, what will be the phenotype of their offspring? All attached earlobes
What is the ratio for crossing a heterozygous parent for ear lobe attachment and a homozygous recessive parent (Ee x ee): 1:1
In crossing two heterozygous parents, what are the chances for a pure recessive offspring? 25%
GENETIC DISORDERS
1. Chromosome Disorders
A. Down Syndrome
Down syndrome is also called trisomy 21 because the person’s chromosome number 21 has three chromosomes joined together instead of just two. The chances of a woman having a Down syndrome child increase rapidly with age, starting at about age 40. The frequency of Down syndrome is 1/ 800 births for mothers under 40 years of age, but women over 40 are 10 times more likely to have a Down syndrome child.
Characteristics of Down syndrome include a short stature; an eyelid fold; stubby fingers; a wide gap between the first and second toes; a large, fissured tongue; a round head; a palm crease (the so-called simian line), and mental retardation, which can sometimes be severe. Their personalities are usually cheerful, good-natured, and pleasant throughout their lives.
Amniocentesis (removing fluid and cells from the amniotic sac surrounding the fetus) followed by karyotyping can detect a Down syndrome child.
Scientists have located genes most likely responsible for the increased tendency toward leukemia, cataracts, accelerated rate of aging, and mental retardation. One day it might be possible to control the expression of that gene even before birth so that at least this symptom of Down syndrome does not appear.
2. Sex Chromosomal Disorders
All of the cells in our body have all of our chromosomes in the nucleus except for the egg and the sperm. Each of these has all of our chromosomes in the nucleus, except there is only one of the two sex chromosomes. Since women are XX, all of her egg cells are X, but since males are XY, a sperm can bear an X or a Y. Therefore, the sex of the newborn child is determined by the father. If a Y. bearing sperm fertilizes the egg, then the XY combination results in a male. On the other hand, if an X-bearing sperm fertilizes the egg, the XX combination results in a female.
All factors being equal, there is a 50% chance of having a girl or a boy. If a couple has 10 children and they are all boys, what is the chance that an eleventh child is going to be a boy? Interestingly, the death rate among males is higher than for females. By age 85, there are twice as many females as males.
A. Jacob syndrome occurs in 1/ 1,000 births. These XYY (an extra male chromosome) males are usually taller than average, suffer from persistent acne, and tend to have speech and reading problems. At one time, it was suggested that these men were likely to be criminally aggressive, but it has since been shown that the incidence of such behavior among them may be no greater than among XY males.
B. Klinefelter syndrome occurs in 1/ 1,500 births. These males with XXY (an extra female chromosome) and they are sterile. They are males with some female characteristics. The testes are underdeveloped, they have some breast development, and there is no facial hair. They are usually slow to learn but not mentally retarded.
C. Triple-X syndrome occurs in 1/ 1,500 births. These are females with an extra female chromosome: XXX. You might think they are especially feminine, but this is not the case. Although in some cases there is a tendency toward learning disabilities, most have no physical abnormalities except that they may have learning disabilities, menstrual irregularities, including early onset of menopause.
D. Turner syndrome occurs in 1/ 6,000 births. The individual is XO, meaning one of the sex chromosomes is missing. These are females and have a short, have a broad chest, and webbed neck. The ovaries and uterus are nonfunctional. Turner females do not undergo puberty or menstruate, and there is a lack of breast development. They are usually of normal intelligence and can lead fairly normal lives, but they are infertile even if they receive hormone supplements.
3. Dominant Disorders (only one dominate allele needs to be present)
A. Neurofibromatosis
Also known as Elephant Man disease, this is one of the most common genetic disorders. It affects roughly 1/ 3,000 people. It is seen equally in every racial and ethnic group throughout the world. At birth or later, the affected individual may have six or more “coffee with milk” colored spots (known as cafe-au-lait) on the skin. Such spots may increase in size and number and may get darker. Small benign tumors (lumps) called neurofibromas may occur under the skin or in various organs.
In most cases, symptoms are mild, and patients live a normal life. In some cases, however, the effects are severe. Skeletal deformities, including a large head, are seen, and eye and ear tumors can lead to blindness and hearing loss. Many children with neurofibromatosis have learning disabilities and are hyperactive. The abnormal gene is on chromosome 17.
4. Homozygous Recessive Disorders (both parents must have rr alleles)
A. Tay - Sachs disease
This disease usually occurs among Jewish people. At first, it is not apparent that a baby has Tay-Sachs disease. However, development begins to slow down between four months and eight months of age, and neurological impairment and psychomotor difficulties then become apparent. The child gradually becomes blind and helpless, develops uncontrollable seizures, and eventually becomes paralyzed. There is no treatment or cure for Tay-Sachs disease, and most affected individuals die by the age of three or four. It is caused by a genetic enzyme deficiency.
B. Cystic Fibrosis
This is the most common lethal genetic disease among Caucasians in the United States. About 1 in 20 Caucasians is a carrier, and about 1/ 2,500 births have the disorder. In these children, the mucus in the bronchial tubes is particularly thick and interferes with breathing, and the lungs get infected frequently.
New treatments have raised the average life expectancy to 28 years of age. The cystic fibrosis gene is located on chromosome 7.
C. Phenylketonuria (PKU)
This occurs in 1 / 5,000 births, so it is not as frequent as the disorders previously discussed, however, PKU is tested for in routine blood screenings of all newborns in the United States. This is the disease that offspring of first cousins are more likely to get.
PKU people lack an enzyme that is needed to break down an amino acid (phenylalanine), and so the amino acid accumulates in the urine. These people have to have a special diet that does not contain that amino acid. If they get too much of it, they will get neurological problems and mental retardation. That’s why nutrition labels have to warn when they contain phenylalanine.
5. Incompletely Dominant Traits