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Chapter : Genetics The Classic Approach. Mendel working in his garden. Pea Plants!. Gregor Mendel. Gregor Mendel combined his farmer’s skills with his training in mathematics. Mendel’s law of segregation states that each individual has two factors (called genes today) for each trait.
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Mendel working in his garden Pea Plants!
Gregor Mendel • Gregor Mendel combined his farmer’s skills with his training in mathematics. • Mendel’s law of segregation states that each individual has two factors (called genes today) for each trait. • Alternative forms of a gene having the same position on a pair of homologous chromosomes and affecting the same trait are now referred to as alleles.
Today we know that alleles occur at the same loci (position) on a chromosome. The factors segregate during the formation of the gametes and each gamete has only one factor from each pair. Fertilization gives each new individual two factors again.
The Inheritance of a Single Trait • A capital letter indicates a dominant allele, which is expressed when present. • An example is Wfor widow’s peak. • A lowercase letter indicates a recessive allele, which is only expressed in the absence of a dominant allele. • An example is w for continuous hairline.
Genotype and Phenotype • Genotype refers to the genes of an individual which can be represented by two letters or by a short descriptive phrase. • Homozygous means that both alleles are the same; for example, WWstands for homozygous dominant and ww stands for homozygous recessive. • Phenotype refers to the physical appearance of the genotype!
Gamete Formation(sperm and eggs) • Because homologous pairs separate during meiosis, a gamete has only one allele from each pair of alleles. • If the allelic pair isWw, a gamete would contain either a W or a w, but not both. • Ww represents the genotype of an individual. • Gametes are represented by W or w.
One-Trait Crosses(let’s breed!) • In one-trait crosses, only one trait such as type of hairline is being considered. • When performing crosses, the original parents are called the parental generation, or the P generation. • All of their children are the filial generation, or F generation. • Children are monohydrids when they are heterozygous for one pair of alleles.
If you know the genotype of the parents, it is possible to determine the gametes and use a Punnett square to determine the phenotypic ratio among the offspring. When a monohybrid reproduces with a monohybrid, the results are 3 : 1. This ratio is used to state the chances of a particular phenotype. A 3 : 1 ratio means that there is a 75% chance of the dominant phenotype and a 25% chance of the recessive phenotype.
One-Trait Crosses and Probability • Laws of probability alone can be used to determine results of a cross. • The laws are: • (1) the probability that two or more independent events will occur together is the product of their chances occurring separately, and • (2) the chance that an event that can occur in two or more independent ways is the sum of the individual chances.
In the cross of Ww x Ww, what is the chance of obtaining either a W or a w from a parent? Chance of W = ½, or chance of w = ½ The probability of these genotypes is: The chance of WW = ½ x ½ = ¼ The chance of Ww = ½ x ½ = ¼ The chance of wW = ½ x ½ = ¼ The chance of ww = ½ x ½ = ¼ The chance of widow’s peak (WW, Ww, wW) is ¼ + ¼ + ¼ = ¾ or 75%.
The One-Trait Testcross • It is not always possible to discern a homozygous dominant from a heterozygous individual by inspection of phenotype. • A testcross crosses the dominant phenotype with the recessive phenotype. • If a homozygous recessive phenotype is among the offspring, the parent must be heterozygous.
The Inheritance of Many Traits • Independent Assortment • The law of independent assortment states that each pair of alleles segregates independently of the other pairs and all possible combinations of alleles can occur in the gametes. • This law is dependent on the random arrangement of homologous pairs at metaphase.
Two-Trait Crosses • In two-trait crosses, genotypes of the parents require four letters because there is an allelic pair for each trait. • Gametes will contain one letter of each kind in every possible combination. • When a dihybrid reproduces with a dihybrid the results are 9 : 3 : 3 : 1.
Two-Trait Crosses and Probability • It is possible to use the two laws of probability to arrive at a phenotypic ratio for a two-trait cross without using a Punnett square. • The results for two separate monohybrid crosses are as follows: • Probability of widow’s peak = ¾ • Probability of short fingers = ¾ • Probability of straight hairline = ¼ • Probability of long fingers = ¼
The probabilities for the dihybrid cross: Probability of widow’s peak and short fingers = ¾ x ¾ = 9/16 Probability of widow’s peak and long fingers = ¾ x ¼ = 3/16 Probability of straight hairline and short fingers = ¼ x ¾ = 3/16 Probability of straight hairline and long fingers = ¼ x ¼ = 1/16
The Two-Trait Testcross • A testcross is done when it is not known whether a dihybrid individual is homozygous dominant or heterozygous for both or one of the traits under consideration. • A cross of a person heterozygous for both traits with a homozygous recessive person produces a 1 : 1 : 1 : 1 ratio.
Pedigree charts represent males as squares and females as circles. Recessive and dominant alleles have different patterns of inheritance. Genetic counselors construct pedigree charts to determine the mode of inheritance of a condition.
Autosomal Recessive Disorders • Tay-Sachs Disease • Tay-Sachs disease is common among United States Jews of central and eastern European descent. • An affected infant develops neurological impairments and dies by the age of three or four. • Tay-Sachs results from a lack of hexosaminidase A and the storage of its substrate in lysosomes.
Cystic Fibrosis • Cystic fibrosis is the most common lethal genetic disorder among Caucasians. • A chloride ion transport protein is defective in affected individuals. • Normally when chloride ion passes through a membrane, water follows. • In cystic fibrosis patients, a reduction in water results in a thick mucus which accumulates in bronchial passageways and pancreatic ducts.
Phenylketonuria (PKU) • Individuals with phenylketonuria lack an enzyme needed for the normal metabolism of phenylalanine, coded by an allele on chromosome 12. • Newborns are regularly tested for elevated phenylalanine in the urine. • If the infant is not put on a phenylalanine-restrictive diet in infancy until age seven when the brain is fully developed, brain damage and severe mental retardation result.
Autosomal Dominant Disorders • Neurofibromatosis • Small benign tumors, made up largely of nerve cells, occur under skin or on various organs. • The effects can range from mild to severe, and some neurological impairment is possible; this disorder is variably expressive. • The gene for this trait is on chromosome 17.
Huntington Disease • Individuals with Huntington disease experience progressive degeneration of the nervous system and no treatment is presently known. • Most patients appear normal until middle age. • The gene coding for the protein huntingtin contains many more repeats of glutamines than normal.
Skin Color • The inheritance of skin color, determined by an unknown number of gene pairs, is a classic example of polygenic inheritance. • A range of phenotypes exist and several possible phenotypes fall between the two extremes of very dark and very light. • The distribution of these phenotypes follows a bell-shaped curve.
Polygenic Disorders • Many human traits, like allergies, schizophrenia, hypertension, diabetes, cancers, and cleft lip, appear to be due to the combined action of many genes plus environmental influences. • Many behaviors, such as phobias, are also likely due to the combination of genes and the effects of the environment.
Multiple Allelic Traits • Inheritance by multiple alleles occurs when more than two alternative alleles exist for a particular gene locus. • A person’s blood type is an example of a trait determined by multiple alleles. • Each individual inherits only two alleles for these genes.
ABO Blood Types • A person can have an allele for an A antigen (blood type A) or a B antigen (blood type B), both A and B antigens (blood type AB), or no antigen (blood type O) on the red blood cells. • Human blood types can be type A (IAIAor IA i), type B (IBIBor IBi), type AB (IAIB), or type 0 (ii).
Incompletely Dominant Traits • Codominance means that both alleles are equally expressed in a heterozygote. • Incomplete dominance is exhibited when the heterozygote shows not the dominant trait but an intermediate phenotype, representing a blending of traits. • Such a cross would produce a phenotypic ratio of 1 : 2 : 1.
Sickle-Cell Disease • Sickle-cell disease is an example of a human disorder controlled by incompletely dominant alleles. • Sickle cell disease involves irregular, sickle shaped red blood cells caused by abnormal hemoglobin. • HbA represents normal hemoglobin; and HbS represents the sickled condition.
HbAHbA individuals are normal; HbSHbS individuals have sickle-cell disease and HbAHbS individuals have the intermediate condition called sickle-cell trait. Heterozygotes have an advantage in malaria-infested Africa because the pathogen for malaria cannot exist in their blood cells. This evolutionary selection accounts for the prevalence of the allele among African Americans.
Many genetic disorders and other traits are inherited according to laws first established by Gregor Mendel. Inheritance is often more complex, providing exceptions to Mendel’s laws but helping to explain an even wider variety in patterns of gene inheritance.
Amniocentesis • Amniocentesis uses a needle to extract amniotic fluid from the uterus of a pregnant woman from the 14th to 17th week of pregnancy. • Up to 400 chromosome and biochemical problems can be detected by culturing fetal cells that are in the amniotic fluid. • There is a slight risk of spontaneous abortion with this procedure.
