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Mendel and the Gene Idea (Mendel used the scientific approach to identify…
Mendel and the Gene Idea
Mendel used the scientific approach to identify two laws of inheritance
Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments, carried out several decades before chromosomes were observed under the microscope
Pea plants are available in many varieties that have distinct heritable features, or characters, with different variant traits
When true-breeding plants self-pollinate, all their offspring have the same traits as their parents
In a typical breeding experiment, Mendel would cross-pollinate (hybridize) two contrasting, true-breeding pea varieties
The true-breeding parents are the P (parental) generation, and their hybrid offspring are the F1 (first filial) generation
Mendel would then allow the F1 hybrids to self-pollinate to produce an F2 (second filial) generation
These alternative versions of a gene are called alleles
If the two alleles at a locus differ, then one, the dominant allele, determines the organism’s appearance. The other, the recessive allele, has no noticeable effect on the organism’s appearance
Mendel’s law of segregation states that the two alleles for a heritable character segregate (separate) during gamete production and end up in different gametes
A Punnett square may be used to predict the results of a genetic cross between individuals of known genotype
An organism with two identical alleles for a character is homozygous for the gene controlling that character
An organism with two different alleles for a gene is heterozygous for that gene
An organism’s observable traits are called its phenotype. An organism’s genetic makeup is called its genotype
A testcross can be used to determine the genotype of an individual with the dominant phenotype
The independent assortment of each pair of alleles during gamete formation is called Mendel’s law of independent assortment: Each pair of alleles segregates independently
during gamete formation
The laws of probability govern Mendelian inheritance
We can use the multiplication rule to determine the probability that two or more independent events will occur together in some specific combination
We can use the addition rule to determine the probability that an F2 plant from a monohybrid cross will be heterozygous rather than homozygous
Mendel’s experiments succeeded because he counted so many offspring, was able to discern the statistical nature of inheritance, and had a keen sense of the rules of chance
Mendel’s studies of pea inheritance are a model in genetics and also as a case study of the power of scientific reasoning using the hypothetico-deductive approach
Inheritance patterns are often more complex than predicted by simple Mendelian genetics
One extreme is the complete dominance characteristic of Mendel’s crosses
Some alleles show incomplete dominance, in which heterozygotes show a distinct intermediate phenotype not seen in homozygotes
At the other extreme from complete dominance is codominance, in which two alleles affect the phenotype in separate, distinguishable ways
For example, humans with Tay-Sachs disease lack a functioning enzyme to metabolize certain lipids. These lipids accumulate in the brain, harming brain cells and ultimately leading to death
Most genes are pleiotropic, affecting more than one phenotypic character
In epistasis, the phenotypic expression of a gene at one locus alters the phenotypic expression of a gene at a second locus
Quantitative characters are usually due to polygenic inheritance, the additive effects of two or more genes on a single phenotypic character
The product of a genotype is generally not a rigidly defined phenotype, but a range of phenotypic possibilities, the norm of reaction, determined by the environment
Many human traits follow Mendelian patterns of inheritance
In pedigree analysis, rather than manipulate mating patterns of people, geneticists analyze the results of matings that have already occurred
Although heterozygotes may lack obvious phenotypic effects, they are carriers who may transmit a recessive allele to their offspring
Cystic fibrosis strikes one of every 2,500 whites of European descent
The most common inherited disease among people of African descent is sickle-cell disease, which affects one of 400 African-Americans
One example is Huntington’s disease, a degenerative disease of the nervous system
One technique, amniocentesis, can be used from the 14th to 16th week of pregnancy to assess whether the fetus has a specific disease
A second technique, chorionic villus sampling (CVS), allows faster karyotyping and can be performed as early as the 8 the to 10th week of pregnancy