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Mendelian Genetics
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Chapter 11 |
I. Fundamentals of Gentics- is the hottest branch of biology. (Genetics studies the ways in which hereditary information is passed on from parents to offspring.)
A. Genotype- is the word that refers to the 2 genes (Mendel's factors) that make up a trait
1. example: TT or Tt, "each letter represents a gene"
B. Phenotype- refers to the physical trait that is expressed as a direct result of the genotype
1. TT is the genotype for tallness, height is the physical trait
C. Alleles- different copies of genes controlling a particular trait. For example, in Mendel's pea plants, there is an allele for tallness & shortness.
1. Homozygous (pure)- refers to the 2 genes that are same for a particular trait
a. TT is "homozygous dominant" for tallness, tt is "homozygous recessive" for shortness
2. Heterozygous (hybrid)- refers to the 2 genes that are different for the same trait
a. Tt is "heterozygous"; the dominant gene will be expressed
b. Tt is called a monohybrid cross because only one pair of contasting traits is being studied
D. Punnett Squares- indicate the expected ratios of genotypes & phenotypes only when there are large numbers of offspring (as in pea plants). When offspring are few (as in humans), the Punnett square indicates only the chances of occurrence of a genotype or phenotype in each separate offspring.
1. each LETTER represents one type of gamete (2 from each parent)
2. each BOX represents a possible union of a male gamete with a female gamete; the law of proability tells us that each of the four zygotes is equally likely to appear if there is a large number of pollinations & fertilizations
3. Law of Probability- if there are several possible events that might occur, and no one of them is likely to occur than any other, then they will occur in equal numbers over a large number of trials
E. The following list are interactive sites using Punnet Squares; they're listed in increasing complexity
Mendel died in 1884, without receiving recognition for his discoveries. The idea that Mendel's "factors" might be carried by homologous chromosomes was suggested first in 1903 W.S. Sutton. At that time, the term gene was used in place of Mendel's "factor."
II. Gregor Mendel- in the 1800's was the first to carry out scientific study of genetics
A. Pea Plants- were his focus of his study; developed laws
1. Law of Dominance- when an organism has 2 genes of contrasting traits, only the dominant trait can be seen in the hybrid
2. Law of Segregation- states that factors (genes) occur in pairs, are separated from each other during gamete formation, and are recombined at fertilization
3. Law of Independent Assortment- Mendel concluded from his experiments of a dihybrid cross that different traits were inherited independently of one another; in modern terms, this means that during meiosis, genes for different traits are separated and distributed to gametes independently of one another. But this fact is NOT always true.
III. Other Concepts of Genetics
A. Incomplete Dominance- blending of the genes
1. No dominant nor recessive gene (roses, Japanese 4-o'clock)
B. Codominance- 2 dominant alleles are expressed at the same time
1. Roan Coat is one example, is a mixture of red & white hair
C. Multiple Alleles
1. In some traits, more than 2 alleles exist for a trait [blood types have three alleles; ABO]
Interactive Quizzes on Mendel's Genetics
return to Mr. Via's syllabus
