Standard set 3. Genetics (Mendelâ€™s Laws)
Breeding of plants and animals has been an active technology for thousands of years, but the science of
heredity is linked to the genetics pioneer Gregor Mendel. He studies phenotypic traits of various plants,
especially those of peas. (A phenotypic trait is the physical appearance of a trait in an organism). From
the appearance of these traits in different generations of growth, he was able to infer their genotypes (the
genetic makeup of an organism with respect to a trait) and to speculate about the genetic makeup and
method of transfer of the hereditary units from one generation to the next. (Probability analysis is now
used to predict probÂ¬able progeny phenotypes from various parental genetic crosses.) The genetic
basis for Mendelâ€™s laws of segregation and independent assortment is apparent from geÂ¬netic
outcomes of crosses.
3. A Multicellular organism develops from a single zygote, and its phenotype
depends on its genotype, which is established at fertilization. As a basis for
understanding this concept:
3. a. Students know how to predict the probable outcome of phenotypes in a genetic cross
from the genotypes of the parents and the mode of inheritance (autosomal or X-linked,
dominant or recessive)
Monohybrid crosses, including autosomal dominant alleles, autosomal recessive alleles, incomplete
dominant alleles, and X-linked alleles, can be used to indicate the parental genotypes and phenotypes.
The possible gametes derived from each parent are based on genotypic ratios and can be used to predict
possible progeny. The predictive (probabilistic) methods for determining the outcome of genotypes and
phenotypes in a genetic cross can be introduced by using Punnett Squares and probability mathematics.
Teachers should review the process of writing genotypes and help students translate genotypes into
phenotypes. Teachers should emphasize dominant, recessive, and incomplete dominance as the students
advance to an explanation of monohybrid crosses illustrating human conditions characterized by
autosomal reÂ¬cessive alleles, such as albinism, cystic fibrosis, Tay-Sachs, and phenylketonuria (PKU).
These disorders can be contrasted with those produced by possession of just one autosomal dominant
allele, conditions such as Huntington disease, dwarf-ism, and neurofibromatosis. This basic introduction
can be followed with examples of incomplete dominance, such as seen in the comparisons of straight,
curly, and wavy hair or in the expression of intermediate flower colors in snapdragon plants.
Sex-linked characteristics that are found only on the X chromosome should also be considered, and
students should reflect on how this mode of transmission can cause the exclusive or near-exclusive
appearance in males of color blindness, hemophilia, fragile-X syndrome, and sex-linked muscular
3. b. Students know the genetic basis for Mendelâ€™s laws of segregation and independent
Mendel deduced that for each characteristic, an organism inherits two genes, one from each parent.
When the two alleles differ, the dominant allele is expressed, and the recessive allele remains hidden. Two
genes or alleles separate (segregate) during gamete production in meiosis, resulting in the sorting of alleles
into separate gametes (the law of segregation). Students can be shown how to diagram Mendelâ€™s
explanation for how a trait present in the parental generation can appear to vanish in the first filial (F1)
generation of a monohybrid cross and then reappear in the following second filial (F2) generation.
Students should be told that alternate versions of a gene at a single locus are called alleles. Students
should understand Mendelâ€™s deduction that for each character, an organism inherits two genes, one
from each parent. From this point students should realize that if the two alleles differ, the dominant allele,
if there is one, is expressed, and the recessive allele remains hidden. Students should recall that the two
genes, or alleles, separate (segregate) during gamete production in meiosis and that this sorting of alleles
into separate gametes is the basis for the law of segregation. This law applies most accurately when genes
reside on separate chromosomes that segregate out at random, and it often does not apply or is a poor
predictor for combinations and frequencies of genes that reside on the same chromosome. Students can
study various resources that describe Mendelâ€™s logic and build models to illustrate the laws of
segregation and independent assortment.
3. c.* Students know how to predict the probable mode of inheritance from a pedigree diagram
Students should be taught how to use a pedigree diagram showing phenotypes to predict the mode of
3. d.* Students know how to use data on frequency of recombination at meiosis to estimate
genetic distances between loci and to interpret genetic maps of chromosomes.
Students should be able to interpret genetic maps of chromosomes and manipulate genetic data by using
standard techniques to relate recombination at meiosis to estimate genetic distances between loci.