Margaret M. Loos
I.
Objectives
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1. To relate the molecular biology to genetics from Mendel and text.
II
Information
:
At this time we consider a
gene
to be a stretch along a chromosome that codes for a functional product. (Either RNA or its translation product, a polypeptide). We know that each parent in humans (and sexual reproduction, of course), donates one of each pair of chromosomes to the genetic makeup of the zygote. We know that the chromosomes that make up the zygote’s complement of genes differ from that of its parents because:
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a. two parents contribute.
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b. crossing over has occurred in the diplotene stage of prophase I in meiosis.
One parent has a pair of alleles AA. This parent can donate either of these to the Zygote. This parent is said to have a genotype of AA and a phenotype (what shows up) of black hair. The other parent (Mom) has a genotype of aa and a phenotype of some other color hair. Both of these parents are homozygous genotypes.
Homozygous
means having identical members of an allelic gene pair coding for the same protein, therefore, for the same trait, or contributing to the same trait.
(figure available in print form)
In every case the dominant gene will be expressed and all four will exhibit the black-hair phenotype. They are all
Heterozygous
. Heterozygous means having an allelic pair that is not identical and which can possibly code for different proteins and therefore traits.
III.
Tasks
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A
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1. Make crosses (like Punnett square above) between two of those heterozygous genotypes. Indicate the resulting genotypes and phenotypes. Does it agree with Mendel’s Law?
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2. Make a cross between a heterozygous genotype and a homozygous recessive genotype. Indicate the outcome in genotypes and phenotypes.
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B If each gene pair contributes one allele and there are 23 chromosomes the possible variation at one site could be 2.
23
. Use a calculator and figure the possibilities. Now, when combined with the contribution of the other parent, what are the possibilities? What would this mean in terms of all the sequences in DNA that constitute genes?
IV.
More information
:
When the coding is from any of the chromosomes other than the sex chromosome, it is called autosomal, and codes in terms of other body functions. Some normal monogenic autosomatic characteristics you may do classes on are:—curly hair, straight hair, long eyelashes, short eyelashes, unattached earlobes, attached earlobes. Some of the abnormal autosomal dominant diseases are Polycystic kidneys, some kinds of dwarfism, (lack of growth hormone) and Neurofibromatosis (elephant man disease). These abnormal congenital defects are caused by mutation of a single autosomal gene.
Since Mendel’s time, we have learned that linkage exists between certain inherited traits because of the proximity of genes on the chromosome and the odds of those genes crossing over together. We also can understand in terms of the coding for proteins accomplished by genes that certain traits are a result of several genes functions.
Example:—Skin color is an excellent characteristic to show how genes still segregate separately and yet determine a strain in combination.
V. Task 1. Set up a square on a full sheet of paper
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ABAB = black
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abab = white
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4
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(ABAB) = black (dominants)
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3
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= dark
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2
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= medium
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1
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= light
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0
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= white
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(figure available in print form)
Task 2. Do a chart of blood type possibilities for offspring of parents AAX AA, AOXAA, AAXBB, etc.
* Before doing this task, read material on codominance.
(figure available in print form)
