In miniunit Gamma you studied how eukaryotic nuclei divide mitotically and meiotically. You learned that through these processes cells are able to pass copies of their genetic information to their descendants. During mitosis cells pass exact copies of the number of chromosomes and genetic material to their daughter cells. During meiosis, cells containing homologous chromosome pairs (diploid) are randomly segregated into cells which contain only one of each homologous pair (haploid). These haploid gametes contain one half of the genetic material as their somatic cells. During fertilization two of these haploid cells fuse forming a diploid cell containing homologous chromosome pairs with an arrangement of chromosomes different from the parent cells (combination of both parents). You also learned that DNA molecules of the nucleus carry hereditary information of all living organism; the sequences of nucleotides in specific stretches of DNA, the genes, encode the information needed to specify the amino acid sequences of proteins. During this miniunit you will examine the patterns of inheritance observed in diploid organisms and how genes (stretches of DNA) regulate this process.
Examine behavioral objective 1. Read the pages in the text indicated by D-1. In your notes write a short explanation between the relationship of genes, DNA and chromosomes. This relationship was presented in miniunit Gamma.
1. Which of the following statements are true?:
a) Several genes are located on a single chromosome.
b) A single gene consist of one DNA molecule
c) A single chromosome contains several stretches
of DNA which make up
genes
d) Genes randomly segregate out into different cells
during meiosis.
e) Chromosomes are made of a DNA molecule and specific
stretches of this
DNA molecule are the genes.
f) Genes are small pieces of DNA molecules suspended
in the cytoplasm.
g) Patterns of inheritance are controlled by genes
located on a DNA molecule.
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Examine behavioral
objective 2 and recall the number of chromosome in the normal human
somatic cells, gametes and zygotes. The figure below was presented in miniunit
Gamma. In miniunit Gamma the meiotic process was important in maintaining
the continuity for the number of chromosomes between the organisms somatic
cell and the somatic cells of their offspring. In this miniunit meiosis
is important in randomly selecting half of the genes of a diploid
somatic cell and placing them into gametes.
Read the pages in indicated by D-1 to learn the pioneering genetic experiments Gregor Mendel performed with which he used to formulate the concept we call Mendelian Genetics.
Much of this miniunit relies on the fact that you are able
to work simple genetic problems by determining the different genotypic
types of gametes (sperm or egg) formed during meiosis and listing
all combinations of the new zygotes produced during
the process of fertilization. This may be learned by using a Punnett
square method. Before learning this method you must be able to identify
the following terms. Fill out the table below listing these terms. These
terms may be found in the readings indicated by D-1
and the glossary D-2.
Examine behavioral
objectives 4 and 5.
| TERM | DESCRIPTION | EXAMPLE |
| gene | ||
| locus | ||
| allele | ||
| homozygous | ||
| heterozygous | ||
| dominant allele | ||
| recessive allele | ||
| genotype | ||
| phenotype | ||
| monohybrid cross | ||
| dihybrid cross | ||
| parents (P1, P2) | ||
| offspring (F1, F2) | ||
| homologous chromosomes |
2. Genes are units of heredity encoding the information needed to specify the amino acid sequence of proteins and hence particular traits and alleles are one of several alternative forms of a gene.
a) true
b) false
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3. In a normal diploid somatic cell, how many alleles are present
for a trait controlled by a single gene?.
a) one
c) four
b) two
d) ten
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4. The physical position of a chromosome or DNA molecule is called
_______
a) allele
c) locus
b) gene
d) phenotype
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Rules to follow when working genetic problems:
Examine behavioral objective 3.
A. Use letters to represent genes (one letter for each allele
and the same letter for
alleles of the same gene)
Example: gene for red pigment in roses =
R ; gene for stem length in roses= L;
Upper case letters represent the dominant allele (R and L in the
example above, where red is dominant to white and long is
dominant to short)
Lower case letters represent the recessive allele (r and l in the
example above)
B. Since chromosomes in diploid cells are in pairs (homologous), genes are also in pairs; each letter represents a single allele.
Example: genes which may
be present in the somatic cells of the rose above
are RRLL, rrll, RrLl, etc.
(RRLL) represents two genes "R and L" and a total
of four alleles (R,R,L,L)
5. How many alleles and genes are represented by the following set-up : AaBBccDd
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6. Study behavioral objectives 4 and 6. The following letters represent pairs of genes. Indicate which of the following pairs of genes are dominant, recessive, heterozygous, and homozygous. You may use abbreviations but in each case include all possibilities. As an example , the first one is completed for you.
a) DD - Dominant/Homozygous
d) ss -
b) mm -
e) Yy -
c) Rr -
f) BB -
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7. Use letters to show the following gene conditions on a chromosome (give all possibilities).
a) dominant - c) homozygous -
b) recessive -
d) heterozygous -
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8. Draw two chromosomes and insert any four letters within each chromosome to show the heterozygous condition.
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9. Draw two chromosomes and insert any four letters with each chromosome to show the homozygous recessive condition.
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10. What percent of genes does each parent contribute to their offspring?
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11. What type of genes determine the phenotypic expressions of an individual?
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12. A rose plant (RR) produces red flowers.
What is the phenotype of this plant?
genotype?
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13. A rose plant had long (Ll) stems and white (rr) flowers.
What is the phenotype of this plant? genotype?
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This is the end of lesson one. Click here to go back to
the home page and lesson two Monohybrid Problems. click
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Created by the Center for Learning Technologies, Academic Technology Services.
Last modified October 22, 1997.