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Patterns
of inheritance
Joanna
George-Johnson, Contributor
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| Students
at Spanish Town High School listen to the message of the entertainers on The STAR's
No Violence School tour, which stopped at the school, recently. |
One of the
most difficult topics in biology is genetics. For some reasons, students seem
to have difficulty grasping its concepts. Therefore, I will go through this topic
in as much detail as possible. Some
personality and physical traits may be passed from parents to their children and
grandchildren. The process is referred to as heredity. It comes from the word
heir. In the early 19th century, it was discovered after much debate that the
structures responsible for our pattern of inheritance were deoxyribonucleic acid
(DNA). In this acid, there are genes. The person who started this research was
Gregor Mendel, sometimes referred to as the 'father of modern genetics'. Terminology
The
following are terms that you will encounter during your study of genes. It is
important that you know the definition of these words and understand them. (See
table below). | Terms | Definition | Example
| | Chromosomes | The
self-replicating genetic structure in cells
containing the cellular DNA. | Chromosome
# 5 | | DNA | Deoxyribonucleic
acid; A double-stranded molecule
that encodes genetic information. | | | Gene | The
fundamental physical and functional unit of heredity. | Tongue
movement | | Allele | Alternative
form of a gene. | Tongue
roller (R) / Non-tongue roller (r) | | Dominant | An
allele that is almost always expressed, even
if only one copy is present. | Tongue
roller (R) | | Recessive
| A
gene which will be expressed only if there are
two identical copies | Tongue
Roller (r) | | Homozygous | The
presence of two identical alleles of a gene. They
may be dominant or recessive. | rr
or RR | | Heterozygous | The
presence of two different alleles for a particular
trait in the cells of the organism |
Rr | | Phenotype | The
physical characteristics of an organism | Person
can roll their tongue or person cannot roll their tongue | | Genotype | The
genetic constitution of of an organism, the combination of alleles for a particular
trait | Rr
or rr or RR |
Points
to highlight - Offspring
inherit two sets of instructions for each characteristic.
- One
set is from the mother.
- The
other set is from the father.
- The
sets of instructions are known as genes.
- Different
version of the same genes are known as alleles.
- If
the dominant allele and the recessive allele are inherited for the same characteristic,
ONLY the dominant allele will be expressed (shown/ displayed/seen).
- Genes
are located on chromosomes.
- Humans
have 46, or 23 pairs, of chromosomes in each cell except for the gametes (sperm/egg)
and red blood cells.
- Gametes
have half the usual number, that is, 23.
- A
red blood cell does not have a nucleus, and therefore does not possess chromosomes.
Exercise:
Underline
the correct word in the parentheses. 1.
Sperm and eggs are known as (sex chromosomes/sex cells). 2.
The expression of a trait is known as (phenotype/ genotype). 3.
The expression of a trait is determined by (phenotype/genotype). 4.
Different versions of the same genes are called (alleles/recessive). 5.
John can roll his tongue. A blood test showed that John has the following alleles
Rr. a)
Rr is John's (genotype/ chromosomes). b)
John is said to be (homozygous/heterozygous) for the trait. Joanna
George-Johnson teaches at Ardenne High School.
Email: Masterbio@gmail.com.
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