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Summary Genetics and Genetic Engineering
Summary of Chapter 3.3 (Genetics and Genetic Engineering) from Mind Action Series Life Sciences Grade 12. IEB Syllabus.
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GENETICS AND GENETIC ENGINEERINGWHAT IS GENETICS? HOW ACTIVE ARE GENES?
- Branch of biology that studies heredity and - Although each cell contains a full compliment
variation in organisms. Tries to explain both of DNA, only genes that are needed are
similarities and differences between parents activated and others are supressed.
and their offspring. (Parents produce offspring - Different genes are activated in different cells,
that look like them in some ways but differ in creating the specific proteins that give a
others). particular cell type its character (brain cell/skin
- Heredity refers to passing on traits/ cell).
characteristics from one gen to next. - Some genes play a role in early development of
the embryo and are inactive thereafter.
FATHER OF GENETICS - ‘Housekeeping genes’ are active in many types
of cells, making proteins needed for basic
- Gregor Mendel, Australian monk.
functions.
- Responsible for major breakthrough in study of
- Only 2% of DNA codes for proteins.
heredity by investigating transfer of
characteristics from one gen to next.
- Realised that something is passed on from
NOTE
parent to offspring and that sexual reproduction - Hox genes are master control genes that
combines these somethings from each parent determine that way in which the body develops
to produce offspring which are unique, yet the from a single zygote.
same. - Epigenetics is the control of genes. The Human
- Published work in 1866, not recognised in Epigenome Project aims to document what
scientific studies till 1900s. switches genes on or off.
WHAT ARE THESE ‘SOMETHINGS‘? ALLELES
- Genes, although Mendel called them ‘factors’. - Genes responsible for controlling different
- Form part of DNA molecules in chromosomes. versions of a trait/characteristic found in the
same locus (position) on homologous
Amount of genetic material does not necessarily reflect chromosomes.
the level of advancement of an organism. - Defined as one of two or more forms of a gene
(allelomorph – alternative form).
GENES - Capital letter ➔ dominant allele.
- Each chromatid is made up of one, helical DNA - Baby letter ➔ recessive allele.
molecule. Each DNA molecule is made up of a - Alleles are passed from parent to offspring by
series of genes. way of chromosomes in the gametes that are
- Gene can be defined as a section of DNA (series made by the process of meiosis in the sex
of nucleotides/bases) that controls a hereditary organs.
characteristic (trait).
- It is the basic unit of heredity in living organisms. SUMMARY
- Each chromosome has between several - Somatic cells are diploid as they have:
hundred and several thousand genes. Total o A pair of homologous chromosomes.
number of genes in a human thought to be o 2 alleles which may be the same or
between 20 000 and 25 000. different.
- Nearly all somatic cells have an exact copy of - Gametes are haploid as they have:
all the genes in that organism. o 1 of a pair of homologous
- Because there are two of each kind of chromosomes (a chromatid).
chromosome (maternal and paternal), each o 1 of each pair of alleles.
cell contains 2 of each kind of gene (before
replication). These versions of a gene are known
as alleles.
GENOTYPES AND PHENOTYPES
- The gene pool is the set of all genes/genetic - Genotype: made up of all the genes an
information in a population of sexually organism carries on its chromosomes which it
reproducing organisms. has inherited from its parents (describes the
- Large gene pool ➔ high genetic diversity and alleles present for a characteristic).
increased chances of survival. - Phenotype: physical appearance of an
- Small gene pool ➔ low genetic diversity and organism (as prescribed by the genotype, but
increased possibility of extinction. also shaped by external factors such as exercise,
diet, and environment).
, HOMOZYGOUS AND HETEROZYGOUS
After fertilisation, the zygote will have 2 alleles for each
gene, 1 from each parent. These will be in the same locus
on each chromosome of a homologous pair.
- Pair of alleles at a locus are the same ➔
homozygous for that trait.
- Pair of alleles at a locus are different ➔
heterozygous for that trait.
DOMINANT AND RECESSIVE ALLELES
In heterozygous pairing, 1 allele of a pair may be:
- Dominant ➔ this trait is expressed in the
offspring.
- Recessive ➔ the trait is suppressed in the
presence of the dominant allele and not
expressed in the offspring.
NOTE
- For each allele, there are 3 genotypes and 2
phenotypes:
o If 1 allele is dominant over another, a
homozygous organism (BB) has the
same phenotype as a heterozygous
organism (Bb).
o The allele b is recessive as its effect is
hidden by the dominant B gene and its
characteristic will only be expressed in
the phenotype when there is no
dominant allele present.
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