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Summary of genetics with a few practice questions
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Genetics & Heredity1
, INTRODUCTION
Genetics is the science of inheritance and studies the principles of heredity and
variation. The hereditary instruction carried within the DNA ensures that offspring
resemble their parents and ensures that genetic variation can take place, resulting
in survival of the fittest.
Genetics and sexual reproduction:
During sexual reproduction, offspring are produced that resemble the parents.
Remember that two haploid gametes are the result of the process of meiosis. The
gametes fuse during reproduction and the result is a diploid zygote, containing a
double set of chromosomes. One set of the chromosomes came from the male
gamete, which contains the DNA from the father. One set of chromosomes came
from the female gamete and contains the DNA from the mother. The child therefore
contains DNA from both parents.
DEFINITIONS AND IMPORTANT TERMS AND CONCEPTS:
Chromatin network: visible as thread-like structures in the nucleus of an inactive
cell
Chromosome: a structure made up of two chromatids joined by a centromere
that carries the hereditary characteristics within the DNA
Gene: a unit of sequenced pieces of DNA that carry the genetic information that
will determine the hereditary characteristics of an organism.
Allele: one of two contrasting forms of a gene that determine alternative
characteristics in inheritance because they are situated in the same position or
locus on homologous chromosomes
Locus: The exact position or location of a gene on a chromosome.
Genotype: This is the total genetic composition of an organism. It is the
information present in the gene alleles, for example BB, Bb or bb.
Phenotype: This is the external, physical appearance of an organism. The
phenotype is determined by the genotype. (phenotype, when both recessive gene
alleles are present e.g.: bb)
Dominant allele: an allele that masks or suppresses the expression of the allele
partner on the chromosome pair and the dominant characteristic is seen in the
homozygous (e.g.: TT) and heterozygous state (e.g.: Tt) in the phenotype.
Recessive allele: an allele that is suppressed when the allele partner is
dominant. The recessive trait will only be expressed/seen if both alleles for the
trait are homozygous recessive e.g.: tt
Homozygous: when two alleles that control a single trait (on the same locus) are
identical.
2
, Heterozygous: when two alleles that control a single trait (on the same locus)
are different.
Gene mutation: a change of one or more N- bases in the nuclear DNA of an
organism.
Chromosomal aberrations: refers to changes in the normal structure or number
of chromosomes.
Genetic variation: this includes a variety of different genes that may differ from
maternal and paternal genes resulting in new genotypes and phenotypes.
Multiple alleles: when there are more than two possible alleles for one gene
locus. e.g. blood groups
Complete dominance: A genetic cross where the dominant allele masks
(blocks) the expression of a recessive allele in the heterozygous condition.
Incomplete/partial dominance: A genetic cross between two phenotypically
different parents produce offspring different from both parents but with an
intermediate phenotype.
Example:
Incomplete dominance in flowers:
Colour key: R (red) W (white)
P1 Phenotype: red x white
Genotype: RR x WW
Meiosis
Gametes R ,R, x W, W
Fertilisation
F1 Genotype: 4:4 RW
Phenotype: 100% pink
Example :
Incomplete dominance in humans:
Curly hair (CC) x Straight hair (SS) = Wavy hair (CS)
Co-dominance: A genetic cross in which both alleles are expressed equally in
the phenotype.
E.g. Parent with blood group A mates with parent with blood group B and can
produce an offspring with blood group AB.
3
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