LIFE SCIENCES SUMMARY NOTES
GENETICS & INHERITENCE
Introduction
Genetics is the scientific study of the principles of heredity and variation. A distinctive
genetic code is found in the DNA of each organism and is passed on to the offspring during
reproduction. Since there are two parents involved for sexual reproduction, genetic variation
will occur to ensure survival of the fittest.
Genetic Terminology
Genetics is a science and specific terms are used. The following terms are important in
understanding the terminology used:
Locus: it is defined as the precise position or location of a gene on a chromosome.
Alleles: A pair of genes that are situated at the identical position on each of the two
homologous chromosomes and signify a precise trait – one from the father and one
from the mother.
Homozygous: Homo = same and zygous = zygote. When two alleles of a pair of
genes are identical for one trait e.g.: both alleles are for red flowers, the cross will
result in a pure breed for red.
Heterozygous: Hetero = different and zygous = zygote. When two alleles of a pair of
genes are dissimilar for a trait, e.g. one of the alleles is for red flowers and one is for
white flowers, the cross will result in a mixture of the two genes called a hybrid.
Dominant trait: A heterozygous offspring will present the dominant trait for the
reason that it will dominate over the other recessive gene of the allele pair, e.g. red
colour will dominate over the gene for white colour, so the offspring will look red.
, The Role of Gametes in Inheritance
DNA molecules on the chromosomes consist of sections called genes
each gene holds the hereditary traits, e.g. skin and hair colour, height, body structure
and blood group are embodied by the genes on each of the two homologous
chromosomes
in the process of Meiosis, haploid gametes are released and the gametes will consist
of one set of genes
during fertilisation, the gametes fuse and a diploid zygote results
one set of genes will come from the female parent and one set from the male parent
the two sets of genes may be the same or different for a trait, e.g. the mother may
have black hair and the father may have blonde hair
the offspring inherits two sets of genes, 50% from the mother and 50% from the father
and will, therefore, be different from each of the parents
the diploid zygote, therefore, contains a double set of DNA and is a combination of
both parents
Connection between Meiosis and Genetics
During the crossing over in prophase I of meiosis and the metaphase I, chromosomes share
information and during metaphase, separate randomly. This determines the combination of
chromosomes and genes that you have as an individual. Genetics determines individual
variation (to be different) and survival of the fittest.
The dominant allele is always written with a capital letter: R = red and r = white
Homologous dominant alleles means that both genes are the same for the same
dominant trait. It will be represented by RR, which represents both the genes for red
flowers. The offspring will be red because two dominant genes are present.
Heterozygous alleles means that one gene is dominant and one gene is recessive for
the same trait, e.g. red flowers. It will be represented by Rr, which represents one
gene for red and one gene for white. The offspring will display red flowers, because
red is dominant over white.
Recessive trait: The recessive allele is the trait that is dominated over by the dominant
gene. It is written with a small letter: r = white.
GENETICS & INHERITENCE
Introduction
Genetics is the scientific study of the principles of heredity and variation. A distinctive
genetic code is found in the DNA of each organism and is passed on to the offspring during
reproduction. Since there are two parents involved for sexual reproduction, genetic variation
will occur to ensure survival of the fittest.
Genetic Terminology
Genetics is a science and specific terms are used. The following terms are important in
understanding the terminology used:
Locus: it is defined as the precise position or location of a gene on a chromosome.
Alleles: A pair of genes that are situated at the identical position on each of the two
homologous chromosomes and signify a precise trait – one from the father and one
from the mother.
Homozygous: Homo = same and zygous = zygote. When two alleles of a pair of
genes are identical for one trait e.g.: both alleles are for red flowers, the cross will
result in a pure breed for red.
Heterozygous: Hetero = different and zygous = zygote. When two alleles of a pair of
genes are dissimilar for a trait, e.g. one of the alleles is for red flowers and one is for
white flowers, the cross will result in a mixture of the two genes called a hybrid.
Dominant trait: A heterozygous offspring will present the dominant trait for the
reason that it will dominate over the other recessive gene of the allele pair, e.g. red
colour will dominate over the gene for white colour, so the offspring will look red.
, The Role of Gametes in Inheritance
DNA molecules on the chromosomes consist of sections called genes
each gene holds the hereditary traits, e.g. skin and hair colour, height, body structure
and blood group are embodied by the genes on each of the two homologous
chromosomes
in the process of Meiosis, haploid gametes are released and the gametes will consist
of one set of genes
during fertilisation, the gametes fuse and a diploid zygote results
one set of genes will come from the female parent and one set from the male parent
the two sets of genes may be the same or different for a trait, e.g. the mother may
have black hair and the father may have blonde hair
the offspring inherits two sets of genes, 50% from the mother and 50% from the father
and will, therefore, be different from each of the parents
the diploid zygote, therefore, contains a double set of DNA and is a combination of
both parents
Connection between Meiosis and Genetics
During the crossing over in prophase I of meiosis and the metaphase I, chromosomes share
information and during metaphase, separate randomly. This determines the combination of
chromosomes and genes that you have as an individual. Genetics determines individual
variation (to be different) and survival of the fittest.
The dominant allele is always written with a capital letter: R = red and r = white
Homologous dominant alleles means that both genes are the same for the same
dominant trait. It will be represented by RR, which represents both the genes for red
flowers. The offspring will be red because two dominant genes are present.
Heterozygous alleles means that one gene is dominant and one gene is recessive for
the same trait, e.g. red flowers. It will be represented by Rr, which represents one
gene for red and one gene for white. The offspring will display red flowers, because
red is dominant over white.
Recessive trait: The recessive allele is the trait that is dominated over by the dominant
gene. It is written with a small letter: r = white.
