Download File

Biology
Continuity Notes
Genes
Each species of living organism has a specific number of chromosomes in its cell, and it’s own variety of genes. In asexual
reproduction, the cells of the new organism are produced by mitosis from the parent cell. In sexual reproduction,
special productive cells called gametes are produce by meiosis. Meiosis produces new cells with half the number of
chromosomes of the parent cell, and with different combination of genes.
Genes are carried on the chromosomes which are in the nucleus of every cell. Chromosomes are long threads made of
DNA and protein. Genes work by providing information about making proteins. Every chemical reaction inside a living
organism is catalyzed by enzymes. Enzymes are protein. So, by providing information for making enzymes, genes affect
all the chemical reactions in an organism’s body.
The two chromosomes of a pair are called homologous chromosomes. One came from the person’s mother, and the
other from the father. Each chromosome of a homologous pair carries genes for the same characteristic in the same
place.
Each cell contains 2 genes giving instructions about which one to express. Two forms of a gene, defining a characteristic
in a different ways, are called alleles. For example, you have genes that enable some of us to tongue roll and other not
to.
Chicken- 78 chromosomes
Chimpanzee- 48 chromosomes
Donkey- 62 chromosomes
Humans- 46 chromosomes
Elephant- 56 chromosomes
Cat- 38 chromosomes
Tongue Rolling
There are three possible combinations of alleles for tongue rolling. You might have two T alleles- TT, two t alleles- tt, or
possibly have on of each-Tt. The two alleles for tongue rolling in your cells that are exactly the same, TT and tt are called
homozygous and the different alleles, Tt are called heterozygous.
Genotype/Phenotype
The genes that you have are your genotype. For tongue rolling, there are three possible genotypes- TT, Tt and tt. The
genotype determines whether or not you can roll your tongue. The effect that the genotype has is called your
phenotype. Your phenotype for tongue rolling is either being able to roll your tongue, or not being able to do it.
Alleles can be DOMINANT or recessive
So, there are three kinds of genotype for tongue rolling, but only two kinds of phenotype. How does that happen?
It happens because the tongue-rolling allele, T, is dominant over the non tongue-rolling allele, t. If you are heterozygous
for tongue rolling, Tt, then it is only the T allele which is being expressed and actually have an effect on the phenotype.
The effect of the t allele is hidden by the T allele and hence the t allele is said to be a recessive allele.
Genotype
Phenotype
TT (Homozygous)
Tongue roller
Tt (Heterozygous)
Tongue roller
tt (Homozygous)
Non-tongue roller
Alleles showing Codominant or Incomplete Dominance
Sometimes, neither of a pair of alleles is completely dominant or completely recessive. Instead of one of them
completely hiding the effect of the other in a heterozygous, they both are expressed and have an effect on the
phenotype. This is called codominance (sometimes you may see is also called incomplete dominance).
Sex determined by the X or Y chromosomes
The last pair of chromosomes in our genome is responsible for determining what sex a person will be. This pair of
chromosomes is called our sex chromosomes. A woman’s chromosomes are XX and a man’s chromosomes are XY.