• Genotype: The two genes an organism possesses for a given trait.
• Phenotype: The actual appearance of a certain trait, i.e. what the feature looks like.
• Homozygous: The genes for a particular trait are the same.
• Heterozygous: The genes for a particular trait are different.
• Trait: A particular feature of an organism which is determined by a particular gene.
• Gene: A small area on a chromosome responsible for a particular trait.
• Allele: Another or alternative form of a gene.
• Gametes: The sex cells of an organism, i.e. the egg and the sperm.
• Zygote: The initial fertilised cell. Directly formed after the sperm meets the egg.
• Fertilisation:The process of the sperm fusing with the egg to form the zygote.
Describing Genetic Features
Genetics has its own language to describe the processes that occur. When fertilisation occurs an egg fuses with a sperm to form a zygote. The zygote develops into the new individual. The individuals traits are determined 1 by the particular genes the individual possesses. These genes come in pairs called alleles. Usually one allele is dominant over the other resulting in a particular trait showing through as a particular feature.
The genotype describes the genes present whereas the phenotype describes the appearance of the trait. If the Alleles are the same they are said to be homozygous. If the alleles are different they are said to be heterozygous.
Dominant and Recessive Genes.
• Dominant: When a gene is stronger than another allele. It will mean that the trait is evident.
• Recessive: This is the weaker gene. It will only appear if the dominant gene is not present.
A dominant gene will always be evident if it is present. A recessive gene will only be evident if the dominant
gene is not present. This means for a recessive gene to be evident and because genes come in pairs there must
be two recessive genes for the feature to be evident.
Dominant genes are expressed as capital letters whereas the recessive genes are expressed as lower case letters.
Example: Tongue Rollers
The ability to tongue roll is dominant. The gene for non-rolling is recessive.
T is the symbol for the tongue roller gene.
t is the symbol for the non-tongue roller gene.
Possible combinations that are the genotype with their phenotype are:
TT Tongue roller
Tt or tT Tongue roller
tt Non Tongue roller
Genes occur in pairs called alleles. The following genetic cross is between a brown eyed male and a blue eyed female. The purpose of this example is to show that brown eyes is dominant over blue eyes.
B means brown eyes
b means blue eyes
Bb Gene Pairs bb
B and b Alleles b and b
This means the 4 offspring are:
• 2 homozygous blue eyes, bb
• and 2 heterozygous brown eyes, Bb.
Male and Female
The human body has 46 chromosomes in 23 pairs. In humans there are 22 pairs plus one pair of sex
chromosomes. In the human female there is one pair of X chromosomes, XX. In the human male there is one
pair of X and Y chromosomes, XY.
Female: 22 pairs plus the XX chromosomes
Male: 22 pairs plus the XY chromosomes
All the sex cells in a female (eggs) carry the X chromosome. Half the sperms in a male carry the X chromosome
while the other half carries the Y chromosome. This means there is a 50:50 chance of either a boy or a girl. The
process of fertilisation can be illustrated in the following diagram:
22 pairs of chromosomes
plus two XY chromosomes plus
22 pairs of chromosomes
plus two XX chromosomes