Genetics & Heredity

Genetics is the study of heredity, the passing of traits from biological parents to their offspring.

Offspring inherit: physical traits e.g. hair and eye colour, nose size and ear shape

Offspring inherit: biochemical traits e.g. blood type, bone density and skin sensitivity

Offspring inherit: physiologic traits e.g. athletic ability, sensitivity to heat and cold and vulnerability to certain diseases

Genetic information is carried in genes, which are found on the chromosomes in the DNA. Parents transmit inherited traits in their gametes: eggs and sperm. The random combination of a particular egg and the sperm that fertilizes it leads to the genetically unique mix of traits.

Each human cell that develops from that zygote has 46 chromosomes in its nucleus, 23 from the egg and 23 from the sperm. Of the 23 pairs, the first 22 pairs are called autosomes; the homologous pairs are numbered 1 to 22.

The last pair of chromosomes is the sex chromosomes, and these determine the gender of the offspring, either male or female. Males are XY, therefore can produce X and Y sperm. Females are XX, therefore can produce only X eggs. The gender of the offspring is determined by whether the sperm that fertilizes the egg is an X sperm or a Y sperm.

The location of a particular gene on a chromosome is called the gene locus. Different forms of a gene are called alleles. Alleles are found at the same position or locus on homologous chromosomes.

All terms will be explained using the example below. Genotype is the genetic makeup of an individual for a particular trait, TT, Tt or tt Phenotype is the expression of the genotype, or how the physical appearance of an individual trait tall or not tall T = tall and t = not tall

Homozygous – created when two gametes with identical alleles unite TT or tt Heterozygous – created when two gametes with different alleles unite Tt Recall: ‘homo’ = same ‘hetero’ = different

Tt = tall, therefore T is dominant Dominant – the allele that is expressed when present in the heterozygous form Tt = tall, therefore T is dominant Recessive – the allele that is masked when present in the heterozygous form. In order to be expressed, it must be present in the homozygous form tt = short. (not tall)

Human Karyotype Chart

Your turn… Complete “Genetics Terms” matching worksheet